Gavins 1st gen MX6 DSP autocross build

[gavin]

Well, I guess I have some explaining to do.

Clearly the autocross car is in some state of completion (finally got it going this past march). Id been starting and stopping with a build writeup to chart my progress, but the car is now running..no more "buildup" for the time being, so Ive been pretty uninspired to get that finished. Some people have spoken to me and mentioned that it would be nice to see what it took to get the car to this point so Im giving it a go once more.

So here it is....what it took to get from there....



to here....




AKA: Gavins SCCA DSP autocross build.

Youll have to bear with me while I attempt to do this writeup as itll be a bit unorthodox. Ive been building the car for the last couple years with a huge push in the last year to get things done. Except for a few folks who knew what I was doing with the car, Ive been keeping things under my hat and keeping the nose to the grindstone. Because of that Ive done a fair amount of things to the car without documentation and to do one big writeup is a bit daunting(maybe one of the reasons I just didnt have the energy to do this before).

The build process was very "organic" as one component choice affected the effectiveness/choice of other components. But I dont think I would be able to make the whole thing make much sense if did a "point a>z" explanation and it would be a mess to try to connect all the dots. With that in mind, Ill be doing the writeup in installments, going from component to component eating the elephant in small bits (so to speak). I think this will focus things a bit better than me going for the whole thing and hoping that it will all be cohesive.

So what am I doing?

PURPOSE:

to build a racecar!! (well duh)

Ill be building what would be considered a “built to the rules, nationally competitive” 1st gen MX6 to be campaigned in SCCAs SOLO. The car will be prepped for competition using the ruleset in “Street Prepared”. The car itself is grouped with other cars with similar performance attributes in D-street prepared. The SOLO2 rulebook will be my guide in the cars .

http://scca.org/documents/Solo%20Rul...lo_Rules_2.pdf this is 2008.

A couple things that should be understood about the reading of the SCCA SOLO rulebook. The rules are allowance based (telling what you can be done) and not restriction based (telling you what cannot be done). It’s a very important distinction in the reading of the rules. Simply put, if it doesn’t say you can, then you cannot.

As with many preparation levels there are limits to what can be done. Street Prepared is a lower prep category (compared to Prepared or modified). The limits can seem restrictive, but for me, I actually like the guidelines. The rules force you into a “box” where you have to be a good reader and then be clever enough to get the full worth of the rule all while being rules compliant.

Another thing about the classing structure for the cars…SOLO2 gives every car a place to play. It does NOT guarantee competitiveness. Got a Renault Fuego ?...theres a place for you to run, but don’t expect a national trophy anytime soon. In any given class theres @2-3 top contenders (1st tier cars) then a small midsection of cars that are close to but not as fast as the 1st tier cars for whatever reason(tires, gearing, whatever), and theres “the rest”. A mass of cars that are either too unknown/unpopular/slow/etc. Trophy fodder in other words.

When building a car for a class, theres 3 types of builders.

1:Theres the ones that look at the existing top leading cars and build one of those. No reason to reinvent the wheel, go with a proven performer.
2: You have people who want to try something different and look for something they believe will “shake up” the status quo (and possibly change perceptions about what the car for the class is). Theres a risk to this as what looks good on paper sometimes doesn’t translate well in actuality. You have more of a chance for a dud than a winner.
3: Finally theres people who build the car they have/like. Generally, not a good idea if you are looking for a “national competitive” car.

Im somewhere between the second and third categories.

THE PAST:

You are going to have to do a lot of reading here(sorry), because Im not going to try to rewrite all of this (Gavin = lazy), but heres the links to what has been done to the car in the past.

1st effort stuff:
http://www.mx6.com/forums/1g-mx6-oth...sion-mods.html (Gavins all out suspension mods) (original suspension buildup, illuminas, cusco camber plates, ground control springs, urethane bushings)
http://www.mx6.com/forums/1g-mx6-oth...elocation.html (Battery relocation) (battery relocation)
http://www.mx6.com/forums/1g-mx6-oth...-endlinks.html (Heim joint swaybar endlinks) (solid swaybar endlinks)
http://www.mx6.com/forums/1g-mx6-oth...t-savings.html (Sunroof Delete and other weight savings) (sunroof removal, seat replacement, weight reduction efforts)
http://www.mx6.com/forums/1g-mx6-oth...-rx7-rims.html (Fun with RX7 rims) (3rd gen RX7 wheels and fitment)
http://www.mx6.com/forums/1g-faq-eng...tallation.html (MSD 6A installation) (MSD igniton install)
http://www.mx6.com/forums/1g-faq-eng...atch-cans.html (Dual Oil Catch Cans) (dual catch cans)
http://www.mx6.com/forums/1g-mx6-oth...-swaybars.html (Fun with swaybars) (swaybar calculations)

2nd effort stuff:
http://www.mx6.com/forums/1g-mx6-oth...treetable.html (Gavins all out suspension mods Version 2.0 (AKA: how to make a car unstreetable)) ( koni suspension, mazdaspeed camber plates, delrin suspension bushings, suspension geometry)
http://www.mx6.com/forums/1g-mx6-oth...lternator.html (Lightweight alternator) (lightweight alternator)
http://www.mx6.com/forums/1g-mx6-oth...heel-post.html (The CCW competition wheel post) (16x9” CCW wheels, fender flares)
http://www.mx6.com/forums/1g-faq-tra...lots-pics.html (Gavins DIY Gurumotorsports Differential install(Huge)(lots of pics)) (GURU torsen diff install)


That’s all of that past. Ill be updating things to get to the present by doing individual chapters and breaking things down a bit. Again, by doing things in smaller chunks, I hope that things wont be too jumbled and confused. Itll also give me time to focus better on those subjects.

