GT550 perfromance, dare I ask

[Vintageman]

OK,

I think the answer will be there is nothing you can do to get more hp from the gt550. No one has really tried… no data exits. Nothing? I got a hold of some used chambers, not sure of make... Chambers should help a bit on power I suppose.

Is there anything else I can do to boost performance just a tadd

My ports match pretty well to casting to liner... I did notice that where the transfer ports at the bottom of the cylinder begin, the liner is still squared off. I see other design taper this liner to make it flow better across it in into transfer port. Does this make sense?

I think I could remove some more material on intake to better align with carb rubber intake boot


What about the transfer window in the piston? It is smaller than the opening in the liner… does making the piston window larger help?


What about taking a bit off the piston skirt intake side to change intake timing some. OR raise the cylinder a bit (extra base gasket, mill head same, the shave intake skirt a bit to lower intake in affect).



I am in the process of rebuilding the GT550 and wondered if anything simple that gain be done to gain just a bit more hp…. I may not dare do anything, But, if anyone knows of something let me know.

[tz375]

How much more power are you looking for and what is the budget?

There are a few GT550s around that have been ported and even more with JEMCo pipes. If you have a big enough budget, 80hp is possible.

A good tune up seems to be the most cost effective way to get a small improvement. After that it gets expensive.

[Barry S.]

While it is apart you could start with smoothing up the rough squared off transfer areas you mentioned, can't hurt. Where the carbs bolts on the intake can be matched to the carb diameter for smoother flow, I think your on the right track.

[Vintageman]

Thanks guys.

What I would like for gain? Well if today peek HP is at 6500 RPM @ 50 hp lets move it to 7500 -8000 rpm. @ 60 hp.

What about just shaving the piston skirt shaved to change intake timing... anyone ever try this on a GT550 alone or any other bike? What about adding extra base gasket as well and then shave head to go with this. What about tapering piston near transfer ports to make them transfers to come on earlier?


Budget$. If you own and RD there are guys that will do Stage I, II, III etc. type porting and cost follows effort. Some of the guys have a design they follow proven out. I don't think the GT550 has such history if they did I would consider a stage II type approach $

I may just do a few simple things (if any at all), but was interested to here if anyone tried something and worked or didn't.

Thanks

[tz375]

A couple of them have been raced and tuned over the years. For example Jeff Kushner over on the KAw triples board ported one. Ron Miller used to race one, so they are around. JAybob on this board has JEMCOs and an electronic ignition on his.

There was a great article in Cycle (?) a few years ago and they mentioned that the GT550 made about 31 at the rear wheel new, out of the box. They did a service and tweaked a few things and got that up to 37.97 at 7,000. That's a little short of the 50 "marketing HP" that we have seen floating around over the years but it's teh only figure I have seen documented.

Machining the piston is a good way to see what happens but is not as efficient as making the same change to the barrels. I have done that in the past as a cheap and nasty way to see what would work and what hurt it.

Shortening the intake or dropping the floor of the intake tends to hurt bottom end more than it helps at the top end. It's better to slightly raise the roof and widen it. I use Bimotion and MOTA software to work out what will work. Bill Tripp uses TSR and together we can usually come up with a set up that works.

Spacers are an easy way to raise all the ports. You could get some 1.5 or 2.0mm aluminum sheet and cut out a set of barrel spacers.

[jaybob]

I have a 1975 GT550 With the aformentioned electronic ignition and a set of Jemco chambers. Additionally I am running a .020 shaved head on somewhat recent .5mm rebore an uni Filter fiam air pod filters. I get 150 measured psi on a compresson test. It feels like around 50 something hp and has some extra POP through the chambers with the higher compression. I turn many heads with the engine sound when I go by them

You can go farther with bigger carbs and porting. Pod Air filters supposedly help. I would love to try a proper squish band head but like Richard says that get expensive.

There is a good article here:


Also I found this you tube video where the guy claims power wheelies on his 550 But I have not contacted hime for details, but he says he used H1 carbs and I am assuming he did some porting. You could also use GT750 Carbs.


I plan to add porting and bigger flat slide carbs to mine ....someday....

[ja-moo]

There is no real "magic" about porting a Suki 2 stroke. It's just that they weren't a bike guys hopped up through the years, so there weren't many performance parts for them. But they can be "woken-up" with porting like any other 2 stroke. The usual hard part, is with porting the heads needs to be reworked to get the compression back, and it's difficult to do without a centrally located/perpendicular sparkplug.

[tz375]

Nothing that a spot of weld and a little machining can't fix.

But the 550 has a one piece head with central plugs doesn't it? Just bandsaw the head into 3 pieces and machine as per normal.

[Cliff]

I have to jump here with my 2 cents. Why do want more HP out of your GT550? I can cruise at 75MPH on the highway with the big V twins and Goldwings without a problem. I've done 13.9 seconds in the 1/4 mile at the strip. I can pull the front wheel off the ground on the 1-2 shift and keep it up. More HP only means a shorter engine life. The GT550 was never built to be a "Widow Maker". She's a heavy bike. The frame might as well be made of cast iron, it's that heavy. The brakes are terrible. The front, solid stainless disc has no friction to it at all and is useless when wet. The rear drum fades out way to soon under hard braking, and the shoes feel like they're are made of butter. Then there's the handling issues. The GT550 IS NOT a sport bike. It will corner well with stock rear shocks/front springs, you can drag the pegs on a tight turn, but be ready for the wobble if you hit a bump or hump!
You could get more HP out of it, but why? Just ride it like you stole it, and keep the shiny side up!
Cliff.

[Craig380]

For what it's worth, my '76 380 is absolutely bone stock, apart from a set of chambers (early UK Allspeeds).

I got the chambers mostly because the factory exhausts on the bike where on the verge of dying: one of the outers was split, and the two centre exhausts have almost no internal baffle plates left, because of age / vibration etc. Obviously no good for optimum performance.

The Allspeeds fitted straight on, and with the stock carbs / jetting / airbox & filters, all I needed was to adjust the pilot air screws 1/4 turn. The bike is transformed with these pipes.

First, it feels and looks a lot lighter and slimmer (all 3 chambers together weigh the same as ONE outer factory pipe).

Second, the performance gets a useful boost: I can now pull 6th gear up gradients and into headwinds at speed. Yet the ability to potter around at 3/4,000 rpm is retained.

Third, the sound. The SOUND.

My 10 pennies / 16 cents worth: a set of street chambers will make a real difference without needing major engine mods.

[Cliff]

I'll agree with Craig. Chambers will make a huge difference. Just in weight savings alone. There's more steel in the stock exhaust system than you'll find in most new cars today!
A set of pod air filters, a set of chambers, some carb work and you have instant HP gain without opening the engine!
Just a word of caution though. Now that you have all this extra Horse Power, how are you going to control it and slow it down?? If you change one thing (increase horse power), you have to improve the handling AND braking as well.
Good luck!

[Allan k]

I about to pull my GT550 engine apart for some tuning. I have also searched the net like crazy to find info about it, but not much came up..

The things im gonna do to the engine will be mild porting, and also in the crankcase, im seriously looking for a set of chambers too..

I will post pictures of it all when i begin.
already there is a few pictures of the engine im working on, on my blog. http://www.allansgarage.blogspot.com" onclick="window.open(this.href);return false;

Stay tuned