Compression Numbers

[TRAIL TAILOR]

Today I decided to get away from taping and masking off the body and jambs on Blue Marley and play mechanic for a little bit.

So, I ran a compression test on my 85 with 189k plus miles on it.

Dry:
1- 120
2- 120
3- 115
4- 120
5- 112
6- 120

Wet:
1- 140
2- 140
3- 136
4- 141
5- 133
6- 142

I re-ran the test on #5 three times with the same results (both dry and wet). It's within the 14psi range but, I'll be watching this one.

Numbers are a little low according to the FSM but, I'm at 6200' elevation. What are everyone's thoughts on these numbers at this higher elevation.

Thanks guys.

J

 

[2mbb]

I think it is more important for the numbers to be close to one another than what the actual value is. What I mean is that there is a lot that can influence the the test--accuracy of your tester, are all the plugs removed, is the engine warm, is the battery fully charged, is the carb throttle wide open, and of course, what is your elevation.

These are my compression numbers, taken over the years, using the same tester. The numbers are higher than the FSM, and I have over 200K miles. But I don't think this means my engine is better than yours.

 

[lehiguy]

Did you perform your test with all the plugs out and the throttle wide open allowing at least three engine rotations per cylinder?

 

[Finnyfam60]

3 and 5 look identical, and only 8% off, so seems like not much to watch.... Cheers on a solid motor!

 

[TRAIL TAILOR]

Did you perform your test with all the plugs out and the throttle wide open allowing at least three engine rotations per cylinder?

Yes, both dry and wet tests were done with all plugs removed and wide open throttle per FSM. Probably 3-5 rotations.

I've read on here that the higher elevation sometimes will lower the compression numbers. Any truth to this?

J

 

[2mbb]

The ultimate pressure obtained during the compression stroke is related to the number of molecules of air inside the cylinder. If you take the worst case condition and performed a compression test in deep space (where there are no molecules of air), there would be nothing to compress, and your "compression" pressure would be zero.

A quick google search reveals this page: PointedThree - 617 A compression test at altitude

You would use the factors to adjust (reduce) the spec by multiplying the factor to the FSM spec.

 

[lehiguy]

I would not worry about those numbers. Keep the valves adjusted and drive it.

 

[TRAIL TAILOR]

The ultimate pressure obtained during the compression stroke is related to the number of molecules of air inside the cylinder. If you take the worst case condition and performed a compression test in deep space (where there are no molecules of air), there would be nothing to compress, and your "compression" pressure would be zero.

A quick google search reveals this page: PointedThree - 617 A compression test at altitude

You would use the factors to adjust (reduce) the spec by multiplying the factor to the FSM spec.

Good read 2mbb. Thank you.

I would not worry about those numbers. Keep the valves adjusted and drive it.

lehiguy, other than a little carb adjustment (need to lean it out a little, running a tad bit fat) the engine runs smooth.

Thanks for the feedback guys, much appreciated.

J

 

[LAMBCRUSHER]

I would not worry about those numbers. Keep the valves adjusted and drive it.

X2. maybe a valve lash adjustment could bring those low cylinders up a bit. HTH

 

[Southeast Overland]

You should be good. Adjust the valves if you havent already and run her.

 

[TRAIL TAILOR]

X2. maybe a valve lash adjustment could bring those low cylinders up a bit. HTH

I'll run the valves and see what I get. Thanks.

J