Subaru80 Posted June 2, 2013 Share Posted June 2, 2013 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? Link to comment Share on other sites More sharing options...
bonvo Posted June 2, 2013 Share Posted June 2, 2013 im watching this ive never heard of altitude effecting compression it will be interesting to hear the replies Link to comment Share on other sites More sharing options...
Idasho Posted June 2, 2013 Share Posted June 2, 2013 Altitude does have an effect upon compression, and should be considered. Altitude Factor500 0.9871500 0.9602500 0.9333500 0.9074500 0.8805500 0.8536500 0.8267500 0.8008500 0.773 Link to comment Share on other sites More sharing options...
Subaru80 Posted June 2, 2013 Author Share Posted June 2, 2013 Yes it actually does.... It is usually lower than at sea level. I just want to know an accurate range. Link to comment Share on other sites More sharing options...
Idasho Posted June 2, 2013 Share Posted June 2, 2013 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. Link to comment Share on other sites More sharing options...
Numbchux Posted June 2, 2013 Share Posted June 2, 2013 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. 1 Link to comment Share on other sites More sharing options...
Idasho Posted June 2, 2013 Share Posted June 2, 2013 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 Link to comment Share on other sites More sharing options...
Numbchux Posted June 2, 2013 Share Posted June 2, 2013 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. Link to comment Share on other sites More sharing options...
Idasho Posted June 3, 2013 Share Posted June 3, 2013 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. Link to comment Share on other sites More sharing options...
Subaru80 Posted June 3, 2013 Author Share Posted June 3, 2013 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. Link to comment Share on other sites More sharing options...
rdweninger Posted June 3, 2013 Share Posted June 3, 2013 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. Link to comment Share on other sites More sharing options...
Subaru80 Posted June 3, 2013 Author Share Posted June 3, 2013 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. Link to comment Share on other sites More sharing options...
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