christallon

Good Morning all.

Do any of you know the formula to roughly calculate the compression difference between a car with 150 Lbs at 5200 ft vs. what that number would be for the same car at sea level. Is it +20%, +30%?????

Thanks in advance

elbeee964

std. altitude check, anyone? You pilots?

edit-
5000 ft: 12.2 psia
sea level: 14.7 psia

btw, 'compression' ratio is only starting volume to ending volume. Should be the same regardless the altitude. Only related to pressure by Boyles law, PV=constant (kinda, sorta...). So the pressures will change by altitude, but not the compression.
So, lbs of air in a cylinder will depend on air density...
...make it about .0625 lbs/cu.ft at 5000 ft. and .075 lbs/cu.ft. at sea level.

All this, and I still don't think that makes your answer... but it's Friday... and my mind is already kicking into weekend mode... Maybe someone brighter will know.
Good luck...

christallon

So the diff between .625 and .75 is about 20%. The car I'm looking at in Denver is somewhere between 145 and 155 lbs of cxompression. I initially thought this was quite low, but at sea level I imagine it would be higher. Applying a 20% differential brings these numbers to 174-186. Am I thinking correctly or just kiding myself?

Thanks elbee.

Indycam

Does the fact that the gauge is outside the motor and at 5200 ft change anything ?

I have a book of automotive math around somewhere , I'll good look for it and see if they have a psi vs ...

Indycam

http://books.google.com/books?id=ahD...um=6&ct=result

elbeee964

Huh. learn somethin' new every day (above).
Theory vs. practice.
http://www.answers.com/topic/compression-ratio

(valve leakage differences between different altitude airs, perhaps?)

Definitely stick with the VW book's chart.

christallon

Very Cool!!!! Thanks Indy Cam. It appears to be more like 16%. So the compression at sea level would calc at 168-180, roughly. Still a little lower than I'd like, but at least they are within 10% low to high.

Cheers

Indycam

I have been thinking about this question , I don't trust the book .
I want you to do a test .
Drive down to sea level ...

christallon

If I buy the car, I plan to do just that I'll let you know the results.

Indycam

And then go back up the hill and retest ...

christallon

Nyaaahahaaa.......Diabolical plan batman