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Old 10-08-2003, 12:18 PM
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Compression

I've been reading a lot of posts here, and gathered that, in a typical hot rod motor, compression above about 9.0:1 doesn't work well with pump gas. And I think I understand that this is due to detonation, that lower octane fuel will combust before spark in a high compression engine.

But then I read Road and Track, and see that some new cars have compression ratios of up to 11.0:1. How is this? These cars are run on pump gas too, what is different about newer engines that the compression can be boosted with no ill effects?

Thanks for enlightening me,

Forrest

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Old 10-08-2003, 01:25 PM
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aluminum heads
better combustion chamber design
computer that keeps fuel mixture perfect at all rpm
port injection so all cylinders have same a/f ratio
knock sensor to sense detonation and "save" engine from detonation
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Old 10-08-2003, 01:52 PM
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Actually, if you build your engine correctly, you can run 10.5 ratios on pump gas.

The cam has alot to do with cylinder pressure, ( not to be confused with compression ratios). If the cam has alot of overlap, you can have 15 to 1 pistons and still have low cylinder pressure, ( rated in pounds of compression)
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Old 10-08-2003, 05:31 PM
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Thanks guys. One more thing; why do aluminum heads help prevent detonation? Thanks again,

Forrest
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Old 10-08-2003, 11:42 PM
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They allow more heat to escape the combustion chamber, causing less pressure.
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Old 10-08-2003, 11:59 PM
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I am somewhere between 10.5:1 and 11:1. My timing is pretty agressive and I can only get 91 octane gas. It was 80o today and I had to drive up hill in the mountains. My car was fine.

You can run higher compression as long as you do your homework first. Don't listen to people that say don't go over 9:1. That is just what someone who has never researched the subject says.

I had to do a few things to live comfortable at my compression. I polished my pistons, deburred and polished my combustion chamvers in my heads. My cam has 234o duration @ .050. My timing is at 34o total. My timing is all in by 2,500 RPMs which keeps it from dwelling in the ping zone unecesarilly. I only use good quality gas (Chevron or 76 only). Don't even think of going to Shell. I have a 2,500 stall converter which keeps the RPMs up and keeps the engine from bogging down out of idle. And I use a 160o thermostat to keep engine temp down.

The higher the compression the better. Just leave yourself some headroom in case you get bad gas, or anything else happens that could put your engine in danger.
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