Compression #'s question

[TacoJonn]

Question, thinking of getting a 40 a ways away from me. Had an independent shop do a compression test for me. 115 to 120 on all cylinders. Dry test.
Where it's at is 500 ft above sea level. I live at 6000 and would be going as high as 9,000 ft.
Should I look elsewhere?

 

[DougAustinTx]

115 - 120 is pretty low. The engines generally have a about 155# with new rings and pistons. I don't know the formula for calculating the loss of compression at 6000' elevation, but I would think that you would be on the ragged edge of not having enough compression to fire the cylinders. I would look elsewhere unless you are confident in doing a ring and valve job (assuming the cylinder walls and pistons are ok). Probably should at least plan on honing the cylinders and maybe installing new pistons with the rings.

 

[3_puppies]

dry test? engine should be warm, all plugs out, throttle open

adjust valves,

my guess is things are OK, test was not done properly

 

[lymerock]

cold engine, incorrect valve lash and low battery voltage can all lead to lower numbers. also who knows how accurate the gauge is in the first place. What is important is that the values are within 10% from lowest to highest, obviously closer the better.

 

[SteveH]

At 6000 ft., a newish 2F tested at 120. Max I have seen at 7000' is 130. Deduct (approx.) 3% per 1000' above seal level for higher elevation readings.

 

[TacoJonn]

He adjusted the valves, ran seafoam through it and got the following revised #'s.

1 140
2 128
3 130
4 130
5 125
6 140

Just wondering how soon itd need a rebuild up here at elevation. I'd be taking it to up to 9000 ft.

 

[Dizzy]

I wouldn't worry too much about the engine needing a rebuild. If it isn't blowing smoke or seizing up, drive it.