Post by stepthrutuner on May 10, 2010 16:39:56 GMT -5
I had said earlier that I would like to marry my existing modified mostly (except for exhaust flange adapter) Technigas NextR pipe with a MetraKit SP pipe for a w/c GP1 Derbi that I had gotten a good deal on at eBay last year. After I had gotten the pipe and made it fit I discovered that my bottom and midrange had gone down the tubes although it pulled like hell on top and easily put me into the mid elevens and close to 65 mph. Also, it caused a soft seize not even wound that tight. I was not a happy camper with that and put the old pipe back on. Looking in the exhaust chapter of A.G. Bell's Two Stroke Performance Tuning I discovered that pipes for water cooled engines used smaller diameter stingers than a/c engines, all other parameters being more or less equal, and therefore attributed this to my soft seize problem as the o.d. of the old a/c Technigas stinger was 22.9mm and the o.d. of the new pipe's stinger was 20.0.... a significant difference. Also there was around a three inch deficit in head pipe length with the new pipe causing the shift, by a couple of thousand rpm to the upside, before the baffle started reverse-charging the cylinder. In addition I had already adapted the exhaust flange spud from my stock pipe to the old Technigas head pipe because the two hole flange plate was free floating (not attached to the pipe) and the spud was a tapered bead fit (sorry no pictures) which eliminated the need for an exhaust gasket (gaskets always leak and burn out eventually I have found). I consider this mating to the cylinder exhaust flange superior to all others.
To make this long story short, I wound up removing everything from around two inches into the diffuser back toward the engine and replacing it with that portion from the old pipe where the two pipes matched in diameters at the diffusers (angles were very close). I cut the narrow stinger from the new pipe and replaced it with the fatter old stinger. I found expanded metal lining the baffle in the new pipe and removed it also(funny how they label these pipes for race only yet still put this stuff in lining the baffles). I gained a lot of volume (up from 88 to 101mm diameter at the belly).
Results are pretty positive. The pipe is starting to hit now probably around 7000 instead of at around 8500 with the old one. This gives me very strong acceleration all the way to top speed which is about what it was before - about 57 mph sitting up. I've done plug reads and have needed to go from 102 to 105 on the main jet. I raised the needle one groove to next to bottom (four grooves). The downside is that fuel mileage has dropped noticeably although I don't know how much. I was getting around 55mpg.
The Old Pipe Already Using New Pipe's Silencer
The Project With Tools and Left Over Parts In Foreground
Main jet and bit is hard to see at end of clamp jaw.
Finished product
One last note. I like to replace exhaust cylinder studs with hex head bolts, drill a hole across the flats and use saftey wire. I hate exhaust bolts coming loose. :riding:
To make this long story short, I wound up removing everything from around two inches into the diffuser back toward the engine and replacing it with that portion from the old pipe where the two pipes matched in diameters at the diffusers (angles were very close). I cut the narrow stinger from the new pipe and replaced it with the fatter old stinger. I found expanded metal lining the baffle in the new pipe and removed it also(funny how they label these pipes for race only yet still put this stuff in lining the baffles). I gained a lot of volume (up from 88 to 101mm diameter at the belly).
Results are pretty positive. The pipe is starting to hit now probably around 7000 instead of at around 8500 with the old one. This gives me very strong acceleration all the way to top speed which is about what it was before - about 57 mph sitting up. I've done plug reads and have needed to go from 102 to 105 on the main jet. I raised the needle one groove to next to bottom (four grooves). The downside is that fuel mileage has dropped noticeably although I don't know how much. I was getting around 55mpg.
The Old Pipe Already Using New Pipe's Silencer
The Project With Tools and Left Over Parts In Foreground
Main jet and bit is hard to see at end of clamp jaw.
Finished product
One last note. I like to replace exhaust cylinder studs with hex head bolts, drill a hole across the flats and use saftey wire. I hate exhaust bolts coming loose. :riding: