GT 550 Porting for more power

[vmxwinn]

I first should introduce my self. Just recently bought a beautiful stock 74 GT550 here from Spamy. I had been looking for a 550 for the last two years and happy now that I found this one.
The bike is in really nice condition and has a fresh motor.. Here is the rebuild link;

viewtopic.php?t=10796" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;" onclick="window.open(this.href);return false;

My first street bike I owned was a ratty 72 GT 550 back in 1982 while in high school which was sold for a 74 CB 550 Four that was in much better shape. Fast forward, now I have to many bikes! I started collecting sixteen years ago while racing MX in AHMRA but I always had the longing for the GT550 from my past.

Thats my short introduction. Now on to my plan and question.

So I have been riding this bike around assessing what I want to do with it going forward. First will be new tires, Avon Road Rider AM 26's. Then some suspension work which I will look into what direction I want to go. Either a complete fork/front end swap or just new springs and damper rod mod. Along with new shocks that are a little longer to gain ground clearance a quicken the steering some.

And the other is more power!! I know I want to put chambers on it and I'm leaning towards Jemco's, or any other suggestions that would compliment port work. Ground clearance is a priority. Since the motor is fresh and the bike purchase came with two extra sets of cylinders and heads, I would like some advice on porting the 550. I am after a fast street tune which I am willing to give a little bottom end to gain as much mid and topend as possible but still streetable.

I ride with my buddy on his nicely restore and hot rodded H1 along with a bunch of other Kawi triples. So I need to save face with this crowd and give them a run for their money. Need to show them there are triples other Kawasaki's. lol

I have done some searching and have not found much. I understand that these motors are limited by the small intake and single transfer port but what can be done? If some one has port specs or map that works and they like, please post them if you will. Or who is the go to, to do porting and head work.

Any and all advice is more than welcome. I look forward to another bike mod/build with this little Suzi. Thanks.

PS
Here are a couple of photos of my bikes and a list.

[tz375]

How much more power do you want? Stock they don't make a whole lot - A good stock motor makes around 37hp at the rear wheel. The road tests back in the day quoted manufacturers claims of 50 HP. A few must have fallen off the boat on the way from Japan.

Jemco pipes should help that a little - say 10% and that would take it to around 41. Still not enough to deal with the average H1 which doesn't make 60hp that KHI claimed but still makes more than a GT550. John Aylor says his H1 makes 84 at the rear wheel, but that bike is well developed.

So the target is probably 60 plus HP at the rear wheel to see off the average H1. That's doable with changes to the heads,porting stock carbs and chambers.

JEMCo are not a full race chamber - they are fairly mild and should work OK with mild porting mods. You will need more time-area particularly on the exhaust and transfer ports and I don't own tools that can get into those small ports. JA could probably do the porting similar to his H1s.

Head should be split in three parts and machined/welded & machined to get a better squish band and combustion chamber shape. JA can probably do that or Kelsey at RK Tek. Scott Clough is another that works with small bore motors.

Above a certain power level, you will need custom pipes but JEMCO should still work up to at least 60 HP with stock 28mm carbs

At the end of that journey your wallet will be lighter and the bike will be very different.

[Alan H]

Fit a 19" rear rim and take off the centre stand. Fit twin front discs.
20thou off the heads and just clean up the ports. A set of chambers are lighter and will give a little more power.
The later crankcases and barrels with Shneurle ports give a couple more hp but they were UK only.

Ride twisty roads, that will give you some advantage as H1s were never famed for handling.
You will win a lot of races as you won't have to stop to refuel as often as your friends.

A few tweaks won't spoil the character of the bike, but to tune the nuts off it and spank it every day will cause unreliability and wallet cramp. Everything is acheivable at a price, but you've still got a 40 year old bike with a 40 year old engine full of 40 year old parts.

[tz375]

Alan, all two strokes with transfer ports opposite each other use Schnurle porting. That applies to twin port motors as well as 4 ports.

Your comments are right on and approach the situation from the other end of the spectrum and it's always good o have at least two opinions. What we don't know is how hard our new friend wants to push. Looking at his other bikes he's probably Ok with higher revs, less bottom end and so on that these bikes are known for.

But with a fresh crank and spare barrels and heads - why not go a little mad.

In reality his RZ350 already makes more power from less weight that the GT, but that doesn't mean it can't be persuaded to pick up its skirts and run a little (or a lot) faster.

