'75 suzuki gt250...Buzzing around the block

Re: '75 suzuki gt250

The riding season sneaked up on me too soon. I had to deal with some mechanical issues with the engine, and by the time I got that figured out, it was too good of weather to just work on it. It is still on my priority list, but will probably finish it next winter. What I ended up doing is ditching the clipons and went with a set of clubman bars that actually worked much better than I thought. The seat is still in primer, but will finish that when I get my other bike that I picked up running.(suzuki gs650). Sorry for the letdown, but now that I'll have a cruiser, I can make some radical changes to this bike. Thanks, Joe
 
Re: '75 suzuki gt250

It's cool, we can understand the rush-to-be-riding sensation. Any pics of the GS?
 
Re: '75 suzuki gt250

I apologize for not updating, too much riding! I've done what I could, and will just enjoy it for the summer. Heres some of the newest pics. I found out that the paint I chose was much darker than the actual paint. However, after seeing how nice it turned out, I'm going to paint the tank and sidecovers this winter. No photos of the gs yet, but its down to some valve shimming and the engine is ready for the frame. Thanks, Joe

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Re: '75 suzuki gt250

Alright, another update. Now that winter is upon us, I'm starting to get the itch to modify it how I originally planned. This is what it looks now, except with the tank emblems on:

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I originally wanted to put expansion pipes on it, along with my fairing. Well, I had to put some stuff aside and I’m ready to dive back in. However, I’m going with a premade, smaller 1/4 fairing: http://www.airtech-streamlining.com/vintage/Dunstall.htm

Its the ajsm4k fairing kit.

I’ve also used Graham Bell’s formulas and designed a set of pipes that should match the bike quite well, in its rpm range. It is also only an inch longer than the stock and about a 1/4” wider in the middle.

I started with some oak and turned it down in the lathe. I’m going to “wrap” 20 gauge steel around it and weld it together on the original headers. Thanks, Joe

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Heres an Idea of what It should look like:

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Re: '75 suzuki gt250

I've made more progress on the pipes. So far it looks promising, and its pretty close to the original dimensions. I still need to get a second pair of donor pipes for the headers, but I can focus on the baffles for now. Suprisingly enough, it only took me about 4 hours to roll and weld the cones together. Thanks, Joe

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Re: '75 suzuki gt250

I would say upswept exhaust doesn't fit with the lines of the bike. Going off the original pic that you are working to it looks great with the level exhaust.

Just my opinion tho... ;)
 
Re: '75 suzuki gt250

I agree, the lines wouldn't flow. I had issues deciding on how I wanted to do the silencer. If I were to make it look like the stock pipes, it would be quite long and look funky. I tried and failed at trying to make a "u" pipe to swing the silencer on top. I stopped by a local tubing bender business and they totally hooked me up with some leftover scrap. I'm now down to welding the silencer onto the pipe and securing it. I also need to get a second set of stock headers. Thanks, Joe

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Re: '75 suzuki gt250

I've now got one exhaust on the bike. All it needs is a bracket to the original mounting spot and its all done. I've also got the other exhaust about 50% finished and I should be close to starting it tomorrow, depending on how much argon I have left. I decided to chop up my original pipes so I didn't have to wait for another set to canabalize. So far so good...also, I apologize for the horrible pics, I should be getting a new battery/charger for my nice digital camera tomorrow. Thanks, Joe

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Re: '75 suzuki gt250

Joe,
Those pipes are pretty interesting. I don't think I've ever seen anything like them. What is the goal with them? And what's the physics behind it? Thanks.
 
Re: '75 suzuki gt250

The goal is to have a better sounding and more powerfull engine than stock. The exhaust, being the greatest factor on a 2 stroke, is the easiest place to get some horsepower. These pipes match the exhaust duration of the engine(130 degrees) and the original powerband(beginning at 5-6000rpm). Theororetically, these pipes should hit around 5,500 rpm at its current tuned length and give me a good boost. What kind of boost? I have no real idea. Its not going to be narrow by any means, but hopefully a wide, easily controled roll on power. The cones of the exhaust reflect the sound wave created by the opening of the exhaust port. The same wave energy is used to not only flush out the cylinder of the used gasses but sucks in a new fuel mixture and "blocks" the exhaust port, thus boosting pressure, almost like a turbocharger. Thanks, Joe
 
Re: '75 suzuki gt250

I got the rest of the exhaust finished up. I ran into a small problem with the kickstarter getting in the way, but I modified the baffle slightly and it seems to work. In the video, you can hear how it sounds. It revs quite quickly, no bogging or lag. I also took it around the block to see how it sounded and it feels great, now just needs to be upjetted and the needle played with. At idle, it sounds like a twin cylinder dirtbike, and really sings at higher rpm, just how it should be. Be ready for a vid of a few flybys once its fully tuned and painted. Thanks, Joe

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suzuki gt250 custom expansion pipes
 
Re: '75 suzuki gt250

Nice looking bike. I'm not sure I like the Euro Scooter exhaust look, but it's not my bike. 10 out of 10 for thinking outside the box and for making it.

