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Old 05-23-2008, 02:37 AM   #1
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Default 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.

The results:
1) 195
2)180
3)165
4)205

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?
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Old 05-23-2008, 09:43 AM   #2
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do it with a tsp of oil in each....
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Old 05-23-2008, 12:30 PM   #3
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That much of a spread is not good, definately do the oil test, or a leakdown test.

Just hope it doesn't look like this.


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Old 05-23-2008, 01:36 PM   #4
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your comp gauge isn't necessarily accurate. you're looking more for variances than actual values.
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Old 05-23-2008, 01:46 PM   #5
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a compression test only shows there's a problem somewhere. it doesn't help much to diagnose the actual problem.

see if you can get your hands on a leakdown tester and run that test. You will find out if the rings are shot, the valve seats are shot, or if it's something entirely different.
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Old 05-23-2008, 03:21 PM   #6
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Toward the bottom is a how-to for building a leakdown tester. You don't really need the regulator since you probably have one on your compressor, you can use the gauge from your compression tester and if you remove the valve core from the hose, you can use that as well.

http://www.xs11.com/tips/misc/misc3.shtml
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Old 06-01-2008, 09:06 PM   #7
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And the leakdown results...

7-8% leakdown on all 4 cylinders. You can hear a spec of leakage from the exhaust on each cylinder. They seemed to leak evenly though the compression test earlier showed #3 lower than the others. Weird. My dad says run it till it drops a cylinder. Looking like that's what I'm gonna do. Motor runs fine and doesn't burn oil, smoke, loose coolant, or anything like that. If it ain't broke, don't fix it I suppose.
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Old 06-01-2008, 09:28 PM   #8
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I would keep it going too. not perfect but not bad enough to tear it down either.
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Old 06-02-2008, 08:56 AM   #9
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Apart from freak accidents or bad tuning, you've got plenty of miles left in that motor. Boost the crap out of it for a year or three. Meanwhile, score a deal on a "new" block and requisite internals to have ready.
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