Inner diameter of stock downpipes?

[Freza]

Thinking of buying Black Widow downpipes, but I'm not sure if I'd get any power improvements over stock pipes?

The guy at Black Widow says their inner diameter is 32mm, anyone knows stock diameter?

[darrsi]

You won't beat stock pipes for giving the bike the smoothest and best running.
When i had my bike Dyno'd years back the mechanic doing it said stock pipes were way better than aftermarket ones.
I can fully understand changing the end can to give a better sound and look though.

[Jules-C]

The black widow pipes probably have a bigger internal diameter but the stock pipes will match the exhaust port better and the sudden step increase in diameter will cause turbulence inside the pipe slowing the flow. 

[Freza]

Ok, but I already have Arrows race slip-on...

[darrsi]

I don't believe for one second that you'll gain any BHP, the main difference i would imagine would be a little less weight, but even then it won't be enough to be noticeable for normal road riding.

[Freza]

I don't believe for one second that you'll gain any BHP, the main difference i would imagine would be a little less weight, but even then it won't be enough to be noticeable for normal road riding.

A long time ago I tried my friend's FZS600 with some aftermarket downpipes and power increase in low to mid rpm was pretty noticeable.
He had a Yoshimura slip-on.
Don't know which downpipes those were, but they were wider than mine (stock).

[darrsi]

I don't believe for one second that you'll gain any BHP, the main difference i would imagine would be a little less weight, but even then it won't be enough to be noticeable for normal road riding.

A long time ago I tried my friend's FZS600 with some aftermarket downpipes and power increase in low to mid rpm was pretty noticeable.
He had a Yoshimura slip-on.
Don't know which downpipes those were, but they were wider than mine (stock).

Noticeable how though?
Noise can be very deceiving, only way to really tell is on a Dyno.

[Freza]

Noticeable by sensing stronger pulling, not by noise.

[darrsi]

Noticeable by sensing stronger pulling, not by noise.

Not disputing it, but even a K&N filter makes the bike feel like it's been massively improved, and i will vouch for that too, the throttle response and pulling as you say certainly feels better, but in reality there's not really much BHP improvement, it's just a feel good factor.
Bit like these 125's you hear that sound like they barely have an exhaust on, they sound like they're going fast, but they're not really.

[Freza]

Haven't really approached too scientifically to it

[midden]

Ive just measured an old set of original downpipes and they are 24mm internal diameter but the first 5mm  (aprox) is 31mm

[Gnasher]

You can grind down the welds where the flange meets the down tube, this smooths out the gas flow, just cleans things up a bit.


Whether it improves anything much is debatable, if anything it would be at higher RPM, as bigger bores increase gas flow, but for mid to low range you want smaller, same applies to carb bores.  From my experience, the Fazer engine without serious mods is pretty much on the money, add a K&N and a good quality end can and you've got a noticeable more usable motor low/mid range.


 

[Freza]

Thanks.
I guess I'll stick to stock

[Paul]

If you change your exhaust headers make sure the restrictor point of the new set is similar in overall opening size to the original set.
 
The original outlet measured:  35mm (narrowest point) to 43.5mm (widest point) externally at its most restricted point.
 
I’m not sure what the wall thickness is but I reckon it must be somewhere between 1.0mm and 2.00mm, my original pipes are still in good condition and I was loathe to saw them up to find out.
 
In any event the Motad replacement header system outlet pipe is has a clear open area of 1364sqm.  Whereas the original outlet is restricted to somewhere between 962sqm and 1075sqm.
 
When I fitted the Motad headers the power band almost evaporated and a gas analyser showed it be running far too weak, this being supported by the plug colour.
 
A further test was done by a lazer temp gauge on the exhaust clamp bracket of both original and new pipes and this showed the exhaust gas temp had risen.
 
To solve the issue the Motad pipe was sleeved down first to 1075sqm which improved things no end, and finally down to 975sqm, which seemed to be better.
 
Plugs were checked and exhaust gas analysed and the results were as good as I could get them.