Much more than that, it simply acts as an extension of the chamber itself, defeating the purpose and losing the benefits. In order to function as a quench area, the clearance at piston TDC should be ~1mm or less from the head chamber surface. The quench area typically occupies at least part of the periphery of an IC combustion chamber and represents the area where the piston most closely approaches the inside surface of the head combustion chamber surface at TDC. While it’s commonly recognized that a ‘tight’ clearance in that area is nice to have, it is not commonly accorded its importance in the combustion process of that chamber, nor is the total area given its due. The quench area is probably the most important design concern in dealing with a given chamber it is the largest determinant of the acceptable compression ratio and ignition timing. 616 chambers need all the help they can get they mimic the "ramp" chambers in the early RR Merlin prototypes, rejected as too prone to detonation. Further, less ignition lead means less pre-ignition, allowing a higher CR, which is ALWAYS desirable.īut an engine’s best ignition lead is not a design value it is a found value, derived of the elegance of the engine’s combustion chambers. A shorter lead means less of the piston’s approach to TDC is pushing against a burning (expanding) mixture, so less fuel is wasted pushing the piston the wrong way trig functions here tell the story that small deltas mean large gains. While time spent on the crank, bearings, case, carbs, distributor, providing clean oil, cam, valve gear and so forth is both necessary and important, all that remains the ‘chorus’ the ‘fat lady’ is in those chambers.Īn IC engine delivers maximum performance and efficiency with the least ignition lead possible. The combustion chambers of any IC engine are the sources of that engine’s power and efficiency. ![]() Quench area influence on compression ratio, ignition lead and power why achieving the last 15% of an engine’s power takes 75% of the time ![]() WHAT I HOPE TO HAVE LEARNED REGARDING C/R AND CHAMBER DESIGN Gonna put this here, since there's no other obvious location: It might be worth plotting out the curve to get a sense for what is going on? IDK, I found it useful.Ĭlick to expand."investigating"!, makes inquiring minds ask: Can you take some time to talk a little about how in a 616 octane, chamber shape, C R, single/dual plug, spark lead and the time of peak combustion pressure (15 degrees ATDC?) interact? Attached is also my reproduction of that curve on my own 031 (again, the caveat is that these numbers are super inaccurate without a proper testing machine). I attached a picture of the 031 spec curve I found (I got it from Marcel at vwnos who rebuilt my distributor). It meant that following Paula's advice of setting max advance and leaving idle as is, I had my idle advance at -3/-4 (or +3/+4 after top dead center) which was causing rough idle. The max advance I'm sure was asymptotic somewhere, but I saw as high as 40 if I really revved high (40 just in the distributor normalized from idle!). What I was seeing was that my distributor was in spec until 3200rpm (3200rpm crackshaft, 1600rpm distributor) then it would shoot up. ![]() Of course, I don't have a Sun tester machine (like I believe Glenn does) in my tiny 1 car garage, but I was able to plot a very inaccurate timing curve just on my car by dialing in an advance on the timing light then adjusting the rpm until the notch lined up and recording the RPM (I did this while warmed up and I repeated each measurement a few times). The reason I mention this though is that plotting the advance curve (even inaccurately) was super informative for me. The root cause being a loose advance stop pin and worn springs. but FWIW I just had my 031 redone (2 weeks ago) because the max advance was too high. I am definitely not an expert, so I don't have anything to add that Paula and Glenn couldn't say with more authority than me.
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