Compression ratio

[rgoodrich]

I'm doing a second top end rebuild in 1700 miles because my head mating surfaces were pitted by an unamed machine shop. New machine shop trued at .011. My mechanic is using a .05 head gasket for safety. The pistons are Euro 3.0's at 9.8:1 compression. Will there be any significant change in net compression ratio?

[snbush67]

You might have a better shot at getting an answer to this by posting in the 911 engine building forum.

You will probably have to cc your heads, pistons and cylinders in order to get your theoretical CR.

I am merely a novice but I have been doing a lot of reading and research on this lately and if you think you are going to be close to 10:1, I have been advised to approach with caution.

Are you single plugged? What size engine?

I don't think you have a Head gasket, he must be referring to the cylinder base gaskets, these can be stacked to reduce the CR. They come in three sizes .1, .05 and .025 I think.

Shane

[Steve@Rennsport]

Originally Posted by snbush67

I don't think you have a Head gasket, he must be referring to the cylinder base gaskets, these can be stacked to reduce the CR. They come in three sizes .1, .05 and .025 I think.

Shane

Shane,....

Just an FYI for you and anyone reading this thread,....

Base gaskets should NEVER be used to lower/raise CR; these are strictly for adjusting deck height. Too little deck height allows the pistons to strike the heads when the engine is at operating temperature. Too much deck height makes the engine more susceptable to detonation.

Piston configuration and chamber volume is what is used to make corrections/changes in CR,.........

[T77911S]

Originally Posted by Steve@Rennsport

Shane,....

Just an FYI for you and anyone reading this thread,....

Base gaskets should NEVER be used to lower/raise CR; these are strictly for adjusting deck height. Too little deck height allows the pistons to strike the heads when the engine is at operating temperature. Too much deck height makes the engine more susceptable to detonation.

Piston configuration and chamber volume is what is used to make corrections/changes in CR,.........

question, why arent cast iron block engine suseptible or adjusted for this? is it due to the heads?

[Jeff Higgins]

Originally Posted by T77911S

question, why arent cast iron block engine suseptible or adjusted for this? is it due to the heads?

They are, just in a different way. I assume you mean something like an American V-8.

There is no stack-up tolerance of parts on one of these motors. Crank centerline to the deck dimensions are easily controled by machining the deck. The nature of the split case, built up air cooled motor is that this dimension varies with the tolerances of the various components used. Not by much, but then again, base shims aren't very thick, either. We are simply trying to compensate for a bit of tolerance build up in the air cooled motor that we don't get in a monolithic engine block like on a V-8.

[911st]

You should be able to re-establish your CR with the base gaskets and come in at spec CR.

I think Steve is saying you should not use base gaskets to lower your CR from 9.8/1.

Shaving the heads on a 930 .040 adds about 1/2 point to compression. If this relationship is right and you do not account for it with base gaskets, .011 would make for about 1/8th point increase and put you near approximately 9.9/1.

Someone could do the math but the only way to know exactly is to calculate the cylinder sweep, measure the piston dome, and chamber volumes to compute the CR. Most 911 motors are reported to come in under there spec CR.

Not sure this helps.

[T77911S]

Originally Posted by Jeff Higgins

They are, just in a different way. I assume you mean something like an American V-8.

There is no stack-up tolerance of parts on one of these motors. Crank centerline to the deck dimensions are easily controled by machining the deck. The nature of the split case, built up air cooled motor is that this dimension varies with the tolerances of the various components used. Not by much, but then again, base shims aren't very thick, either. We are simply trying to compensate for a bit of tolerance build up in the air cooled motor that we don't get in a monolithic engine block like on a V-8.

yes, V8's. but wouldnt you have to do a dry build, measure then take apart to have any machining done? i guess i have never heard of any V8 guys doing this or worrying about deck height.

[Jeff Higgins]

Originally Posted by T77911S

yes, V8's. but wouldnt you have to do a dry build, measure then take apart to have any machining done? i guess i have never heard of any V8 guys doing this or worrying about deck height.

Yes, this is how it is done. Much easier on a V-8 than a 911 motor, though. You put together much less of it to get that far, at least as far as setting deck height is concerned. This is a seperate step from checking piston to valve clearance, which involves darn near as much "dry assembly" work as it does on a 911 motor.

[Steve@Rennsport]

Originally Posted by T77911S

yes, V8's. but wouldnt you have to do a dry build, measure then take apart to have any machining done? i guess i have never heard of any V8 guys doing this or worrying about deck height.

Yessir, thats correct,...

Measuring and correcting deck height requires assembly of one piston assembly in a V8 (assuming the decks have been straightened and the main bearing bores are aligned).

Been doing such things since 1963,....

[snbush67]

Originally Posted by Steve@Rennsport

Shane,....

Just an FYI for you and anyone reading this thread,....

Base gaskets should NEVER be used to lower/raise CR; these are strictly for adjusting deck height. Too little deck height allows the pistons to strike the heads when the engine is at operating temperature. Too much deck height makes the engine more susceptable to detonation.

Piston configuration and chamber volume is what is used to make corrections/changes in CR,.........

Thanks sir, I did mention I am a novice.

So just to clarify the CR of a piston as listed is based upon a stock unmolested head. IOW if I am putting a 9.8:1 piston in a shaved cylinder I could come up with a higher CR ?

And rather than using base gaskets to increase the cylinder volume and decrease the CR I should select a lower CR piston.

...I probably just wrote something really stupid

[RoninLB]

air cooled 911's are very heat sensitive in general from top piston ring to above unlike water cooled bock and heads

even to the point that oil carbonizing may become an issue

afai know