GT750 Phat Trakka - new video revised exhaust
- Thread starter teazer
- Start date
Re: GT750 Phat Trakka - Almost ready to rock
I'm negotiating to get my hands on a Tommy Crawford Exhaust of the Phat Trakka to see how that works. TC has a reputation for being THE 2 stroke exhaust man. It's a short fat 3 into 1 set up and I may just have to make up my own pipe just for comparison.
Just finished cranking numbers for a GT250 project - ports and pipes. That was interesting. Maybe it will appear here sometime (not mine)
I'm negotiating to get my hands on a Tommy Crawford Exhaust of the Phat Trakka to see how that works. TC has a reputation for being THE 2 stroke exhaust man. It's a short fat 3 into 1 set up and I may just have to make up my own pipe just for comparison.
Just finished cranking numbers for a GT250 project - ports and pipes. That was interesting. Maybe it will appear here sometime (not mine)
Re: GT750 Phat Trakka - Almost ready to rock
So the dyno sessions were educational as always. I did some more research and plotted tons of simulation runs to see what was happening with that 3 into 1 exhaust and concluded that there are three areas of concern.
First the headers are too long
The collector is all wrong
The body of the chamber is too small
& the muffler/baffle arrangement is way too restrictive. Oops that's 4. Ah well.
The first three of those require a completely new design, so that can wait, but the tailpipe can be fixed.
Inside the pipe is a closed ended tube and the baffle slides into that, but it's all too small and too restrictive. So I cut the end of the pipe off and surgically removed the obstruction.
You can see how restrictive it appears to be here where that tube is on top of the chamber, roughly where it used to live.
Next I fabricated a replacement end cone and welded that in place.
Then made up a baffle tube/stinger from Chrome moly 4130 thin wall tube.
Wrapped that in muffler packing and inserted it into the revised tailpipe.
Fired it up and the noise isn't much louder but it has a sharper note now and it picks up revs much faster
http://www.youtube.com/watch?v=6L6N_MYfuNY
So the dyno sessions were educational as always. I did some more research and plotted tons of simulation runs to see what was happening with that 3 into 1 exhaust and concluded that there are three areas of concern.
First the headers are too long
The collector is all wrong
The body of the chamber is too small
& the muffler/baffle arrangement is way too restrictive. Oops that's 4. Ah well.
The first three of those require a completely new design, so that can wait, but the tailpipe can be fixed.
Inside the pipe is a closed ended tube and the baffle slides into that, but it's all too small and too restrictive. So I cut the end of the pipe off and surgically removed the obstruction.
You can see how restrictive it appears to be here where that tube is on top of the chamber, roughly where it used to live.
Next I fabricated a replacement end cone and welded that in place.
Then made up a baffle tube/stinger from Chrome moly 4130 thin wall tube.
Wrapped that in muffler packing and inserted it into the revised tailpipe.
Fired it up and the noise isn't much louder but it has a sharper note now and it picks up revs much faster
http://www.youtube.com/watch?v=6L6N_MYfuNY
Re: GT750 Phat Trakka - Almost ready to rock
Apropos of not very much, I originally uploaded that video to Photobucket and could not get the HTML to link correctly. I just uploaded a copy to Youtube and that link seems to work fine. Maybe I needed to convert the file format to work on PB.
Apropos of not very much, I originally uploaded that video to Photobucket and could not get the HTML to link correctly. I just uploaded a copy to Youtube and that link seems to work fine. Maybe I needed to convert the file format to work on PB.
That would be mighty handy. I'll have to do the same.
Couple quick questions for you. I just read over your carbon work a few pages back. I'm doing it a little differently, with molds and such, but since I'm not vacuum bagging them, air pockets and pinholes worry me quite a bit. Brushing on resin like you did seems to be a good fix, especially cosmetically, but did you have any problem keeping the resin even, or sanding too far? I would hate to hit the fibers and mar that nice weave. Any tips or things you would do differently?
Secondly, I see I'm using the same fork brace as you. The front fender I'm building will mount directly to the brace. I was thinking it wouldn't be much trouble at all to pop a second fender out of the mold, if you'd like one. You would have to drill two holes in the center piece of the brace though. I know trackers don't normally run front fenders, but just thought I'd offer, plus it would compliment the carbon covers! Least I could do for all the tech help.
Couple quick questions for you. I just read over your carbon work a few pages back. I'm doing it a little differently, with molds and such, but since I'm not vacuum bagging them, air pockets and pinholes worry me quite a bit. Brushing on resin like you did seems to be a good fix, especially cosmetically, but did you have any problem keeping the resin even, or sanding too far? I would hate to hit the fibers and mar that nice weave. Any tips or things you would do differently?
Secondly, I see I'm using the same fork brace as you. The front fender I'm building will mount directly to the brace. I was thinking it wouldn't be much trouble at all to pop a second fender out of the mold, if you'd like one. You would have to drill two holes in the center piece of the brace though. I know trackers don't normally run front fenders, but just thought I'd offer, plus it would compliment the carbon covers! Least I could do for all the tech help.
Thanks mate.