Next installation will be about what my competition looks like.

Gavin

[gavin]

THE COMPETITION

Im lucky to live in an area where the local competition in DSP is well represented. I will have well prepped and well driven cars in my class to compare the cars potential/performance with.

The class leader in DSP is the E36 chassis 3-series BMW. BMW makes an interesting car. Although its performance potential isnt as transparent as a Porsche, when prepped for competition they are excellent handling machines that are much more than the sum of their parts.

This is one of those cars.




Gordon Whites Edge Mortorsports "RED"

Its main strength is that er…it works? Well balanced chassis. Responsive to performance upgrades. RWD crunchy goodness dynamics. Ability to run wide/tall tires without adverse penalties to the driving dynamics(285/30/18s F/R are the hot ticket for the car). Wide range of rear end ratios to use. 2.8l I6 is a smooth, powerful and torquey engine that responds well to bolt ons. Massive go-fast knowledgebase from BMW community.

The Bad?…. its not light. Low 2800lb to high 2700lb weights are whats to be expected out of a prepped E36. RWD can be a hinderance on some surfaces (low grip asphalt/rain).

So far for the last 4-5 years this has been the car to beat.

The contenders…

Subaru Impreza 2.5l RS.




The Good: AWD grip*. Boxer engine is pretty torquey. Can(has to?) run 285s.

The Bad: Struts all around(struts suck), short gearing makes 2-3shifts necessary, not light (@2800lbs fully prepped?). Non-turbo engine doesn’t like to rev high.

*(This is important as the lots we use are low grip asphalt and the new surface of the national championships site is a low grip, sandy, asphalt surface. The old nationals surface was high grip concrete which equalized the differences between drivetrain configurations..IE: everyone had grip.)

Locally the De-Los Rios brothers have been showing the way to the front and currently lead the local point standings in DSP with numerous 1st place finishes.

DC3 Acura Integra RS/ITR

(sorry, no picture)

A recent change to the rules has added the Integra Type-R to the previous listing for the integra RS. Basically owners can mix and match parts between both chassis. The decision was made because all of the things that made the ITR a superlative stock car compared to the RS (engine, diff, chassis mods) were all negated by SP rules anyway.

The Good: Light weight (2300lb weights?), Honda engine that’s powerful and a screamer, Honda aftermarket, Honda knowledgebase, real working suspension.

The bad: Low torque, wrong wheel drive.

Locally there isn’t a DSP integra that I can compare to. There was a fellow in the past but the Acura curse struck and his car was stolen. There is another fellow in the northeast that’s currently building an ITR for the class that I am watching carefully (his local competition is the current 2 time national champion who has a BMW).

E30 BMW 3-series…..see “E36 BMW”




Lighter than the E36, but with a “worse” rear suspension.

Alex Shchipkov has been runner up with his car at the nationals for the past 2 years and 3rd 3 years ago. Its not a slow car. Unfortunately theres no one locally with a car to compare to.

The Dark horse


Ben Martinez has a gift for car control and loves german Fords (for some odd reason). He has finished as high as 3rd nationally with his 1974 v6 Capri. With a recent change in the rueleset regarding the relaxing of turbo restructions, he has changed to this car to something he believe will take the fight to the BMWs…a 1986 Merkur XR4Ti. 2.3l Turbocharged I-4, RWD, strut front and semi-trailing arm in the rear. Suspension wise its basically a Ford(of germany) BMW 3 series.

The good: 2.3l lima engine is built like a brick outhouse(solid aftermarket as well). T3 turbo power. RWD. Light weight (2600lbs?) for a RWD car.

The bad: small front wheelarch means difficult fitment of 285s in front. Big T3 is laggy off boost. Not as many rear end ratios as BMWs. Limited front castor on front struts.

Ben is fast, the car is fast and promises to get faster.

Theres other cars for the class, but those are the current top contenders.

EDIT: Ill add another possibility...The Rookie



Mazdaspeed Protege.

This is Jeff Wong who works in southern california for Tri-point engineering (The same people who used to run the Speed World Challenge Proteges and now the Mazda6s...they also do the work on my struts). Jeff recently started autocrossing and has moved his car from street tire class (STX) to Street Prepared (DSP). He is in the same boat as I am as no one really has attempted a national competative build of a MSP. The car has a lot of potential though and Jeff is a quick learner.

The good: Most modern/refined F-series engine that likes to rev. Small/quick spooling turbocharger using modern technology. Long legs from transmission. Doesnt hurt to be working in a race shop with knowledgable people.

The bad: struts all around. limited update/backdate means its kinda heavy for FWD. G-type transmission doesnt like heavy shock loading.



