400 compression readings at sea level and 5500 ft elevation differences... am I nuts?
 

I'm getting some goofy readings on my 400 (still troubleshooting the carb situation) and I need some help.

The 400 was rebuilt with KB 346 pistons which are supposed to produce 9.7:1 compression with 72cc cylinder heads. I've switched to the KRE aluminum heads with the as cast 65cc combustion chambers, and had the heads milled .030 to get a lower cc volume, which should be around 63/62cc now.

The block was decked .010 to clean it up when it was rebuilt 12 years ago.

So it should have roughly a 10.634:1 compression ratio at 14.7 (sea level air pressure).

My mechanic tells me it's got 132/134 compression readings consistently at 5500 foot elevation. At 5500 feet the air pressure is 12, not 14.7.

My question is if it's a 10.6:1 compression ratio at sea level (100/200 feet above) with the same setup, is the lower AP at 5500 feet making the compression test read 132/134?

Because if I run the calculator it comes up with 132/12= 11:1 compression.

If I stick with 14.7 AP, then 132/14.7 comes up with 9:1 compression, which is way off from the UEM/KB piston calculator.

So what the hell am I doing wrong here?

Jim aka ScumOne

Short and Sweet Version
 

Static compression ratio is figured on paper and doesn't change with altitude.
It's also based on full stroke.

Actual compression (dynamic/effective) comes from stroke based on intake valve closing point. Which is always less than full stroke.

Air pressure would change actual compression while testing cranking compression and running compression.

Clay

Going from sea level to 5500 feet will loose about 30-35 on a pumping test.

That makes more sense to me with the numbers I’m seeing. Thank you.

Jim

I'm just curious how many digits past the decimal point changes your CR from "roughly" to "precisely"? ;) Might be a better gauge for answering your question regarding your sanity.

LOL…well it turned out the Piston CC was 17 instead of 12. So the exact compression ratio is 10.416. I must’ve done a typo when I plugged it into the calculator.

NON of this has anything to do with you being CRAZY!Only how HIGH you are!Tom

I can always count on you buddy! L O L