For years I was aware that Bud Aksland had a publicly expressed preference for low exhaust ports in two-stroke racing engines. On the other hand, it was also a matter of historical record that Yamaha, his traditional employer, had continually rung the changes on exhaust port height, swinging from the 79 degrees ATDC of the original TD1-B of 1965, back to something more like the 83 ATDC of the 1976 TZ250-C, and then back again in an unending cycle.
To me it had seemed there was good reason to have the port high; after all, there must be something between 30 and 35 degrees of blowdown -- the period during which only the exhaust(s) is open, and spent cylinder gases blow down into the pipe. Then there must still be room for an adequate transfer period, during which crankcase compression and exhaust pipe suction combine to flow fresh mixture into the cylinder, sweeping out (diluting out might be more accurate) exhaust residuals and recharging the cylinder for the next power stroke.
Some race engines -- such as those on snowmobiles -- have traditionally very high exhaust timing - like EO 76 ATDC - and I believe the production Morbidelli 125 twin had EO 78. On the other hand, the 1994 production Aprilia 250 that Chris D'Aluisio is running this year in AMA 250 races comes with EO 84, which is only one degree "racier" than a stock RD350 of nearly 20 years ago (despite the fact that the company's printed literature calls out EO 81).
Nevertheless, Bud likes something more conservative, like EO 81 or later, I suppose. Why? I always wrote it off as coupled to his preference for the forged (which means soft at temperature) Mahle pistons, over the cast stock Yamaha parts. While the stock Yamaha part retains its shape under pressure and at operating temperature, the Mahle pistons, being thin in the dome and made of low-silicon forgeable material, sagged easily. Aha, I thought, another of these light piston enthusiasts, willing to make any sacrifice to be able to run paper pistons.
But maybe there's more to it. Someone who runs engines on the dyno constantly, as Bud does, would see detonation all the time, and would view it as what it is -- the critical factor that often prevents use of a higher compression ratio or earlier ignition setting. Lots of development goes on all the time to get optimum cylinder head squish height and width, but what stops all this is ultimately detonation - AND DETONATION IS USUALLY TRIGGERED FROM THE HOTTEST PART IN THE COMBUSTION CHAMBER -- THE PISTON CROWN.
The earlier the piston uncovers the exhaust port, the hotter are the gases that then rush past the exhaust edge of the piston, heating it vigorously. Open the port a little later and you expose the piston to cooler gases. Hmm. Good old Bud. He knows what he's doing, even if armchair theorists don't. By opening the exhaust port later, he is lowering the piston's maximum dome temperature, thereby allowing himself to reach a higher compression ratio before detonation comes knocking. More through less. Machine the cylinder to lower all the ports, then perhaps add back some extra blowdown by filling the tops of the transfers (because when the port opens later, there is less pressure inside to push the exhaust out) a millimeter or so. Now there's not much time left for the transfer event, so perhaps the diffuser angles of the pipe have to be increased to compensate. Could be good.