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I think there is relevance,,, but someone trying to get a 13to1 engine to run on pump gas will tell you why you are exactly right. They open that intake valve earlier, while the exhaust is open, letting the air charge partially escape so the cyl pressure stays in the range where pump gas octane will fire and not detonate. A possible 260psi goes can go down to 185psi real quick. The overlap is whats used to control when that air enters,,, if the overlap keeps the valve closed longer, you will have higher manifold vacuum, open it sooner, and you have lower vacuum, cause its going out the exhaust pipe!! Race cars, street cars been battlling how to get the big hp engines to run on the street for years on low octane fuel. Thats how they do it,,, Thats the arguement that makes you right as rain when you said,, too many factors to consider, especially if pump gas is concerned, and a high mathmatical ratio, and a low cyl pressure at cranking speed. Same goes with turbo charged engines, no one can estimate how much pressure the cyl builds under 20psi of boost,, even though it maybe rated for a 7to1 mathmatical ratio. Its not readable. The cyl pressure could be 200-250+ psi!! Is it possible to belive, that a turbo can boost an engines cyl pressure up to 12.5 to 1 compression ratio? If you anserwed yes,,,, then you have to belive that there just maybe a chart relative to cyl pressure and mathmatical ratio. the chart,,,it did not come from the racing world. It came from the farm tractor world. IH proved this theory,, when they built the H and the M. In their parts book, I&T shop manuals, said that their kerosine engine, was a 5to1 ratio, and 100 psi of compression at cranking speed. John Deere proved this as well, when they said a Dual fuel A JD was 4to1 and 80 psi at cranking speed. Ive had bone stock H farmalls with the kerosine heads, flat top pistons, checking 100 psi and coming out to 5to1 mathmatical ratio,, thats what the book said,,, so I belived it from that day forward.
I had a 1943 JD A, old hand cranker, book said 4to1, and the compression read 80 psi. Bone stockers,,, low compression, low rpm engines. From the book stats from IH, JD,, etc etc,, field testing,,, teardowns and checking by hand to know what your dealing with,,,, CC'n heads and cyl bores, checking the math,,, leads me to belive this much,,,, tractor engines were about as honest as you can get, with what they gave us. I belive 1 to 1 is 20 psi
I belive 5 to 1 is 100 psi
I belive 10 to 1 is 200 psi
I belive 15 to 1 is 300 psi every 10 psi is a half point
110 psi is 5.5 to 1
every 20 psi is a full point Tractor cams dont have overlap to lie,, so it will run on cheap grade fuel,,, the math ratio is not out in left field,, it was real specs and real time, and real power, no tricks to try to lessen the cyl pressure. if there was a cam grind that helps build cyl pressure, if the math ratio is low, your doing a good thing in a tractor engine,, and IH proved it when they built the Super M. John Deere proved it too,,, but,, can you tell us what year, or model it was that made the difference and changed what we belived was not possible,,, or not existed, these engines were here long before the science of tricking the valves to dump air out the exhaust so cyl pressure is not peak, to run on cheap gas,,,, shoot,,, tractors have been doing that for years. ChadS
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