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Compression test results - elevation

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ErikTande

Supporting Member
1,505
560
Dec 17, 2012
Centennial, Colorado
Here's my results (dry test, engine warm)

113
119
117
120


I'm at 5300 feet elevation though, and I found this with a google search saying test results will be lower at higher elevation:

Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
8500 0.773

So applying .86 for my elevation, the corrected numbers turn out to be:

129
136
133
137

So am I ok on compression or should I be worried about the low numbers?

Thanks!
 
It's probably best to do a leak down test as well to get a complete picture. 135 average isn't terrible but it's a little on the weak side. A rebuild is in your future.
 
It's been a while since I've done a compression test on my car, but from what I remember I've always seen no higher than about 120 to 125 on my 2 1gs (including my 90k mile one now). Low compression by itself doesn't hurt anything, especially when all the cylinders are fairly even like yours. As long as the car is running well and you aren't burning a ton of oil, I wouldn't worry about it.
 
You've got an early 6-bolt from a 1990, so your compression ratio is 7.8:1. The above stated 2g's motor (which has an 8.5:1) is going to have a slightly different result. The service limit for your motor is 121 psi, so you're on the low side of normal, even adjusted for elevation.

Like Type1DSM said, follow this compression test up with a leak down test. The compression test only tells part of the story. A leak down test will show where exactly you're losing compression and maybe it'll be an easy fix. For example, on my n/t Laser I used to own, there was a big loss of compression on the rubber fuel injector seals. Replaced those and the compression went up a few points.
But, the motor is likely just worn, so there's going to be minor blow by on the rings, valves won't seat like they did new, etc.

I wouldn't worry though. You've got the perfect excuse to schedule a rebuild now :)
 
= For example, on my n/t Laser I used to own, there was a big loss of compression on the rubber fuel injector seals. Replaced those and the compression went up a few points.:)

Injector seals have nothing to do with cylinder compression. Pretty much the only things that affect compression is rings, head gasket or leaky valves.
 
You're right. Sorry, that was worded poorly, the fuel injector seal causing a loss of compression (or compressed air) referred to the leak-down test part of my comment.
As in, pumped air into cylinder, air escaped (passed intake valve?) and out from the seal around the fuel injector. So not a loss of compression in the direct sense, but a vacuum leak none-the-less.
 
at least the numbers were close to each other throughout the board, wouldn't worry to much, specially on a stock motor. Run it till it blows!
 
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