Tapered FXR header. Ideas?

[omamari]

Hi guys, it's winter and I want to mess about with a good thing at night - naturally my header came to mind. I'm keen to extract a bit more power out of my standard FXR engine. Does anyone have some good dimensions for me to fabricate a higher flow header?

Currently i have an OKO28 and the muffler is under slung below the belly. The header is shorter but stock diameter. The muffler is still just as muffly, but i shortened it a each end to remove the chambers (not having any idea what this would do to the flow, but I assume the most important characteristics come from the header shape). Engine internals are stock (worn).

I found this from an old post on www.fxr150.co.nz: "Approx 460mm overall header length, 32, 35, 38 mm o/d approx equal lengths worked well on my previous FXR." This advice sound good enough to go waste a whole lot of time doing?

Once I make a header, i hope to correctly jet it and maybe dyno it.

Cheers,

[AllanB]

32, 35, 38 - you'll find each slot over each other most likely in standard exhaust pipe so the easiest way to make a progressive one is start with the smaller as the head outlet then step up to the larger size each third.

[richban]

Easy and cheap way to make a tapered header is to buy a wide U bend from auto bend. It should be the biggest size so you can then cut a small wedge down the length. Then all you do close it up in the vice and weld it up. Some hammer action will also be necessary. You want all the taper change to happen in the first 200mm of the pipe then straight from there. Also you want the inside of the pipe to match up nice with inside top of the port but a 3 mm step down at the bottom of the port (bigger at the bottom). The exhaust port is a D shape and quite good so don't go messing with it.

322818

322819


Can't remember why I made 2. Maybe one was for someone else.


322820

[omamari]

Brilliant, thanks both of you. And is there an ideal diameter to get to over that 200mm? Is it 38mm?

[Henk]

Brilliant, thanks both of you. And is there an ideal diameter to get to over that 200mm? Is it 38mm?

Anything Rich tells you will only apply to a 200cc engine.

[omamari]

Anything Rich tells you will only apply to a 200cc engine.
Cool, I'll just make it 75% smaller. [emoji52]
Surely someone has done the complex math for the FXR, that I'm unwilling to spend a month to learn because I don't know the engine variables, or is it trade secret.

[richban]

Cool, I'll just make it 75% smaller. [emoji52]
Surely someone has done the complex math for the FXR, that I'm unwilling to spend a month to learn because I don't know the engine variables, or is it trade secret.

He is trying to be funny. 450 form the valve to the big open muffler. Then play with bellmouth length till it goes really good. Not many engines are the same due to cam timing, exhaust diameter intake port size blah blah. I would go with 450mm form the valve to muffler. That should give you peak power at 12k ish. The happy place for all FXR engines. I can't remember the pipe diameter I used. It was quite big. Can find out tomorrow.

[omamari]

He is trying to be funny.
Bastard, I thought he might have.

I would go with 450mm form the valve to muffler
Cool, thanks.

I can't remember the pipe diameter I used. It was quite big. Can find out tomorrow. Look forward to it! Thanks Richban

[timg]

Bastard, I thought he might have.

Cool, thanks.
Look forward to it! Thanks Richban There's quite a few "successful formula's". The length and not going too large in the initial section of header seem to be the critical things. My one quoted in your first post worked well for an engine putting out good hp. Rich gets good hp from his engines as do GPR using something a bit different again. Cheers

[omamari]

Also you want the inside of the pipe to match up nice with inside top of the port but a 3 mm step down at the bottom of the port (bigger at the bottom). The exhaust port is a D shape and quite good so don't go messing with it.

Hi Rich, really appreciate you getting the diameters for me. I'm sorting out this header now. What did you mean by the 3mm step down?

Do you mean the initial section of the header,i.e the internal shape of the small flange that locates in the head, shouldn't be circular? It looks to me like the exit of the port is symmetrical top and bottom.