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Real compression ratio using 2.4 heads

Hey guys,

Ií starting to rebuild the engine of my 2.4S with a QSC/JE set of 92 mm bore and compression ratio 9.5 :1. Using the 2.4 heads of the original engine, what is the expected CR?

Considering the prevalent street use of my car with 95-98 octane gasoline, will be necessary the twin ignition?

Finally, using the original valves (In 46mm, Ex 40mm) can you suggest the best port size for a prevalent use at 2000-5000 RPM?

Thank you in advance for your time,

Paolo

Old 06-10-2008, 11:32 PM
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Paolo, not enough information.

Do you know the dome volume of the pistons? You say they are 9,5:1, this assumes what combustion chamber volume? Most later heads are 68cc but you should measure.

95-98, is that research octane or motor octane? I think Europe uses Research octane. At 9,5:1, twin plug probably not required for street use.

Keep the ports the same, port enlargement only necessary for high-rpm power for racing applications.
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Old 06-11-2008, 08:22 AM
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Real compression ratio using 2.4 heads

John, the dome volume of JE pistons (CR 9,5:1) is 19,7. In another thread of this forum i have read that using piston with CR 10,3-10,5:1 with 2,7 heads leads to a final CR of 11,5-11,7:1. So, my question about the final CR using piston with CR 9,5.
Old 06-11-2008, 08:55 AM
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Paolo, according to my calculations 92mm on a 70.4mm crank gives you 2808 cc (a 2,8), with 19,7 cc dome volume, you end up at 9,5:1, assuming a 68cc combustion chamber.

I think what you are reading from the other thread is using 906 pistons intended for a 2,0 liter twin plug engine with later 2,7 heads with modification to the cylinder to make them work. Going from a 72cc combustion chamber to a 68cc chamber achieves the increase in compression you describe.

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Old 06-11-2008, 10:16 AM
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The published 9.5:1 ratio, with the 92mm JE pistons, was probably referenced to using them on a what was a 2.7L motor. The 2.7L motors used the same basic head and crank as the 2.4 motors so no change in compression ratio should be seen.

If the motor has been rebuilt before and the case and heads were decked then there might be a slight increase in CR after you have them decked for this rebuild but I still don't think you would need to twin plug because there is still room on the table for a slight compression increase without need for twin plugs.

As John suggested stay with the stock ports.
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Old 06-15-2008, 10:28 PM
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real compression ratio using 2.4 heads

John, Bobby, thank you very much.

I have read the discussion between YTNUKLR and Steve Weiner in a past thread:

“Originally Posted by YTNUKLR:
I like! They make a GREAT engine with a 66mm crankshaft. 2.63L short-stroke. "10.3:1" was advertised with the RSR heads, which had bigger head volume than 2.7 engines. On a 2.4/2.7 70.4mm crankshaft with 2.7 heads (usually about 76cc chamber volume), compression is probably around 12:1. With a 2.0/2.2 crankshaft it drops to about 11:1, maybe even a little higher..no other modifications needed with a 2.7 case. VERY snappy! Twin spark plugs are a MUST”

Hi Scott,
The 92mm RSR P/C's make around 11.5-11.7:1 with the 70.4mm crank using the normal 2.2-2.4-2.7 heads. With a 66mm crank and the same heads, they make around 10.5-10.6:1.

B. Anderson, page 139, figure, comparing 2.8 RSR and 2.7 heads, showed the effects of the head volume on the compression ratio.

So, my error.
Concluding, 92 mm bore pistons by JE have a compression ratio referred to 2.4/2.7 head, differing by mahle 92 mm bore….



….wonderful forum!!

Last edited by 304065; 06-16-2008 at 04:39 PM.. Reason: removed line breaks to make easier to read
Old 06-16-2008, 04:17 PM
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3.6L pistons/cylinders with 3.2 crank

I too have a similar question.

Have a supercharged 911 carrera which had 3.2L pistons machined. Replaced them with lower compression 3.6L turbo pistons and cylinders. I am trying to figure out the real compression ratio i have, seeing as that the crank is still from a Carrera. I believe the factory specs on the turbo pistons are 7.5:1 Thanks for the help!

Tauran

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Old 06-19-2008, 11:48 AM
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