When I signed on to this motorbike racing business, the Yamaha TD1-A had just achieved a reputation as a good way to examine the responsibilities of leading a race, without the obligation to win. They would usually go about five laps at a clip good enough to hold off all but the most built AerMacchis and those Ducatis whose tuners had overcome impossible odds. Those machines were (optimistically, I'm told) rated at 32 BHP at 9500 or so. Engine stoppage would then cancel the program -- piston seized to anodized cylinder wall, piston ring broken, crank snapped on left-hand end, &c. Another aspect of the charm was the search for the powerband, using only five trials-inspired ratios.
And now the contrast. Last season's top GP 250s are reputed to have made 96 BHP somewhere between 12 and 13,000 (except the Aprilias, making their power way up past the vocal cords' elastic limit). Now it's reasonable to expect there to be a 100 BHP 250 shortly. That's more than triple the power available to historic figures on their fragile A-models in 1963. What is more, these new super-power bikes are reliable and they have generous powerbands. Look at the numbers, which are frightening indeed; brake mean effective pressure is the stroke-averaged combustion pressure. At one time, a BMEP of 100 PSI was considered a decent goal for a two-stroke engine. Then it was 125 PSI, and we all admired the three-cylinder Suzuki and wondered how they could get all that pressure through two dinky transfer ports 49 mm down the wall from TDC. Then it was 150 PSI. Not so long ago it was 175 PSI -- penetrating deep into four-stroke territory.
Here are some comparative figures for four-strokes. A newly-designed street-bike engine makes 145 PSI or a bit more. A Ferrari 312T GP car engine from the 1970s achieved 190 PSI across a broad range. The classic Manx Norton was credited by its builders (but not by independent dyno testers) with 200 PSI at the power peak. A crushingly superior Harley XR, fully-attended-to by the best people, makes 219 PSI at its torque peak. A Cosworth DFV Formula One auto racing engine might make 230 PSI, also at its torque peak, where it is benefiting from maximum intake and exhaust wave action.
Now the two-strokes are at 200 PSI and still climbing -- but they are contriving to make this pressure every time the piston comes to TDC. Back in the foggy, seminal days of the 1960s, the top engines made 120 PSI and had 300-600 RPM powerbands in some cases -- vanishingly small, really needing, begging for, those ridiculous lathe gearboxes with rows of countershafts and stacks of slowly-tapering gears that looked like the child's toy called Tower of Benares.
The modern engine has six speeds and a solid 2500 RPM of power, with a fringe of usable extra extending 500 RPM either way. BMEP parity with four-strokes has seemed a faraway goal, not worth thinking about, but here it is. And it has been achieved without compressors, mechanical valves, gears, cams, and linkages. All it has taken is incremental detailing and practical use of all that exhaust energy. And thirty years.