Rd400 barrels on gt550

[arneg]

Before I go off and try this (the stroke matches, the crank mouth is the same) I was wondering if anyone has tried this swap? Are the gt rods strong enough? Would rd rods work? The case could be welded or a packer used with shaved cyl bases.

[tz375]

I want to try this on a bike I sold to a mate of mine. I did not check stud patterns or rods yet. I figured I'd start with simple stuff such as can an RD400 pot fit in the middle position of a GT? If that's a yes, I figure we are good for 100hp on race gas in full race spec.

[arneg]

I checked and the studs are much wider plus the transfers much smaller on the gt. For the pipe I would think standard or jemcos would work with very little modification.

[tz375]

You could be right but that's not what was in my mind. Transfers can always be opened up with either weld or Devcon F and studs can be moved but that requires skilled welding and machining.

[arneg]

yes weld up and machine case -- or get 3 ~10mm packing plates, machine down barrel bases by 10mm, bolt packers on original studs, and set new studs in packing plate. that would be a totally reversible conversion. the transfers might not open the rear ports well but should be ok otherwise.

[tz375]

That's another good way to do it. I think that's how someone fitted H2 barrels to a GT750 bottom end, but the adapter/spacer was more like 30mm thick.

[arneg]

i will have to measure the compression height on RD vs GT pistons and con rod length; maybe with a bit of luck it could be done by having higher pistons and/or longer rods and no mods to the barrel.

[D&D CHASSIS]

Would`nt it be easier to get the burr warmed up on the Gt550 cyls? instead of trying to reinvent the wheel...........
Probably make close to 80-85 hp with good pipes (not jemcos), 34-36mm mik`s , weld up the heads and cut a modern combustion chamber........

[arneg]

the transfer area on the GT is too small, the exhaust tiny, there is no forged piston available with wire rings, so limited exhaust width, and there is no space for a boost port or reed. plus no squish. that's all much more modern on the RD cyl.

[tz375]

Good point Dave. I agree that a 550 could be made a lot hotter. The US cast iron barrels are rather restricted in the transfer area and it really needs better pistons with thin rings. That was my first idea - bore it to take TR3/TZ3250G pistons with single thin rings so I dragged out a couple of TZ barrels and an R5 Barrel and some pistons and there is some scope to improve things.

Playing with the numbers suggests it should be possible to get more out of a 550 than my 750 currently makes, but that's not saying much because my 750 is supposed to be a low rev torquey sweet riding street bike. But while I was playing with the numbers, I need more transfer area and I started thinking in terms of epoxy and weld and that brought me to the logical question of why not bolt on a whole lot more port area. I did manage to avoid the obvious illogical conclusion with Mattoon triple cases, CPI barrels etc and 150 HP motors. I'm just playing with ideas for next year or the year after if the guy who now owns that bike starts to get serious.

I have convinced him to buy a spare set of stock cylinders to play with, but we need to drag him to the track and see if he catches the infection, and pounce when he's hooked.

It was those Yamaha barrels on the bench that made me do it. That little voice inside my head kept goading me to take hjust one more step into the darkness.

The fact of the matter is that there is no simple way to make modern type HP with these old motors, but that doesn't stop us trying.

[Vintageman]

The only frusting thing is most want to go from 4x horsepower to 8x horsepower


Why so much?

OK if your hobby is drag racing road racing I get it.
Do like some other

Level !

Level 2

Level 3

I ask has anyone ever got 15-20% more horsepower out of the GT550 beyond just a blueprinted engine. If so do tell.

Price for the simplest least expensive mod

[tz375]

Stock GT550 around 40 HP.

Ported plus pipes 80HP 2x

RD400 top end (if it were feasible) 120HP 3x

Why? Because you can. Because it's fun to see what can be done with an old design and intellectually stimulating.

4x would be almost 160 - not happening in this solar system I suspect

Remember though, a modern 600 twin sled can make $150 without a whole lot of trouble, but the technology requires metal to me moved to very different places than a GT550 has it. 180- 240 is feasible from a triple Arctic Cat or Polaris, but it isn't cheap. Check out drag race sleds with over 1000cc motors and crabs carved from solid lumps of billet because no one makes carbs that big and the HP they make is ridiculous. And the pipes. OMG you should see how large their exhausts are.

I spent a few hours at AAEN Performance yesterday chatting to the boss and some of the pipes they have are huge. I take that back. Huge is too small a word for those pipes. They have stingers that are almost as large as some headers. B U T they don't care about power much below the peak. Fort that application it's all what we would call top end. For street bike not so useful, but always something new to learn.

