Inside view of a FMF powerbomb/megabomb, fab your own?

[TooFast]

Has anyone checked out the inside of a YZ450F powerbomb or megabomb? or seen any pics?

Would like to try making a SS one, wonder how critical the dimensions are?

My 1977 WB TT500 pipe had one, they called it a mid muffler and it was just a repackable perforated tube section

[KJ790]

The original powerbomb had a perforated core like a silencer. Then the megabomb came out which has an inside that is a straight through pipe with about 4-5 holes drilled in it that are each about 1/4 inch in diameter. Then the powerbomb was redesigned a couple years ago to have the same internals as the megabomb, just a smaller expansion chamber. It is like they drill a couple holes in the normal pipe, then weld the expansion chamber over it. I wonder if you could get a smashed up powerbomb free, cut the bomb out, and weld it into a stock header? That might be a cheap alternative if you are good at welding.

[grayracer513]

I saw a cutaway of a Mega Bomb at the races once, but I don't see the same thing on their web site.

It is similar to a "glasspack", but it isn't packed at all, and the holes in the pipe that runs through the enlarged section are much larger (about 8-10mm) and fewer than in a perf core for a muffler. In fact, the Mega Bomb had a couple of slots about an inch long cut into its center pipe along with a small number of holes.

FMF claims the configuration of the core and the volume of the chamber are important. They would say that even if it weren't, though, so until you start fooling with it yourself, or get hired by FMF R&D, you won't know for sure.

The principal is this: What makes a 4-stroke exhaust "tuned" is the timing of pressure waves within the system, and having them synchronize with a certain range of engine speeds. The timing is controlled by varying the diameter and length (mostly the length, as the usable diameter is limited). The range tends to be narrowly focused, and you normally have to choose top end over low end, or vice-versa. The chamber in the 'Bombs has a damping effect on the pressure waves within the exhaust, and the result is that the engine "sees" the header as not having a fixed length. This in turn has the effect of widening and flattening the power curve, sometimes quite remarkably. Most such FMF exhausts will seldom produce as much peak power as other pipes, but quite often produce a wider power curve than most others.

Other companies have recently produced systems that employ pressure damping chambers connected to the header to accomplish similar effects (Akrapovic and GYT-R for example) without building the concentric 'Bomb section. DRD has also experimented with another design.

[KJ790]

I just tried to snap some shots of the inside of my spare megabomb. I can shine a light down in and see right to the front of the bomb. I can easily see 3 holes, but I'm sure there is at least one more that I can't see. The rest of it is just a straight pipe. It is hard to take a picture of, but I managed to get this one:

[rbn14]

You could get ambitious and do a patent search, it will include schematics...

[dforbes67]

I think it was dirt rider magazine that was at a show and fmf had a cut out of their pipe on it .May do a search there but it was a pipe with a few holes drilled in it then a cover welded over it .

[TooFast]

Thanks for all your replies -

* The patent search is a great idea, gotta figure how to do it

* Page 478 of the current Rocky Mtn Atv/Cycle catalog has a pic of the

Powerbomb and it looks just like my 77 WB TT500 pipe. Must be the old

style since it looks like a perforated tube, not the bigger holes

* KJ790 thanks for the pic, so it looks like the pipe is indented with an oval

hole four times (?). I might be able to measure the oval hole off the pic

since it's going to be to scale...do all the holes face the oulet end of the

header? or maybe two facing the exhaust port and two facing the

muffler? I would greatly appreciate if you could measure the diameter and

length of the center section of the megabomb chamber and also the

length of each taper although they look symetrical. With the wall

thickness also this should give me the dimensions and volume of the

chamber. Now how far from the exhaust port is the chamber?

I had been thinking the volume of the holes within the chamber might

work it's magic (resonance) if they totalled the cross section volume of

the pipe but obviously this is not what your pic shows....maybe the slots

G513 mentions do though.

With some more of your measuring and pics I will have my Tig welder busy

and have a custom SS Megabomb

[grayracer513]

* The patent search is a great idea, gotta figure how to do it

* KJ790 thanks for the pic, so it looks like the pipe is indented with an oval

hole four times (?).

I had been thinking the volume of the holes within the chamber might

lik work it's magic (resonance) if they totalled the cross section volume of

the pipe but obviously this is not what your pic shows....maybe the slots

G513 mentions do though.

The patent is a non-issue if you don't produce them for sale. I would venture that there is at least a US patent, since Yamaha puts "bomb" headers on most of their YZ450's exported to Europe and other parts of the world, but not here. Google the US patent office.

The Mega Bomb cut away that I saw looked as if it had 2 to 3 slotted holes roughly 9mm wide and 20-40 mm long or more which were at an angle to the pipe axis, and arranged around the circumference in the center of the chamber. Between these slots and the ends of the chamber there were 1 or two more round holes like the one in KJ's picture.

I seriously doubt that it makes any difference where the holes are located on the pipe diameter, since from the standpoint of the exhaust, they are all facing in toward the center of the pipe.