And then theres the 1988-1992 Mazda MX6 GT



The Good: Im pretty sure I can get mid 2400lb weights out of the car legally. 2.2l F2T is pretty bombproof. Tons of torque. Turbocharger equals easy power gains. Small turbo is pretty lag-less in autocross setting. Strong H-type transmission.

The bad: Tons of torque in a FWD car is a lot to ask of the front tires. Not the lightest FWD car of the bunch. Engine doesn’t like to rev/low redline. Short gearing means 2-3 shifts. Struts all around. Limited aftermarket/knowledgebase.

The plan naturally is to work with the cars advantages and minimize the limitations. Its not as light as the Hondas but is much more powerful in the lower RPM regions. Struts aren’t ideal, but that means the car is no worse off than the Subaru or MSP. Its much lighter than the RWD cars(and arguably with more torque potential as well).

There have been three new changes to the rules and a change in tires that (I hope)will beneft my car as well.

-Boost controls have been released on turbocharged cars. With the advent of ECU controlled boost on newer cars and an existing rule that allowed for aftermarket ECUs, it became problematic to verify stock boost levels. Instead of fruitlessly chasing electrons the rule was changed to allow open boost controls in the STOCK turbocharger(stock turbo can be easily verified by something as simple as a set of calipers). Its become known as the “unlimited boost” rule.

-Adjustable cam gears have been allowed to cars that have no electronic or mechanical means to alter cam profiles (IE: vario-cam, VANOS, VTEC). Theres no way a static adjustable cam gear can be as effective as an infinetly variable intake/exhaust double VANOS, but it’s a way to bridge the performace gap a bit.

-Alternate bypass valves have been allowed. Under the old definition a bypass valve was considered to be part of the turbocharger and not a part of the intake system(which was free to be changed). A recent change in the rules interpretation has the bypass defined as a part of the intake system and thus can be altered.

-Lastly a new crop of tires have been released by Hoosier in response to calls for a wide 22” diameter 15” tire and a wide 20” diameter 13” tire.

Without these rules and tire changes I believe the MX6 wouldn’t really have a chance to compete, especially with the previous restrictions on turbocharged cars. Now that changes have gone into effect…and love for ones car notwithstanding (its easy to fool yourself into believing that your car of choice has some innate superiority)…think that the car has a legitamate chance of being close to the top. Can it be top dog? Maybe not, but I think it can get close…close enough to be a real handful.

Anyway, lets go on to what Ive done so far.

Gavin

[Mazda4ever]

Damn, looks like somebody has been doing their homework!. I have a question though, even though these new rules make you more competitive dont they make your competition even harder to beat if they find ways to take advantage as well?

[gavin]

The rules changes really help turbocharged cars(which have been hamstrung for years). All those turbo cars will be able to take advantage of the rules equally. But you are correct, it doesnt mean that everyone gets the same gain. EX:I wont be able to realize the same gains as another car with a T3 (for example) that can utilize the "unlimited boost" allowance, but the numbers dont tell the full story of what is "too much" or "not enough" for a particular car.

As for the N/A cars, the ones that have variable cam technology have already taken advantage of that. The ones without variable cam technology get the same benefits that I can also take advantage of so, its somewhat of a draw there.

Just FYI: heres the full list of the cars classed in DSP.

http://sccaforums.com/forums/storage...solo_rules.pdf

There are bookmarks on the left (go to "Classing SP") pages 177-178

Gavin

[gavin]

I wanted to do things "right" and in my mind that meant that I had to find a way to get the car out of daily driver status. You dont treat your daily driver like a racecar and you dont treat the racecar like a daily driver. horses for courses.

Theres a lot of people who get pride in the "daily driven" moniker, but Im not one of them. You get half the drivability/comfort, half the performance potential and all the cost of maintainance. I dont see much sense to that, but thats just me. (In truth I think the "daily driven" thing is a hoax anyway, because the cars tend not to be really daily driven anyway. A trip around the block every year doesnt equal daily driven to me. Meh..whatever)

After the initial build of the car I did actually daily drive it for a while. The car was a handful on the streets. Its suspension was stiff and the roads were not the best of shape. It made for long braking distances and twitchy handling in the rain is always a thrilling experience. There was also much wear and tear on the car as bushings needed to be regreased constantly and spherical bearings needed to be replaced in short order.

Those issues were simply annoying for the most part. The thing that really got to me when I started the second stage of the cars life was the potential for loss while driving the car.

It really got to me that all of what I was doing could be lost in an instant because of a mistake on the road. I dont know how things are where everyone else lives, but the drivers here in california arent the best. What would happen if I were to be in an accident? What would insurance pay? Whatever it was it wouldnt be able to pay for all that I had done to the car. I had custom this and custom that, discontinued this and discontinued that. Id worked had to get quality parts for cheap that if they were lost I couldnt pay full retail value for them a second time around.

In other words, I couldnt afford to replace the car for what it really would cost at full value.

I *never* thought of the possibility of trailering my car when I started. A trailer seemed too expensive, out of my league, too ostentatious.... but faced with the (very real) possibility of a catastrophic event with the car....the cost of a trailer to preserve my investment was looking like the most sensible thing I could do.




A local racer was upgrading to a 20ft enclosed trailer and needed to sell his old open bed 16ft texas bragg. I was lucky that I was up late at night when he posted it on the local autocross forum.

