Whats the problem with 10.5:1 comp ratio?

[farleyd]

My JEs pistons have been identified as 10.5:1 CR by EBS. They mention not to use this on a street engine with regular gas. Could someone explain?

Also, can i - and to what - lower the comp ratio with thicker cylinder gasket??

Please help me save the pistons... and the 200US$ i paid for them...

[TimT]

You should be able to use 10.5:1 cr on a street car provided you have the required clearances( piston/head and valve/piston), and you use PREMIUM fuel,93-94 octane. 10.5:1 is about the limit before you have to use race type fuels.

Im guessing EBS said not to use regular gas ie 87 or 89 octane.

[ChrisBennet]

Tim: I don't know, I think 10.5:1 is really pushing it. The last time I was in Quebec I didn't notice any really high octane pumps. I know they didn't have any 94 octane near Mt Tremblant. I have 10.5:1 JE's in one of my motors but it is twin plugged and I run 94 octane.

Faryleyd: I would look into having a machinist remove some material from the piston tops. I think it would depend on how thick the top of the piston was. Perhaps JE or EBS could give you some advice in this matter? If it was my motor I'd go for no more than 9.8:1. For comparison, a Euro 3.2 is supposedly 9.5:1 but actually measures 9.2:1.
While thicker base gaskets will decrease compression they will also increase deck height. I've been told that a larger deck height can actually precipitate detonation along the edge of the piston.
What kind of motor is this anyway?
BTW: $200 for a set of JE's is really a good deal.
-Chris

[farleyd]

Hi Chris, its for use in a 2.2S engine. My idea was to use a 2.4 crank but that would take the comp ratio up to 11.2:1!

We do have some 94 octane pump in some area, mostly in big cities, where you dont go in a 914/6.....
Id go along the removing material, form the pistons. here they are:


Does someone has an idea on how to do this, remove material? And how much?

[snowman]

Do not go above 9.5 to one for a street engine on pump gas. You will regret it and your engine WILL break if you do not. Air cooled engines need almost one additional point for compression vs gas engines. People with 10:1 gas engines seem happy, With 10.5:1 not so happy, thats with water cooling. I just replaced the 10:5 to ones on my BMW with 9.8 to ones and have not regreted it one bit.

[911pcars]

I believe aftermarket piston manufacturers build enough "meat" in the piston so they, the machine shop or builder can "customize" the compression ratio to suit the engine. In this case, I think a call to JE will answer your question about the amount of material you can safely remove.

Radical cams wll lower the effective compression pressure. However, I'd follow EBS's recommendations. Are they indirectly saying it's okay to run with premium fuel?

Sherwood Lee
http://members.rennlist.org/911pcars

[TimT]

I keep forgeting 94 octane isnt available everywhere, guess im spoiled.

That being said, Faryleyd should find out the "actual" cr JE made them as 10.5:1, but what is the actual cr with the pistons installed? measure deck hieght , dome height etc. calculate cr.
The difference in dome hieght between 9.5/9.8 and 10.5 isnt that great. The pistons could easily be machined to reduce the cr.

[Mr Beau]

If you're running "S" cams, you can get away with bit more compression than would normally be assumed, and the smallish bore doesn't hurt either. The factory ran 9.5:1 with premium (91?) fuel, and with ignition timing at 32-36 degrees. If you can regularily find 93 octane fuel and keep ignition timing to more than 32 degrees, you'll probably be OK.

Safest thing to do would be to have some material removed from the pistons though. If you get down to 9.5:1, you'll be emulating exactly what the factory did. Are your sure they're JE's? You could send them back to JE, but probably easiest to use someone local to you. Just call around some engine shops and let them know that you need some pistons flycut. If you provide all your measurements (displacment, head cc, etc), they can figure out how much material to remove to yield your desired compression ratio.

If you're sticking with the 2.2 crank to avoid too high a compression ratio, you'll get more power from the added displacement. Shoot for the same 9.5-10.0:1 range, and you'll be very happy with your "2.4S" engine.

Hope that helps.

Matt Beaubien
'73 911E

[Wayne 962]

Anything above 10:1 with single plug on street gas will not perform too well. You'll have to adjust the timing to prevent detonation and that will affect power...

-Wayne

[Wayne 962]

Oh, as I mentioned in the email to you, you need to inspect and check those pistons for wear before you reuse them again. In the photos, you still have the rings on them - you need to check the ring groove for wear (as shown on Page 56 in the Engine Rebuild book).

-Wayne