Maybe I'm not getting this but here goes.

A guy at work and I got in a discussion about high mount exhaust on bikes, not the undertail but the side high mount.

Is there more piping involved in these setups?

And if so how does that affect the back pressure and horsepower.

I would think the difference would be small but I'm still curious as to how this works because he claims I would lose back pressure, along with horsepower.

I thought low back pressure caused increased low end power and decreased high end, and vica versa, not sure if it's right but it's what I've been told.

 

A high mount uses more piping and typically there are more bends so you'd probably end up with more "back pressure" by conventional wisdom.

The problem with back pressure is that it's not a simple "more does this and less does that" answer. You also have to worry about resonance frequencies, which affects scavenging.

Your airbox creates a certain frequency in which the air moves through it which can be represented by a sin curve (the actual value is arbitrary for this point and I'm not very good at math so we'll leave it at that). Now let's say that as the air is sucked into your motor and is used to combust the fuel it leaves the exhaust ports on your header at some frequency as well. Now, a perfectly engineered exhaust will be designed to carry on a frequency that runs on a similar harmonic to the "sin curve" that is going through your intake. What CAN happen, however, and what usually happens with aftermarket exhausts that haven't been properly engineered is that you can get reverb or conflicting frequencies which can cause turbulence. So the reason I bring up the sin curve is let's say you got a tan curve being reflected back because the length of your piping 'causes the frequencies to hit the back of your muffler in such a way that it bounces all over the place. In essence you're working against yourself with the piping you're running.

It's not necessarily back pressure from the exhaust that affects "low end power". The 2011+ CB1000R is essentially a 2007 CBR1000RR motor that was tuned to have higher "low end torque" and a lower HP rating. The way they achieved it was by changing the camshafts. So in a sense, back pressure was changed, but not by restricting the exhaust ports directly.

You're right about the power difference being negligible compared to other aftermarket exhausts so long as the piping was engineered correctly.

My boss explained this stuff to me, but I never studied Engineering so I kind of ad-libbed pseudo engineering speak. This really is barley the tip of the ice berg, however, when it comes to trying to calculate HP ratings in relation to exhausts and back pressure and whatever have you.

 

That definitly clears things up a bit, thanks man. Ive decided not to go with a high mount. Not worth the trouble for something that looks cool. Can I do an undertail setup with the 02 F4i or is there a lot of modifying involved? Also are there any "not so maninstream" setups for this year? It has a two bros slipon but I want a full system and I want something off the beaten path, something excellent. Anyways thanks again.

 

The problem is that the F4i is a bike that's been around for quite a few years now so it's gonna take some doing to find something "off the beaten path" as people have tried and done all sorts of crazy things with those bikes. If you know how to bend pipe and weld, the under tail exhaust wouldn't be too difficult, just time consuming.

That being said, I was always a fan of shorty pipes on the F4i.

 

You can do an undertail exhaust but it will have to be custom made & fit. You'll need a heat shield btwn the pipe(s) & seating area as well. Plus, you'll loose your under-seat storage space & most likely the OEM tail light for clearance. IMO, while an undertail system might look good, its a lot of work for not much benefit other than those looks.

As for the high-mount system vs traditional, any output dif is negligible.

 

Alright guys its been awhile since I checked up on this thread but thanks for all the help and info. I agree with raylee after seeing many pics of these bikes on this site with shorty style exhaust I think I am going to go that route. Im looking at a full system erion exhaust thats fairly shorty and looks real nice. And ive heard good things and at the same time dont see many people with them.. any thoughts?