joeschuit said:
The goal is to have a better sounding and more powerfull engine than stock. The exhaust, being the greatest factor on a 2 stroke, is the easiest place to get some horsepower. These pipes match the exhaust duration of the engine(130 degrees) and the original powerband(beginning at 5-6000rpm). Theororetically, these pipes should hit around 5,500 rpm at its current tuned length and give me a good boost. What kind of boost? I have no real idea. Its not going to be narrow by any means, but hopefully a wide, easily controled roll on power. The cones of the exhaust reflect the sound wave created by the opening of the exhaust port. The same wave energy is used to not only flush out the cylinder of the used gasses but sucks in a new fuel mixture and "blocks" the exhaust port, thus boosting pressure, almost like a turbocharger. Thanks, Joe
Click to expand...

Joe, that exhaust is rather long for that motor in terms of tuned length. It will probably choke it at the top end because it's designed for a low 6k peak and that motor as stock peaks at around 10k. Fortunately you can always pull length out of the belly.

But try it first and see what it's like and then you can decide if you want to tweak the design.

We developed a pipe for a reed motor that should make about twice stock HP but I hate to see how long it lasts at 11,000 revs plus. What we do is to develop a pipe using BIMOTION software and then input all the parameters into MOTA and run the simulator software and that gives us a place to start tweaking port timings and width and pipe dimensions to get the widest powerband. I suspect that pipe will need to be shortened to let it rev. The long cones mean it shouldn't be too peaky and maybe it will have enough overrev to keep pulling to the red line.
 
Re: '75 suzuki gt250

Thank you for your input. I'm not sure what I exactly did, but I goofed on the exhaust duration. I knew 130-132 degrees was way too low, and it turns out that I was right. I got out the degree wheel again and triple checked it, turns out its 170 degrees. I went back through the calculations and it now says the tuned length is for about 7,400 rpm. I used this formula: (exhaust duration x 42545) / tuned rpm = tuned length. That is from Alexander Graham Bell's notes. That explains why it gets a decent boost upwards of 5,500-6,000 and keeps going. Unfortunetly, I don't think I'm going to get the "on the pipe" feel unless I give the diffuser pipe a larger angle than where I have it now(5.5 degrees). It does spread out the power quite well, possibly too well. Damn, I need to make a dyno next so I can start to understand this stuff better. I was able to get some video of it moving. Sounds pretty good doesn't it? Thanks, Joe

suzuki gt250 flyby with pipes
 
Re: '75 suzuki gt250

That sounds really good. Way better than most pipes. Well done.

170 degrees is about right for a 7500 rpm peak, for 9000, it should be around 180 degrees. AGBell's book is right on with the cone angles etc. That long convergent nozzle should stretch the powerband but flatten it. Of course that calculation makes assumptions as to the speed of sound which would probably be lower in that pipe than in a race bike.

I ran some more complex numbers and your lengths and angles are fine for a 7500 peak, but that could be raised to 9 or 10 without too much pain, but you will lose some at the bottom end.
 
Re: '75 suzuki gt250

How do you think the bike would react if I raised the exhaust to a 180 degree duration? If I then matched the exhaust length to the new peak rpm, would I develop any more power or just move the powerband up further? Maybe keeping the rpms low saves my crank, ha. I need to pick up a new chain and sprockets, maybe I can play with the gearing and work with that. I'm also going to check my squishband and reset that to an acceptable amount(.7-.9mm). I'm assuming suzuki kept the figures high for production and I can benefit from modifying it a bit. Thanks for you knowledge, Joe
 
Re: '75 suzuki gt250

If you have a port map, I can run a few numbers for you. The extra time area should allow the bike to make more power at higher rpms rather than just shifting the powerband. That of course assumes that we have enough intake and transfer time area to support the revised exhaust flow.

The last model GT250 with 4 transfer ports had exhausts at 29mm IIRC versus 32mm down on the early ones. How much difference that made and how much of the improved performance came from the extra transfers I can't say. People tend to believe that because they are an old design, they can't make as much power as newer designs, so they don't bother. In reality that old design works - just not as well. It will never be a 60HP RGV for example, but racers are getting 50hp out of that motor at 12,000 with TZ cranks etc, so you should be able to get a few more horses out of that without it exploding.
 
Re: '75 suzuki gt250

I've got an extra pair of cylinders I can scratch some numbers off of for you. I spent some time on the exhausts, and really made some improvements. I made the baffles how they should have been, and it is just as powerfull as before but with a deeper note and less obnoxious noise. Its That, and now I can modify the tuned length without running into clearance issues. I'll look through the calculations again and try to get it to rev up a little more, probably knocking off a couple inches should do it. Only problem is that I've ran out of argon. Oh well, tuning on moday. Thanks, Joe

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Re: '75 suzuki gt250

If I had your welding talent I'd tuck them under the sump and dog leg out.... or even one under the engine and one over!
Lovely work!
 
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