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Discussion Starter · #1 ·
Tonight I was on my way back to school and I passed by a Probe with a Supra wing and checkered flag paint waiting to pull out of a gas station. The road I was on is one lane each way, but in some of the straight aways, it splits to 2 for about a half mile and lets you pass. I was doing 67 with the probe right on my ass before a split. He got on it and I downshifted to 4th. The guy went flyin right by me. Unless he had some serious mods, we shoulda been pretty even. I'm always gettin beat at rolling start races because I feel like i'm gonna beat on my car and don't downshift to a low enough gear. What is the max speed that you guys will downshift from 5th to 3rd? from 5th to 4th?
 

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Depends on how fast you are going in 5th.. 3rd gear is good for about 90mph.
 

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Discussion Starter · #3 ·
Cmon, I'm not askin a whole lot here. What is the max speed at which you will downshift from 5th to 3rd? 5th to 4th?
 

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bigjeff said:
Cmon, I'm not askin a whole lot here. What is the max speed at which you will downshift from 5th to 3rd? 5th to 4th?
Okay, I'll try. As a first-order approximation for the V6 MTX:

Speed = EngineRPM / (15.706 * Gr * Fr)

where Gr is the current gear ratio (1st thru 5th) and Fr is the final drive ratio (4.105 for the MX6, 4.39 for the PGT IIRC...)

(BTW, I'm aware this isn't exact...add fudge-factors as you please.)

Also, you can calculate the engine speed in a "new" gear by:

NewRPM = OldRPM * (NewRatio/OldRatio)

With these, you can figure out what you want to know.

For example, you ask at what speed in 5th I would allow myself to downshift to 4th. Assuming you do this to maximize the acceleration rate, you want the engine RPM to fall somewhere near the power peak which, in the V6 is near 5600RPM. So 5600RPM in 4th gear will be:

5600 * (0.717/0.914) = 4392RPM

in 5th gear. This corresponds to:

4392 / (15.706 * 0.717 * 4.105) = 95MPH

So, if you're going 95 in 5th, feel free to downshift to 4th.

Keep in mind that you're targetting for an RPM that will maximize acceleration rate in a given gear, which will be in the 5500 to 6000 range. You aren't necessarily shooting for the maximum safe physical RPM the engine can withstand.
 

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here's what i know

Okay...I have a 95 LS bone stock as far as performance. The downshiftng goes like this in my car...there seems to be locks on gears after I hit certain speeds on my car. The dealership I go to said this is about right b/c these locks act as rev limiter type thing (sorry... I'm not real techy) I can't downshift into:
2nd after 40mph
3rd after 60mph
4th after 80mph
If you want to downshift: (I assume you have a standard) when you have the clutch pushed in, rev the engine up to a higher rpm (like 4000 or 5000 rpm) before letting out on the clutch that way tranny doesn't jolt leaving you with no tranny after a while. That is what I was told to do and it works...the car reacts to it really well. And if I'm wrong someone please inform me so I don't keep screwing up my car.
 

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Discussion Starter · #6 ·
Thanks a lot Mike. It's about time someone gave me useful information on gettin phat ass power, smoke hondas, yeah!!! Seriously though, for those of you who want to shift more aggressively, check this out. If you are in 5th gear, your RPM in 4th will be 1.274(0.914/0.717) times what it was in 5th. If you are in 5th, your RPM in 3rd will be 1.720(1.233/0.717) times what it was in 5th. I believe that these values are a good estimation of where you will wind up after you downshift. Agree Mike? Now I have to remember to keep a calculator in my car.

And aggiedj02, that sounds like something is wrong with your car. It should allow you to shift at higher speeds than those which you have stated.
 

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Without all the tech talk isn't it pretty simple... You know what speeds you feel comfortable upshifting from so it's just the reverse, all you do is use good judgement.
 

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bigjeff said:
Thanks a lot Mike. It's about time someone gave me useful information on gettin phat ass power, smoke hondas, yeah!!! Seriously though, for those of you who want to shift more aggressively, check this out. If you are in 5th gear, your RPM in 4th will be 1.274(0.914/0.717) times what it was in 5th. If you are in 5th, your RPM in 3rd will be 1.720(1.233/0.717) times what it was in 5th. I believe that these values are a good estimation of where you will wind up after you downshift. Agree Mike? Now I have to remember to keep a calculator in my car.
The numbers look about right. So at 5000RPM in 5th, you'd be at 6370RPM in 4th and 8598RPM in 3rd. I point this out only to ensure it's understood that it's not just about gear-to-gear ratios and where your RPM will end up when you downshift. If the goal is to accelerate after the downshift, then it's important that the downshift will occur only if the engine will fall into the fat part of the power band.

As far as keeping a calculator, this exercise was intended only to give you an idea. You can use the theory to establish a baseline and use experience and judgement behind the wheel to decide when it's a good idea (or a not-so-good idea) to downshift for acceleration.

And aggiedj02, that sounds like something is wrong with your car. It should allow you to shift at higher speeds than those which you have stated.
I agree, either something with the car or the shifting technique. For example, aggiedj02 says he can't shift into 3rd after 60MPH is reached. Off the top of the head, 60MPH is 3000RPM in 5th (in my PGT at least...the MX6 may be at a slightly lower RPM.) Using the equation, this is "only" 5160 in 3rd...you should easily be able to shift down to 3rd at 60. And there are no physical interlocks built into the car to prevent these downshifts, not on MTX cars anyway. Aggiedj02, what is your shifting technique? Are you shifting like this:

- clutch in
- begin moving shifter out of current gear while simultaneously revving the engine
- smoothly re-engaging the clutch with the RPM already matched

or are you doing something like this:

- clutch in
- moving gearshift to next gear
- revving the engine
- smoothly re-engaging the clutch
 

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well...

...I shift out of gear and when in neutral I rev the engine and then let it into the lower gear while slowly letting out on the clutch as to not to get a sudden jolt out of the transmission. Quick story: I was going about 45 once and I tried to shift into second and when I did I heard that "dreaded gears getting stripped" sound that we all hate. I took it into my dealership and they told me what I told you above. If there is any thing wrong with my car? I'd like to know. I'm about to get a pacesetter short shift kit put in as soon as I get it in the mail. Will this fix it? I'll try to downshift today from higher speeds and I'l let you all know.
 

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at 67mph, i would go into 3rd. third brings me to about 80 or 85 and 4th after that. i race mostly in 4th. peak torque on these cars are at 4800rpm and peak hp is 5600rpm. there isnt much power up above 6500rpm. so even though 3rd can get me to 90mph, id upshift before though. i find that its faster on my car to upsift too early then to upshift too late so when im into a race i use that as a rule. these arent hondas, highest rev doesnt mean highest power. anyone have the dos program cartest ?? it can show u your optimum shiftpoints exactly as calculated by computer. its free. i just dont remember the website.
 
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