How about building a true piston port?

Any dirt bikers out there? Remember the original Honda Elsinore 125 cc dirt bike? Super light, cheap, and fast for its time--but you had to fan hell out of the clutch at low speed or that sucker would load up like crazy! It was a piston port design (common at that time); the timing for the intake was when the piston skirt covered/ uncovered the crankcase opening. Nice and simple, with massive flow rates when set up for high performance. (Honda used that design for their GP road racing bikes as well.) A dog at low rpms, though.

These were air cooled motors, so the water jacket didn't get in the way, but there's not that much need of a water jacket at the low point of piston travel and on the intake side. And they were simpler than a reed/ rotary valve set up.

Interesting?

Jeff

Konig buit 4 cylinder piston ported engines in the last '60s, and then converted/redesigned to rotary valve. A much better engine!

A couple years back we built a 44 Merc with piston ports. Doug Kay also fabricated the four carb crankcase. The engines pushes our D MOD about 90. Hope you like the photos.
Alan

Back in the 70's, the first Yamato Alky A and B motors were piston port. They would run very well once you go them on plane and up on the pipe. They used to call it the Yamato shuffle to try to get them on the pipe. Guy would go out and when they would break over, they would not be anywhere near the power band and bog down. They would go to the infield and start doing donuts to get the prop to cavitate and get the boat to a speed where the powerband would allow them to straighten out. Made for some really messy starts due to the churned up water.

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Originally Posted by Fastjeff57

Any dirt bikers out there? Remember the original Honda Elsinore 125 cc dirt bike? Super light, cheap, and fast for its time--but you had to fan hell out of the clutch at low speed or that sucker would load up like crazy! It was a piston port design (common at that time); the timing for the intake was when the piston skirt covered/ uncovered the crankcase opening. Nice and simple, with massive flow rates when set up for high performance. (Honda used that design for their GP road racing bikes as well.) A dog at low rpms, though.

These were air cooled motors, so the water jacket didn't get in the way, but there's not that much need of a water jacket at the low point of piston travel and on the intake side. And they were simpler than a reed/ rotary valve set up.

Interesting?

Jeff

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Piston Ports were/ are a very old idea, The problem with them is for go fast motors when you make the opening duration longer, it kills the starting quality and low speed torque. If you get it going it will scream top end but very little drivability. Later manufactures added reeds to improve this but as time went by more power was needed with longer still close time and early open times. This is why the reed was moved to the case so it lets the motor take what it wants based on intake and exhaust events. The old Konigs were sometimes a bitch to start when people cut the windows or the skirt of the piston. The rotary valve fixed this problem and is very reliable, but limited as to what it can do compared to the modern reed type motors. Steve

yep reed valves for wide torque band
I remember the Elsinore and in those days there were many kits to add a reed valve to a piston port engine to help the bottom end

Thanks for those great photos, Alan! (Into the Keeper File.)

So, getting them on plane was a dog--kinda like riding an old Honda Elsinore. (Ah, memories!)

I note that--in Alan's impressive motor--the air entered via those 3 holes in the piston. Dirt bike engines used the bottom edge of the piston as their "valve", which opened up a nice round hole for better flow.

But--as stated above--adding a reed made a dirt bike out of the Elsinores; we could even do alittle trail riding with them (something that was unthinkable without a reed setup!)

Ah, memories!

Jeff

PS: That was some beautiful machine work on those ports!