Dynamic compression ratio, cams and turbo's

[witko999]

Hi.

I have a bit of a technical question about dynamic compression ratios and turbo's (and cam's).

This article:

http://cochise.uia.net/pkelley2/Dynam

suggests that the optimum dynamic compression ratio for an engine is usually 7.5 - 8.5:1. Now I understand everything written in the article but am wondering what the situation is when a turbo is added to the engine.

For example, I have deduced that my engine, with a static compression ratio of 8.5:1, will have a dynamic compression ratio of 6.01:1 with the standard cam that closes the inlet 78 degrees ABDC.

If I change the cam to one which closes at 63 degrees ABDC, the dynamic compression becomes 6.85:1. This is the mildest cam I can fit without having one ground.

The question is will I gain performance by switching to the milder cam? I know that the dynamic stroke will be greater and therefore less of the air/fuel mix will be pushed out of the (still open) inlet valve. Also, since my cylinders will be more than 100% full (due to the turbo), there will be less (or no) ramming effect with the longer duration cam than there would be with natural aspiration.

Therefore, it is my opinion that I'd be better changing to the milder cam. Does anyone with experience in this area agree/disagree?

Cheers

[Boosted LS1]

I've not read the link but have been pondering about this myself. I think (and I stand to be corrected) that the basics are the same whether blown or atmo. You compress whatever you put in the cylinder to, up to 9:1 dynamic c/r. The extra fuel in the blown engine will give a longer burn and more torque.

Now, I'll read the link and see where I've gone wrong

Boosted.

[Mave]

witko999 said:

Hi.

I have a bit of a technical question about dynamic compression ratios and turbo's (and cam's).

This article:

http://cochise.uia.net/pkelley2/Dynam

suggests that the optimum dynamic compression ratio for an engine is usually 7.5 - 8.5:1.

I don't think it quite suggests that; I would interpret the article as suggesting that you want as high a DCR as possible, and that 7.5 - 8.5:1 is about the limit for 91 octane fuel. You could aim higher if running on 95 or super unleaded.

witko999 said:

For example, I have deduced that my engine, with a static compression ratio of 8.5:1, will have a dynamic compression ratio of 6.01:1 with the standard cam that closes the inlet 78 degrees ABDC.

If I change the cam to one which closes at 63 degrees ABDC, the dynamic compression becomes 6.85:1. This is the mildest cam I can fit without having one ground.

The question is will I gain performance by switching to the milder cam? I know that the dynamic stroke will be greater and therefore less of the air/fuel mix will be pushed out of the (still open) inlet valve. Also, since my cylinders will be more than 100% full (due to the turbo), there will be less (or no) ramming effect with the longer duration cam than there would be with natural aspiration.

the arcticle acknowledges that cylinder pressure is the most important parameter. If you have a constrictive head / valve design, then you I would expect the longer duration cam to offer benefit. If you have a free flowing multivalve head, then the opposite would be the case. Also depends on where you want the power; mid range or top end.