can bad compression effect idle running
 

I have a question following:
Will bad compression effect the idle characterisitics of a car?
Specifically, if one or more cylinders are 10-20% less compression than the others, will this show up as poor idle on the car?

What is the limit to expect on compression results between cylinders before idle is effected?

Lastly, if compression is poor on all cylinders, will the idle be bad or I feel ok just the car will be severely down on power?

Bad compression is most noticable at idle and when trying to start the car. If it's on just one or a some of the cylinders it shows up with a rough idle or rythmic shake of the engine. It high RPM it is difficult to detect unless it's so bad that the engine misfires. You get very little power loss with low compression. One of our competitors had all his top compression rings broken and was still competitive on the track. Of course he got black flagged after only a couple of laps due to all the blowby.

You can do a rough check of compression by cranking over the engine with the starter motor and listening for faster and slower rotation as the engine turns. A "flat" cylinder will make a skip sound every 6 beats on a 6 cylinder engine.

-Andy

Fred,
If you suspect that your motor may have a compression issue, my best advice is to check the compression.

" You get very little power loss with low compression."

Perhaps you mean you feel little power loss with low compression. That is, the engine wouldn't run rough at high speed. However, HP/torque output requires equal equal work from each cylinder, and low compression from one or more cylinders means they aren't contributing. You need to squeeze the air/fuel to get energy out of it.

Obviously, one can isolate a weak cylinder with a compression test. Another way is to short out or disconnect the plug wire one cylinder at a time and observe if the engine idle speed changes. It should if each cylinder is healthy. If not, it's not helping.

Sherwood