Further note on exhaust

To elaborate on the spreadsheet from last night... in my mind two items which have a major impact on the shape of your torque curve are cam geometry and exhaust manifold geometry. As per series rules I can't play with cam profiles, but exhaust is free.

With regard to header geometry there are multiple schools of thought as to calculating what effect the lengths, diameters, etc will have on performance. Helmholtz resonators are mentioned though I believe are generally out of favor for high-rpm application. I don't really have much to back that up, but it's a simple model. Sam Zimmerman did some real good research into Development and Validation of an Impedance Transform Model for High Speed Engines. Good read, and it's something I'll be taking a look at later down the road. Hopefully I still have the PDF around. It is a good bit more involved analytically and shows both resonant (peak) and anti-resonant points for a manifold. Mid-way between the two is the "wave reflection" theory. This is Caldwell's method that he has a lot of experience with.

The idea is as follows: At the end of the power stroke the combustion gasses in the cylinder are still under fairly high pressure. EVO (Exhaust Valve Opening) occurs at some angle before BDC so the cylinder has time to "blow down" and the engine isn't working against so much pressure during the exhaust stroke. At EVO you have a rapid pressure pulse that's start to travel down the exhaust primary at the speed of sound in the exhaust gas at EGT. When the pulse meets a junction such as a merge collector, you can treat the change in tube cross-section as a change in the medium in which the wave is travelling. As painful as Physics 3 was, you'll note that when you have a change in medium a portion of the wave is transmitted (to go further down into the exhaust secondaries) and a portion is reflected with negative sign back up the primaries. Now you have a negative pressure pulse traveling toward the still-open exhaust valve. The idea is that you tune your header lengths so that the negative wave reaches the exhaust valve right before it closes. The extra pressure differential across the port helps scavenge out more exhaust gas to give you a boost in volumetric efficiency (a tuning peak).

My spreadsheet takes all that into consideration and gives you tuning peaks given the length of primaries, secondary (if they exist) and tertiary (if they exist) tubes.

The other rule of thumb Caldwell had was to aim for about 300 ft/s flowrate in the exhaust at high-rpm peak power. "It seemed to work the best." I can imagine you'd want the exhaust to be open and free-flowing, but with enough velocity to get the gasses out while they're hot and inviscid and carrying some momentum.

Some people try to use the same methodology for intake tuning ("ramcharging" the intake) but I've never been able to conceptualize where you would get a strong negative pressure pulse at IVO, and from practical experience playing with intake runner and exhaust header lengths, the exhaust seemed to have a much greater impact. But for all I know that could just be a fallout of cam geometry...

But what do I know. I'm a tire / suspension guy.