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#1
10-31-2022, 04:00 PM
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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 |
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#2
10-31-2022, 05:54 PM
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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 |
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#3
10-31-2022, 06:43 PM
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Going from sea level to 5500 feet will loose about 30-35 on a pumping test.
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#4
10-31-2022, 07:06 PM
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Quote:
Jim |
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#5
10-31-2022, 07:24 PM
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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.
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#6
10-31-2022, 07:35 PM
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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.
Last edited by ScumOne; 10-31-2022 at 07:40 PM. |
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#7
10-31-2022, 08:13 PM
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NON of this has anything to do with you being CRAZY!Only how HIGH you are!Tom
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#8
10-31-2022, 10:40 PM
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Quote:
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