Join Date: Mar 2002
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Re: Low cyl. compression = Low int. vacuum?
I posted this in my other Pony vacuum thread, and am cross posting here, since it may be significantly important to both threads:
I didn't get around to working on the Pony tonight due to working on the cylinder compression mess, but I was re-reading an OLD (like 60s or 70s) engine test kit instruction booklet that said that you can "expect a drop in vacuum of one division per each 1,000 feet above sea level".... (Edited: the newer vacuum gage booklet I have, nor the FSM mention any difference in vacuum, compression or timing due to altitude...... but then the "computer" is supposed to adjust automatically for altitude, so I can see why they would not bother to mention it.)
Well, if that is correct, and "one division" means one inch of vacuum, then even with the leak around the secondary throttle shaft, I'm within specs, or at least really close, because I'm at 7,000 feet altitude and pulling 11-13 inches of vacuum at ~800 RPM depending on where I set the timing and idle mixture.
Now, I'm wondering if cylinder compression also measures less at higher altitudes? hmmm, a quick web search does indicate that cylinder compression does decrease with altitude, although I haven't been able to find a specific formula or table....... Is 7,000 feet enough to drop compression by 40-50 PSI or so?
I suppose I could test this out myself by driving Suzuk out to Los Angeles or to Death Valley, but if you guys can confirm this theory for me you'll save me gas money [img]images/graemlins/grin.gif[/img]
(Edited... now that I think about it, if the altitude does actually drop the vacuum measurement by ~25% it would make some bit of sense if compression also dropped by a similar percentage.)
And I also just saw on a Saab mechanics website a recommendation that on older Saabs, you should increase timing advance about 1 degree for each thousand feet, up to a maximum of 4 degrees extra..... Does that apply to normally aspirated, non ecm/non-emissions Suzukis too? Darn sure performs better when I have timing set at 14 degrees BTDC.