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Discussion Starter #1 (Edited)
The following is meant to be a GUIDLINE for choosing a good cam for a turbocharged vehicle. Sources are cited.

First Step is to remember what Corky Bell, the author of Maximum Boost, has to say about cams and forced induction:

Camshafts

Make no mistake in the fact that the turbo performance cams are very different from atmospheric performance cams. The characteristics of long duration and high overlap for atmo cams are unwelcome in the turbo system. The street turbo, which is generally small, operates on exhaust manifold pressure somewhat higher than intake boost pressure. This situation, when presented with long-duration, high-overlap cams, creates a huge amount of reversion. Thus the "turbo cam" tends to become a low duration, very limited overlap camshaft..

RULE: It is hard to find a turbo cam that works better than the stock item.

So remember, overlap= bad. Although it is hard to find a better cam than stock, we can still try...

http://www.forcedinductions.com/help.htm
How to select a turbo cam

Duration:

Duration is critical to a turbo setup since its probably the single most important event of a turbo motor (i.e. time valve sits open and closed). Since the air is being forced instead of drawn into and out of the combustion chamber, duration will be your largest variable on how that incoming/outgoing air is managed.

Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration (226/220, 240/234). This is mainly because a manifold/log design will typically see higher then a 2:1 pressure ratio in the exhaust ( as high as 4:1 with some logs). By using a reverse split duration this will somewhat help prevent from getting exhaust gas reversion.

Duration when using an efficient header setup with most turbo cams will usually be (230/230, 224/224) or better known as a dual pattern cam. The thinking is with the exhaust backpressure being only 2:1 you can leave the exhaust valve open a little longer then if the exhaust backpressure was 3:1 or higher. Also some of the new turbo designs produce a much lower backpressure with the advent of better flowing turbine wheels and housings which further decrease the total amount of backpressure created by the system.

Overlap:
Overlap definition, is the time period when both the exhaust valve and the intake valve are open at the same time. The exhaust valve needs to stay open after the piston passes TDC in order to use the vacuum created of the exiting exhaust gases to maximize the amount of exhaust gas drawn out of the cylinder. The intake valve opens before TDC in order to use the vacuum created by the exiting exhaust gases to start drawing the intake charge into the cylinder.

This sequence of events above are controlled by the duration and LS (Lobe separation) of the cam. On a typical N/A motor this is essential since you have no pressure being developed on the intake side to push the charge into the combustion chamber. The problem with this event is a turbocharged motor will create a larger amount of backpressure on the exhaust side. Due to this event the above definition will not apply. Reason being is, when the intake valve opens at BTDC, the burned gasses in the chamber will exit out the intake since the pressure is lower than the exhaust. Since this is true you would not want to open the intake valve until the piston has started going down, ATDC. This will lower the combustion chamber pressure till it's below the intake manifold pressure.

To calculate the overlap of your cam simply follow these steps below:
**Example turbo cam:**

Duration @ .006 218/212

Lift .544/.544 lift

Lobe Separation (LS) 114

Add the intake and exhaust durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2

Overlap is -13 Degrees of overlap

Above was the process on how to calculate your cams overlap. As you can see, the overlap in the 2 cams differ greatly. Running the N/A cam example on a manifold setup would be a horribly in-efficient setup and the engine would be operating well below its potential output. While running the example turbo cam would work well even with the most in-efficient of the header systems out there.
Typically a overlap spread of -8 degrees to +2 is a safe bet. Of course this will differ with whatever combination header, turbo and exhaust is used, so those #'s could be higher or lower.

Lift:
How much lift should I get in my cam? Well that will depend on your heads' flow characteristics. To choose the correct turbo camshaft, you really need to know how your cylinder heads flow. Reason is if your cylinder head flows X amount of air at X amount of lift, choosing a cam that has a lift much greater then that will gain you nothing except extra heat and premature wear of the valve spring. Airflow through a head reaches a peak as the valve is opened, then starts to drop off as the valve is lifted beyond that peak. Most of this of this will hold true to definition, but with a forced induction motor, valve lift is not as critical since the incoming air is pressurized.

A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point.

So be the definition above if your head flows best at 0.500" of lift, use a cam that will lift the valve between 0.600" and 0.625". The reasoning behind this is, if you lift the valve only to its peak flow point, then the valve only flows best when it's wide open. The cycle is brief and would only happen once per stroke. So to benefit from you peak flow the most, you want to lift the valve past its peak. That way the valve will pass its peak flow twice in the cycle. The result is more flow during the opening and closing event of the valve. You do not want to raise the valve much past the peak flow though, or you lose total flow by going too high.
Calculating the best lift:

0.500 X 1.20 = 0.600
0.500 X 1.25 = .0625

Conclusion:
There are way too many factors to just say XX cam will make XX power with your combo. Things like "114LS is best, or 117LS, or ..etc", are just blanket statements. Backpressure, RPM range, boost level, target horsepower, A/R of turbo, turbo frame (T3, T4, T6/Thumper), head flow, cubic inches, and even location of turbo...etc. All of these factors are extremely important in determining the cam that best suits your needs. There is no rule of thumb with a turbo cam. There are too many variables and the only way to get the right cam is to take all of those your parameters into consideration, and only then can a proper cam be selected. All of the points of reference above are just to get you on your way to building the best and most powerful turbo system for you. Study your design and ask questions along the way and you will be smiling the next time your opponent lines up next to you.