16ft low boy (it had the axles flipped so it was taller to clear driveways better), dual axle, closed steel bed, welded castors on the rear, hand winch added, spare tire, removable drivers side fender with handles, new electric brakes, new hubs, wheeled front casor, manual dolly.......$850(!!!)

5min later he had a PM, email and phonecall to his cellphone. I was at his house the next day to put a check in his hand(he mentioned he had 50 other offers by that time).....I didnt even have a truck in my disposal. i figured it all out later, but there was no way I could pass this opportunity up.



I borrow a friends Ford F150 to tow the car to events. He has a Spec Miata and a truck, but no trailer and I have my car and trailer, but no truck. We swap back and forth with the rig and this arrangement has worked out for both of us. I have more events than he does so if there is a schedule conflict he gets the trailer. I figure thats fair since there will always be another autocross the next week.

My thought on all of this trailer nonsense? Its fully worth it.

If theres anyone out there who has thoughts of a "full build" anything I highly reccomend getting the project vehicle out of daily driving duties ASAP. Working yourself to death on weekends to get the car back in shape to get to work on monday will get old fast. Get a new daily driver (I did).

Is a trailer the answer for everyone? Maybe not. For me it absolutely was. I can tweak the car to its limits and can go to events with the confidence of knowing that if something goes wrong(engine, transmission...plug in the exhaust manifold blowing out because of EGR pipe removal..this did happen) I can still get home.

Gavin

[gavin]

SUSPENSION

Two things that matter the most to an autocross build(and for a road-race build as well), suspension and tires. Ive worked on this portion of the car since the beginning. The prior links should explain the bulk of it, but here are some changes and explanations for some of the pervious changes..

Struts:

15.5 C :Any shock absorbers may be used. Shock absorber mounting brackets which serve no other purpose may be altered, added or replaced, provided that the attachment points on the body/frame/subframe/chassis/suspension member are not altered. The installation may incorporate an alternate upper spring perch/seat and/or mounting block (bearing mount). The system of attachment may be changed. The number of shock absorbers shall be the same as Stock. No shock absorber may be capable of adjustment while the car is in motion, unless fitted as original equipment. MacPherson strut equipped cars may substitute struts, and/or may use any insert. This does not allow unauthorized changes in suspension geometry or changes in attachment points (e.g., affecting the position of the lower ball joint or spindle)

European 2nd gen Koni single adjustable strut inserts (revalved by Tri-Point Engineering) with ground control coilover sleeves. Theres no better combination for the money in terms of performance. Factor in servicability, adjustability and the vast knowledgebase and things get even more lopsided. There was an outside chance that I would go with KW and their variant3 setup for the 2nd gen, but with limited knowledgebase in the US and questions about revalving services and turnaround time I went with the more well known konis. Ground Control has been making coilover sleeves since the dawn of time and are the US distribution point for Eibach race springs.

I have shock dyno charts for the work done to the konis. Ill have them scanned one day and include them. Suffice to say I have more than enough rebound damping to deal with the spring rates I intend to run.

The european 2nd gen fitment is about 2" shorter than the 1st gen struts. Ill be using a spacer to bring the rear struts to within 1" of stock length. As the car will be running on smooth surfaces I simply do not need that huge amount of downtravel like the stock length struts. The fronts will be 2" shorter than stock. The shortness will be to accomodate a modification made to the geometry in the upper mounts. The overall downtravel when assembled will be within 1" of stock. Uptravel will not be affected mainly due to the high spring rates.

Springs are 7" 500lb fronts (with 50lb/in tender springs) and 7" 650lb rears (with 50lb tender springs). Its marginally a two-stage setup(as seen in the 2nd and 3rd pictures). The main springs do the bulk of the work. The tender springs are there to extend the strut to the ground and keep traction if I find myself in some silly cornering scenario where the wheel otherwise would be off the ground.

These will not be the final struts the car has. I am currently working on the successor to the konis.





These are Penske 8760 series triple adjustable remote resevoir struts. They were originally made for a 993 Porsche RS-R GT car. The owner of the car had two full suspension sets to sell (Porsches use struts in front and shocks in the rear). I bought the two front sets of struts. I took a guess on the struts as I didn’t know their dimensions, but as it turns out the body lengths are close to that of the stock MX6 front struts and the strut shafts are similar to the stock MX6 rear shafts. Theres a bit of work to do to convert them for use in my car. Ill have to lengthen one pair for the rear of the car, remove the stock Porsche mounting ears and replace with new ears that fit the mazda. One of the pair of shafts will need shortening for the front and then theres misc seals that need replacement. Then Ill figure out valving.

Did you know that all of Penskes piston valves are/were patented? Its mindboggling to try to figure out the different permutations of what can be done with their pistons. It seems that they can do anything. In any case, Ill be sorting the car out on the Konis. When the car is set and I feel Im ready(3 knobs?), Ill make the change to the Penskes. They should be the last strut Ill ever need to install on the car.

