GT 550 Porting for more power

Getting your blazingly fast Suzuki powerplant to perform even better!

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tz375
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Re: GT 550 Porting for more power

Post by tz375 »

JA - good question. I have been given three sets of port dimensions and they differ by up to 1mm in height and not a lot of width measurements.

Con rods are 117mm ctr to ctr.


V'Man: The usual reason for a dip or double peak is a mismatch between exhaust timing and port timing. For example your pipe may be designed to work at say 8000 but the exhaust port resonates at say 6500, that would create your issue.
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Re: GT 550 Porting for more power

Post by Alan H »

When I had my original GT550J back in '72, all I did was clean the ports up (not polished - just deburr and match) 20 thou off the heads and up a grade on the plugs. IIRC, I didn't rejet, just lifted the needle a notch and all was well. I was 21 at the time and tended to ride with more throttle than not, but it used to spin to 8K+ without issue in all gears except top. It was faster through the gears and I went down a tooth on the rear and fitted a 19" rear rim for extra clearance. It idled well, didn't oil up and went well.
Standard exhausts with shortened baffles. I took advice from a well known tuner at the time who was production racing a 550.
The main advantages were a bit more power, but better handling than most other similar sized bikes.
I also tried a disc 'upgrade' but was so disappointed (especially in the rain) that I took it off and went back to the 4LS but with Ferodo green racing linings.

Rather than do lots of porting mods that will completely change the character of the engine (and it's still 40 or so years old!) just smarten the insides a bit and see what it goes like, you might be pleasantly surprised. Save some $$ too.
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Re: GT 550 Porting for more power

Post by tz375 »

Try telling that to Brett deStoop who has gone over 200MPH on his GT750 or Kevin H who is into the nines on his GT750 drag bike or the rest of us for whom more is never enough. :shock: :lol:
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Re: GT 550 Porting for more power

Post by Vintageman »

I have read a few periodic articles about shorting baffle in Yam and Kaw to ship a few sections of the stock muffler baffling scheme. I tried that on some bikes. No comparison (Power boost and extended RPM reach) as when you put the right pipes - chambers. My r5 for example has stock ports and the D&G chambers are well getting into play (no dip) helping pull out fumes where the stock Ex was not be able to do on its own at the previous outlet pressure. With D&G It climbs much higher than it did stock exhaust and same port timing

I see you point about the pipe. I think of Ex pushing out fumes until it can't savage any more spent fuel (not enough time) and drops off quick: Unless it get some help from the pipe at the right time creating lower pressure

So if my Ex runs out of breadth before the pipe can help pull I either have to raise my EX more for example or get pipes tuned lower down. I could try my stock exhaust again. Or buy a set of Jemco's just for another reference point. I think Jeff Kirscners (sp?) mod to his GT550 is similar to mine and he liked it very much. I also widened and lowered my intake some… https://www.youtube.com/watch?v=sxxS2CR1Lxc" onclick="window.open(this.href);return false; see @ 1:30

I don't want a rev montser. I want to shift from peak end from 6500 to near 7500

I hijacked this post but, the gent asking the question can see the challenge in his request. I also know of no Suz or aftermarket article to make the GT550 peak say 1000 rpm higher like the H1 you/we reference

But I now better respect Bell’s book where he mentions not too much on ports at first for the pipe is key and that tuning should be the focus foremost (I paraphrase)

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Re: GT 550 Porting for more power

Post by ja-moo »

Allen,

I can see you like to keep things rather mild, but the little doodling you recommend is not going to do much for power. I have a feeling the 550 is ported like a enduro, so very mild. I had one for a short time, and the power was very lackluster. I'm betting a lot of porting can be done to get it to just mild for other motors. And stock pipes weigh a ton, and cutting baffles is doing nothing for weight or power. There is no chamber effect.

And any port specs with at least get a ballpark to start with.
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Re: GT 550 Porting for more power

Post by Vintageman »

I put Iriduims plugs versus standards that were in it. I verified timing and I was maybe off <1.0 degree too retarded on a couple cyls. Took it out for ride around the block tonight and this has cleared things up very nicely. It climbs RPMs very well. no more dip. :up:

I suspect it was the plugs more so than the timing that fixed the issue.

