ALTITUDE & COMPRESSION READINGS

[hugh]

O K I HAVE HINTED AT THIS QUESTION A COUPLE OF TIMES ON OTHER THREADS, BUT… I LIVE AT 4,000 ELEVATION. MY HP IS REDUCED ABOUT 12%. MOST LARGER OUTBOARDS I GET NEED TO BE REPROPED TO RUN AT FULL RPM.
HOW ABOUT COMPRESSION TEST? I HAVE BEEN TOLD BY A COUPLE HOT ROD PEOPLE THAT IT IS AFFECTED BY ELEVATION. ANY OPINIONS? ANY CALCULATIONS?

[aquasonic]

That’s an interesting topic. My first thought was… "What difference does it make? It’s inside a piston and it will compress the same anywhere."

But then I realized that the air going in is thinner and less dense, so maybe the compression would be affected.

Improvise-Adapt-Overcome

[SCOTT BOGUE]

It’s reasonable to expect lower compression readings, all else being held the same.

Let’s say you have a 10:1 compression ratio. At sea level, at room temperature, you would expect to see 147 psi on the gauge.

At 4000 feet the atmospheric pressure would be 12.7 psi at room temperature, so you would expect to see 127 on the gauge.

[david-bartlett]

Hugh,

A quick Google search brings up info that would support your case!

[frankr]

What auldscott said would seem to be true. Except. Compression readings on the gauge are the pressures inside the cylinder AS RELATED TO pressure outside the cylinder—–not as related to the absence of air pressure as in outer space. In other words, the gauge totally ignores the ambient or barometric pressure. It doesn’t even know what the ambient pressure is. Now if you were to put the gauge on the moon and run a long hose to the cylinder, his theory may be correct. I dunno, nobody has come up with a hose that long yet.

Just my uneducated opinion.

[Steve A W]

Ok, time for someone to make a road trip! 😉

Member of the MOB chapter.
I live in Northwest Indiana

[billw]

When I first started reading this thread, I was thinking along Frank’s lines, having taken a just enough physics courses to be dangerous. However, what if you took the engine and compression gauge to the moon, where there is no atmosphere at all? Then what would the gauge read, when there was absolutely nothing to compress?

Long live American manufacturing!

[frankr]

zero. Well actually somewhat below zero because the spring inside the gauge makes it read zero at normal atmospheric. But then it wouldn’t really matter because there is no water to run it.
EDIT: Reconsidering, and this is incorrect. It would read zero because, as always, it is indicating the difference between the pressure inside the gauge and the pressure outside the gauge.

I better shut up now.

[wiscoboater]

I believe we have a Mythbusters episode in the making here. Better hurry, I hear this is their last season.

[billw]

Yeah, I don’t know why I let myself get sucked into these things. I guess I thought it was interesting to talk about. Whatever.

Long live American manufacturing!

[Author]

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