(on to part 2)

Gavin

[gavin]

Camber plates:

15.8 F. The following allowances apply to strut-type suspensions: Adjustable camber plates may be installed at the top of the strut, and the original upper mounting holes may be slotted. The drilling of holes in order to perform the installation is permitted, but the center clearance hole may not be modified. Any type of bearing or bushing may be used in the adjustable camber plate attachment to the strut. The installation may incorporate an alternate upper spring perch/seat and/or mounting block (bearing mount). Any ride height change resulting from installation of camber plates is allowed. Caster changes resulting from the use of camber plates are permitted.

I used a combination of the mazdaspeed camber/castor plates in front and the custom rear plates made by long time ago board member Adamturbo.




I wanted as much castor as I could possibly gain from the car. To do that I designed an offset castor spacer to be used in tandem with the mazdaspeed plates. The spacer offsets the camber plates 1" further towards the firewall and relocates the camber plate 3/4" lower. Camber adjustment is still on the top. Castor adjustment is done under the fenderwell. The new result is a max of -4.5 camber and a max of 5.5deg castor.








Why so much castor?

The camber curve (amount of negative camber gained in compression) for struts is almost nothing. Id roughed out the camber curve calculations for the front of the 1st gen at @.97 to 1 and the rear at @.98 to 1. Basically youll be seeing about -.03deg to -.05deg of camber for every inch the suspension compresses. This isnt completely linear for the full range of motion(things actually start going back to positive camber at a certain point), but for the important bits youll be lucky to get -1deg total camber at full compression. So whats the big deal about that?

Since the car wont gain negative camber when the suspension compresses, I have to “pre-add” a fair amount of negative camber to the car to keep the tire patches flat when the car is trying to roll in a corner. However, because of the angled contact patch, when the car is going straight acceleration and braking are negatively affected since the full width of tire isn’t touching the ground completely. In an ideal world one would have the tire flat to the ground when accelerating/braking and gaining negative camber when the car rolls in a corner. The tire patches would always be optimized for maximum grip. This is generally what happens with a double a-arm (wishbone) suspension....but I dont have a double a-arm suspension like a Honda/Acura, I have struts...and struts kinda suck in this regard.

Because of this issue, the traditional solution to get struts to work is….prevent them from working!(no, Im serious). Up the spring rates high enough that the suspension doesn’t move. If it doesn’t move then you never have to deal with the camber gain issues. This ,however, poses issues as well. Mechanical traction goes out the window. Off line traction will be abysmal, the car will “work” the tires harder and because of the lack of compliance in the system you will have handling issues on less than smooth surfaces.

The biggest help in this is castor. When viewed from the side of the car, castor would be defined as the angle at which the strut top points towards the firewall. For me, the more I can get the strut tops moved to the firewall, the better. Castor has the effect of adding “dynamic” camber when the car is cornering. The more castor I can run, the less static camber I need to affect the same performance in the corners.

Nothing comes for free though. Castor has a self straightening effect (think about the front wheels of a shopping cart). There are plenty of RWD vehicles that have strut fronts and lots of castor (BMWs, Mercedes) without issue, but in general FWD vehicles tend to run low amounts of castor to get away from the self straightening effect on the powered front wheels. Basically, youll get heavy steering as the car tries to just go straight.

There will be increased steering effort from the "jacking" effect of high amounts of castor… but this is actually something that I want, heres why. Looking at the strut on a side view, you see two planes. One plane is the surface of the road where the wheels sit. The other is the axis defined by the castor angle. If the castor angle is 10deg in a turn the wheel (since it is no longer pivoting perpendicularly with the surface of the road but instead at a 10deg angle) will at a certain point be pivoted above and below the roads surface. The “jacking” effect. I want this effect because of what it does for the inside wheel. If the car is making a left turn, it is now rolling to the outside right.. the inside wheel is running left and is now being pivoted outwards and DOWN into the roads surface keeping contact and traction to be maintained longer.

If a car was running lower castor angles the same action would have the wheel being pulled off the surface and wheelspin/lack of traction would be the result. Id need heavier springs/swaybars to prevent the car from rolling and maintain that traction, but that only works so much (as mentioned above). With a lot of castor I can have the inside wheel produce more forward traction longer than with low castor angles. I hope that will allow me to run reasonably stiff spring rates without having all the penalties that come with overly high spring rates.

Suspension Bushings:

15.8 C. Suspension bushings may be replaced with bushings of any materials (except metal) as long as they fit in the original location. Offset bushings may be used. In a replacement bushing the amount of metal relative to the amount of non-metallic material may not be increased. This does not authorize a change in type of bushing (for example ball and socket replacing a cylindrical bushing), or use of a bushing with an angled hole whose direction differs from that of the original bushing. If the Stock bushing accommodated multi-axis motion via compliance of the component material(s), the replacement bushing may not be changed to accommodate such motion via change in bushing type, for example to a spherical bearing or similar component involving internal moving parts. Pins or keys may be used to prevent the rotation of alternate bushings, but may serve no other purpose than that of retaining the bushing in the desired position.

Custom machined delrin/UHMW polyethelyne.




Effectively these may as well have been machined out of steel, they are that hard. The UHMW is a litle softer and springier than the delrin and thats to give a bit of motion to the rear arms. The rear arms do not move in single planes. They instead move in wavy arcs that require bushings that give a wide range of motion. Rubber is the ideal material for this. It is durable, long lasting, weather/contaminant resistant, an excellent isolator of noise/vibration, compliant to absorb shocks to the cab.

