A forum for owners of Yamaha TTR250 trail and enduro bikes!

Markvan · Guru

Small tailpipe outlet

noeyedear · Senior Member

Maybe not the same ?? In my youth we rode vespas, and a friend of mine put an after market exhaush for a 50cc onto a 125cc, after a short time it heat seized, the local mechanic said it was the wrong back pressure ?? But looks good

Markvan · Guru

We also had a step through vespa in our youth and thrashed it in the bush tracks. Collector bike it would have been now. Measured my outlet tailpipe. Its 11mm. Ran it for 50kms today on the road and went well. I think it still breathes easier than the standard tailpipe And i can always increase the outlet size , but will be louder. May have a 20mm for the tracks and the 11mm for road use.

Markvan · Guru

A reason why im thinking i do not have to much back pressure with the small tailpipe outlet is the restrictor in the header pipe. With it in and the standard tailpipe full of baffles it must be more back pressure than what im running with now. My complete exhaust is straight through With no restrictions except the pee hole at the rear. tiny smokes rings will be emitted on cold mornings.

ttboof · Super Guru

I recon you may find that the small outlet will slow the clearing of the exhaust gas which also carries a lot heat away .leaving the engine hotter than normal

Another problem with a restricted exhaust is clearing the cylinder on the exhaust stroke leaving burnt gas behind loosing a bit of the avalable space for the new mixture to enter leaning the bike off and adding a bit more heat

In the end it's all about a balance of in and out .

Markvan · Guru

Seen this tailpipe mod on a TTR for sale in Australia. I guess it may help the machine to breath a little better , but i wont be doing it to mine unless i had an old rusty spare one.

petenz · Super Guru

A Vespa is a 2T is a totaly diffrent animal to a 4T..
2T requires back pressure to return the fuel mixture
back the the cyclinder before the piston closes the exhaust port..

very basicly.............
4T runs with very little back pressure... to much back pressure
in the pipe can stop the motor getting full charge during the
valve over lap section of the cycle by slowing the burn fuel
from exting the cylinder... If you get the pressure right it will
produce a vacume for the following exhaust cycle to be drawn
into...

..

locky · Guru

I would guess at it being to small and would create to much back pressure and heat.
My exhaust ttr250.activeboard.com/t60286245/exhaust-againwhite-brothers-e-series/ can be adjusted to change back pressure and they warn not to fit less than 4 discs . 4 discs is equal to a 25mm outlet.