I was just being a lazy bugger with those side covers. I had intended to use the modified stock covers as plugs and take a mold etc, but then I looked at them and decided they were only side covers, so I decided to wrap them instead.
Bagging is the way to go though. You end up with a lot less resin in the finished part and it's thinner lighter and stronger. Bagged parts typically are not very shiny and teh weave is close to the surface, so they hit them with a couple of coats of clear.
Mine have some horrible weaves and ripples in them where the resin didn't flow out as nicely as I had hoped and it has tiny pin holes - I guess from gas out. I did wet rub them after the first coat of resin dried and in a couple of spots the weave was starting to show through at corners but the second coat covered that up. In the end it would still have been more work or cost to build a mold and bag it but the parts would be even nicer.
If you do decide to go baggless, take extra time to get the mold perfect and to mix the resin without bubbles - fast mixing leads to lots of bubbles. Spread the resin in the mold carefully and watch out for bubbles before you lay in the C/F. Minor pin holes will fill with clear when it's finished.
A CF front blade would be awesome. I can't wait to see how yours turns out.
I was just being a lazy bugger with those side covers. I had intended to use the modified stock covers as plugs and take a mold etc, but then I looked at them and decided they were only side covers, so I decided to wrap them instead.
Bagging is the way to go though. You end up with a lot less resin in the finished part and it's thinner lighter and stronger. Bagged parts typically are not very shiny and teh weave is close to the surface, so they hit them with a couple of coats of clear.
Mine have some horrible weaves and ripples in them where the resin didn't flow out as nicely as I had hoped and it has tiny pin holes - I guess from gas out. I did wet rub them after the first coat of resin dried and in a couple of spots the weave was starting to show through at corners but the second coat covered that up. In the end it would still have been more work or cost to build a mold and bag it but the parts would be even nicer.
If you do decide to go baggless, take extra time to get the mold perfect and to mix the resin without bubbles - fast mixing leads to lots of bubbles. Spread the resin in the mold carefully and watch out for bubbles before you lay in the C/F. Minor pin holes will fill with clear when it's finished.
A CF front blade would be awesome. I can't wait to see how yours turns out.
Compared to counting trees - absolutely.
Mine needs to be re-set. You enter wheel diameter into the setup and if the speedo reads high or low, adjust that number accordingly. My problem is that it's a touch too stealthy for use on the track but it's OK on the street. At the drag strip at night there's just too much else going on and the small size makes it hard to read/interpret at speed.
When I rode on the street with it, it was fine. Things happen far more slowly as a rule and the times that they don't speedo accuracy is not top of things to watch. For an LSR or top speed run it should be fine, but take time to calibrate it.
When I bought it they came in silver so I stripped that and painted it. I believe they come in stealth black now.
Mine needs to be re-set. You enter wheel diameter into the setup and if the speedo reads high or low, adjust that number accordingly. My problem is that it's a touch too stealthy for use on the track but it's OK on the street. At the drag strip at night there's just too much else going on and the small size makes it hard to read/interpret at speed.
When I rode on the street with it, it was fine. Things happen far more slowly as a rule and the times that they don't speedo accuracy is not top of things to watch. For an LSR or top speed run it should be fine, but take time to calibrate it.
When I bought it they came in silver so I stripped that and painted it. I believe they come in stealth black now.
I'm still not happy with the sound and want to make it more mellow with less of a crack. Typically aluminum is a more suitable sleeve material because it resonates a lot less and takes some of the ring out of it. So I bought a couple of lengths of .125" wall thickness aluminum tube from on-line metals.com which is where I usually but aluminum and chrome moly.
Now I need to fabricate a transition and new inner baffle with more holes/higher percentage open. Maybe that will make it a touch more mellow.
Now I need to fabricate a transition and new inner baffle with more holes/higher percentage open. Maybe that will make it a touch more mellow.
Texasstar
Can't is a four letter dirty word
Teazer i'm trying to read the roadsigns once again we were given a woodcraft sheet metal roller last week and now Gt750 has presented itself. Who was the California Highway Patrol bike. It has the big drum on the front. I'm not familiar with parts availability for this model so I thought I'd use the phone a friend. Any thoughts? Good compression #'s?
Late to the party.Awesome build man.
Texasstar said:
Talk about resurrecting a thread.. I have been drag racing it wit Jemco pipes and now have too many pipes to test and no time to test them.
GT750J with big drum brake ie a 72 model Patroller. Highly collectible. Brake is the one that HRMA racers like. I am on my third and so far I have not been able to fit one to a bike despite lots of machining - tyhey are sooooo heavy.
Good compression numbers 140. Mine is around 165 with raised exhaust ports and milled head and barrels. Test it for crank seal leaks 6 psi for 6 minutes is the old rule of thumb. Bill Bune in Anoka MN is probably the best guy to do crank rebuilds and I may have a spare crank you can split for parts if you need anything.
Mine is lighter than stock and runs around 107 in the 1/4 mile in about 13.1 TR750 were good for around 180 on 110-120 HP and you can make much more than that. Kevin The Hatchett Hutchison runs 9.99 at 135 and change on his "Modified" We are aiming for a little more on the latest "Altered". How fast do you want to go? I have a couple of GT750 projects if you want to think about going heavyweight.
