Tech question vacuum vs altitude

[old_man]

I am in the process of troubleshooting a vacuum problem in driving my heater/AC. I hooked up my vacuum gauge to my intake and got 15" at an idle. IIRC it should be closer to 17". My question lies in the fact that at higher altitudes the vacuum will be less. Does anybody remember the correction factor per 1000 feet? The vacuum is steady and does not pulsate so a burnt valve or ring is out. There are no leaks to be found. I tested out the intake/exhaust gasket and the brake booster. I am also wondering if my cam on the stroker could be causing the lower vacuum. Power feels a bit down from when I first built the stoker but I was running a stick back then,

I plan to do a complete compression check and plug reading soon. The exhaust is in good condition and the cat and muffler are only a couple of years old.

I'm starting to wonder if the one way valve on the vacuum bottle is crapped out. If I hit a steep hill, the air all goes to my defrosters and the hot coolant valve seems to close.

 

[Fred]

For compression, it's about 3% per 1000, maybe about 15% down your way.

I forgot all about it at first when I was testing the compression to check for a head gasket leak. My highest compression reading was 122. I'm thinking, damn I need to rebuild the whole engine. Finally, my brain kicked in and I realized I was at 6000 ft. Hard to compress air that isn't there.

Fred

 

[old_man]

Let me correct that for you.......

It's hard to breathe what isn't there!

 

[JJacobs]

Tom, sea level standard atmosphere is 29.92" at 59 degrees F. The lapse rate is 1" per thousand feet and temperature varies too.. so our standard day is 24.92 @ 40 F. So 15" sounds reasonable, and I suspect a check valve problem or leak in the vacuum canister system.

 

[philip_g]

correct, 1"HG on a standard day, this of course varies. You'll have to convert to PSI.