High Compression Engine

[Ronald Sieber]

I have what I suspect is a high compression engine (11:1) that was used competitively and I'd like to detune it for street use without breaking it down and changing its internals. I've read its build sheet, and the engine only had a few hours logged on it since its build in 1988. The next owner, from whom I purchased the car with this engine in place, drove it on the street, and I suspect that nothing was changed, as it always seemed to run like an engine that wanted to go-go-go.

I'd like to measure the internals to confirm what's actually in the engine, but I'm not sure how that's done short of a complete teardown. Therefore, my question below, which sort of resembles a bandaid approach.

My question: is it possible to de-tune the engine with street carbs, street plugs, and a mild timing curve? Would it run okay, or would I need to open it up to do a street-engine reconfiguration in order to make it run smoothly?

[C J Murray]

If it is truly a 11:1 compression engine there is nothing that you can do other than tear it down. I am guessing that it will need pistons and a cam if it was built for competition. If you can spin it over with the starter, carbs wide open, and get compression readings on all 4 cylinders you might tell how radical it is. The ports may also be too big now for a milder cam and lower CR.

[Martin Benade]

If you have any doubts about the story you could degree the cam and pull off a head to look at the piston dome. I imagine CJ is correct about what it will need.

[Harlan Halsey]

Why not just shim the barrels to a 9.5 compression ratio and see how you like it on street gas?

[C J Murray]

Why not just shim the barrels to a 9.5 compression ratio and see how you like it on street gas?

Unlikely to be successful. It probably has a long duration cam and reducing the CR will result in zero torque in normal driving and no power at high rpm. Shimming the barrels will also kill the squish and slow the combustion. You can't fool Mother Nature!

[Ronald Sieber]

Thanks for the comments, guys. I was anticipating this news; here comes the headache....

[Harlan Halsey]

C.J.
11:1 is a low CR for a racing engine, (Even vintage racing 356s are north of 12:1 these days.) and Ronald said it was run on the street by the previous owner. It may not be far from a street engine. That is the reason I suggested that simply reducing the CR to allow it to run on current street gas might be acceptable. Were it mine, I would simply disassemble it and build what I wanted from some of the parts, but that was not what was asked.

[C J Murray]

It is not possible to take a high performance engine and simply reduce the CR and attain a good result. This is especially true when the method of reducing the CR results in an excessive clearance between the piston and the head. Just reducing the CR doesn't address the need for a shorter duration cam to regain low rpm torque so now the engine works poorly throughout the entire rpm range.

There is an update. I know Ron somewhat and he sent me an email with a little more information, maybe. He is not certain that the engine he has is the engine that he was given build data for so we really don't know any more than before. Having read the "maybe" specs I wrote him back.

Even in a race engine, CR is not a free lunch. Joe Cogbill once told me that 11.5 was his highest CR and that when he tried to go higher the power was reduced. I don't know what cams he was running, maybe they were slightly shorter duration than some, but I do know from building motorcycle air cooled race engines that too high a CR gets you one or two fast laps and then you slow down dramatically due to detonation. You can't just throw a lot of CR at an old school air cooled engine. I have found the mid to high elevens to be good even when using a very long duration cam.

There are ways to estimate what you have for CR and cam without tearing down the engine, as I suggested in my email.