Megabomb header explanation

I own a crf450r, powerbomb header.

 

Now what exactly do the megabomb and powerbomb headers do? From what I understand, they quiet the exhaust while producing more power compared to a header of continous diameter but how?. What are the actual physics behind them? Maybe they provide a little more back-pressure or more volumetric flow?

 

And is it true that fmf holds a patent for their header designs, which is why other manufactures have tried to make similar yet different  expansion chamber designs, like yoshimaru?

 

Just food for thought

I own a crf450r, powerbomb header.

 

Now what exactly do the megabomb and powerbomb headers do? From what I understand, they quiet the exhaust while producing more power compared to a header of continous diameter but how?. What are the actual physics behind them? Maybe they provide a little more back-pressure or more volumetric flow?

 

And is it true that fmf holds a patent for their header designs, which is why other manufactures have tried to make similar yet different  expansion chamber designs, like yoshimaru?

 

Just food for thought

 

Anti_ReversionChamber.jpg

 

 

 

the power bomb is an anti reversion chamber patented by the late Jim Fueling

Hytech exhausts ended up with the patent and builds all kinds of headers incorporating the design.

the Anti-Reversion chamber can do two things,

 

1 It can be placed a distance from the exhaust valve that will aid blow down. Blow down is the part of the exhaust cycle between when the exhaust valve opens and the piston hits bottom dead center.  The area increase of the bulbous area and also usual pipe diameter increase on the outlet (not shown in the drawing)  create a low pressure signal (scavenging). If the AR chamber is placed the proper distance from the exhaust valve for the cam timing, pipe diameter, and EGT used the low pressure signal will arrive at a proper time to help evacuate the cylinder before the piston starts to ascend in the cylinder.  This will help reduce pumping losses (work the engine has to use to sustain itself)  this will occur at a specific rpm for a given length (and sort of somewhat multiples of that rpm) but work for a few hundred rpm on either side of that specific number just a little early or a little late but still helpful.

 

2 the function it gets its name from anti reversion. There is two types of reversion. Mass flow and acoustic.  Mass flow or gas flow will have a harder time flowing upstream than down stream, down the flow will be unaffected, up the flow will be impeded by vena contracta.  In a properly designed (not too large for application) exhaust system mass flow reversion should not be a problem.  Acoustic reversion is pressure waves traveling inside your exhaust.  That same low pressure waves used for scavenging are accompanied by positive pressure waves at other rpms.  So say you have an exhaust system tuned for 6000 rpm, well this exhaust system for example might have beneficial scavenging effect from properly timed low pressure waves at 3000, 6000, and 9000rpm and have positive pressure waves traveling back to the cylinder during overlap at 1500, 4500, 7500 and 10500   If the positive pressure waves are strong enough it will actually push mixture right back up the intake manifold through the open intake valve.  The sharp edges on the pipe protruding into the AR chamber are intended to attenuate these positive pressure waves.

 

The mega bomb is a large chamber with a tube continuing all the way through.

there is a few large holes drilled in the inside tube.

the mega bomb probably functions as a blow down device and not so much an anti reversion device.

the mega bomb also acts as a pre muffler softening the pressure (sound) wave.

I have a better explanation for that but i cant make it come out of my head at the moment :facepalm:

The powerbomb and megabomb are both Helmholtz resonators. These have been used in car exhausts for decades. Yes, Don Emler (owner of FMF) does hold a patent on its use in a motorcycle exhaust system, it is patent #7510050. http://www.google.com/patents/US7510050

 

The megabomb is simply a powerbomb with a larger chamber. These are used to acoustically deaden sound waves in a pipe using resonance within the larger chamber. Through the center of the chamber is a straight pipe with a couple small (about 1/4" diameter) holes to allow sound waves to expand out into the chamber. The majority of exhaust gases pass right by these holes, as the gas has nowhere to go within the chamber.

 

Anti-reversion chambers like englertracing mentioned look very similar to the powerbomb on the outside, but they are different on the inside.

if they are just Helmholtz resonators they are loosing out on the anti reversion effects and just working to aid blow down.

Stupid IMHO,

but they probably couldn't use the AR chamber because hytech has the patent

The cross section of the anti-reversion chamber is not representative of either a MegaBomb, nor a PowerBomb.  Four-stroke exhausts are not tuned to assist blow-down, anyway, but to assist in the end-of-cycle extraction of the last of the exhaust and to boost the initialization of the following intake cycle.  Anti-reversion chambers interfere directly with that.

 

The central tube runs the entire length of the chamber and is perforated with large (10-15mm) holes.  It's function, as stated by KJ, is as a resonator.  Pressure from the exhaust forces gas into the chamber through the holes, and that pressurized gas empties back into the header as the pressure there drops, diluting the low pressure wave that follows. 

 

True resonant tuning of an exhaust with a "glass pack" muffler on the end of it is not as effective as with an unmuffled system because the muffler interferes with the establishment of a definitive end point to the tract as far as pressure reversions go.  The concept is that by damping the pressure waves in the header, the engine is fooled into seeing the header as having no defined length, theoretically resulting in a wider torque curve.  Does it work?  The Power Bomb I had on my '03 YZ450 did definitely take the peaky spot out of the power curve.  Otherwise, I can't say.