Rubber is pretty awesome, but in a performance application its benefits work against it. Because of its compliant nature it tends to be vague and lacks the crispness needed for precision driving, thus the change to the rules compliant(no pun intended) hard-as-steel delrin. Its a pain in the arse as they transfer every bump in the road to you but its precice.

When you have the tires that I am currently running and have limited clearance, precision is what you need or things start banging into each other. The tires are next.

Gavin

[gavin]

15.2 A. BODYWORK
Fenders and bumpers may be modified for tire clearance. This includes the portion of hood which serves as a fender/wheel well, where applicable. This does not permit modifications to the chassis or bodywork inboard of the vertical plane of the hub/ wheel mounting face (at rest, with front wheels straight ahead). Flares may be added although tires may extend beyond the bodywork. Replacement of complete hood, flared fenders, or quarter panels is prohibited. Plastic and rubber wheel well splash shields may be modified for tire clearance and for installation of fender flares as allowed herein.

15.3 TIRES
Tires must meet the eligibility requirements for Stock Category with the following exception: the list of non-eligible tires in Section 13.3.A is replaced with the following list, which may be altered at any time by the SEB upon notification of membership.

(section 13.3A goes on to describe the eligability of tires available (when they can be available for purchase, in how many sizes available, non-zero measurable tread depth, etc)

15.4 WHEELS
Vehicles may only exceed the allowances of 13.4 as specified herein.
A. Wheels of any diameter, width, or offset may be used. Aftermarket wheels may be modified to install OE tire pressure sensors.
B. Wheel spacers are permitted. Wheel studs and knock-off wheel drive pegs may be changed in length and diameter. Wheel bolts may be replaced with studs and nuts.

Effectively the three above rules allow me to use any DOT (Department of Transportation) certified tire (IE: DOT race-compound tires), any wheel width/diameter tire and flaring to fit them. The harder part is figuring out what the best combination will be for the application since tire size will change gear ratios. Since I cannot change gearsets or final drives, changing tire diameter is an effective tool to changing gear ratios.

In autocross (much like drag racing) the fewer shifts one has to make during the run the better. Typical speeds in autocross is in the 60mph range. Ideally one uses 2nd gear for the majority of the run with no need for shifting. A car needs to have enough bottom end grunt to get out of slow turns without having to downshift to 1st while having enough legs to get to @60mph in the faster sections without needing to upshift to 3rd. Its a lot to ask of an engine and transmission, but cars that can do this are usually a leg up on the competition.

The 1st gen MX6 has an abundance of low end torque so the need to shift to 1st in slower sections wont be necessary. The nature of the engine and transmission gearing however have it a bit close at the high end reaches of 2nd gear.

Heres a couple nifty tools to sort all the numbers out.

Gear ratio calculator http://www.f-body.org/gears/
wheel/tire size calculator Wheel / tire size calculator / comparer - RIMS-N-TIRES

And heres the gear ratios for the GT transmission.

1st: 3.250
2nd: 1.772
3rd: 1.194
4th: 0.926
5th: 0.711
Rev: 3.461
Final:4.105

The stock tire size for the GT is 205/60/15. This tire has an overall diameter of 24.7". Assuming a 6000rpm redline and plugging in the rest of the numbers into the gear calculator chart, youll see that the top speed in 2nd gear will be at 61mph. Not bad at all. Looks like for my purposes Ill need a tire that has a diameter of 24.7".

There are two top tier DOT R-compound manufacturers that have proven themselves to be perennual winners. Hoosier tire and Kumho tire. Their best offerings are as follows.

Hoosier A6


Kumho Ecsta V710


In general, the Hoosier is the tire that gives better feedback because of its sidewall construction. Its a "snappier" tire that turns in well. The compound is also a bit grippier than the Kumho giving better braking and acceleration. Its a more expensive tire (sizes being equal) and the softer compound wears much faster than the Kumho. The Kumho on the other hand has a high tollerance for high slip angles than the Hoosier (you can slide it a bit more without completely breaking traction).

I need a tire thats 24.7" and nets me the most rubber I can stuff under the tire wells(and beyond since I can flare the fenders). The best candidates for this are the Hoosier 245/45/16 (24.6") or the Hoosier or Kumho 285/30/18 (24.8"). Initially I chose the 245. Heres why.

Theres actually more factors in choosing a tire for a car than diameter. The tire width also has an affect on the car. The wider the tire the wider the car becomes overall. In a game of inches you need a car that is the narrowest it can be to navigate the course without needing to turn the steering wheel as much. The more steering angle needed, the longer the distance travelled. Imagine the difference between a corvette and an elise navigating a 6 cone slalom. Use the centerpoint of the cars to draw a line through the course as they make their way through the slalom. Because of the sheer width of the corvette, the line it needs to make it through the slalom will be further away (and ultimately a longer line) from the cones than the line of the much narrower elise. Longer distance traveled equals slower time.

