A couple of very talented guys

[Suzukidave]

A very custom triple using a 700cc snowmobile engine and top of the line parts , i hope he posts some completed video with it running and riding https://www.youtube.com/watch?v=kjJ1iYtg_OM" onclick="window.open(this.href);return false;

[GT750Battleship]

Yes looks good....a little primitive with the lawn mower rope start I thought
Cheers,
Roger

[tz375]

And what's really odd is to see 8 second 1500cc triple sleds making 200+ hop started that way. It just doesn't seem right.

[Suzukidave]

Yes looks good....a little primitive with the lawn mower rope start I thought
Cheers,
Roger

They must have just used the stock pull start that comes on the snowmobile engine .

[celt_rock]

Just because you can... does that mean you should?

[tz375]

I'm probably not the right guy to ask that of. How does that old saying go? I can resist anything - except temptation....

[D&D CHASSIS]

And what's really odd is to see 8 second 1500cc triple sleds making 200+ hop started that way. It just doesn't seem right.

The 1500 that Doug Flannery
built makes340 hp...............

[tz375]

Sheee it

That's a lot of horses. If I manage to work out how to slip one of those into my GT750 can I still run it in the "Stock" class?

How many horses do we think that Bob's Twin cylinder sled motor makes? That would have to be in the 175 range I guess.

[D&D CHASSIS]

600 twin is 159-160 hp @ the crank

[ja-moo]

For some reason it annoy's me that these big 2 strokes are "wowed" with their hp. The last GP 500's were tame enough to be streetable, but still had over 200 rwhp. To me, that would be the pentacle. 400 plus rwhp per liter. And that's without any of the developments in the last 2 decades. It just seems they went the 4 stroke road, and didn't do development, just increase displacement.....

[D&D CHASSIS]

...................yes john you`re right, 200 rwhp out of 500cc`s I agree.......... streetable......that's a skeptical maybe.........
The factorys threw very bit of technology they could at the 500`s and they were built from a clean sheet of paper.........the 1500 is done with aftermarket cyls that are still a conventional transfer layout and a PRODUCTION case that's modded.........not exactly ideal, a compromise at best........... 340 hp out of a 3 cylinder ......... 113 hp per cyl, a KX500 or a CR500 dirtbike couldn`t come close to those numbers even wildly ported.......

[tz375]

Good points. 500cc is just 4 125 singles and a good RS125 makes around 50 on a really good day, and an Aprilia RS125R made over 50, so 200 seems reasonable for a 500 at a 1991 level of development.

So a 1500cc motor at the same level of development could mathematically make 600HP and would probably need to be 12 cylinder motor to get enough wall area for the required port area. It would also have to be 50-54mm stroke for the same reasons.

A 500cc cylinder with say a 80mm stroke and 89 mm bore would have a cylinder wall with a 279mm circumference available to carve ports into it. By comparison a 125cc (54*54) would have 170mm per cylinder or 680mm of wall available to be turned into ports for 500cc of capacity. Please check the math, but the simple answer is that a 500cc 4 cylinder can rev higher and has more port area to drag gases in, around and out again.

At the same level of efficiency, power is proportionate to RPM and at the same mean piston speed, a54mm crank can push pistons up and down over 50% higher revs.

Imagine a 1500 triple with a 54mm stroke and 108mm bore. leaving aside the obvious design issues, that 108mm bore would still only have 339mm of bore circumference because circumference is pi*d and volume is pi r^2, so there is a lot more wall surface with small cylinders.

So the best we should reasonably expect out of 1500cc triple is say 200*3/1.55 or around about 380HP.

Of course those simple calculatiuons assume a lot of variables remain constant which is over simplistic, but that at least puts it in the ball park.

[ja-moo]

I understand the design variables, and it's not a true "comparative". It just annoys me for some reason...........