[Craig380]

Hi and welcome, VMX. Love those Maico crossers, the one sound I like as much as a twin or triple with chambers is the honk of a big stroker single

[titan performance]

I had a 550 many years ago too, found them a bit clumsy compared to the 500 to be honest, though the motor was nice and smooth. But your dirt bikes are beautiful. Slap some super motard wheels on the crosser, that'll put you in front, or as Alan said...sort the handling on the GT and keep off the highway. Kawa 500's I have ridden handled rubbish !

[vmxwinn]

Thanks guys for the replies. I do want to take this from a diifferent approach than the norm. More like what TZ is saying. I would like to get close to the 60hp range as possible.

Basically with two more complete top ends to play with I would like to see what kind of gains can be made. Welding up the combustion chambers and machining back in a proper squish band would work wonders I believe. Along with giving it more port timing and area with in what is reasonable and doable.

I know that the GT won't be more powerful than a well tuned on Kawi triple but at least hold its own against stock or mildly tuned ones would be fine.

Hell the RZ surprises many of those Kawi triples but that is a completely different bike. Much lighter, power valved etc... Mine has very mild porting, bigger 28mm PWK's carbs, reed blocks and chambers and it is dyno'd right at 60 rwhp. But in no way am I trying to compare or replicate that with the GT.

I have planned on doing suspension and brake improvemnts as I said and as Alan suggested. Alloy rims laced up, dual disk with either a newer Suzuki setup or GT750 stuff, shocks and tires. Swing arm has new bronze bushings already.

[Alan H]

Alan, all two strokes with transfer ports opposite each other use Schnurle porting. That applies to twin port motors as well as 4 ports.

You're correct - I meant the later barrels with two additional ports in Nikasil (SCEM) plated bores.

[Vintageman]

I think late 1976 GT550 had significant rear brake change. There is a service bulletin that tells you what you need to upgrade older bikes. I did it and can say that increased rear braking power significantly. I think the service bulletin can be found at oldjapenesebikes.com for example.

I did some engine mods for more power on my 73 gt550
I took 0.020" off stock head (T350 gasket versus GT550 head gaskets) and I made my EX port height 38mm were I think it was +39.5mm stock. I widened Ex 1mm. I run Bassani's 3 into 3 pipes..... I am very far away from 60 RWHP. I possibly gained 10% more power and these mods preserved its original personality and remains a great street bike.

[tz375]

Alan, all two strokes with transfer ports opposite each other use Schnurle porting. That applies to twin port motors as well as 4 ports.

You're correct - I meant the later barrels with two additional ports in Nikasil (SCEM) plated bores.

I figured that's what you were referring to. Funny how often people talk about the later 4 transfer port barrels as Schnurle ported though. Not sure where that sprouted up - Brussels perhaps

I have not seen those barrels in the flesh so to speak but have to believe that they have a lot more potential with the extra transfer area built in.

[vmxwinn]

TZ I saw in another thread that you might have the Bimotion or MOTA software. If you have the stock port map and timing specs would you mind running a tuned port map with matching chamber? Thanks

[Vintageman]

Suz extra ports are not like Yams or Kaws back then.There are in the rear as we know more like the extra extra port you see on reed valve engines. I think this also allowed them to keep the larger schnurle placed ports the same width and not reduced to fit the others. I am curious if Suz changed the Ex or intake port timing on the GT550 cyls with extra ports.

Yes ,TZ if you have modeled GT550 ports, what is needed to make its peek at 7500 rpm for example, anymore would be difficult to run in a civilized manner

I have another set of cylinder where the Ex is now 36mm height no change to transfers. They have a fairly fresh bore @ 0.5mm OS with new pistons to match.

I am embarrass to say I bought them and raised ports 2 mm, but never checked (not the first time either) what they were to start and looks like someone already raised them 1.5mm for a total of 3.5mm. I knew this once I tore my engine down and compare to what was stock ~39.5mm. I milled a stock head 0.035" to match with 0.02" head gaskets as another option for more compression but, I decided not to use them 36mm Ex cyls for maybe too much Ex timing (Like 76-79 RD400 though) I think RD Daytona is 38mm

I raised the 39.5mm Ex cycls to 38mm as stated earlier. I think I only gained a few hundred rpm on peak.. I expected maybe 500 rpm to move from 6500 RPM peak to 7000 RPM?

If stock is 39.5 mm Ex height do you agree peak is 6500 RPM per Suz?

What does 38mm Ex give you? 6800-6900 rpm?

What does 36mm Ex give you? 7500 RPM?

Doe the transfer matter much if we only change Ex a few mm?

How much money do I owe you for these answers ?

[tz375]

Last night I wrote a long answer to those questions and just as I was making a last minute edit, my PC went all Blue Screen on me and it was lost, so here's the short version.