The thought you had on pipe volume is very likely well off the mark. The function of the chamber is not the same as that of a two-stroke expansion chamber in any way. If it were, it would not have the perforated pipe running through it. Picture the chain of events in slow motion. The exhaust valve opens the relatively very high pressure combustion chamber to the relatively low pressure header, and as this happens, a pressure wave begins traveling down the pipe. The "wave" occurs because gas is compressible, and the entire pipe does not rise to the pressure at the exhaust port instantaneously. Rather, the pressure starts there and moves along the pipe very much like the compressed coils of a "Slinky" spring toy will when quickly compressed a short way. The wave travels at about the speed of sound, and when it reaches the end of the pipe, it produces a negative pressure (partial vacuum) that is "reflected" back toward the port, and will help draw out the last bit of exhaust and draw in the first bit of air/fuel, but again, only within a narrow range of operating speeds.

As this pressure wave passes through the 'bomb section, the wave is modified by having some of it bled of into the air space in the chamber, outside the walls of the pipe. The rate at which the gases flow into the chamber, how much will flow in during the life of the wave, and how they rebound outward into the pipe again will affect how the pressure in the pipe is changed, and in general, has the effect of extending the life of the negative wave, making the pipe more effective over a wider range of speeds.

[rbn14]

I never said anything about patent infringement, just that you could find a schematic there.

[grayracer513]

I never said anything about patent infringement, just that you could find a schematic there.

Point taken.

[WR250F Captain]

it works though, who cares how, all I know is it adds alot of bottom end and mid, don't really notice the top end so much with the stock, but once you put a better flowing muffler. man, what a sweet set up. a bit off topic sorry.

I think it works like this. but please correct me if i am wrong.

at low exhaust pressure, the front holes allow exhaust in and then they exit the back wholes in the expansion chamber giving it a long header type feel.

at higher exhaust pressure, the champer fills up and doesn'want to take in very much more, so exhaust flows throught the strait pipe and acts like a short header.

anyone else have any idears. thats my opinion

[grayracer513]

Your error is in thinking of the exhaust as a steady flow of gases through a pipe at a rate varied only by the engine speed, when in fact, nothing could be further from the truth.

[WR250F Captain]

thats why I am asking, so how does it work? any opinions?

[grayracer513]

Did you read my previous post?

[WR250F Captain]

gotcha, thanks.

BTW, fmf claims it adds peak power, mostly mid power, and bottom end. in there catologue they sent me.

what I got from that dyno was there advertised 3-5hp gain in the bottom-mid(on my bike, where I got the most power)

but noticabley better on top

again, great guy. worth it IMO.

thanks for the infor grayracer, but I don't see any slots in mine, just holes. but i am not about to cut 'er up to see if your right or wrong.

"you can stick your head up a cow's a$$, but wouldn't you just take the butchers word for it"

[grayracer513]

BTW, fmf claims it adds peak power, mostly mid power, and bottom end. in there catologue they sent me....

... I don't see any slots in mine, just holes. but i am not about to cut 'er up to see if your right or wrong.

The one that was opened up on display had holes near the ends of the chamber, and was slotted at mid span. It may well be that there have been different versions of it made, or that it varies from one application to another. The holes in the tube inside the Power Bomb have certainly changed somewhat.

In regard to FMF's claims, with all due respect to them, they are tests they themselves run, and have full control over. The FMF dyno and the people that run it are among the best in the business, which is why Dirt Bike magazine goes to them when they want to include dyno tests in their reports. However, I see evidence that the stock bikes used for the baseline comparison against their products in their ads are not prepped as well as those in DB's comparisons, since the latter always seem to throw up bigger numbers on the same equipment. It would appear that an as-delivered stocker is compared against a dialed in bike with their exhaust, thus "fudging" the results. But I could be wrong.

Either way, the "bomb" concept clearly does work to broaden the power curve.

[WR250F Captain]

very true. but me jetting had been dialed in. it now needs to be changed, but I left my carb tool in my buds toyhauler.

but as for the megabomb power increases, i can attest to it, Idk about the top end increase, i think its about the same, might be a little better, but I had a short header on my bike stock and I think it was ti, as well, which doens't do anything just weight saving, i've been told.

like i've said, your probably right about the inside, its kinda hard to see inside, and its nice, cause all you have to do is pull 2 springs and you can remove it, no need for a new exhaust manifold.

it is definately worth it. great design by fmf, and the performance gain I felt of bottom-mid, went exactly what the dyno showed, theres might have been a bit amplified, but I think my bikes go tons of toque down low now

[Small_Booger]

For what it's worth

Patent NO:07510050

http://patimg2.uspto.gov/.piw?docid=07510050&PageNum=1&IDKey=C00B7AAF90E3&HomeUrl=http://pimg-piw.uspto.gov/

[Axel43]

just found out about this thread now, so sorry for the delay I guess (?)

Edited September 23, 2012 by Axel350