From Kevin to Jim:

British racing singles of the 1930s started life with flat-topped pistons and compression in the 4.5 - 6-to-one range. This allowed them to pretty much copy the hemi 2V chamber pioneered by Fiat in their 1922 GP car engine, setting the two valves at a 90-100 degree included angle. But fuel octane number rose through the 1930s as Britain legalized use of the violent poison/antiknock tetraethyl lead and combined best available leaded gasoline with 50% "benzole", which was a by-product of coke production. Benzole was a catch-as-catch-can mixture of benzene, toluene, and xylene - all highly anti-knock aromatic compounds. With all that new ON, compression ratios could go up. It was a lot cheaper to make a new piston than a new head, so up, up went piston domes. Combustion had never been all that fast in OHV engines - there was no way to have squish as in flatheads/sidevalves. Along came Harry Weslake with a little help from the tangential intake port, which converted high intake velocity into rotary swirl around the cylinder axis. THis "stored" intake energy for later use as turbulence to accelerate combustion. Now a little math - making a true hemi chamber exactly doubles the surface area of the combustion bowl, as compared with the area of a disc whose diameter is the cylinder bore. And adding the matching piston dome did something similar to piston crown area - increased it a lot. This put piston temperature up, discouraging people from increasing bores and decreasing strokes for a long time. In 1950, here came Pole Leo Kuzmicki, working for Norton. If you imagine the tall piston dome as being made of fudge, he essentially pushed it down, forcing it outward, closer to the head surface everywhere except where valve clearance was needed. In those regions he brought the piston as close to the head as mechanically possible, creating OHV squish for the first time. As he pushed the top of the piston dome down, he created room above the now-flat piston top in which intake motion could persist all the way to TDC without being damped out by friction between moving gas and close-by metal surfaces. He transformed the old, slow "half an orange peel" combustion chamber into a faster-burning, much more compact chamber that was basically just the valve cutouts plus spark-plug area. The greatly improved Norton would have defeated the new Gilera-4s in GP racing that year, but Dunlop tires came apart on a couple of fast tracks and prevented what would otherwise have been runaway wins. Norton came back in 1951 and did the job - beating a potentially much more powerful Gilera. People today, in the 4-valve era, forget this great lesson - that just cramming a bunch of mixture up into a tight, badly-shaped combustion chamber and setting it off does not equal power. Or, as the late Keith Duckworth put it, "People are mesmerized by airflow, never reflecting that they must burn all that air and fuel they are getting into their engines." When Duckworth applied the 4V version of Kuzmicki's concept, the result was a flat-topped piston, a narrow valve angle, and a strict separation between as-close-as-possible squish and the most open, roomy combustion space. When Duckworth applied this concept to his DFV V8 GP car engine of 1967, it was able to defeat higher-revving V-12s. In place of Weslake's tangential intake, he biased his intakes to produce downdraft so that air flowed from the intake valves, across to the far cylinder wall, then down to the pison, across its crown, and back up the near cylinder wall. This, which he called "barrel motion",, is now called "tumble". The problem today is that too few builders realize there must be room in the combustion chamber for the turbulence needed for fast combustion. They just add material to the piston wherever it is easiest until they get the 13.8-to-one or whatever ratio their buddies told them they had to have. The piston now comes so close to the head that there really is NO combustion space. Any tumble-generated turbulence is damped out as the piston rises close to TDC, so they are having to use very long ignition timings for best torque. To a certain extent, this compromise must be tolerated, but the Kuzmicki/Duckworth idea has to be kept in mind at all times; make room for combustion turbulence. In some cases, like the truly terrible 5V Yamahas, the compromise really bites, so you can have either acceleration (from high compression that kills flame speed on top, causing weak peak power) or top-end (by lowering the compression enough to get back some top-end flame speed), but not both. When I asked Claudio Domenicali at Ducati how they have been able to shorten stroke again and again and still have competitive engines, while both Suzuki and Kawasaki have made new, shorter-stroke models that were slower than previous longer-stroke versions, he replied, "I cannot speak for other manufacturers, but in our case, we use a device like a small anemometer, placed in the cylinder. Then we vary the intake downdraft angle and port sizes until we get the tumble motion that our experience shows to be necessary." Sure, nothin' to it! Anyway, that is the modern combustion chamber conundrum in a nutshell. It really hurts in F1, where bore/stroke is 2.5, and they end up with ignition timings up in the 60s. Another problem is a social one. Racers don't mind being considered "advanced", but no one like to be thought "retarded". But where combustion is concerned, the more ignition timing your engine needs, the worse its combustion is revealed to be. Some people just can't get past the old idea that needing a lot of ignition advance is good. The reverse is true. A classic example of a bad engine is the old Honda 450 twin of the 1960s. Its tall piston dome and 78-degree valve angle made it into a heat-gatherer, and air just hates to go into a burning hot cylinder. It is delightful to be rid of air cooling at last! I have to go on another trip weekend after this, but am resolved to write the vaporization article you have asked for thereafter. I've just finished writing a "50-engines book", so there is more time available for other things. KC