Muffler Porting

[Dago]

While my exhaust is off I'm planning on either redoing it or buying another unit.

Porting came to mind, if any of you play paintball you know what I mean with the barrel porting.

If I use straight line porting on the side of the muffler that faces away from the bike . . . or maybe even faces down toward the road depending on how it's gonna look . . . porting should reduce backpressure, reduce noise (depending on where the porting starts in the pipe and how large it is), reduce overall exhaust temperature, and reduce weight. I'm going to refinish the exhaust, so the drilling appearance won't be an issue when I'm done.

Can anyone think of any reason why I shouldn't do it? It definitely seems beneficial to me at this point.

[steves]

Why do you want to reduce back pressure? You need a certain amount of it for your motor to run correctly.

[Dago]

Same thing an aftermarket exhaust does. It won't reduce it to the point of eliminating torque, but it will allow greater flow at higher rpms.

As far as I know the headers alone will actually make enough backpressure for the engine to run great, which is why so many race machines used to just use headers.

[Rok_Stars_Bars]

Theres a exhaust mod already done!!!

http://katriders.com/vb/showthread.php?t=74710

[Dago]

Hmmm . . . it doesn't address porting at all.

My concern isn't sound, but the dual benefit of decreasing backpressure and noise.

Keep in mind here I'm talking about drilling a series of holes from the outside casing of the muffler to the inside of the exhaust pipe along the weld at the bottom of the pipe.

Something like the porting which can be seen here :

http://www.paintball-online.com/paintball/view_big.asp?number=BRCIMP15&variation=&picnum=1

[Rok_Stars_Bars]

If you look at one of the member who posted. He did a cut away. The muffler has 3 chambers time you go through all that might has well just cut the can off...

[Dago]

If you cut the can off the exhaust gas will be exiting all at once, thereby increasing noise. When you port as such, gas exits a pipe progressively at lower pressures (all of this assuming an identical pressure source)

[kjcmetal]

i did that with some pre 750 headers, made them come out right after the foot peg, man, it sounded like a weak drag bike lol. any and all baffling will be in the very end of the can, about 5ish inches long if i recall, and the the rest of it is just straight empty can into the pipe. i cut the pipe down past the baffling then cut the end of the can off at the weld, gutted it so it was just the tip into hollow can. i guess you could "tune" it but leaving in half of the baffling, but i'm a fan of loudness. took me about two days o' fabrication... and a welder, and about a dozen cut-off wheels on a 4" angle grinder...

[steves]

The point of greatest constriction is the collector anyway.

[Rothman]

. . . porting should reduce backpressure, reduce noise (depending on where the porting starts in the pipe and how large it is), reduce overall exhaust temperature, and reduce weight. I'm going to refinish the exhaust, so the drilling appearance won't be an issue when I'm done.

It will only reduce the backpressure so slightly that there will be almost no benefit if any at all. Also, considering that you are going to refinish it so the holes are not visable, what have you even accomplished really? It's still the same amount of exhaust coming through the same size openings. In fact there's a chance that backpressure may be increased due to the fact that you will have changed the way the gases flow and introduced otherwise nonexistant turbulance inside the can. As far as lowering exhaust temp... I don't even see that as a mere possibility. How could that even help cool the exhaust at all? It will reduce weight but, we're only talking about grams here so there is really no advantage there. Could it reduce noise? Yes it's possible, it's also possible that the noice could be increased due to the possibly added turbulance. Basically, it's pointless.

[gobigorange189]

so are you trying to make your stock exhaust quieter??

[Dago]

It will only reduce the backpressure so slightly that there will be almost no benefit if any at all. Also, considering that you are going to refinish it so the holes are not visable, what have you even accomplished really? It's still the same amount of exhaust coming through the same size openings. In fact there's a chance that backpressure may be increased due to the fact that you will have changed the way the gases flow and introduced otherwise nonexistant turbulance inside the can. As far as lowering exhaust temp... I don't even see that as a mere possibility. How could that even help cool the exhaust at all? It will reduce weight but, we're only talking about grams here so there is really no advantage there. Could it reduce noise? Yes it's possible, it's also possible that the noice could be increased due to the possibly added turbulance. Basically, it's pointless.

Theres a chance that changing your oil could cause the bike to increase wear as well. Without any actual study that's ever been done, how does one know that it will decrease backpressure ever so slightly or increase noise?

Porting will decrease exhaust temp because less exhaust gas will be traveling through to the end of the muffler. As a result, the large end of the muffler will act much more like a heatsink to the headers than it did as a whole cylinder. Air from the outside will also have more of an effect on the muffler and cause it to cool the overall exhaust assembly. As for making it louder, air gun barrels with porting are always significantly quieter at the same velocity than when using barrels without.

The only possible issue is turbulence, and that's the main reason I'm asking the question. If someone who has a working knowledge of airflow physics could chime in, that would be cool.

[p8ntbill2]

I Play paintball and you will reduce your back pressure and increase the noise from your exhaust. Just my 2 cents, -Bill

[Rothman]

Theres a chance that changing your oil could cause the bike to increase wear as well. Without any actual study that's ever been done, how does one know that it will decrease backpressure ever so slightly or increase noise?

Porting will decrease exhaust temp because less exhaust gas will be traveling through to the end of the muffler. As a result, the large end of the muffler will act much more like a heatsink to the headers than it did as a whole cylinder. Air from the outside will also have more of an effect on the muffler and cause it to cool the overall exhaust assembly. As for making it louder, air gun barrels with porting are always significantly quieter at the same velocity than when using barrels without.

The only possible issue is turbulence, and that's the main reason I'm asking the question. If someone who has a working knowledge of airflow physics could chime in, that would be cool.

If your exhaust can is sealed than the same amount of air will be moving through the end of it. As for the barrels being quieter, I could see that. The thing is, the barrels are not sealed inside a can and therefore have a direct relationship to ambient air. As the paintball passes the porting holes even though there is pressure pushing the ball, the holes actually create a venturi effect drawing ambient air into the barrel and decreasing backpressure to a degree. When the unit is sealed inside a canister, you lose that effect entirely and have pretty much accomplished nothing. If you are hell bent on doing it then go for it. I just personally think that it would be a waste of time.

[scottynoface]

I just personally think that it would be a waste of time.

+1, it will net you no performance gains, gases will not run cooler, it will most likely sound pretty awful, the collector is the restriction point so your backpressure will remain almost the same. You don't have to be a physicist to see this as a wild goose chase...