The cross section of the anti-reversion chamber is not representative of either a MegaBomb, nor a PowerBomb. Four-stroke exhausts are not tuned to assist blow-down, anyway

Well actually some are ;)

According to jerold at hytec the placement does effect pressure at blow down and they place them 7-11 inches from the exhaust valve because of this.

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

This isn't a Bomb or AR.

But shows that an area change near the flange effects blow down.

The effects of a header with a step anywhere near the head are all because of how they affect blow down.

If a stepped header Has enough of an area change,

Well so does an AR chamber, power bomb and megabomb.

" The illustration shows a single 10-mm step spaced approximately 125 mm from the flange. However, experts say that in 2008, two smaller steps (5 mm each) in the primary are more commonly seen, depending on the research and beliefs of the developers. The first step is typically between 100 and 200 mm from the flange. If there is a second step, it is typically another 100 to 150 mm beyond the first step, and in general, tubing sizes range from about 50 mm to 65 mm. (1.97" to 2.56"), although the specific designs seem to vary dramatically from team to team.

My first impression, which was shared by a number of experts with whom I spoke, was that, since these engines are operating up to 19,000 RPM, then the primary length required to achieve the negative pressure pulse during overlap was so short that, due to packaging constraints, the location of the collector would be too far away from the valves to initiate the properly-timed reflection. However, a bit more thought and a quick calculation revealed quite a different theory.

For purposes of approximation, assume that the mean temperature of the exhaust gas in the primary up near the head is 1500°F (815 °C). The speed-of-sound-in-air equation (close enough for approximations, according to Professor Blair) produces a sonic velocity of 661 m/s (2168 feet per second). At 18,000 RPM, (300 RPS) one crankshaft rotation takes 3.33 milliseconds (ms) or 3333 microseconds (μs). Therefore one degree of crank rotation takes 9.26 μs (3333 ÷ 360). If the first step in the primary is 200 mm from the back of the exhaust valves, then using the calculated speed of sound as an approximation of the propagation speed of the finite pressure wave, the 400 mm round trip from the valve to the step and back takes about 600 microseconds, or 65 degrees of crankshaft travel.

Assume that, in an 18,000 RPM engine, the establishment of enough exhaust valve opening to allow meaningful flow would occur in the neighborhood of 100° after TDC. Therefore, it is clear that this first reflection is timed to arrive back at the valves even before the piston reaches BDC. For what purpose? Recalling that during blowdown, there is sufficient pressure ratio in the cylinder to establish choked (sonic) flow through the exhaust valve orifice, then it would certainly be advantageous to maintain that gas velocity for as long as possible.

A noted engineer in the world of Formula-One confirmed that this is exactly the reason for the one-or-more large-magnitude steps in the primary: to place a negative pressure at the back of the exhaust valve timed so as to extend the duration of the critical pressure ratio. "

Edited by englertracing

from what I've seen on the fmf production pipes the "bomb" is placed where it's convenient to package it for the application. Not so much this many or that many inches out.   I see it as an anti-resonant device, killing or smoothing any natural resonant effect from the pipe.  Any gain from resonant effect will have a loss at either side of it.  For example a head pipe tuned for top end, it hurts low,  or visa versa,  Add the bomb and it'll fuzzy the top end resonant, making less loss at the bottom. Why you can take bikes with weak low end and pick up some bottom using a bomb. The trade off still exists, you don't gain anything, just shift it around.

from what I've seen on the fmf production pipes the "bomb" is placed where it's convenient to package it for the application. Not so much this many or that many inches out.   I see it as an anti-resonant device, killing or smoothing any natural resonant effect from the pipe.  Any gain from resonant effect will have a loss at either side of it.  For example a head pipe tuned for top end, it hurts low,  or visa versa,  Add the bomb and it'll fuzzy the top end resonant, making less loss at the bottom. Why you can take bikes with weak low end and pick up some bottom using a bomb. The trade off still exists, you don't gain anything, just shift it around.

In that case, does that mean by running an exhaust with either bomb on it, you could block the holes off some how and have your standard, peaky power curve. Then when you wanted to smooth out the power, at the sacrifice of some peak power, open the holes up and turn the bike into a tractor?

from what I've seen on the fmf production pipes the "bomb" is placed where it's convenient to package it for the application. Not so much this many or that many inches out.   I see it as an anti-resonant device, killing or smoothing any natural resonant effect from the pipe.  Any gain from resonant effect will have a loss at either side of it.  For example a head pipe tuned for top end, it hurts low,  or visa versa,  Add the bomb and it'll fuzzy the top end resonant, making less loss at the bottom. Why you can take bikes with weak low end and pick up some bottom using a bomb. The trade off still exists, you don't gain anything, just shift it around.

 

That was the stated purpose as expressed by FMF.

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