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Old 12-23-2013, 11:51 AM   #50
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Originally Posted by InvalidJohnny5 View Post
I honestly don't think this engine was built to be an NA engine, it just reacts too well to any properly installed/tuned FI system.
Originally Posted by archer View Post
I dont know if this is true or not but i heard before that this car was actually developed to have a turbo originally but they dropped the turbo to keep the car more affordable. If this is true, then your spot on saying this engine wasn't built to be NA.
It's a happy accident that the engine doesn't tend to blow up when you boost it. There was a significant safety margin in the stock components, and the ECU/control system is forgiving. The heads and cams certainly were not designed for boost.

[QUOTE=Ice;1406884]I disagree with this. People always assume that because the static compression is high that there isn't much left before the engine goes bang. The reality is that it's the final figure after cam bleed off, engine age etc that determine whether the static compression ratio works. (My engine for instance starts of with 12.4:1 static, its dynamic is about 10.3:1).[quote]

I've never seen specs on the stock intake intake cams. When do the intake valves close at full retarded cam phasing, at say 1mm lift?

There are two factors at work. One is based on simple geometry, and one is based on airflow. First is the effective volume at intake closing timing. Once you close past about 10-20 degrees after bottom dead center, your effective compression ratio decreases in the geometric sense. Your AVCS tuning changes your dynamic compression ratio in that way.

The other factor is filling efficiency. This is based on the closing timing, engine speed, and pressure drop across the intake valve. This exact relationship is unique to every engine, but basically at higher speeds you get better cylinder filling at later intake valve closing. This is due to the inertia of the air. Beyond that there is also dynamic effects of the intake manifold.

Don't forget too that there is compression ratio, and there is expansion ratio. The actual compression ratio is going to be determined by geometry and intake valve closing timing, and the expansion timing is determined by the exhaust valve opening timing. Retarded exhaust valve opening gives a greater expansion ratio and thermal efficiency, but it also requires more pumping work because of a weaker blowdown pulse.

Part of the reason why this geometric compression ratio debate isn't so important is because most of the builds we're talking about here are running E85 and thus knock resistance is not a big deal.
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