[Alan H]

I ask has anyone ever got 15-20% more horsepower out of the GT550 beyond just a blueprinted engine. If so do tell.

Price for the simplest least expensive mod

Not sure what increase there was, but in 1972 Crooks Suzuki recommended 20thou off the head, clean, match and polish the ports, shorten the baffles and when I did that I geared it up and still easily outaccelerated a standard 550 with a better top end too. 19" rear wheel and throw the centre stand away so it corners better, 20w50 in the forks and Ferodo linings in the 4LS.
That's what the A will be getting when I've finished the K - except for the 4LS!
Not peaky and still well tractable around town.

[Vintageman]

Tz375,

You guys want the max. Love the challenge that brings I am in the minority want my 15-20% more out of my old air cooled girls and hope I can still street ride OK.

I also got some help this year from AAEN on a Polaris 600 HO on tuning. Guy was spot one in his suggestions for clutching. Years ago I put their vintage single pipe on a Ski Doo Blizzard 550 Ski doo and it alone added power I could really feel.

Alan H.
I agree with cleaning up ports. I suppose this is what one calls blue printing. Not changing the design, just getting out what the engineers intended. My 73 ports looked decent… transfers well matched versus some I have seen but, did not polish up exhaust or intake. I think I have seen pics of your work on this site and nice.

Shortening the baffles was also popular for other 2 stroke like the Kawasaki line too. But, I have chambers that added some mid boost but, no more top end range really. I suppose it may be my pipe design, but bet it is something else like port timing.

I am wrong. Does anyone’s pipes really add to top end?

I did shave 2mm off piston skirts. With that trick it did not gain as much I hoped. On my T350 and T500 when I changed intake timing I gained top end along with stronger middle range. The GT550 still had same exact top end feel with that change.

I have not tried shaving 1/2mm of head. My bike pinged (was carbon build though, for no more ping since de-coke) so worth a try. I don’t think that would help top end though? I would like to see +500 rpm shift in peak power point with a little over rev.

Does anyone know what the stock intake and exhaust timing is for this bike?

I think this one could stand a little off the exhaust port ~ 2 mm. Wonder what the timing would be after that?
Anyone have the specs for rod length piston specs to run the numbers in a timing software program?

When I have raised the exhaust port or chamfer piston top, I always question if I should restore some, all, or none of the compression lost. Seems like no one does return it all back? If not restoring the compression is so one does not melt down for sustained WOT maybe I want to restore it all back and not WOT too long thus not loose to much mid and bottom end power. Maybe not an issue if restore it all back since I am only taking a little off? 5-10 degrees?

I did recently note that a Yam R5 has a 32mm exhaust port top height to cylinder where the RD350 has a 29.5mm. Wow 2.5 mm on a 54mm stroke (same change early to late GT250 made). I think the R5 head and early RD350 head have similar volume. The R5 states 6.9:1 and RD350 is less at 6.6:1 compression (is this the higher exhasut port difference?). R5 = 36 hp at 7000 and rd350 39 at 7500 hp. I suppose they did not restore to 6.9:1 just for reliability at high rpm?

I am thinking 2 mm may be what I need for GT550. But, don’t want to be too greedy. That 600 Polaris I mentioned earlier. The Power Valves are not allowed to open until 6800 RPM and the engine stock has peek HP at 7800… so exhaust port change only added to very top end to maintain the sled’s ability to run well low and midrange. I don’t want to F it up. Can I get 15-20% from gt550 without F’ing it up for the street?

How about port timing info on the GT550?

[D&D CHASSIS]

I am a proponent of the "KISS" method of engine development...........only after every stone has been overturned, only then look to exotic ideas such as "cross-dressing" an engine (rd cyls on a gt550). Everyone used to think the tr750 (107-109HP?) was the end all, be all as far as porting for power was concerned. Kevin Hutchinson`s gt750 makes 127 RWHP .......The difference? MODERN PIPES, A squish band head setup that actually aids in power production unlike the stock (junk) crap from the factory. It doesn`t hurt to have a good ignition also ....... Getting back to the GT550, if all you did was weld/machine up the heads with a semi tight squish band (maintaining stock compression), it`s going to wake that motor up.........add what I said in my above post (Porting and Pipes) 80-85hp is feasible.....and I`m assuming the transfers are similar to the GT750...... something to keep in mind...the Kawasaki H1R 500 made 90hp @9000rpm

Arneg, what are you looking to do with the bike? street rider? Track bike?