Applying the Formulae.

Remember, the only good way to judge a cam's duration or timing is at .050 lift - b/c thats where it really starts to matter the rest is ramp-up.

So, now that we understand the guidelines, lets look at a couple examples of the math in action:

STOCK CAM
Cam Lift (Intake): .210"
Cam Lift (Exhaust): .197"

Duration at .050''
Lobe duration (Intake): 197º
Lobe duration (Exhaust): 208º

Valve timing stock:

IN: Open BTDC 10º
IN: Close ABDC 49º

EX: Open BBDC 55º
EX: Close ATDC 12º

Ok, good stuff. Now lets start crunching numbers with the right formula(s).

To make sure that the number are right we will calculate in to ways.

#1 Obvious, look at the pic method.


comes out to 22* overlap at .000 lift.

#2

to find duration at .000 lift - In= 10+180+49= 239* Ex= 55+180+12= 247*

instead of gust guessing the lobe seperation we'll find the real thing.

{[(in. dur / 2) - dist BTDC] + [(ex. dur / 2) - dist ATDC]} / 2 = Lobe Separation

{[(239/2) - 10] + [(247/2) - 12} /2 = 110.5* Lobe Separation

ok, now we can find overlap:

239 + 247
---------- - 110.5 = 11º x 2 = 22º of overlap at .000 of lift
4

Now remember that we are supposed to look at cam specs at .050 lift? But we don't have timing numbers for that lift so we need to do it with some math...

Ok so now that we know the math method works, we can find out what our overlap at .050 is. Using:

Duration at .050''
Lobe duration (Intake): 197º
Lobe duration (Exhaust): 208º

197+208
--------- - 110.5 = -9.25 x 2 = -18.5 overlap at .050 of lift
4

OUT OF ROOM...
 

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Discussion Starter #3 (Edited)
Thanks Sick, lets put the Crower to the test:

#1
"Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration (226/220, 240/234). "........ FAIL

#2
"Typically a overlap spread of -8 degrees to +2 is a safe bet.".....

254 + 264
---------- - 110 x 2= 39 deg of overlap!?!
4

........ FAIL

#3
"A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point."

well, since no one to my knowledge has ever flowbenched our stock head, we don't know what that lift our "peak flow point" is at. Do we even know what our stock lift is?
........ Incomplete
 

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You missed your own conclusion:
There are way too many factors to just say XX cam will make XX power with your combo. Things like "114LS is best, or 117LS, or ..etc", are just blanket statements. Backpressure, RPM range, boost level, target horsepower, A/R of turbo, turbo frame (T3, T4, T6/Thumper), head flow, cubic inches, and even location of turbo...etc. All of these factors are extremely important in determining the cam that best suits your needs. There is no rule of thumb with a turbo cam. There are too many variables and the only way to get the right cam is to take all of those your parameters into consideration, and only then can a proper cam be selected. All of the points of reference above are just to get you on your way to building the best and most powerful turbo system for you. Study your design and ask questions along the way and you will be smiling the next time your opponent lines up next to you.
 

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horsefluff alert!!

n2o_matt said:
Thanks Sick, lets put the Crower to the test:

#1
"Duration when using a manifold or log design on most turbo cams is usually about 6 degrees more intake duration than exhaust duration (226/220, 240/234). "........ FAIL

#2
"Typically a overlap spread of -8 degrees to +2 is a safe bet.".....

254 + 264
---------- - 110 = 19.5 deg of overlap!?!
4

........ FAIL

#3
"A good rule of thumb is to select a cam that will lift the valve 20-25% past its peak flow point."

well, since no one to my knowledge has ever flowbenched our stock head, we don't know what that lift our "peak flow point" is at. Do we even know what our stock lift is?
........ Incomplete
I don't know if you understand the numbers you're manipulating there, but I suspect not.

The over lap of the Crower cam shown above is 25° at .050" lift. At zero lift it would be more than 25°. It would not be 19½° except in your dreams and your fluffy calculations.

I have not checked your other calculations but I suspect either your calculations or your formulae are equally fluffy. In either case, you should not be spreading untruths here.

Please stick to facts. Do you even know what overlap is?

If you did, I suspect you would not post such bonehead conclusions.
 

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Discussion Starter #6
I'll be moving our PROVED findings in to the top post so that this can be made a FAQ - with no assumptions.
 

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Discussion Starter #7

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