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930gt-40r's Avatar
 
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compression ratio

On a 3.3 motor with stock pistons (97 mm) that have had the edge milled (to clear head) acting on a 3.6 crank, what would the compression ratio be? Keep in mind that the 3.6 crank is 2mm longer stroke.
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2008 C6/Z51 Corvette
Old 03-31-2007, 07:01 PM
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Just the displacement increase alone would yield 7.63. However, the unknown is the volume remaining that was not machined off to clear the head. Chances are, the actual result would be higher.
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Old 03-31-2007, 09:35 PM
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so you dont think that it would be high compression like 9 or 10:1? The pistons only had the edges milled down to clear the heads.
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Old 04-01-2007, 06:15 AM
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My 3.4 with 98 mm pistons with the new 3.6RS crank resulted in a compression ratio of 7.72
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Olli Selander

1989 930 - 3.5l - M48 - G64/51
Old 04-01-2007, 08:18 AM
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Oms, What piston crown hieght did you use? Did you get custom pistons?
I need to get the exact formula in order to calculate it.
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Old 04-01-2007, 12:19 PM
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Quote:
Originally posted by 930gt-40r
Oms, What piston crown hieght did you use? Did you get custom pistons?
I need to get the exact formula in order to calculate it.
The pistons are 98mm Mahles which have flat tops but I'd need to get the details (numbers) going into the formula from my builder. You may have seen this already, but just in case, here's a thread dealing the compression ratio calculation:

http://forums.rennlist.com/rennforums/showthread.php?t=325101
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Olli Selander

1989 930 - 3.5l - M48 - G64/51

Last edited by oms930; 04-01-2007 at 03:08 PM..
Old 04-01-2007, 02:54 PM
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Thank you, that was very informative- Im thinking the C/R will not be that high which will leave nice room for boost to be squeezed in there.
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Old 04-01-2007, 03:27 PM
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probably a dumb question, but doesn't the effective C/R change when the chambers get boosted?

anyone know an equation?

Bill K
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Old 07-18-2007, 08:28 AM
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Yes, the effective - or dynamic - compression does change under boost. It is not directly proportional to the increase in absolute pressure as the effeciency of the engine will cause a reduction. More air forced through the same size holes (vavles, intake, etc.) will result in less air overall. This is one of my favorite links for approximate calculations;
http://www.turbofast.com.au/javacalc.html
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'77 930 turbo Garretson I/C 1 BAR spring, (2) '82 Triumph Bonneville Royal Wedding Edition
Past rides: '74 914 1.9 liter twin plugged track car, '83 928S, '87 924S, '75 911S w '78 ROW 3.0, '72 911T, '70 911T and various other insignificant domestic examples. Happiness is a grey tailpipe! Turbo lag......it's worth the wait!
Old 07-19-2007, 12:54 PM
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thanks, pretty cool link.
I estimated VE at 70% and the numbers came very close to what I was expecting for the 3.0 L.
Is the Volumetric Efficiency estimate black magic?
Bill K
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Old 07-19-2007, 01:42 PM
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