the last time i did a compression test i ran about 270 across the board. yesterday to make sure my car was up to par after crazy detonation on the dyno i thought i would run some #s again. nothin has changed on the car at all. but i ran 300+ across the board. the car was warmed up and tested as b4. the motor hasnt put out 300 compression since i first put it together. could someone explain or offer a eductated guestimate??? id like to understand what the heck is goin on inside there.

 

well, when you ran the compression #'s before, was the engine freshly built? if so, then you can have a lot of carbon build ups which increase the compression, now, i'm not sure if that increase is normal, or if you have WAYYY too much carbon build up.
Maybe another reason might be the gauge broke, those things are very fragile and you have to be very carefull with them.

Where your spark plugs really black? when you took them out? How about your tailpipe? is it really black?

 

i appreciate the response. the guage is the same one ive been using for 2 yrs. this is my second yr on the same motor. spark plugs r light brown like they should b. lookin down into the cylinder u can see the pistons r black but u can also see they are still shiny. last time i compression tested it was 2 months ago. thanx again.

 

It's just carbon buildup in your combustion chambers. You might wanna try this to loosen them up before a compression test: Loosen the spark plugs slightly and start the engine for just a few seconds; then turn off engine and pull all plugs and continue with the compression test as usual....this should loosen up some stuck carbon in there. See if that helps any in bringing the readings back down but as long as the readings are consistent across all cylinders, I wouldn't worry about it.

 

Just out of curiosity, how does loosening the spark plugs help clear buildup?

 

Its probably a few different things, your car may be warmer than before, causing the rings to have a better "seal" and the addition of carbon. The gauge could also be a bit off, or the battery could have a stronger charge, causing it to spin faster since compression test relies a lot on the charge of the battery.

You said you has some crazy detonation, the carbon buildup can cause excessive detonation, so its a good idea to do some decarbonizing and see where that takes you. Some things you can do to decarbonize: SeaFoam, ATF or any other similar decarbonizer through intake vacuum line.

Edit: Warning: The car will smoke heavily when you do this too.

 

You can also get a bottle with a squirt top on it, and just squirt some water into your intake. Also make sure to keep your revs up as your engine will tend to bog until the water has run through it.