Header dimension recommendations for a 143 cc single

[arcticXS]

Hi,
This is definitely not XS related, but you guys seem to know pretty much everything about any bike.
I have this long dormant Suzuki GN125 "mini street tracker" project, that I'd like to wrap up.
The engine was/is used in many Suzuki models, GS, GZ, DR, DRZ and GN125, and also in the Kawasaki KLX 125. So is almost as long lived and widely used as the XS. My engine has a 61 mm 11:1 Wiseco, stock camshaft (guessing 230 degree or so duration), 2 mm oversize valves (28 mm exhaust valve), 3 angle valve job, light port blending, and a 25 mm DellOrto carb.
When I was working on it, I could only get 1 1/2" tubing for the headpipe, which in hindsight definitely is way too big for best performance. Although it ran way better than stock.
So my question is what size tubing I should go for the second time around? 1 1/8 , 1 1/4 or 1 3/8?
Maybe a stepped header, like 1 1/8- 1 1/4- 1 3/8, maybe all the way to 1 1/2 before the megaphone and muffler?

 

[Jim]

Have a read here.

 

[arcticXS]

Great! That gives 26 to 28 mm pipe ID, which means 1 1/8 or 1 1/4 tubing.
Thanks!

 

[Jim]

Great! That gives 26 to 28 mm pipe ID, which means 1 1/8 or 1 1/4 tubing.
Thanks!

Math rules the world.

 

[arcticXS]

Yes, math is a great thing

 

[GLJ]

Have a read here.

From Jims very good link. Bigger is not always better. There are times it is.
"Again, remember the basics: Smaller-diameter tubes cater to low-end torque, while larger-diameter tubes tend to favor peak torque at a higher RPM band. In terms of primary tube length, shorter tubes favor higher engine speeds while longer tubes favor lower engine speeds."

 

[arcticXS]

Now muffler or open mega with reverse cone is the next question.
Any experience with the flat black mild steel Supertrapp type?
Or should I just get some cones and build a reverse cone megaphone instead?
If so, it would be around 400 mm (16") long overall, with a large diameter of 3" and 1 1/2" outlet. And then maybe some sort of alloy muffler can at the end.

 

[Signal]

Supertrapp's are great but can be a bit loud and are not cheap for what they are. If you have a roller you can make your own megaphones to suit your performance requirements. If its too loud you can fit a muffler after the megaphone it wont alter performance.

 

[grizld1]

If you have access to a swaging tool you might try making an insert to correct the torque peak. Swage both ends of a ~12" piece of 1-1/8" or 1-1/4" pipe until it's a snug fit to the ID of your present header and either weld the insert in place or secure it with a couple of set screws. Michael Morse sells similar inserts as "torque peak optimizers" for XS650s with fat pipes.

 

[arcticXS]

If you have access to a swaging tool you might try making an insert to correct the torque peak. Swage both ends of a ~12" piece of 1-1/8" or 1-1/4" pipe until it's a snug fit to the ID of your present header and either weld the insert in place or secure it with a couple of set screws. Michael Morse sells similar inserts as "torque peak optimizers" for XS650s with fat pipes.

I might do that too, so I can compare a 1 1/4" pipe with the 1 1/2" I already have. I did machine something resembling an "Anti reversion cone" that sits at the flange end of the 1 1/2" header. Thanks for your tip!