Quote:
Originally Posted by Walperstyle
As for the compressor, I already figured out I can easily make 500hp, but boost would kick in after 4000 rpm. Looking to set these cams at a good midrange power, and let the ECU and turbo help with the top end. Mid to top is important for me.
HP goal is 500hp, 94 octane. The BSFC suggests going E85, but we simply don't have that many stations up here in Canada with corn. So I will probably have a 94 and E85 tune.
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1. The goal for most street cars should be 4,000 RPM of usable powerband. So if you are shifting by 7,500RPM you want peak boost (and hopefully peak torque) around or before 3,500. The more power you want to make, the higher you need to shift this curve to take advantage of the displacement, when you intend to run pump gas. Otherwise, it simply takes more boost, and that means higher octane fuel.
So your option for pump gas comes back to the spool characteristic of the turbocharger given the displacement of the engine. You must select a compressor that can meet your power goal (55lb/min +/-5lb/min) and do the math to figure out where 18~psi is going to land you 500 horsepower, at what RPM, or better to say this: when will just over twice the volumetric efficiency of the engine give 500 horsepower?
If we are talking 122 cubic inches then (122x7000/3456=247CFM *.069= 170horsepower @ 7000rpm given 0psi of boost (using atmospheric pressure) assuming 100% VE. Double the effective pressure and you have 340 horsepower, and in fact, most SR20 engines running 15psi on a compressor that can support (40lb/min) tend to produce about this number on pump gas- again assuming 100% VE (that means aftermarket cams), notice OEM camshafts on SR20 engines tend to produce much less power? Because VE is dropping by that 7,000rpm, simply.
You want headroom in the compressor for overlap losses and drivetrain losses. Giving some up to overlap can be a good way to clear out the cylinders between events, although it is wasteful. On the path to power, sacrifices must be made.
That is where you are going with this "cam degree" talk. You are trying to maximize only the VE of the engine- regardless of the boost pressure used, or compressor flow sacrificed. Although in fact, as boost is factored into the equation, the effects of pressure and the relative sizes of the exhaust manifold plumbing and anything after that- including the way the wastegate vents the exhaust gas- all comes into play... but it does not directly alter the breathing characteristics of the engine, so cam timing that yields 100% VE with atmospheric pressure is going to give the engine that same opportunity when boost pressure is applied, plus or minus the effects of the turbocharger being in the way, and the routing of the exhaust gas by the wastegate, and the additional pressure/heat contained within the plumbing of the exhaust system.
Traditionally, in the search for the best possible VE across the board, you could run the engine at atmospheric pressure or low boost, and dial the cams in for best torque area under the curve.
edit: You may need to adjust the exhaust cam afterwards, reducing overlap, when using a small turbine. (while I was sleeping I thought of this possibility to include)
Forget magic numbers, there arn't any, unless you find the exact same combination of parts somewhere else with paper results, and even then its still not a sure thing.