Cylinder Head Temperature and Performance

[Admin]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

If you’re nerdy enough to notice, you see that in general, compression ratios on air-cooled engines (especially if they have large bore) tend to be lower than on liquid-cooled engines, especially those with moderate to small bores.

The higher the compression ratio you can safely run in an engine, the greater the torque it will produce. Why? Because as a rule of thumb, peak combustion pressure is one hundred times the compression ratio. Engine torque results from that combustion pressure pressing your pistons down to turn the crankshaft.

What sets the limit here is the degree of heating the fresh fuel-air charge suffers as it enters the cylinder and is heated by contact with it, then by compression, and finally, as yet-unburned mixture out near the cylinder wall is compressed and heated by the expanding combustion gas.

If that unburned mixture out at the cylinder wall—which the engineers call the “end-gas”—is sufficiently heated before the flame can reach it, bits of it may autoignite, burning at or above the speed of sound in a phenomenon called “detonation” or engine knock. The knocking sound we hear during detonation is sonic shock waves, hitting the metal surfaces of the combustion chamber.

Compression ratio has to be set low enough that even on the hottest day, with the worst gasoline you are likely to find at the pump, your engine will not detonate. Higher compressions heats the fuel-air mixture more and produces higher peak pressure, so the higher the compression ratio, the more likely detonation becomes.

Because it’s harder to cool well and consistently with air (summer air? winter air?), head temperature of air-cooled engines tends to be higher than in a liquid-cooled engine, so fuel-air mixture gets heated more, possibly leading to detonation.

Steady detonation (as opposed to the “occasional tinkle”) is destructive, causing overheating, piston heat softening, and finally, erosion of piston metal.

Because in general, the larger the cylinder bore the longer it takes to complete combustion, detonation is more likely in big-bore engines, which expose their mixture to heat longer. This is why air-cooled engines and many engines with large bore are given lower compression ratios than liquid-cooled and/or smaller-bore engines.

There can be exceptions. I spent the summer of 1963 in Denver, Colorado, where I worked in a bike shop. One of our customers was a very serious night warrior who had built an air-cooled Triumph 650 twin with a sky-high 12-to-1 compression ratio (that’s not sky high today, when every sportbike engine has 13:1, but in 1963 it was crazy high).

Why didn’t his engine detonate itself into aluminum gravel? His mother had a back shed, and in it he kept a secret 55-gallon drum of purple aviation 115/145 gasoline. It was rich in highly knock-resistant alkylates and contained 6 grams per gallon of the powerful anti-knock compound tetraethyl lead. That was the fuel that powered many supercharged and air-cooled World War II aircraft engines, all of which had huge bores between 5-3/4 and 6-1/8 inches (146 to 155.6mm).

On this wonderful but now unavailable fuel his Triumph did not knock, but left the competition for dead. Drag racers today pay $60 a gallon for racing gasoline having similar knock resistance.

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[Tinkicker]

Indeed.  Back in the early 1990s I used to be the mechanic for an amateur racing team in the Auto 66 125 superstock club series.

We were running a Cagiva Mito 125.

Ported this, polished that, gas flowed the other.  Stock Cagiva squish clearance was 1.2 mm, so we ran that.

Rider and team owner said more power required scotty.

 

Squish reduced to 1.0 mm.  More power required scotty.

Squished reduced to 0.7mm with advice given that it was on the verge of becoming a handgrenade on pump fuel.  Advice ignored.  More power required scotty, getting 1st and 2nd place podium finishes was not enough.  He wanted world domination.

 

What a conundrum.  Rules stated that cooling system must have water only and only standard pump fuel could be used.  I told the owner he needed to run glycol  to carry more heat away and run avgas if he wanted more power.  Getting rid of heat was becoming the major problem, despite every available cooling passage being widened with a dremel and the thermostat removed.  Without these mods the thing would overheat in minutes.

 

More power required scotty.. Oh hell.

 

Took it to 0.4mm squish and well into detonation territory on pump fuel.  Asked the owner if he had his avgas... Err no, I have decided to stick to the rules.....

 

All I could do was advise that it needed nursing carefully, just enough abuse to stay out in front, no more and pull on the choke at the end of the full throttle straight to cool the piston a little.  At least it was early spring and cool weather.

Results came back in after race.  Bike went through the speedtrap at 108 mph and blew the 125cc lap record at Elvington into the weeds.

Wealthy owner peeled his rather nice wristwatch from his wrist and gave it to me as a reward.  I reckon it was putting out 35+ bhp.

 

Unfortunately the weather heated up and the owners ego took over.  He did not just want to win, he wanted to rub everyones noses in it.  He used new tyres every race weekend, and the other racers used to come by his caravan and buy his old rubber for £20 a set.  It was like he thought he was Team Harris or something.  He stopped nursing the bike and it needed a new piston after every weekend.  It was detonating badly at full throttle and it was also showing signs of heat seizure because he kept forgetting to pull the choke lever out.

Because we were running no thermostat, as soon as the throttle closed and the engine was producing no heat, the cylinder cooled faster than the almost glowing hot piston and nipped it.

 

We had words.   I told him if he did not nurse the bike, I was out.  Next race, watching him pull 25 yards in front on the back straight, I was seething.

Next lap it was 50 yards.  Next it was 75 yards.  Next it was 100 yards. Next lap the rest of the field came around and he didn't.  Engine had blown big time.  It was toast.  I refused to rebuild it and walked away.

 

I heard he rebuilt the motor, but the next race at three sisters circuit was a wet one.  He binned it on the first lap, broke his leg badly and totalled the bike.  Last I heard was it was sold cheap to another racer who was going to try to rebuild it.  The owner had a lot of problems with bone infections with his broken leg and gave up on motorcycling.

 

There endeth my chance to earn a good reputation in racing circles,  leave the motorcycle workshop and become a racing technician.  Total wages for a couple of hundred hours work.  One secondhand watch.

 

 

Edited May 13, 2023 by Tinkicker