The 285s while a lot of rubber will make the car very wide. Add to that the cost of wheels to fit the tires (18x10.5 or 18x11"), the amount of spacing needed to have adequite clearance in the fender wells and finally....with the car to be in the 2400lb range thats not a lot of weight to generate enough heat int he tires for them to be at maximum effectiveness.(theres no warmup lap in autocrossing, the tires have to be up to temperature in a couple turns). The BMWs and other rear driven cars can make use of this tire, but in general they have more weight and upon acceleration transfer that weight to the tires, not so in a FWD car where the weight shift will be to the undriven wheels. There is such a thing as too much tire, at least in this case. The 245s, while not as much rubber as the 285s, seemed the better choice.

This was the first go at things.







The flares were for datsun 240/260z cars. The wheels were 16x9" CCW 3piece wheels using a 1" spacer and 245/45/16 Hoosier A6s at all four corners.

A funny thing happened while I was in the middle of finalizing this setup, but first a trip down memory lane.

on to part 2..

Gavin

[gavin]

In the past Kumho had two top contenders the V700 victoracer and the Ecsta V700. Hoosier countered with the A3 series culminating in the brilliant bias ply A3S03. In the Hoosier vs Kumho tire wars, its always been a Hoosier advantage. The Kumhos are Good, but if you wanted that extra tenth of a second and you were willing to sacrifice cost/wear it was always the Hoosier, no questions asked.

At the same time many Subaru owners were looking for a wider tire than the 245/45/16. Kumho had a 255/45/16, but (while good for their short gearing) made the cars too tall, raising the already tall CG. With a lage ownership and online community they petitioned both Kumho and Hoosier for a shorter, wider 16" tire. Something like a 265/40/16 (24.3"). With the subarus being strut suspended cars, I was hopeful that they would be successful with their petition as I could also use such a tire. Unfortunately their requests fell on deaf ears. The consumer car market had long since gone past 16" wheels as standard fare...17"s, 18"...even 19" were more commonplace. Kumho, being primarily a passenger tire maker couldnt see a 16" race tire on their radar. Hoosier is a race tire maker and is smaller and more flexable, but with a barrage of championships and tires that worked, an oddball 16" size wasnt at the top of their list of things to do.

A couple years passed and a few things happened. The Subaru WRX STi came to the shores of the USA. The previous folks who had non-turbo and WRX model subarus graduated to the new car and left thoughts of 16" wheels in the past. Kumho introduced the Ecsta V710 to replace the aging Victoracer. Hoosier introduced the new radial A3S04 to replace the old bias ply S03....it was a disaster for Hoosier. The grip was actually worse than the S03 and the tire wore like soft cheese. Many stock class cars went to the cords in mere runs. The V710 however was proving to be a success. The grip was phenominal and the wear seemed impossibly low. Its didnt hurt that the tire was cheaper than the Hoosier offering as well. Hoosier sales stumbled a bit while Kumhos stock rose. Hoosier countered the next year with a revised offering the A3S05. It had improved grip, better construction and more rubber in critcal areas that tended to cord quickly. Still, it wasnt the answer to the V710. The hoosier still wore quickly and was too easy to run to the cords in fast order. Finally with new construction and compounds, Hoosier introduced the A6 series. This was the answer hoosier was looking for. It was still more expensive than the V710 and still wore faster, but it had more traction than the V710 and the cording issues were finally solved....but the damage to hoosiers reputation was already done. Once there was no question to hoosiers superiority, but with 3 years of solid V710 performance their stock had slipped a bit.

Its my opinion that at this time Hoosier needed something to regain the marketshare it lost...but how? Hoosier and Kumho both had similar size offerings. Hoosier is a smaller and more flexable company, it could make up the difference in "tweener" sizes, but which ones? The subaru voices had dimmed down as time had moved them to STis. Right about this time a fellow by the name of Craig Wilcox started a campaign for a tire that would benefit his Mini Cooper S for Street prepared. He wanted a wide 15" tire, something like a 245/40/15, a tire neither manufacturer had to offer. Its,again, my opinion that Hoosier saw this opportunity to regain marketshare by making a tire that only it offered. They went to their factory and added a spacer to the mold for their 225/45/15 tire and created a 275/35/15.

Gavin

[gavin]

The 275/35/15 is a 22.6" diameter tire. A full 2" shorter than the size tire I needed but with the width I had wanted. My inital feeling for this was "thanks for nothing Hoosier!".....and then I thought about it a bit.

Truth of the matter the 24.7" diameter tire was barely keeping the car up to speed. Yes, the numbers seemed to work, but only at 6000rpms. Frankly the drop off in power after 5500rpms left the car out of breath. The loss of power at high RPMs in 2nd and a very strong 3rd gear(shifting to 3rd from 5500rpms drops the tach in a good portion of the torque curve) found me in between gears a lot on course. Too long in 2nd gear and not long enough in 3rd. It wasnt ideal. I was airing my frustration of my gearing woes and my annoyance of the new 275 tire and got a unusual suggestion.

If the car is in between gears now, why not completely go for 3rd gear? (Damned the torpedoes and full speed ahead). Go for the 22.6" tires and instead of using 1st and 2nd gears, the tire will dump the gearing so I could use 2nd and 3rd as the primary gears.

With a 22.6" tire 2nd gear will top out at 55mph @ 6000rpm. 3rd gear tops out at 82mph. Thats a bit much for my needs, but 55mph in 3rd is @4000rpm. I could short shift 2nd and still be in fhe fat of the torque curve in 3rd, which pulls to 68mph @5000rpm. Still a bit too much, but better. Im liking this.

