Two stroke triple street bikes

Port timing and port time-area are the 2 stroke equivalent of cams with more lift and larger valves on a 4 stroke.

A simple approach to improving performance is always to compare the motor in hand with another with better performance. Sometimes that reveals the futility of the dream, but is often a good source of information. I use Bimotion software to determine target time area and develop a starting point and then explore a range of possibilities in MOTA. Of course the software allows us to model ports that could not be machined without a lot of welding but that does help in understanding the limitations of a particular design.

A modern 2 stroke like say an Aprilia RS250 cup bike makes around 60hp at the rear wheel and with careful assembly can top 65BHP from 250cc. If we could scale that up we could get something spectacular.

Hold that thought.

Matoon 3 cylinder billet banshee cases with a triple cylinder CPI Cheetah or Serval top end would make enough hp to make your frame curl up and cry in the corner of the shed.

Take say a Polaris XCR800 and port it for 180 plus HP or an Arctic Cat triple and mate it to a modern 6 speed trans in a custom machine case and you could be looking at teh same HP as a MotoGP bike.

Those all have a lot more port area than any of our old triples and more potential to make power, but we all have a responsibility to make the most of what we have.......
 
guernz said:

whats your handle over on vintage sleds? I hang out over there too some
 
teazer said:
Port timing and port time-area are the 2 stroke equivalent of cams with more lift and larger valves on a 4 stroke.

A simple approach to improving performance is always to compare the motor in hand with another with better performance. Sometimes that reveals the futility of the dream, but is often a good source of information. I use Bimotion software to determine target time area and develop a starting point and then explore a range of possibilities in MOTA. Of course the software allows us to model ports that could not be machined without a lot of welding but that does help in understanding the limitations of a particular design.

A modern 2 stroke like say an Aprilia RS250 cup bike makes around 60hp at the rear wheel and with careful assembly can top 65BHP from 250cc. If we could scale that up we could get something spectacular.

Hold that thought.

Matoon 3 cylinder billet banshee cases with a triple cylinder CPI Cheetah or Serval top end would make enough hp to make your frame curl up and cry in the corner of the shed.

Take say a Polaris XCR800 and port it for 180 plus HP or an Arctic Cat triple and mate it to a modern 6 speed trans in a custom machine case and you could be looking at teh same HP as a MotoGP bike.

Those all have a lot more port area than any of our old triples and more potential to make power, but we all have a responsibility to make the most of what we have.......

You're dangerous. But in a very good way.
 
No no no...not dangerous at all. And yes...in a good way. The feel of a well built 2S coming on the pipe is so addictive. I miss mine. But I had to quit, or go to rehab, so I cold turkey'd it.
Teazer knows his stuff. Respect.
 
Shoeman, I respect your self control, but in this post PC world, it's time for you to treat your inner two stroke junky to another two stroke.

And remember that owning a two stroke is like malaria. You think you are cured, but the symptoms keep reappearing and the most effective treatment is re exposure to the sound smell and exhilaration that only a two stroke brings.
 
teazer said:
Shoeman, I respect your self control, but in this post PC world, it's time for you to treat your inner two stroke junky to another two stroke.

And remember that owning a two stroke is like malaria. You think you are cured, but the symptoms keep reappearing and the most effective treatment is re exposure to the sound smell and exhilaration that only a two stroke brings.
I admit I got a little choked up when my last ring-ding drove away in the back of the buyers truck. It was a real misbehaving machine and I had much fun developing it.
After more than a decade away from bikes I'll be happy with my little KZ400 project in my old age.
 

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Sonreir™ said:
I always found it to be a bit strange the the aircooled Kawis were quicker than the GT750. What accounts for the difference? Presumably, someone could tweak the engine on the GT750 to put out some better numbers?
like teazer said but he forgot the why the suzuki basically has same porting as the rl trials bike it uses one piston from the trials and ts250 and old tech bigger bore than stroke which only helpful in a 4 stroke a longer stroke for a 2t motor allows a much better transfer port taller transfer port giving a much better area/time deal which gives a porting tuner a better canvas to work with
that said we are talking a 250 now 125's have been square forever and it is the best formula for them 54x54
i dont think you can compare the 125 to a 250 or bigger jug
 
