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What's typical compression on GM 3.8L?

09-07-2003, 09:26 AM
Can anyone tell me what compression is typical for an 87 Pontiac Bonneville 3.8L? Mine is running 175-180 on all six. My Haynes manual only says a minimum of 100.

175-180 seems high to me. Looking in the spark plug holes the tops of the pistons seem clean so I don't think it is carboned up any.

Also, is this an interference engine? I just had to replace the timing chain and gears because the nylon teeth stripped off the original gears. I was worried about bent valves until I got these compression readings. I don't think a bent valve would allow this kind of compression. At least not this even of compression.

The compression test is the first test I have done since replacing the gears and I am concerned that I may have gotten the valve timing off. Could that make the compression high?


09-07-2003, 01:26 PM
at some point or another, I'm fairly sure every motor is interference. If the chain snaps with the valve fully open I can't imagine a motor having enough room to let the piston travel all the way up without hitting it. The 3.8 isn't an interference motor whatever good that definition does anyone, and it seems you lucked out not having any fully open when it snapped. Your compression is higher then I've seen a 3.8, but that won't hurt anything, if anything you're making more power, but just to make sure, are you confident in whoever tested it? You certainly didn't bend a valve with that ocmpression across the board, so unless the tester was off you're just fine, better then most actually.

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