Hey guys, just wondering about that- say I rebuild this motor, and shave the heads a bit, to raise the compression ratio. (Stock is 8.9:1.) Will I confuse the computer? Will I raise emissions? I'd imagine it would. I always hear of people upping the compression ratio, but never hear of the computer/emissions results.


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
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The computer could care less (it cares about vacuum). The fuel you use would matter more (when increasing CR) than the computer issues. When raising the static CR, you also increase HC (IIRC, might be NOx, but doubt it). This is because with higher compression pressure more fuel is squeezed into the tiny gaps where the spark can't ignite it. It won't throw you out of the allowed range though.

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George P. Lara
1994 Z28 LT1 T56
1984 Z28 High Output
Member: SCCA, SCFB, SC3GFB

[This message has been edited by MRZ28HO (edited November 16, 2000).]

George is right, for the most part the computer has no way of being aware of, or affected by, the CR. Things that affect vacuum, and more specifically the vacuum at idle, are what gives the ECM fits, because so many of its decisions are based on MAP. In fact, higher CR with all else constant may actually make the computer happier in some cases, because it will make the vacuum go up.

With increased CR, NOx emissions go up slightly up to a point, then rise rapidly; the reason is that the combustion temperatures reach a point where the nitrogen in the air starts to burn (combine with oxygen). In general you'll have major pinging problems before that will happen, at least with pump gas.

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"So many Mustangs, so little time..."

I cannot add anymore on the ECM issue and higher compression, George and RB answered it completely.

On the emission issue, I agree with the NOx as RB described. In fact, as HC and CO goes, higher compression will tend to reduce it as the higher temperature will combine it more into H20 and C02. The only problematic one is NOx as RB described.

Where high compression and HC and CO become a problem is when you get longer duration cams that bleed off some of the pressure from the cylinder, especially at lower rpms. And this is where your vacuum drops and THEN you have an ECM problem.

But, excluding longer duration cams, higher compression is actually a good thing as emissions go (overall). That is why all the car manufacturers are creating higher compression engines today.

Thanks for that information! I think I'll be putting in a computer-friendly Compucam camshaft instead of stock, so I don't think I have to worry too much about a loss in vacuum. So you think if I shave it all a bit, and maybe went to 9.1 (can they do increments that small), I'd be okay? I run 93 octane gas right now anyway, since my ADS Superchip requires it.

Won't I also lose some compression if I chamfer the edge of the head's combustion chamber?

I was also considering a different set of heads; GM put aluminum heads with a splayed valve arrangement in the "Generation II" 2.8 v6's. Going by what you guys said, it seems that a swap to these parts (requiring the heads, pistons with different valve reliefs, and a different intake manifold) wouldn't raise emissions either... right?

Thanks!


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-Tom P (Hot rodded 1986 Firebird 2.8l) from http://www.f-body.net/mailbag/3rd/3rd_mailbag.html message boards
---Think your car could be pic of the week? Visit http://www.f-body.net for details!