Several weeks ago I decided to put new plugs in my 79 motor and at the same time do my first compression test. It tested between 140 and 150 on all but number 4 which read in successive tests: 130, 140, 128, and 115. I went ahead and installed some platinum Bosch (at .034, seemed to run rougher than the old plugs, hmm.). Finding out that I could put some oil in the cylinder to try to isolate where the compression loss is coming from, I decided to retest number 4 tonight. Well, It tested 145 then 145 then 145 and again. I decided to test all the rest of the cylinders in case my inexperience could have caused issues with either testing sessions but got readings between 140 and 150 on the others. I set the compression hose more than once on the number 4 in both sessions. Both the old and new plugs in number 4 were oilier than the other cylinders. Any insight?

throw your bosch plugs away, you do not own a german sports car. honestly though i recomend that you use only NGK plugs. never had any problems and if you ask anyone on this board im sure they will tell you the same.

two are you doing the test with the engine warm and throttle open. are you sure your vavles are properly adjusted and timing is in spec.

make sure that your car is in tune to the best of your knowledge and then do your tests again, like i said before, adjust valves time engine, check compression when engine is warm with the throttle open.

good luck and let us know the results

German parts are for German cars.

Japanese parts are for Japanese cars.

American parts are for American cars.

Harley parts are for Harleys.

and throttle open

Will this make a difference? I would have thought that at cranking speed the engine will draw juat as much air with the throttle wide open as closed... Are you saying that you can alter the effective cylinder pressure (not CR) by cranking with a wide open throttle.? On a naturally aspirated engine I thought the only way you could to that was with some sort of ram-air system...and that would only tend to work at higher road speeds..

I was testing warm and with WOT, valves are due for adjustment, though. Still, quite a bit different results between the two tests. Sticking valve?