I did port work for a racing buddy with a GT750 who was building a TR replica out of it. He was an Englishman living here in Maine at the time, and he was a huge "Sheeney" fan as he referred to Barry. He had connections back home that got us a lot of Suzuki GB race spec info. We took some huge slice off the top of the block, he machined a spacer for underneath it, I matched that to the transfers and raised and widened and set angles on everything to the specs he had settled on. It was the most metal removal I ever did on any cylinder I ever worked on. My hands were toast by the time all three holes were finished. He got Swarbrick kit pipes that he fabbed to have one do the up and behind the motor under the seat trick, big carbs, a trick ignition, welded up the heads and cut some squish into them, and in the end it dyno'd at about 96hp at the wheel. And that was without a lot of carb and ignition tuning or further development. And it looked awesome in blue and white with replica 'glass and all the trimmings. He raced it with the USCRA at NHIS for a bit. I have not seen him in years, I wonder where it ended up?
LOng story short...the GT750 can make serious power if you build it right, and it was reliable as a rock too.
 
guernz said:
Same handle
That's another good forum.
I'm not as active there, just lurk alot. Dad is more active, he goes by boa ski
 
Shoeman said:
I did port work for a racing buddy with a GT750 who was building a TR replica out of it. He was an Englishman living here in Maine at the time, and he was a huge "Sheeney" fan as he referred to Barry. He had connections back home that got us a lot of Suzuki GB race spec info. We took some huge slice off the top of the block, he machined a spacer for underneath it, I matched that to the transfers and raised and widened and set angles on everything to the specs he had settled on. It was the most metal removal I ever did on any cylinder I ever worked on. My hands were toast by the time all three holes were finished. He got Swarbrick kit pipes that he fabbed to have one do the up and behind the motor under the seat trick, big carbs, a trick ignition, welded up the heads and cut some squish into them, and in the end it dyno'd at about 96hp at the wheel. And that was without a lot of carb and ignition tuning or further development. And it looked awesome in blue and white with replica 'glass and all the trimmings. He raced it with the USCRA at NHIS for a bit. I have not seen him in years, I wonder where it ended up?
LOng story short...the GT750 can make serious power if you build it right, and it was reliable as a rock too.

Mine doesn't have a spacer - just mild porting and makes close to 80hp at the rear wheel. Kevin H was making about 127 at the rear wheel last time I checked and on nitrous it's obviously much more.

Spacer plates are just the ticket if you want to raise the exhaust port far enough. Raise the exhaust port roof too far and it breaks through into the water passageway unless the roof is curved. That's true for late or early blocks. A Spacer gets you to 6mm taller than stock quite easily and over 100hp. Transfers are also a problem the way the barrel is cast and three of the 6 transfer ports break through into the water jacket with any high HP porting. I have a few 3mm lift plates in stock and with the extra gasket, require 4mm to be machined off the top of the block.

GT750 is 70mm x 64mm and a 550 is 61mm x 62mm stroke. Bore it to 64mm and you have almost a three cylinder piston ported RD400.

Banshees are 64 x 54 and with large barrels and stroker cranks, 72 x 64 is common. I have seen up to 14mm cranks so that's 68mm stroke with the longer RD400 type rods of course.

Shoeman, Nice RZ. Are those Spec 11 pipes? I have those on mine . Super quiet and clean running and a nice step at 7,000
 
Spec II indeed. Back in the '90's when I was into vintage racing I had Gary make me that set and asked him to leave them bare so I could use the VHT hi-temp clear for that factory GP team look. They were the race spec and man did they work well.
After years of riding the AC RD's, feeling the YPVS do it's thing was a revelation even before I put the SpecII's on it. You suddenly had a wide (well, sort of, but you know what I mean) powerband and throttle response even with VM34's on it. Just twist and go. On that motor I tweaked the transfers and exhausts, enlarged the intake ports, did the Boyeson trick of drilling from the intake to the transfers, cut the head a bit and ended with a street bike that was crazy fun.
 
i think the ultimate 2 stroke for street riding would be a twin with 2 cr250 barrels a real narrow package without heavy vibes maybe a v tandem or vtwin
i dont think you could stay out of jail tho
 
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