92-93 pistons - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


1992-1999 Series I L27 (1992-1994 SE,SLE, SSE) & Series II L36 (1995-1999 SE, SSE, SLE) and common problems for the Series I and II L67 (all supercharged models 92-99) Including Olds 88's, Olds LSS's and Buick Lesabres Please use General Chat for non-mechanical issues, and Performance and Brainstorming for improvements.

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Old 04-07-2005, 10:05 PM   #1
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Default 92-93 pistons

So, I go to www.seriesoneperformance.com and check out pistons and find that 92 and 93 have different pistons available. I know that the rings are different, but when I asked DrJay about it he tells me 93'* will work in a 92 but will change the compresison ratio. I haven't got around to asking why. I suppose the compression ring (top one) is at a different height. Anyone know if it'* higher or lower? And what was different about the engines to keep the compression ratio the same between the two despite the ring difference?
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Old 04-08-2005, 12:31 AM   #2
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Ring height does not change compression.

Increasing or decreasing combustion chamber size or piston domes does.

I suspect, but can't say for sure, that the compression went up in '93. One point in compression is worth about 2% in horsepower and about 4% in fuel mileage, and GM was hurting for CAFE improvements that year.
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Old 04-08-2005, 01:33 AM   #3
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However, compression is usually lowered to compensate for an increase in boost.

So if the boost increased in 93 (although I'm uncertain if it did) then I would liken the compression was decreased.
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Old 04-08-2005, 09:55 AM   #4
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Neither boost nor compression changed. And top ring height DOES have a small effect on compression ratio. I know I'm not mistaken on that.
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Old 04-08-2005, 10:02 AM   #5
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The top ring moved down in order to prevent cracking the top of the piston.
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Old 04-08-2005, 10:10 AM   #6
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Ok. Thanks.
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Old 04-08-2005, 11:45 AM   #7
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So if the engines had the same compression ratio, but swapping pistons changes it, what else is different? Something has to be.
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Old 04-08-2005, 01:54 PM   #8
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Are the heads (and thus combustion chamber size) a different size?
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Old 04-08-2005, 02:05 PM   #9
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The heads differ, but as far as I know, it'* only the rocker pedestals that are different. 92'* don't have roller rockers.
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Old 04-08-2005, 02:40 PM   #10
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I thought the chamber might be a possiblity. So, when I put the 93 heads on, will it affect my compression? That is the real important question here. Will it go up slightly because the chamber is just a little smaller? How far did the top ring move?
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