I'm in process of freshing up my VFR750 and i'm going to build a set of headers.

My overall concept is a 4-2-1 stepped header based loosely on a Nascar header. As of right now it will be 24-25" primaries (limited by packaging on V4 to get right cylinders to merge), 12" of secondary, a 1.875 or 2" merge into a 2.25" hindle muffler which i already have.

I'm asking if anyone has an opinion on primary diameter. I was thinking a stepped header of 1 1/4 to 1 3/8 or 1 3/8 to 1 1/2.

Most bikes regardless of displacement or power are 1 3/8" primaries, but a stepped header normally allows the use of a smaller primary to the step then you would normally use in a 4-1 or non stepped 4-2-1, so i'm leaning on 1 1/4 for the first 8" or so of the primary.

Stock I had 1 3/8" primaries 26.5" average length. My exhaust port width is 1.18" wide but it's not as tall, so 1 1/4 is still an increase in area.

I'm not all that much of an engine guy, but what I hear all comes from a 35 year experienced, fairly well known race engine builder. He had given me some advice when we were designing our exhaust last year.

His rule of thumb, was for where you want that peak power at high RPM, to size the pipes for ~400 ft/s flow rate. He had the best results from that. So whatever that works out to be for your bike...

And keepin that rule of thumb you get a nice increase in cross-section area at your merge junctions for wave reflection.

I made a cool Excel worksheet from his original set of equations for sizing header diameters and lengths.

I guess it depends on which RPM range you are looking for. I did some length testing on the dyno for the formula car but it didn't really line up with any predictions. Even then, the trends we saw were kind of like "Well, maybe it sort of looks like it might..."

I think once you put a muffler on there, it limits the effect of tuned length and size because it dampens the waves so much. I'm sure there are gains, but i can't imagine them being nearly as significant as something like a muffler change.

Well, i decided to go all out and get custom je pistons 4mm overbore which will get me to 837cc. There are a couple of kits available, although I'm going to spec my own pistons.

That plainly puts me in the 1 3/8 to 1 1/2 category as my rear wheel power should go from 85 to 115!!! With a proper exhaust to match I'm hoping for 120, as 115 is with stock header tubes and slip on mufflers.

You know, if you take the fairings, seat, and gas tank off a VFR750, it kind of looks like...



Just an Idea for your next project.

I wish there was a 'kick someone in the taco' smilie

The worst thing i see in either picture is the chrome spray bomb on the clutch cover, which should have the same gold 'sandcast' as the valve covers.

[QUOTE]Originally posted by VFR750R:
I wish there was a 'kick someone in the taco' smilie

there is!