Compression Ratios

bynummustang said:

What thickness gaskets? That will change it a bunch. If you use a .05" compressed thickness gasket, you'll be at 9.22:1.

Engine Size and Compression Calculator

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bynummustang said:

I'd go a little lower than that. Run a google search for static and dynamic compression ratios.

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FastDriver said:

My thoughts/rambling:

I'm not sure if you intend to do so on pump gas. If so, I personally wouldn't be interested in running that much boost on pump gas with my car, but there are plenty of '03 Cobra owners who do it. Their Static Compression ratio is 8.5:1. However, there are a lot of important factors to take into account, and static compression is only one of them. Total timing is very important, and if you retard it enough, you can prevent detonation at any boost pressure (within reason). However, hotter exhaust and coolant temps, less power, and burned exhaust valves will result as this envelope is pushed. Far more important than static compression ratio is your dynamic compression ratio, which has much more to do with your camshaft than your static CR. The efficiency and material of the cylinder heads makes a difference too.

In my case (a turbocharged, intercooled 331 with AFR heads, 8.5:1 SCR, a custom turbo cam, and more), I'm sticking to 15 psi, though the car has momentarily seen 17-18 psi and lived through it. I'm glad I had the built in wiggle room. If I check to see what psig-max is and it is more than 15 psi, I know that I have lost control of the boost, and can make adjustments accordingly. If I were already at 18 psi and got a boost spike, it might be too late to do anything about it at that point. However, with a blower, boost spikes are extremely improbable, I think. So, I can understand how one with a blower might be more inclined to push the ragged edge.

On the other hand, with those blowers, despite their relative efficiency, you don't have an intercooler to keep IATs down. I don't know man, my attitude is that even an extra 100 rwhp would still not be worth living with the worry that I'm going to break stuff.

Chris

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