Tay-Tay

I'm wondering b/c I got about 90 PSI across all cylinders - Harbor Freight $9.99tester. 951 has about 93k, engine warmed up, WOT, good battery. When I added oil they came up to 100 PSI.

Thanks,
Taylor

LS1Porch

Damn, 9100'! I thought i was high (between 6200 and 7200).

Anyways, with a compression test the most important thing is that you're even across all cylinders. I wouldn't speculate on how worn that engine is based on the numbers, but what matters most is that you don't have any bad cylinders.

And yes, elevation does affect compression tests, but i'm a little surprised it's that much (the cheap tester probably doesn't help either).

JohnKoaWood

compression test even accross cylinders is Key, leak down test will highlight conditions needing to be corrected...

youre good man...

DanaT

900 feet is about a 40 power loss (just out of guessing based upon 5000ft) so expect your pressures to be much lower due to lower atmospheric pressure.

-Dana

JohnKoaWood

Huh? Atmospheric pressure at 9000 feet on a perfect day (weather wise) would be 10.5 psia (where at sea level it would be 14.696 psia).

100/10.5=9.52:1 compression ratio... variation allowed for weather...

OP, your motor is fine compression wise, now go do a leakdown test to check its true health if you are concerned, OR just enjoy the ride!

Ronin-951

Trying to get my head wrapped around why compressed pressure would be less due to atmospheric pressure? Other than a little bit less resistance to bleeding at the rings and valves.

theedge

Atmospheric pressure drops with altitude. Less pressure outside the engine = Less air going in = Less mass in the cylinder to be compressed = Lower compression as he is seeing.

Edit: At sea level atmospheric pressure is about 101 kPa (some variation for weather and temperature but standard is 101). At 9100 ft as the OP is, its about 74 kPa.

I think 90PSI compression is good for that altitude, but if the OP really wants to open a can of worms then a leakdown test is the way to go

DanaT

10.5/14.7 = .71.. so you are down 30% on air pressure (if 10.5 is right..i haven't calculated that...guessed at 40% on an earlier post).

Now, assume you have 10.5 * 8:1 CR...that means 84psia or 94.7psig (g for gauge pressure because you have to add atmospheric pressure to gauge).

At sea level = it is (14.7*8)+14.7 = 132.3psig

So at 9000ft (if the 10.5psi atmospheric pressure is correct), then the reading will be 30% low.

Ronin-951

Thanks for the lesson, I needed that.

michael2e

I was seeing around 115-120 PSI in Aspen at 6000 feet last time I checked so I'm guessing your readings are pretty normal for the additional altitude

Tay-Tay

Thanks all, that's what I was thinking. My quick math had me thinking 105 was about as good as I could expect, 150*.7=105. 150 being great compression and .7 for the 30% loss.

I also purchased the HF two dial leak down tester. I'm quite sure I messed up the % dial. As I've read elsewhere the % dial only works up to 15 PSI, I unknowingly went up to 70 PSI, and clocked the spring inside, it now zeros at about 80%. As far as leaking, I only heard it leaking in to the crank case. I can only go by sound, but at 50 PSI is was barely noticeable.

Taylor