I didn't start this thread but as I'll be going TT in a few months I find this thread very informative. This thread could save someone a lot of phone calls. I've spoken to a lot of shops in the last couple of months regarding this very subject.
Thanks to all that responded so far.
How about a few more responses from the rest of you with turbos? It would be very beneficial for anyone going the TT route ... and me of course
I didn't start this thread but as I'll be going TT in a few months I find this thread very informative. This thread could save someone a lot of phone calls. I've spoken to a lot of shops in the last couple of months regarding this very subject.
Thanks to all that responded so far.
How about a few more responses from the rest of you with turbos? It would be very beneficial for anyone going the TT route ... and me of course
Wish I could say the same. I have tried two browsers and both have the same problem. Several pages of the Turbo 101 section are overlaid on each other and end up as garble.
Wish I could say the same. I have tried two browsers and both have the same problem. Several pages of the Turbo 101 section are overlaid on each other and end up as garble.
BJK
Bruce, here you go buddy...hope this helps some.
Turbo Cam Selection 101
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 -6.5 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. Feel free to contact us for your needs. Also once you have read this and want to know the theory behind turbo charging, check out our advanced look at the engineering behind turbochargers
Stay in touch and keep us posted when you start your TT project.
Will do but it won't be near the beast you've created. I followed your build ... very nice!
My goals are pretty modest compared to your TT setup but I figure to be around 550-600 rwtq with the cam and stock heads and somewhere between 7.5-9.5 psi.
Again, thanks to all ... very informative.
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2007 LeMans Blue Coupe Z51 MZ6
Grand Sport widebody, APS Twin Turbo & exhaust
Will do but it won't be near the beast you've created. I followed your build ... very nice!
My goals are pretty modest compared to your TT setup but I figure to be around 550-600 rwtq with the cam and stock heads and somewhere between 7.5-9.5 psi.
Will do but it won't be near the beast you've created. I followed your build ... very nice!
My goals are pretty modest compared to your TT setup but I figure to be around 550-600 rwtq with the cam and stock heads and somewhere between 7.5-9.5 psi.
Again, thanks to all ... very informative.
Thanks!
My street setup is approx. at 640rwhp/680rwtq (8psi), anything above and beyond can get pretty scary on street rubber and unnecessary unless I'm in a track environment. The most rewarding part of a TT setup I have discovered is adjustability....your desire for 7.5-9.5psi is as simple as a push of a button.
Feel free to send me a PM if you have an questions about the APS kit, I'm no expert but here to help.
Corvette Store
Post your Turbo cam specs
Here is the vid link 
