FMF vs. stock header

The PB has several diameters. It is smallest at the head like you said, flares into the "bomb", then comes out of the "bomb" at a larger diameter than the inlet (and larger than the stock header I believe), then swells up further to the muffler mount diameter.

I'm not saying it isn't possible the stock header may flow better/worse, especially at a given RPM, just pointing out that there is more to the PB than just a smaller diameter inlet. It tapers, expands, etc. probably as an attempt to get the most low end & top end as possible rather than just one or the other with a constant diameter piece of tube.

Really....the colored text is tough on some eyes...especially the red.

Can you just use the default font color and font?

It would be appreciated.

Sorry motokid, done.

Quote :

The PB has several diameters. It is smallest at the head like you said, flares into the "bomb"

I thought the PB had a larger inner diameter where it flares into the bomb also. When I prepared my bike for the DynoJet PC V, I dropped my PB header off at the welder's so he could drill it and install the O2 sensor bung. When I went to pick it up, the welder had not drilled anything and told me that the pipe was double walled, with the inner diameter substantially the same all the way through, with no flare. It would be very difficult to drill and mount the bung because while you can weld to the outer pipe, you can't weld to the inner pipe and there would be leakage into this chamber.

The outer bulging wall makes it look like it is a single wall that flares out, but it's not. I don't understand how the second outer wall has any effect on the exhaust flow. It's not physically linked to the exhaust flow.

So, I had the welder install the O2 sensor bung on the stock header and put the PB on the shelf.

sswrx wrote:

Sorry motokid, done.

Thanks...much better. At least for us older guys...

The thing is....the header pipe on our bikes is so damn short. I find it hard to believe any aftermarket header can make much difference at all.

What I really need to see is a before and after dyno run.

I wish someone would spend the cash on that. But I understand why they probably won't.

I was reading through the EXUP article at TT & it seems that the PB header makes better exhaust flow through changes in pipe diameter at specified places which simulates changing header length without actually doing it. Something that can only be proven on a flow bench though. I guess that's where all the R&D costs are at.

Interesting find on the double-wall "bomb"... didn't realize that. Still, I think there are couple of points worth thinking about:

1. FMF is known to make high quality products. I doubt they spent the time developing the Power Bomb to just make it look like it's doing something. I am willing to bet some cash that it does make an improvement, even if your Butt Dyno can't tell the difference.

2. The FMF programmer is meant to work with the PB and the FMF pipe. Once again, I'm going to go out on a limb here and assume that FMF developed these products to work together. I say, it you're dropping ~$500 on a pipe and programmer, you might as well go all the way and get the header too.

3. The PB makes the bike sound better and it looks better. In addition, there is a tiny weight saving. These, to me, are good points too.

The above is my opinion. I am supper happy with the FMF combo, the stock header and pipe now live in a box.

No doubt that the combination of exhaust system & programmer makes a difference in performance. I agree on that 100%. Since doing the air box mods, FMFsystem & PCIII, the bike runs stronger & sounds better.

ramz wrote:

Quote :

The PB has several diameters. It is smallest at the head like you said, flares into the "bomb"

I thought the PB had a larger inner diameter where it flares into the bomb also. When I prepared my bike for the DynoJet PC V, I dropped my PB header off at the welder's so he could drill it and install the O2 sensor bung. When I went to pick it up, the welder had not drilled anything and told me that the pipe was double walled, with the inner diameter substantially the same all the way through, with no flare. It would be very difficult to drill and mount the bung because while you can weld to the outer pipe, you can't weld to the inner pipe and there would be leakage into this chamber.

The outer bulging wall makes it look like it is a single wall that flares out, but it's not. I don't understand how the second outer wall has any effect on the exhaust flow. It's not physically linked to the exhaust flow.

So, I had the welder install the O2 sensor bung on the stock header and put the PB on the shelf.

Sounds like a bunch of hoey to me. Show me the double wall, should be easy if its on the shelf--the welder saw it.

After about 5 minutes of research my thoughts are confirmed. The inlet & outlet pipes are not double walled at all.

The "bomb" alone had a perf core in older versions.

Then when they came out with the megabomb they switchesd to an inner tube (in the bomb only) with drilled holes instead of perf & no packing.

There is no way the inlet & outlet tubes are double walled. You can see it in any pic of the outlet tube & this pic of an inlet tube that broke:

I think the "double wall" that the welder was referring to was the section where the tube & bomb section is. Putting an O2 there is not likely due to the air gap between the 2 sections. It would have to go before or after the bomb section.

i love my megabomb........ ok, back to your discussion.

Introduction of restriction in exhaust pipes is proportional to introduction of silencing effect (assuming tube wall thickness and tube length are the same). With restriction, it can also negatively affect heat dissipation from the engine (as relatively more hot gas is kept at bay and not released immediately after every exhaust stroke). Ideally, the restriction should be as far away from the engine as possible (which is the opposite of what this aftermarket header is doing). It gives the engine more room to compress the exhaust gas with sudden bursts.

But slight restriction is also good with increasing low end torque (at the same time killing top end hp). No restriction means no low-end torque but increased peak hp. Balance is critical.