Triumphrat

FuryFour · 12-30-2006

ok... i want to put a new pipe on my bike, kind of a custom thing.

the exhaust piping is 2.5"

would it have a negative impact on performance or anything to run a reducer
and put on a slip-on with a 2" inlet diameter?

pete · 12-30-2006

I've always kinda wondered about the S4 exhaust diameter, since it's a bit on the large side. When I planned for the bonkers mad exhaust about a year ago, I concluded that most probably building a collectior with a 2" out pipe is the best way to go. Then I thought it was too much work and binned the project.

This I base on assumptions, which I will explain below:

1. Smaller diameter = faster gas speed = better (To a degree. A crazy straw has a small diameter, but it wouldn't provide enough gas flow)
2. 2" is most likely not too small for an S4, afterall, for instance my R6 has a 2" piping, and it pumps a lot more exhaust gas through it than the S4

Then as for using a reductor somewhere after the collector, with a part of the 2.5" pipe remaining:

3. N/A rally cars often use a reduced diameter pipe for a bit after the header. This is to increase gas speeds. However, if you use a larger diameter piping after the collector, and reduce the diameter before the can, the reductor can act as a blockage, making it harder for the slower moving gases to enter the smaller hole, requiring more inertia. This is what is done on certain aftermarket car intake manifolds to increase torque - there's a new charge ready and waiting in the intake for the valves to open, which in certain applications result in more low speed torque. Mind you, there is a reductor in a Speed Four can anyway, but the inside shape of the can is such that the gases that are left in the large diameter part of the can, can't easily distract the next charge, and to my understanding actually wiil be drawn out by the following charges. So, basically if you use a reductor after the collector, to my understanding it would be beneficial to build also some kind of anti-turbulence/backward movement of gases construction.

And as I said, I'm not an engineer or anything. This is just my own reasoning.

FuryFour · 12-30-2006

Quote:

On 2006-12-30 10:47, pete wrote:
anti-turbulence/backward movement of gases construction.

this all made good sense to mee, but i dont know what that means haha

pete · 12-30-2006

Well, something that prevents the remainder of the previous charge to start going backwards to the engine, and/or creating a turbulence, which distracts the next charge. Look inside the can you have, and you'll see a sort of thing I'm talking about. The gases on the sides can't easily go back, or move to the middle on the bolt side of the can.

Will · 12-30-2006

I am not an engineer, either, so this is a shade tree mechanic's view of it. In very simplistic terms, you restrict the diameter of the header as it exits the engine to get more torque, and you open up the system to flow more gasses to get more top end. Drag racers sometimes use stepped headers for that reason. Road racers generally run wide open pipes, since they need top end more than low end torque. Adding restrictions well down the line might reduce both horsepower and torque, but that will have to do with the pulse frequencies, length of pipe, and all kinds of other things that I don't know how to calculate, or even begin to calculate. The EXUP valve in my bike is at the end of the individual header tubes, in the collector. As it opens and closes, it presumably aids low end torque by creating some backpressure. Depending on where you put the restriction and how you do it, you might increase torque a bit. If your can is wide open after that, you might end up with a power curve similar to what the stock can produces because you have a restriction earlier in the system, but no baffle at the end.

Really, the only way to find out is to try it and see what happens.

SteveLarson · 12-30-2006

If I may inject a couple "thoughts", you guys can discuss them as you wish.

1) You never want to "disrupt" the flow, anything abrubt is bad no matter what it is. This goes for "steps", "bad welds", "reducers", etc.; anything that interferes with the exhaust gas getting from the port to the exit in the most efficient and quick way is not good.

2) It is never as simple as tube size. You have to consider the entire design of the header to know if small tubes, big tubes or "whatever" is going to work for what you are trying to achieve.

3) In general, you want the pipe to taper from start to finish, every mm along the way including inside the bends and thru the collector, not just "welded in" sections at some point in the tube. There is a huge cost in trying to achieve this, but it is probably the true "holy grail" of flow design.

4) Lastly, the only thing that matters is what you are looking for the power curve to look like, and what the pipe you have designed actually does. If somebody uses a garden hose, attached to a section of your roof gutter exiting thru a coffee can and it makes 40 extra HP, do you really care? :-D

There are MANY ways to design an exhaust, many "tricks" that get used or added on for no reason. No "one" of them matters. It is all about the complete design and what you end up with when attached to the real motor...

Steve Larson