GT's have extremely conservative port timing and increasing port-time are should bring immediate benefits, but past a certain exhaust duration value, it is the pipe that dictates where peak rpm occurs. If you take a bike with say JEMCO pipes and increase the exhaust opening point progressively from say 100 degrees ATDC to say 80 degrees, peak power changes as does shape of the curve but peak RPM doesn't rise as much as you might expect because the exhaust pulse timing moves out of synch.

The other issue is that all the other ports are not necessarily capable of supporting higher RPMs. For instance if the transfer ports are too small or the wrong shape that may be the limiting factor or perhaps the intake port is all the wrong duct shape or is timed inappropriately, that could be the limiting factor.

What I usually do is to work out a range or possible port time areas that would yield the target HP at target revs through a "Basic" version of Bimotion. Then I look at the actual port dimensions to get a starting point or baseline. From that I explore those ranges in Bimotion and that gives me a more nuanced starting point for MOTA simulations. Mota is not a design tool as such. Its power is as a simulation tool and that is used to run multiple runs exploring alternative port widths and heights. Bimotion also has an exhaust design module that can be used to explore exhaust options. That usually generates better pipes than Wizard or MOTA's own tool.

It's not a quick process where you plug in targets and out pops the perfect, optimized, answer.

For example I designed a championship winning Kart exhaust that must have take 100 MOTA runs to develop first the "best" and then the "best with as few changes to the existing pipe" and ended up with an optimized design that worked best in the real world. That was months of experimentation and days of fabrication.

For a street GT550, the process is somewhat simpler and if I did this for as living I would start with what works on an RD400 and see what could be adapted for this particular motor. That would produce a faster response and would be in the ball park pretty quickly. You can't make a 550 into a 1 1/2 RD400s because it is too different, but it's one place to start. There's not a lot of point in me spending time on potential outcomes if the porting is to be done by Scott Clough or someone else. He will have his own way to approach the situation, as would JA or Lowell or others. Some use TSR and some use EngMod 2T and some use an experiential approach.

[ja-moo]

Had a good talk with VMX today. More power is doable, as was stated, they are pretty mildly ported. It's not the best design for a "lot" of power, but I don't see why it would be hard to give it some more "kick". I don't have any Suki specs, so does anyone have the stock port specs, with bore stroke and rod length?

[Vintageman]

Ja-Moo

I do not have that info. I do know stock ex on my 73 GT550was ~39.5mm. I could measure the other ports on my spare cycls. I would assume 1mm offset rod. But, what is the length of the rod?

Tz375
I do think it’s my pipes. I did this Ex mode late last year, breaking in fresh top end same time but, was suspecting it to change more than it did. Be careful what you ask for.

I had not rode the bike since (stored winter seasons) but, recently did two short rides this year with stock head gasket and after I changed the head gaskets to T350 to gain a little compression to compensate for the EX to 38mm as stated. Verified timing. It was about 60F when I road again yesterday. Was maybe 50F first ride this season?

Compression truly helps upper mid. This may seem an exaggeration but, I was turning slightly ( a corner I ride a lot and know and enjoy well) and the bike at about 6000 felt like the rear wheel got loose . Last year I had a head with almost 1mm shaved off and I knew that helped but, also sure it was pinging so put stock head back on. T350 gasket splits the difference and seams OK. May be wise to run 93 vs 87 now (bike has last years 87in it)

I got to a wide straight road and accelerated and the bike pulled hard and linear to 6800. Above this the power was flat and maybe even dropped a little but, definitely still there though. So, I let it climb to 7000, versus shifting (like I would have last year) and then from 7000 to 7300 the power band climbed again. I did not feel like over rev. (yeah I am using butt dyno, but it seams to work and I am being very critical)

I did this test a couple more times and behavior repeated. I even let it go passed 7300 to +7500 but here the power is dropping fast. 8000 rpm is unlikely unless low gears

Hmm a sort of “suck out” between 6800 and 7000… annoying … not enjoyable to make it go to 7200. enjoybale to just shift at 6800

So my guess I did move my peak to near 7000 versus 6500, but some weirdness between 6800-7000 RPM?

I need to save up and try a set of Jemcos.

I suspect that the 36mm Ex port would be suited for 7500. I think for me that would change the bike too much...well it sure would start to be a bike that's riden more excitingly then relaxed.

Its best I focus on the Pipe to get the power to climb continuous up to and a little passed 7000. Well I am not a pipe marker and not too many options out there.

If anyone gets a chance/ port info I wonder if the simulation agrees 38mm Ex is suited for 7000 RPM peak

Any ideas as to why my power band has the shape it does from 6800-7200?