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I like these "for conversation sake" topics. My experiment is a 262.5 ci 4 cyl. 112.5cc head, and a 24 cc dome, and a 14.5 cc dish. Head gasket is 7.5 cc I came out to 13.4 to 1. The block and head deck surfaces are stock, not milled. Im "expecting" 268-272 psi of cyl pressure at testing speed for that machine. I read old tractor repair and I&T shop manuals, and I see compression ratios at 5 to 1 and 100 psi at testing. Well, for anybody who has a M farmall, with flatops and a 8060 gas head which is still pretty common, Ive tested em myself, torn into em to know what they had, sure enough, the book specs I found were dead matched to the base model M engine. So, lets say there is a relative between compression ratio and cyl pressure,, we can pretend for a moment or two,, right?? LOL same book spec, 5 to 1 and 100 psi. broken down, 1 to 1 equals 20 psi going off this book spec, and I feel I can stand by it by seeing the "book speced" tractor engine in front of me, that Ive ran tests for comparissons. Ive gone thru alot of compression guages!! LOL!! Sure, cams will throw off cyl pressure, different ratio rocker arms affect cyl pressure, head milling, gasket thickeness, etc etc, but take one thats not been messed with,, and compare it, spec it out and when I did, I seen this come before me,,, a formula for cyl pressure related to compression ratio. 1 to 1 = 20 psi
5 to 1 = 100 psi
10 to 1 = 200 psi
15 to 1 = 300 psi Question,,, conversation sakes now,,, a small block V8, naturally aspriated running 14 to 1 compression ratio. Would it run all day long with a camshaft profile that did not deflate the cyl pressure by overlap?? Say at 5000 rpm on a dyno?? No playing with cams,, what it came with stock is what you get to use,,, I promise you, it would not run, and if it did, it would not be for long before it breaks. a perfect 14 to 1 is 280 psi on the scale, but with autos and cams with overlap,, you get to see a 14 to 1 engine test 180 psi and run on 93 octane, but it sure sounds cool with the rumpty rump sound the cam gives em. Also, thats why I think there is no scale ever related V8's and cyl pressure from the performance spectrum. This is just something I thought of, and in about a month, Ill know if my scale works when I fire up my engine, and get to do some homework on it. Ill test it, and report back with my results. Its just info Ive taken from old tractor repair manuals, and sat down with a tractor, did all the cc's and all the math, which matched the books, and that info gave me the impression, that a tractor engine and compression ratio and cyl pressure can be very accurate????? Why would a tractor at 5 to 1 need an overlap camshaft profile? and why would a 14 to 1 V8 need a cam with overlap? ChadS
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