Maybelline

Maybelline

TDC

Kevin Cameron

CADILLAC PULLED UP TO A HUNDRED AND four/My Ford got hot and wouldn’t do no more.

So sang Chuck Berry in his memorable song "Maybelline," which concerns the pursuit, in a V-Eight Ford, of an elusive lady in a Cadillac. The V-Eight is surely the classic flathead, a 1930s design that was long in the tooth at the time of the song (1950s). Romance is the substance of the song, but its subtext is detonation.

Detonation is a kind of abnormal combustion in which, as the flame front sweeps through the fuel/air mixture, compressing and heating the unburned charge ahead of it, chemical changes occur in the fuel. If there is time enough and temperature enough, those chemical changes will go far enough to cause parts of the unburned mixture to go off by themselves-to autoignite-before the flame front actually reaches them. And when this chemically altered unburned mixture does autoignite, its combustion occurs at or above the speed of sound, creating a shock wave of pressure that hits the piston and combustion chamber walls like a hammer, generating the familiar tinkle of engine knock-detonation.

Light detonation cannot be easily heard, but its supersonic blows still have effects. Normally, a layer of stagnant gas clings to inner combustion-chamber surfaces, acting as an insulator that slows heat flow from hot combustion gas to the metal parts. The hammering of detonation scours away this stagnant layer, thereby increasing the flow of heat to the engine’s cooling system.

The flathead combustion chamber is still familiar on lawnmower engines, and was last seen in motorcycling on Harley-Davidson’s 1950s K-model and through the 1960s on the K.R racer. Flatheads position their valves beside their cylinder bores, stems downward, not above the cylinder with stems upward, as in all current fourstroke bike engines. The head gaskets we used to see hanging from nails in the walls of old garages were from flatheads, outlining each combustion chamber with the familiar one big circle/two little circles shape. When originally conceived just after the turn of the century, the flathead chamber was an open invitation to detonation. Being much bigger than the cylinder bore, it took a long time to burn, and time is the first ingredient in detonation. Flaving a very large surface area in relation to its volume, the chamber’s hot walls also rapidly heated whatever was put into it-and temperature is detonation’s second ingredient.

In Chuck Berry’s V-Eight Ford, detonation was suppressed by a concept still in use today: rapid agitation of the fuel-air mixture by squish. A gasoline/air mixture burns quite slowly at rest, with a flame speed measured in inches per second. But if you stir up the mixture, adding turbulence, the flame front is wrinkled and convoluted, multiplying its surface area hugely, enabling it to gobble volumes of mixture in thousandths of a second. In any engine, the entering jet of fresh mixture from the intake valve(s), during the intake stroke, provides some turbulence. In Berry’s pursuing Ford, extra turbulence was provided by bringing part of the cylinder head close to the piston crown at TDC, so that the charge trapped between was squirted out at high speed into the rest of the chamber as the piston rose. This is squish, a concept still in wide use today. Squish further agitates the mixture, speeding the flame, burning the charge before detonation can develop.

However, it’s also possible to have too much turbulence. An engine with enough turbulence at low and medium speeds may have too much at high speed as the scouring action of excess turbulence increases heat flow from combustion gas to chamber walls. This is why many modern engines have means of boosting turbulence at low and medium speeds, but not at high speed-technologies such as closing off one of a pair of intake valves at lower rpm to provide intake swirl. There is even a current alternative theory of detonation, which proposes that flame in highly turbulent conditions may spontaneously accelerate to supersonic speed. Berry’s Ford, its combustion turbulence optimized for legal highway speeds, probably had too much at Maybelline-pursuit speed. It overheated and began to detonate.

The sky got cloudy, and it started to rain/So I tooted my horn for the passin ' lane.

Old Chuck knew about the detonation-suppressing qualities of humidity. Anything that will reduce the temperature of the intake charge will make detonation less likely. In Chuck’s case it was water. In the case of supercharged aircraft engines in WWII combat, it was a 50/50 mixture of water and methyl alcohol. In the case of current turbocharged engines, it is an intercooler between the heat-generating turbocompressor and the intake manifold.

Rainwater blowñn ’ all under my hood/I knew that was doin ’ my motor good.

There may have been some extra cooling effect at the radiator too, but the major effect was that detonation was stopped by the water, and with it, a major cause of the overheating.

Motor cooled down, the heat went down/And that’s when I heard that highway sound.

I’ve always been curious about “that highway sound,” and have assumed it was just the roar of open intakes on an engine that is getting all the air it needs. The hot-rod air cleaners of the 1950s probably absorbed little of the sound coming up from a row of three Stromberg 97s. When I was a boy of 10, my mother demonstrated to me that our family car (a flathead-Six) would climb the mountain at Elizabethtown, New York, in top gear on foggy nights, but had to be shifted down to second otherwise. Foggy or not, we never heard that highway sound.

Cadillac sittin ' like a ton o’ lead/Hundred and 10 a half a mile ahead/Cadillac look like it’s sittin ’ stili/’n I caught Maybelline at the top o ’ the hill.