Rd400 barrels on gt550

[arneg]

Steering this back on topic

I received 4 RD400 barrels last week and here's a first comparison. The RD's compression height is about 2-3mm taller than the GT, potentially allowing this conversion to be down with a steel plate instead of welding the cases. That would cover large parts of the rear transfers. Not yet sure which route to take.

The RD and GT barrel, view from below:



The GT port layout:



The RD port layout:



The RD is 2 generations more modern, with a reed valve, split transfers and a boost port. Odd for me is that the rear transfers are angled like the boost port!



The exhaust port is shorter, so pipes will have to be custom made for sure. A part of me was hoping GT pipes would almost fit but this definitely needs a different stub. Plus the exhaust is angled more which I would consider more modern for the reverse pulse.

The RD barrel on the case:



The mouth is maybe 1/2mm too narrow. Very little work required to get this to fit.



The fins aren't much larger than the GT, this almost looks like it belongs there.

And there is also enough space for the middle one to squeeze in. This will require a lot of machining on the middle barrel.

[Alan H]

Remember that the later 550 barrels have Shneurl porting too. The early ones don't.

[Vintageman]

Easier challenge if you get the later Euro engine cases with dual transfers maybe. was one recently on eBay NOS.

Not sure if center cyl will cool enough running aggressively, if you have to remove much of the side fin area. Do you think you do? However, wasn't the rd400 fins blessed longer than rd350/R5 on exhaust side? Else fin extensions. Make mine blond.

Yes the outside one looks nice setting there.

[campbell45]

any updates?

[Cragdog]

This is a great thread for the 550! Love it, keep the updates coming.

[Alan H]

Just bumping this thread for interest.
Just shows what can be done.....

[tz375]

Out of interest, why do people think that later GT550 have so called Schnuerle porting, also known as loop scavenging. Early and late models all use that principal of directing flow to the rear of the cylinder. later cylinders have an extra pair of ports and that can be more effective, but they are all using the same principal. The system is named after its inventor, Adolf Schnürle I believe.

RD250/350/400 porting is a bit different than modern 2 strokes. Back then, the idea was to add a small amount of extra flow to go up behind the main rising column of gas. Modern theory is to have the ports point in different radial directions to the old idea and with the same upwards angle around 17-19 degrees to create one rising column rather than two on different paths.

On a basically stock 550, we managed to make almost 60 rear wheel HP with just the exhaust and intake ports cleaned up with Jemco chambers. With changes we made to the transfer ports we expect to gain another 10HP. With RD400 cylinders it should be possible to make around 90 hp without going completely bonkers on the ports. Bore the liners out and weld and machine it like an OW31 and it should be possible to get close to 100 hp. And at that level, everything will start to break.

[dollydog]

i know a couple of people that have put aircooled rd250 top ends on gt250 crankcases, so i did a rd250lc top end on my ramair gt250 bottom end. pretty easy really and if i can do it, anybody can
cheers, dd.