Hi Guys!

 

I hope you can help me here! I heard that compression levels change from sea level to high altitudes.  I am in about 8,612 ft.  What would be healthy compression range in my EA 71 engine?

im watching this ive never heard of altitude effecting compression it will be interesting to hear the replies 

Altitude does have an effect upon compression, and should be considered.

 

 

 

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

 

Yes it actually does.... It is usually lower than at sea level. I just want to know an accurate range.  

Furthermore,

 

With everything being perfect, it is a simple math problem.

 

14.7  x  altitude correction  x  CR

 

 

One atmosphere of pressure is 14.7PSIA

 

The compression of a EA71 IIRC is 9:1

 

 

Now consider the altitude table I provided.   

 

Your at 8600 feet, so use the 8500 feet entry.

 

 14.7 x .773 =   11.4

 

Now enter the supposed compression ratio.

 

11.4 x 9.1  = 104PSI

 

 

 

If I were you Id be happy with anything over 95PSI on every cylinder, just so long as every cylinder is within 10% of each other.

 

 

That sounds low. At "normal" altitudes (I live at just over 1k ft, or so), a healthy engine can easily put out 160-175psi. I find it hard to believe that it would be less than 60% of that!

 

 

 

 

But, either way. What's more important than what the number is, is that all 4 are pretty close. When you have 1 or 2 that are drastically out of whack from the rest, that's when you have a real problem.

I find it very hard to believe 160+ PSI out of any EA

 

 

Id suspect the gauge.

 

 

 

The math is simple.  Obviously the main variable is cam timing/overlap

I find it very hard to believe 160+ PSI out of any EA

 

 

Id suspect the gauge.

 

 

 

The math is simple.  Obviously the main variable is cam timing/overlap

 

I should have been more specific. But I didn't say EA for a reason. I've done lots of compression tests, never on an EA. But that's sort of my point, I don't think the math is simple.

 

 

But by your math, the only engine variable that matters is compression ratio, and 9:1 is on the high end of what I've done. I've seen Turbo EJs with 8:1 or less put out 150psi, which by that equation (14.7x8=117.6) is not even remotely possible.

Ill say it again....  the one primary variable is cam timing and overlap.

 

 

 

That alone can and will toss all kinds of odd numbers. High and low.  Even at a constant test RPM.

 

Do the same test on two different size motors with similar cam timing/overlap and at the same RPM and the same compression ratios, and you will get the similar results.

I should have been more specific. But I didn't say EA for a reason. I've done lots of compression tests, never on an EA. But that's sort of my point, I don't think the math is simple.

 

 

But by your math, the only engine variable that matters is compression ratio, and 9:1 is on the high end of what I've done. I've seen Turbo EJs with 8:1 or less put out 150psi, which by that equation (14.7x8=117.6) is not even remotely possible.

 

But keep in mind......high altitude does effect compression readings. In all my Subaru manuals it says so.  I just did not know the exact range.  I just rebuilt my motor and the compression for each cylinder is about 102psi. so garage junky is dead on.  

My first question is 8600' elevation in New York?

Google tells me Mt. Marcy at 5343' is the highest point in NY state.

Did you hone or bore your cylinders?  Where'd you get the ring set?   Valves reground?

I would drive down to the ocean shore and re-test.   Let us know the results.

My first question is 8600' elevation in New York?

Google tells me Mt. Marcy at 5343' is the highest point in NY state.

Did you hone or bore your cylinders?  Where'd you get the ring set?   Valves reground?

I would drive down to the ocean shore and re-test.   Let us know the results.

 

I am from New York..... but I am currently living in Bogota, Colombia.  8,612 ft.