Rider Rich Schlachter and I were crossing the Rocky Mountains from the west. We had been in the van a long time and stopped at the Continental Divide at dusk, 12,000 feet up. “Race you to that monument!” one of us said. We sprinted, but within 100 feet we wheezed out. No air! No wonder. Standard sea-level air pressure is 14.7 psi, but up at 12,000 feet it has dropped to only 64 percent of that. In the climb up Pikes Peak (which Don Canet will be doing Sunday on the prototype Victory Project 156 Hill Climb Special), internal combustion engines with atmospheric induction (no supercharging, no turbocharging) wheeze out in the same fashion. The Pikes Peak race begins at about 9,000 feet, where atmospheric density is just 71 percent of sea level. That means engine torque will also be 71 percent of sea level torque. Pretty feeble. And at the 14,110-ft. summit, torque will be down to only 59 percent of the sea level value. This is not much better than running on half-throttle. Electric bikes have no altitude problem—they make the same peak torque in the vacuum of space as they do at sea level.
It’s even worse for an internal-combustion engine with a carburetor fuel system. Carburetors deliver fuel in response to the difference between local air pressure and carburetor venturi air pressure. As we climb into the sky, air density falls steadily but the carburetor-metering signal does not, so our engine becomes ever richer, losing even more power. Electronic fuel injection overcomes this problem by linking fuel delivery to an air-pressure sensor. During Word War 1, German engineers realized that engine knock, or detonation, sets an upper limit to engine compression ratio. Anyone who has driven a car on cheap gas and heard its engine knocking knows that the knock will cease if we throttle back enough. In similar fashion, a compression ratio that produces detonation on WOT (wide-open throttle) at sea level will operate knock-free at 20,000 feet, where air density is less than half of that at sea level. To gain back some of the torque normally lost at altitude, German makers built “overcompressed” engines, giving them high compression ratios that would cause destructive knock at sea-level WOT, but preventing knock by linking maximum throttle opening to altitude. In this way they could optimize performance for a specific altitude. Using this concept, Franz-Zeno Diemer in 1919 set an altitude record of 32,000 feet behind an over-compressed BMW aircraft engine.
Stock car racing teams today have sealed dyno rooms in which air density and temperature can be set to any desired value. This allows them to tailor engine performance precisely to the altitude and expected air temperature at specific tracks. The higher the altitude, the higher the compression ratio that can safely be used. Pikes Peak racers do the same, giving their engines higher-than-normal compression ratios that increase torque and run knock-free up on the mountain, but which would knock destructively if run WOT at sea level. Dr. Sanford Moss, the “father of turbocharging,” portaged a Liberty V-12 aircraft engine and a dyno to the top of Pikes Peak in 1918 to pursue his research (there were no sealed dyno rooms in his day). Why not just recover altitude performance loss the same way—with a turbocharger? The reason is that while aircraft engines operate mainly at constant throttle, the throttles of a motorcycle engine climbing Pikes Peak are in constant motion as the bike is braked for turns, then accelerates, again and again. The output of a turbocharger increases as the square of impeller rpm, giving sudden and very peaky torque that is a really bad match to human reflexes and a motorcycle’s limited tire footprint. Snowmobilers on turbocharged “mountain sleds” have it much easier because of a sled’s monster footprint.