Valued reader Mr. Hustknuckle writes to propose that detonation or materials set the limits for two designs that I recently wrote about, Rob Muzzy's best 1982 build of the 1,000cc Z1-based Superbike and the estimated 230 hp of the current World Superbike-spec Kawasaki ZX-10RR.

Yes and yes. When conditions leading up to combustion become hot enough—intake air temperature, combustion-chamber surface temperature, time required for combustion, compression ratio, etc.—combustion may no longer smoothly consume the compressed fuel-air charge from spark plug right to the cylinder wall. Instead, near the very end of combustion, as fresh charge out at the edges of the chamber reaches a critical temperature, tiny volumes of it can auto ignite. This is detonation, also known as combustion knock.

Mr. Hustknuckle notes that in both of these engines, the brake mean effective pressure (BMEP), or stroke-averaged net combustion pressures, are very similar: 212 psi and 205 psi.

Note that detonation is not pre-ignition, which is combustion that begins before the ignition spark, triggered by the presence of something hot in the chamber—an overheated spark-plug electrode or glowing carbon deposit. Detonation occurs only near the very end of an otherwise normal progression of flame from spark plug to cylinder wall. This is why detonation damages the outer edges of the piston, while pre-ignition typically punches a hole through the center of the piston's crown.

Those last bits of charge out at the edge, heated to a critical temperature, have been chemically changed in ways that transform the mixture into a sensitive explosive. If it auto-ignites, it burns at the local speed of sound (2,500 feet per second or more), thereby forming a violent shock wave that can splash aluminum off piston or head, and which hits metal surfaces with audible impact, hence the term “knock,” scouring away the normal thermal protection of a boundary layer of stagnant gas there and accelerating the transmission of combustion heat into those metal surfaces.

Kawasaki’s World Superbike-winning ZX-10RR
You’ve come a long way, baby. Kawasaki’s World Superbike-winning ZX-10RR can trace its roots to the four-cylinder Z1 of the 1970s. About those early air-cooled powerplants, Rob “Mr. Superbike” Muzzy once said, “Things go bad really fast when you get to 11.”Nic Coury

Riders of later 1970s Yamaha TZ250/350 roadracers, noticing an unexplained sudden 5-degree Celsius rise in coolant temperature, knew their engines had probably begun to detonate. Had those bikes carried exhaust-gas temperature gauges, they would at the same time have seen a fall in EGT. Why? With more combustion energy going into hot piston and chamber surfaces, there was less left over as exhaust gas temperature.

For best information on engine operation, riders or tuners pulled the head after every practice, and some even pulled the cylinder as well, allowing careful top-and-bottom examination of the pistons for any possible sign of detonation or abnormal operating temperature.

When the lead-based anti-detonant additive tetraethyl lead (TEL) was banned in Grand Prix racing gasoline in the late 1990s, Yamaha responded by making its inner cylinder heads of copper, which conducts heat significantly better than the original aluminum. Engineers also doubled the thickness of piston crowns, lowering the resistance to heat flow from the hot crown center to the much-cooler cylinder wall. These measures, by reducing the temperature of metal in contact with the fresh charge, reduced the amount by which compression ratio had to be reduced to cope with the mandated less-knock-resistant lead-free fuel. TEL acted as a negative-rate catalyst, slowing the heat-driven chemical transformation of those last bits of fresh charge into a sensitive explosive.

Pro Stockers run for just seven seconds, but Superbikes had to go 200 times farther.

In 1944, as the US B-29 bomber force was going into action in the Pacific, engine fires and failed takeoffs were a serious problem. In an effort to make loaded three-engine takeoffs survivable, higher supercharger boosts were tested. No dice; the marginal air cooling of those Wright R-3350 engines was putting them into detonation at 54 inches manifold pressure. The development team wanted to give aircrews 2,600 or even 2,800 “war-emergency” horsepower, but even with 20 percent fuel enrichment to evaporatively cool the incoming charge and with 6 grams of TEL per gallon of 115/145 fuel, detonation set the limit much lower.

Can’t you just push past the limit, as athletes so often describe their winning performances? There is no pushing past the bearing-destroying hammering of detonation and the steady loss of material from pistons, which can leave piston rings jutting out into empty space that was solid aluminum just minutes before.

There are other limits. Muzzy told me that in his Z1-based Superbike engines, “Things go bad really fast when you get to 11.” Those engines had pressed-together roller-and-ball-bearing cranks that were reliable at 10,250 rpm but would shift at their press fits just 750 revs higher, even if those press fits were welded in approved Pro Stock drag-race fashion. Pro Stockers run for just seven seconds, but Superbikes had to go 200 times farther.

Life is not automatically perfect just because the present ZX-10RR engine has a modern one-piece forged steel crank running in super-rigid plain bearings. To make those cranks live at 15,000 rpm, the oiling system has to guarantee that zero air is pumped with the oil and that took some intensive engineering. The alternative can be a spun bearing in first practice (a spun bearing is the result of the bearing shells seizing onto the crank journal, spinning with it, and destroying their supporting saddles in the rod or crankcase). Showstopper.

Can the stock crank handle the nearly 60 percent increase in stress level that results from boosting redline from 12,000 to 15,000 rpm? Or do cracks appear dangerously soon? Every part in a racing engine must have a guaranteed known life in hours, especially now that sanctioning bodies require engines to last through more than one race weekend. Maybe it’s time to switch material to low-internal-defects vacuum-remelted 300M steel for the crank. Just send us the bill.

Time also for minute attention to surface finish on every radiused fillet that joins a crank journal to the adjacent crank cheek. Time for those fillets to be rolled or nitrided, putting them into local compression that protects them from crack-propagating tensile stress. If quality-assurance inspection is rejecting half the cranks, that doubles the price.

When you handle one problem that's blocking the way to greater power and reliability, another problem is revealed. When time is shorter than money, all possible solutions have to be tackled simultaneously, not one after the other. That's really expensive, and is how the giant F-1 rocket engines on Saturn V were qualified for flight in time to accomplish that historic 1969 manned mission to the moon.