A 22.6" tire also means that the car automatically drops 1" in height and CG without the suspension geometry being comprimised. 15" wheels will be lighter than 16" wheels, the tires will also be lighter leaning to lower unsprung weight (the struts will have less work to control the weight on the hubs of the car). The smaller diameter will also shorten the gearing of the car leading to improved toque multiplication and acceleration. Sure more torque could lead to tire spin...but 275/35/15 is a LOT of tire. That lot of tire will be an asset to a front weight biased FWD car. The weight per sq/in of tire on the road will be less leading to less worked tires. Except for the need to do a 2-3 shift, everything seemed to be an improvement.

The E36 BMW wasnt always the dominant car for the class. It was classed in DSP for many years and wasnt a top contender. It was too heavy and too under tired (245/45/16 were the best tire for the car at the time). And then a miracle....Kumho introduced the Ecsta V700 in a 285/30/18 size. It took a while for the BMWs to adopt this new tire, but it was the catalyst that catapulted the car from 2nd best to top of the heap in short order.

Many people looked at the new 275/35/15 offering and wondered if this was the answer that could also make a difference for their car like the 285 had done for the E36. I thought It was definetly worth the risk to find out.

I sold the 245/45/16 setup and redid the tire/wheel combo. The king is dead...long live the new king.






The wheels are custom made by Kodiak racing wheels. They are 15x10.5" and 15x9" 3-piece wheels. Forged outer shells and billet CNCed centers. 5x114.3" bolt circle with a 60deg taper, 5.5" backspace, 69mm hub center holes, non-hubcentric(I wanted them that way). The 10.5s are @12lbs apiece and the 9"s are @10lbs.

The rear tires are 225/45/15 Hoosier A6s. I had mentioned before that one of the reasons for not going with the huge 285s was the lack of weight to generate heat in the tires. Well the same applies here. The weight of the car is biased to the front about 75/25%). I can make use of the 275s in front as theres enough weight to generate heat and grip, but the rear of the car lacks that weight. Theres no sense in me using the same size tire in the rear as I believe the tires will not get to temperature and generate decent grip. Hot front tires and cold rear tires doesnt sound like a good setup. I went with 225s because I feel that thats "enough" tire for the rear of the car to get heat in them. The theory is that both ends of the car come up to temperature about the same time. So far this seems to have worked out well with a 15deg split in front and rear temperatures.


I did have to redo the rear flares of the car. Well, I didnt "have" to, but the old rear flares were big for even the 245s as they were made for a RWD car that sees the larger tires in the rear. With the new staggered setup I was to run, the flare simply looked silly, too big for a tire that small...like a beach umbrella covering a toddler. The wheelarch was already cut so I simply went for smaller flares made for a toyota corrolla.





Drivetrain next?

Gavin

[gavin]

Good to see that I have people riveted to this series. Positively unable to respond it seems

15.10.Q. Limited slip differentials are permitted. This permits locked differentials, either by design, welding, or mechanical means. Differential cases, internal differential parts, and axle stubs may be machined as required for clearance and installation to the extent that material may only be removed, not added, and the exterior of the case may not be altered in any way. This machining may serve no other purpose.

This is pretty straightforward.







Installation was challenging. But after a few tries it seems as if I have gotten the preload correct. The Guru Motorsports torsen works brilliantly. Between the choice of tires and the ability to have traction at both wheels, the car can make full use of the torque produced. I have a bit of concern about the load the transmission gears will take because of this. Its been my observation that torque tends to break things. Its been said that the H-type transmission of the GT is the strongest FWD box that mazda has made. I hope this to be the case.





The axles are From Raxles of Florida. I cant say enough good things about the product. The cups and associated hardware are NEW and not remanufactured. The axles run about $125 per item and there is a lifetime guarantee. Both of my cars have a set on them.

15.10.O. Any metal clutch assembly, metal flywheel or metal torque converter that uses the standard attachment to the crankshaft may be used. Non-metallic friction surfaces (e.g. clutch disks) are permitted. Dowel pins may be added. Any hydraulic clutch line may be used. Minor repositioning of the clutch slave cylinder is allowed to accommodate the alternate clutch, but the unit may not be relocated and the repositioning may serve no other purpose. This allowance does not permit the substitution of slave cylinders or the use of non-original methods of clutch actuation (e.g. pull type versus push type).

Again, pretty straightforward in what is allowed. Initially I went with a conventional route.



11lb fidanza aluminum flywheel and a stock exedy/daiken clutch disk and pressure plate. This was a very easy bolt on change. The stock flywheel was 23lbs so the aluminum flywheel shaved a good 12lbs from the rotating mass.

While I had the transmission out of the car to check on the torsen diff, I looked at the clutch assembly I had. I felt I could do better and started looking at competition multi-plate racing clutches. They are compact, lightweight, durable, rebuildable and hold a stunning amount of power for their size. The only issue (naturally) is no one makes a fitment for the 1st gen MX6 and no one has previously attempted to have one fitted (even in the old IMSA support series where the MX6 competed used full face clutches). I thought I could engineer a solution so, I would (again) have to do this myself.