05-23-2008, 01:37 AM
Join Date: Aug 2007
Location: Houston, TX
Total Cats: 119
My Compression Results
Ok so for starters I know I need to do a leak down test and plan to.
When I bought the car it had 185 PSI of compression on all 4 cylinders. After about 7 bottles of nitrous it had 170 on all 4 cylinders. Not good but not bad either. Nitrous cleans the combustion chambers and removes carbon, which can reduce compression. Course wearing the rings does to.
After blowing a headgasket at the drag strip, it had 170 on cylinders 1&4, 150 on 2&3 (blew between middle cylinders). When I had the head off I poured mineral spirits down the runners and sure enough, the exhaust valves leaked a bit. Well, they leaked a fair amount. Not shot, but definately worn and could have used a rebuild. I had to make the car drive by Sunday so it was reassembled without rebuilding.
So now I run a compression check again. I'm figuring ok, compression will probably be down since I heavily unshrouded my valves. I removed a good bit of metal from the chambers. Just guessing I'm sure it dropped compression 1/2 a point, if not a full point.
I even ran the test again and got the exact same numbers. All test were throttle open, hot motor, hot battery, etc.
Ok, so they're all over the place, and strangely enough, one, two, and four are HIGHER! I suppose more air in the cylinders is causing the guage to read higher compression, but not sure. Still, 3 is lower and that's quite a spread of numbers. I'm pretty sure it's just the exhaust valves leaking. Car runs fine and doesn't burn any oil. (Have 3500 miles since I changed the oil, and it's still a bit over half way in the safe zone. I haven't added any yet) Plugs look very clean with no signs of burning oil. But of course a leak down test is in order. I just find it so odd compression is up after unshrouding the combustion chambers so much and all the bowl work that I did. Guess that extra air is doing it?