I did compare my pipes to Jemco’s ala nice pictures (ctluba) on the internet: left and right sides. My pipes are not Jemcos and was told Bassani’s… (can’t verify) but, the chamber sections and their diameters look the same as Jemco’s. :o

I referenced to key points on the bike as cones begin and end. The difference is mine don't sweep up as high in the back. And, Jemco’s first cone starts to taper a little early as they come off the engine down pipe. Mine has separate down pipes then the chamber section slips over this down pipes: two pieces per pipe, not one like Jemco's

On the left and right pipes I can see the first cone tapers on Jemcos start 2-3 inches sooner than mine. Mine doesn’t start the taper until after the two pipes overlap. I suppose Since Jemco’s are all one pipe it can taper sooner since no two pipes overlapping constraint as mine. But, as I said after this the dia and lengths of remaining cones and section are very very similar!

:idea: Hmmm, must have been a common design both used or one person copied the other.

Also mine are chrome and feel like the material is a thicker gauge (comparing to other Jemcos I have) and the silencer can be removed after removing a circlip not a screw. The middle pipe (which must have a different routing at least) can be installed preserving the center stand, Jemco’s cannot: could not see that one to compare cones.

Well I don’t think Jemco’s would be very different then what I have now if I wanted to try something different.

I think the ignition coils are limited on these bikes and you need modern plugs to help if you hop them up a little. I am still running last year’s 87 octane with a little oil, dry gas, and preservative in it. I’ll burn through more of that and drop in some fresh 93 and see if it gets even better.

I am good with 38 mm Ex height. I think 36mm would create a more exciting bike and very curious but, that may be too much for me… I need some more GT550s to try various ideas :twisted:
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Re: GT 550 Porting for more power

Post by vmxwinn »

I just to some measurments and drew up the stock port map.
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Re: GT 550 Porting for more power

Post by vmxwinn »

Any progess on this?
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Re: GT 550 Porting for more power

Post by GT750Battleship »

tz375 wrote:Try telling that to Brett deStoop who has gone over 200MPH on his GT750 or Kevin H who is into the nines on his GT750 drag bike or the rest of us for whom more is never enough. :shock: :lol:
:up: Hi,Brett did my '76 GT750 motor....more than happy with the performance...could have gone a whole lot further !!! but it's a club bike,so building a "rocket ship" wasn't what I had in mind at the time 8)
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Re: GT 550 Porting for more power

Post by Alan H »

tz375 wrote:Try telling that to Brett deStoop who has gone over 200MPH on his GT750 or Kevin H who is into the nines on his GT750 drag bike or the rest of us for whom more is never enough. :shock: :lol:
Fair comment, but not really road bikes, and I would think gas mileage isn't too good.......
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Re: GT 550 Porting for more power

Post by tz375 »

Alan, That's just not so. :lol: I get at least 8 miles per gallon at the strip - or something like that, but at 1/4 mile at a time, that's not bad.

We grow to love our old bikes with their lack of power and flat torque curves, but they are at a lower state of tune than a BSA Bantam or MZ250. The old girls could use a little more pep in their step.
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Re: GT 550 Porting for more power

Post by Alan H »

tz375 wrote:Alan, That's just not so. :lol: I get at least 8 miles per gallon at the strip - or something like that, but at 1/4 mile at a time, that's not bad.
:clap: :lol:
We grow to love our old bikes with their lack of power and flat torque curves, but they are at a lower state of tune than a BSA Bantam or MZ250. The old girls could use a little more pep in their step.
My first bike back in '67 was a D3 bantam and with HC 'Todd' head which peaked at 55mph, head down and arse up.
Then I got a Suzy 50 sport M12 which did 60+ with a Yam YL1 100 spanny and a little port work..........

I do like the 550s though and soon I'll let you know the difference between a 550K 'standard' and a 550A 'not standard'.
A few weeks yet, but hopefully running and well sorted by the end of August when I go to the Classic TT on the Isle of Man.
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Re: GT 550 Porting for more power

Post by vmxwinn »

Alan H wrote:
tz375 wrote:Alan, That's just not so. :lol: I get at least 8 miles per gallon at the strip - or something like that, but at 1/4 mile at a time, that's not bad.
:clap: :lol:
We grow to love our old bikes with their lack of power and flat torque curves, but they are at a lower state of tune than a BSA Bantam or MZ250. The old girls could use a little more pep in their step.
My first bike back in '67 was a D3 bantam and with HC 'Todd' head which peaked at 55mph, head down and arse up.
Then I got a Suzy 50 sport M12 which did 60+ with a Yam YL1 100 spanny and a little port work..........

I do like the 550s though and soon I'll let you know the difference between a 550K 'standard' and a 550A 'not standard'.


A few weeks yet, but hopefully running and well sorted by the end of August when I go to the Classic TT on the Isle of Man.
Thats funny. Growing up my older brother had a 66' Batam, D10, 3 speed, that he bougth from my best friends dad. I seized that thing two up on the freeway. Man was my brother pissed at me. He still has it and now it is retored.
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