Engine Tech: How Much Power, For How Long?

By now we’ve all seen the email that lists all the intense action taking place aboard a AA Fuel dragster in the course of a run, including the estimate that the engine makes 8,000 horsepower in the process. Since NHRA limits the class to 500 cubic inches, that is nominally 16 hp per cubic inch. This naturally leads us to ask, “Why are all other kinds of engines such slackers?”

If you've been to a Top Fuel meet, you know that these engines are quickly disassembled and inspected between runs in an amazing and specialized process. Any suspect item is replaced. Let's make some comparisons. One of the power giants of production motorcycling is the Suzuki Hayabusa, whose 1300cc (79.3 cu. in.) make close to 200 hp. But at 16 hp/cu. in., it would make 1269 hp. Why the shortfall?

First thing is, AA Fuelers are supercharged, which stuffs them with more fuel-air mixture than their cylinders would draw in by themselves. Supercharging roughly triples those engine’s horsepower per cubic inch. Second thing is, they burn a high-energy fuel that contains much of its own oxidizer—nitromethane. It potentially multiplies power by 2.5. Thus the gain from just these two features is 3 x 2.5, which equals 7.5. Taking these away leaves us with roughly 16/7.5 = 2.13 hp/cu. in. or “only” 1065 hp.

That performance from a 500-cubic-inch V-8 turning 8,300 rpm is starting to sound almost reasonable. Such an engine, unsupercharged, but with race-level stroke-averaged net-combustion pressure, could make about that power at that speed. If we multiply the Hayabusa’s 79.3 cubic inches by that same 2.13 hp/cube, we get around 170 hp, which is like the numbers measured from stock ’busas in 2005. So it all sort of makes sense.

Anyone who’s been to the Reno Air Races knows it takes about 4,000 hp to make a clipped, smoothed-off P-51 Mustang competitive. That power comes from a 1,650 cubic-inch liquid-cooled V-12. Again comes the creeping question: Why so little power from such a big engine? Is it because the P-51’s Rolls-Royce engine was designed 75 years ago? No. What it comes down to is the question of how much power you need for how long. In Top Fuel, the engine gives big power for roughly four seconds (remember, they’re running 1,000 feet now, not 1,320) and must then be torn down, inspected, and built up again.

In air racing, the engine must survive not only the 51 miles of actual racing but also a week of heat races, so lifetime must be of the order of one hour (that is, 900 times longer than in Top Fuel). These engines run long enough to require effective cooling systems, for if something does not continuously remove waste heat from the engine, it will be instant toast. That being so, the crews turn up supercharger boost as much as the fuel will permit without detonation.

Stock power on these engines was about 1,750 hp at 3,000 rpm. Because these engines had to be made light enough to fly, even without detonation, adding power tends to blow the heads off the top or the crank out the bottom, requiring that engines be reinforced by steel girdles. Then they run the rpm up another thousand; any more than that and rods and pistons come through the cowling. So some 33 percent of the extra power comes from the higher revs, taking the stock 1,750 hp up to 2,300 or so, and the remaining extra power comes from higher supercharger boost—cramming in extra fuel-air mixture.

Well, okay, but how come they didn’t make that 4,000 hp in WWII? Wouldn’t that have given pilots a valuable edge? Again it’s a question of how much power for how long. To be useful in combat, engines had to be lifed for 250 to 300 hours, and that imposed tighter limits on power and rpm. When skilled enemy aviators are trying to kill you, the last thing you need is engine worries (“What’s that funny noise?”). Once WWII was over and aircraft piston engines were operated more carefully by commercial aviation, engine life rose by a factor of 10.

Ship engines—giant two-stroke diesels of 10–14 cylinders, each 110,000 cubic-inch cylinder of which makes 7,800 hp at maybe 80–100 rpm—are at the other end of this scale. These 100,000-horsepower monsters are twice as fuel-efficient as the average car or bike engine (in pounds of fuel used per horsepower-hour). They have to be, because the international shipment of goods is a tough, competitive business (how do you suppose all those Chinese products get here so cheaply?). Such an engine is expected to run 20,000 hours before overhaul or 18 million times longer than the four-second TBO (Time Before Overhaul) of the Top Fuel drag engine.