ls7 compression ratio 11.8:1?
09-06-2006, 12:11 AM
#1
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ls7 compression ratio 11.8:1? Hey folks,
I'm doing some math to try and figure out how much chamber volume I have to add an ls7 to get the cylinder pressures down to an appropriate level for a forced inducation app.
I've been out of the gm gen-iii/iv loop for a while so I had to do some research to get the figures I needed.
As all the readers of this forum know, In order to calculate compression ratio you need to not only need to know how much volume the head and the piston contribute to the combustion area, you also need to know how much volume the gasket and the piston to block phasing contributes.
I was able to get basic measurements (cylinder displacement, compression ratio, etc) for ls1/ls2/ls6 engines to calculate these figures.
Here's basically what I have:
LS1: 5.66L, 708 CCs/cyl, 10.1:1 CR = 77.8 CC combustion area
LS6: 5.66L, 708 CCs/cyl, 10.5:1 CR = 74.5 CC combustion area
LS2: 5.96L, 745 CCs/cyl, 10.9:1 CR = 75.3 CC combustion area
LS7: 7.00L, 875 CCs/cyl, 11.0:1 CR = 87.5 CC combustion area
I also scrouged on the web and found out that LS1 heads are 66.7CC, LS6 are 64CC, LS6 are 64CC, and LS7 are 70CC.
So for a LS1, the 77.8CC combustion area, minus its head volume of 66.7CCs, means the gasket + piston phasing (whether manufacturing has the piston zero deck or not) = 11.0CCs.
I calculated this figure for the LS2 and LS6 as well, all roughly 11 CCs of volume for the head gasket and the piston phasing.
However for the LS7 with its 87.5CC combustion area and an 11 CC gasket, there is 5.5CCs missing. Even calculating based on a 4.125" bore gasket there is still a 5.0CC discrepancy.
So either the LS7 has dished pistons, or the gasket is thicker, or the compression ratio is actually 11.8:1 rather than advertised 11.0:1. Or I made a math error.
I checked for dished pistons, according to the article I read (and the associated pictures) they are not.
Does anybody know what I'm missing?
I'm doing some math to try and figure out how much chamber volume I have to add an ls7 to get the cylinder pressures down to an appropriate level for a forced inducation app.
I've been out of the gm gen-iii/iv loop for a while so I had to do some research to get the figures I needed.
As all the readers of this forum know, In order to calculate compression ratio you need to not only need to know how much volume the head and the piston contribute to the combustion area, you also need to know how much volume the gasket and the piston to block phasing contributes.
I was able to get basic measurements (cylinder displacement, compression ratio, etc) for ls1/ls2/ls6 engines to calculate these figures.
Here's basically what I have:
LS1: 5.66L, 708 CCs/cyl, 10.1:1 CR = 77.8 CC combustion area
LS6: 5.66L, 708 CCs/cyl, 10.5:1 CR = 74.5 CC combustion area
LS2: 5.96L, 745 CCs/cyl, 10.9:1 CR = 75.3 CC combustion area
LS7: 7.00L, 875 CCs/cyl, 11.0:1 CR = 87.5 CC combustion area
I also scrouged on the web and found out that LS1 heads are 66.7CC, LS6 are 64CC, LS6 are 64CC, and LS7 are 70CC.
So for a LS1, the 77.8CC combustion area, minus its head volume of 66.7CCs, means the gasket + piston phasing (whether manufacturing has the piston zero deck or not) = 11.0CCs.
I calculated this figure for the LS2 and LS6 as well, all roughly 11 CCs of volume for the head gasket and the piston phasing.
However for the LS7 with its 87.5CC combustion area and an 11 CC gasket, there is 5.5CCs missing. Even calculating based on a 4.125" bore gasket there is still a 5.0CC discrepancy.
So either the LS7 has dished pistons, or the gasket is thicker, or the compression ratio is actually 11.8:1 rather than advertised 11.0:1. Or I made a math error.
I checked for dished pistons, according to the article I read (and the associated pictures) they are not.
Does anybody know what I'm missing?
Last edited by Slart; 09-06-2006 at 09:57 AM. Reason: minor clarification
09-06-2006, 09:56 AM
#3
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No, I assumed they were 0 cc pistons. But if they are not, then the question becomes if the ls1/ls2/ls6 pistons also add combustion area. If so, then are their advertised compression ratios wrong?
Steve
Steve
