Lowering High Compression

[dimps8]

Anyone ever heard of lowering the compression ratio on a 912 engine using shims under the cylinders?

My high comp (10.5 to 1) runs 98 octane fuel here in Oz, and having to detune it as to keep detonation to minimum, I want to try something else. (not twin plugging)

[camgrinder]

Sometimes shimming the barrels out will not cure the problem. Did you use a performance cam when you built the engine? If so what are the specs?

[dimps8]

John,

I'm not sure about the specs of the engine. It was built over the pond in the US. The PO said 10.5 To 1 race engine was built, Isky cam, arias pistons etc. This seems consistant with the slight detonation even at about max 20 deg BTDC advance.(i'd rather have 35 deg). The cam is very agressive, really hits power about 4 grand. May have to find some race gas.

[camgrinder]

Have you done a cranking compression test?
A camshaft with a 4000 and up rpm band should work well with 10.5-1 compression ratio.

[dimps8]

I remember doing one a while ago. the numbers were quite low, around 100 psi. An agressive cam can cause low figures I believe.

When i set the dynamic advance at 35 deg@ 3500 rpm the engine screams as you would expect, but it's the hot dry days we get here in Oz, when the air thins out, I get detonation. I just want to ensure i'm not melting pistons.

[cnavarro]

Depending on the forging or casting used for your pistons, you could cut deeper valve reliefs and still maintain an optimal .040-.060" deck height, rather than shimming the cylinder out. It should be reasonable enough to get an extra few cc's out of the pistons without compromising the strength, but you will have to take a few crown thickness measurements to be sure.

Charles Navarro
LN Engineering
http://www.LNengineering.com
Aircooled Precision Performance=

[camgrinder]

If you only have 100 lbs cranking compression, I suspect someone might have shimmed the cylinders out allready. When the deck height is increased the tendancy for detonation can increase as well.
Can you extend the advance curve in the distributor? Have the full advance come in at 5000 instead of 3500?
Any idea how your 98 octane fuel compares to the U.S. spec fuel?

[cnavarro]

The 98 would be comparable to our 93 octane here in the midwest. I don't know what to compare California gas or any of the 91 octane premium formulations to.

[bigbrotherjohn]

I remember from when i built my 912 engine that i had to shim the cylinders for proper valve clearance and the compression didnt drop all that much. You could probably reasonably drop your compression to somewhere in the high 9's. I think it would be better to change the pistons if you can, either way the heads and the cylinders have to come off. As far as the 100lbs cranking compression I agree that a very agressive cam could cause that.

[dimps8]

Thanks guys,

Charles I think your right with the 93 rating compared to out 98 in Austaralia. Looks like I may open her up and find out exactly what pistons were put into this engine. Charles, can you describe what evidence of detionation(slight) I'm looking for?

John,

I'll try and set the advance to peak at 5,000 instead of the lower 3,500 rpm and see if it makes a difference.

I've also heard that very slight detionation is no real problem on a well constructed engine? Can this be true?

[adomakin]

sorry to hijack the thread but camgrinder can you explain how shimming can increase the the chance of detonation?

andy

[cnavarro]

This goes back to the desirable .040" deck height. The idea that if you shim out the cylinders to decrease the compression (yielding a taller deck >.060"), you kill quench, which is a very bad thing to do. Quench pads provide for more efficient combustion. If you shim the cylinders out too far, you end up with combustion occuring within the cylinders, not the combustion chamber, which as you could imagine, is a very, very bad thing.

[911pcars]

I agree with Mr. Navarro.

Many aftermarket pistons (JE, Arias, etc.) build enough material in the piston crown so the builder can take off-the-shelf pistons and machine to the desired compression ratio. At 10.5:1, you should have more than enough material to drop the compression below 10:1. Contact Arias. They should have the info. you need, or measure yourself.

Sherwood

[adomakin]

cheers for the lesson blokes

Andy

[camgrinder]

Cnavarro has it right. The extra clearance in the quench area is bad. A tight quench area causes the trapped mixture to be forced into the center of the chamber creating turbulence. This is a good thing.
If you do a search on google you will easily find more than 100 articles on the quench to detonation relationship. Silvolite piston has/had a really simple explanation, but I couldnt find it just now..

[dimps8]

Thank guys your comments have taken on board. I want to keep my deck height, so i'l try and tune out the detonation, maybe remap the advance curve. If this is to no avail, i'l pull her down and inspect the hight dome pistons to see if i can machine deeper valve reliefes. Thanks again for the usual informative help.

[dimps8]

John,

I just found the link you suggested. Looks like i'm not going to get much work done today!

http://www.kb-silvolite.com/feature.php?action=read&F_id=29