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Turbo/cam question

This is a discussion on Turbo/cam question within the Forced Induction forums, part of the LSx Technical Help Section category; I know when running FI your cam should have an LSA of 115 or better but what would happen if ...

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    Bye Bye Officer woodymaro's Avatar
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    Turbo/cam question

    I know when running FI your cam should have an LSA of 115 or better but what would happen if I installed a turbo on the cam I am currently running? Spec's are: 230/226 duration .591/.566 lift on a 112 LSA.

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    Is it because you'll lose boost with a closer LSA???

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    Or at least that's what I'm thinking...

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    Moderator 35th-ANV-SS's Avatar
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    I think there is a lot more to it than just a cam's LSA. It would also depend on your set-up. What size turbo? DP?

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    1999 TA M6 stock 853 heads, LS6 intake manifold and LT headers. It is a STS kit with a 67mm turbo.

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    Moderator 35th-ANV-SS's Avatar
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    This is a copy and paste I found that I thought had some good information.

    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.

    So remember, overlap= bad.

    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


    **Example N/A cam :**

    Duration 236/242
    Lift .568/.576
    Lobe Separation (LS) 112

    Add the intake and exhaust durations
    Divide the results by 4
    Subtract the LSA
    Multiply the results by 2
    Overlap is 15 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.

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    Senior Member Lunatikgixxer's Avatar
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    thats some good info

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    Wow, thanks!!! That really helps!!! I'm 4 degrees overlap according to that calculation. I am going to be doing a lot more research before I choose the cam. One more question... When running methanol injection, are you injecting it before or after the intercooler and is there a formula that I can use to figure out the injection rate? Something like 600cfm requires 4 liters per hour of methanol(just an example of what I'm looking for). Thanks for all the help.

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    On my supercharger set-up, I am injecting the methanol right after the MAF. This would be on the charge side of the intercooler (after), between the MAF and throttle body.

    I am not sure on a specific formula, my tuner figured out the amount

    I can tell you though at 8psi (500rwhp), I am hardly using any methanol at all with my set-up.

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    what heads, cam, displacement and exhaust are you running? Stock bottom end?

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    The motor is stock. Stock heads, cam and bottom end. I did replace the pushrods and valve springs.

    Exhaust - Kooks 1-7/8" LT's, 3" Kooks ORY and LMII.

    I have a thread in the FI section here about my build with lots of pictures if you want to check it out.

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    Moderator 35th-ANV-SS's Avatar
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    Here is my thread just for reference: http://www.ls1.com/forums/showthread...er-Build-Kinda

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    Nice build man!!! I can't wait to get mine done... Turbo kit should be in next Tuesday.

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    Moderator 35th-ANV-SS's Avatar
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    Awesome! Keep us updated and as always, feel free to ask questions. It was a long time coming, but now that I have it...I LOVE it

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