A question of back-pressure?

[mark5lam]

I have no intention of running a strait pipe but I’m curious for other reasons. If you had an exhaust shop remove the cat, install a Y and run a strait pipe to your liking (under the seat or along the trellis frame or what ever) would the 1200GS suffer any ill performance effect due to the lack of back-pressure provided by the cat and exhaust can?

 

I’m doing some day dreaming about making a custom exhaust (with a muffler) that looks good and allows the installation of two full size Jesse bags.

[steve.foote]

Mark,

 

Al Jesse is supposed to be working on an underseat modification for the 12GS which would allow two full size bags to be installed.

 

It could be kind of cool.

[mark5lam]

Mark,

 

Al Jesse is supposed to be working on an underseat modification for the 12GS which would allow two full size bags to be installed.

Steve,

I know his site has been saying "coming soon" for a long time. Aside from the fact it will probably be 900 bucks, rumor is that he's having issues with heat management and all the plastic the R12GS has under the seat. But that's just rumor. Still, I'll be interested to see what they come up with. The set-up they have for the 1150 seems good. In the mean time, I'm trying to think of how I would do it myself if I could.

 

Not to hijack my own thread but, I'm seriously considering replacing my BMW bags with a set of Jesse's but I don't want to spend the money and still have the left bag reduced in size and overheated by the exhaust running through it.

 

Anyway, in addition to any comments anyone would like to make on this thread, I’m also, still hoping someone will chime in with knowledge of how exhaust back-pressure affects engine performance.

[David]

Isn't there some valve in the exhaust that opens and closes above certain RPM boundaries? I'd need to look it up, but I seem to remember something like that in the original sales literature. If so, you might need to account for that.

[R4ND0M_AX3]

...Anyway, in addition to any comments anyone would like to make on this thread, I’m also, still hoping someone will chime in with knowledge of how exhaust back-pressure affects engine performance.

 

The whole backpressure thing is kinda sensitive for some people. It's not really the backpressure that helps. It's the ability of the exiting gasses to pull out the gasses behind it (scavenging).

Less old exhaust leftovers in the combustion chanber means more fresh air can enter during the intake stroke.

 

The least restrictive exhaust is none at all. But that has no scavenging effect.

 

Others will step in to correct me.

[mark5lam]

Isn't there some valve in the exhaust that opens and closes above certain RPM boundaries? I'd need to look it up, but I seem to remember something like that in the original sales literature. If so, you might need to account for that.

David,

If you're referring to the valve built into the stock muffler that opens at higher RPM's to allow more gas to flow through, I’m also not sure of the significance of it’s function. I don’t know if the valve being closed at lower RPM creates more back pressure and if so, for what reason. Nor do I know if opening the valve at higher RPM’s increases horsepower.

 

Never the less, BMW decided to do away with that muffler valve on the newer 1200 GS's. As I understand it, you can tell which mufflers have it by the color of the end cap. The mufflers with the black end cap have the valve. The mufflers with the silver end cap do not.

I'm not sure why BMW decided to do away with the valve. Probably one or a combination of reasons including: not being really necessary, cheaper build cost and or lighter weight. But I'm just guessing.

[ShovelStrokeEd]

Go ahead and toss your cat and muffler as far as that goes into the middle distance. You won't hurt performance in the slightest so long as you make air/fuel mixture adjustments to compensate for the small increase (maybe) in flow.

 

As has been mentioned, the scavenging effect is more important than flow. In fact, the factory exhaust provides very little restriction as it is. The whole "useful backpressure" thing is bull crap. There is no such thing. Internal combustion engines don't want back pressure, they want, as has been mentioned, a nice clean combustion chamber in which to draw new mixture.

 

Now, an improperly designed Y transition and poorly executed straight pipe can hurt flow and may well introduce some major holes in the torque curve due to reflected sound waves within the system. That is a topic best left to engineering texts though.

[mark5lam]

Now, an improperly designed Y transition and poorly executed straight pipe can hurt flow and may well introduce some major holes in the torque curve due to reflected sound waves within the system. That is a topic best left to engineering texts though.

"Reflected sound waves within the system" Interesting! I never thought about that but I'm thinking about it now. Nice input.

[ShovelStrokeEd]

Mark,

Just some more food for thought. The pressure pulses in an exhaust system actually move, within the system, at the speed of sound. Pulses occur at the ends of pipes with the sharpest pulses coming at pipes with squared off ends. They also occur, albeit to a lesser extent, at pipe size transitions, small to large. It is the position of these that allow the tuning of exhaust pipe length to benefit, or hurt, scavenging in the cylinder. The extreme case is illustrated in the old 500-cc GP bikes like the Norton Manx which, with certain exhaust configurations, could exhibit such a bad case of "megaphonitis" that the motor would hardly run at some areas in the rev range. Incidentally, a diverging cone, megaphone, can amplify the effect at the expense of a narrower range, this can be mitigated somewhat by a sharply converging cone at the end of the divergent section (reverse cone). The good news is this is all old hat and well known technology. A nice Y transition involves a pretty long, straight merge over say 2 to 3 pipe diameters and an increase of 1 pipe size over the original head pipe. This is not strictly necessary on a 180 degree motor but gives a little boost in the mid range provided the final pipe is about as long as the headers.

 

Have fun.

[JohnJC]

How refreshing. The back pressure myth dispelled immediately, and by two people. Always seems to be an argument in this area.

 

I agree with above. If you keep the same diameter headers, with mandrel bends, and a good Y pipe (and possibly a good X-over as well) you won't see any difference in performance. Perhaps a bit more top end if you design a better flowing system. The only restrictive part of the stock system is the can, and it isn't bad. A straight through muffler can be a little better while keeping noise at reasonable levels.