Quote:
Originally Posted by R/TErnie
Port velocity based is based on the kinetic energy of the gas leaving the port and the volume leaving the cylinder. If you improve the VE of the engine by tuning it... add several degrees of timing in the known dip and give it more fuel you're going to increase the combustion temperature, cylinder pressure, VE, and your blow down pressure. This will all change how your header scavenges the port. Just saying you may be compromising your top end power to support the mid range, which will be fine once the engine is able to be tuned. I've done a lot of Engine Dyno work and designed a bunch of header, intake manifold, and cam packages... It's hard enough to pick the right compromise when you have good data right in front of you. My suggestion would be don't throw away any of your old designs... and break them out and re-test them after you're able to tune the engine.
So I guess all I'm saying is... keep making new designs. Try making a single change to the header you have with the assumption that it will hurt performance and validate your theory. I always find the most interesting stuff when I do counter-intuitive work... like working on TDI swapped Vanagons.
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Alternatively the current scavenging is not as good in that range hurting VE and the ability to get good fuel/air mixing. This causes a reduced flame speed and the need to advance timing to make peak cylinder pressure at ~14 degrees ATDC.
If the cause is local pressure differentials caused by intake or exhaust acoustics, the AVCS will probably be adjusting in that range. Everyone is calling this a 'dip' based on relative to the rest of the curve. But realistically it is still making good torque there for a 2.0L NA motor. So it's possible that this is just a zero scavenging zone, as opposed to bikes where pipe acoustics can create their flat-spots by 'bad' scavenging like double carburetion and whatnot...