Riddle me this: exhaust backpressure

[MaineShark]

Okay, another "riddle me this" topic:

Why is there such an issue of exhaust backpressure? On the one hand, too little backpressure hurts your low-end torque, and on the other, too much hurts your top-end power. Any setup you go with seems to be a trade-off, with one end losing out in favor of the other. Either that, or it does both moderately well, but gets beat at one end or the other by the others.

So, maybe this is crazy, but why not have a low-backpressure exhaust, and just add some backpressure when you need it? I can think of a dozen-odd ways to add and remove a restriction in the exhaust. Simply have a computer, which would react to certain conditions and add or remove restriction to give quick low-end response, as well as a lot of top-end power.

I'm thinking the simplest way to experiment would be to take a heat riser valve, drill a lot of holes in the butterfly, and use a shift light box, rigged up to a solenoid valve to trigger it at a various RPM points. That should give it restriction when it needs it, and then open up once the engine wants to breathe.

I think the benefit would be even greater on turbocharged cars, where a lack of backpressure can increase lag by quite a lot. Imagine a boost-sensitive system that closes the valve when the turbo is spooling-up, then opens it once the turbo starts making real boost.

Am I crazy?

Joe

 

[Ken]

SuperTrapp has been doing that for years:

Dr. Paul Moller, the inventor of the SuperTrapp, reasoned that if you could scavenge residual gas this would allow a stronger charge to enter the combustion chamber, resulting in an increase in performance.

The SuperTrapp uses a patented radial diffuser system. A series of discs with a precise gap form an exhaust outlet. By adding or subtracting discs, you can virtually wave tune your engine and carburetion. You can increase or decrease the exhaust outlet size; adding discs opens the system and removing them reduces it. The fewer the discs, the quieter the system; the more discs, the nosier it becomes. SuperTrapp dispenses exhaust gases radially rather than straight out, it is greatly effective in reducing noise levels. This opening and closing of the exhaust exit is very important to tuning. Allowing you to adjust your powerband, according to the type of track you are running.

• Remove discs to decrease the exhaust opening and increase back pressure.
• This decreases the powerband to give you more low-end torque.
• Decreases the exhaust tone.
• Enriches the carburetion.
  
  Add discs to increase the exhaust opening and reduce back pressure.
• This increases the powerband to give you more top- end.
• Increases the exhaust tone.
• Leans out the carburetion.

I know that doesn't quite answer your question Joe, but it starts it in the right direction I think.

 

[MaineShark]

Yeah, that's kind of what I was thinking, except with the ability to change on-the-fly

Joe

 

[XS650]

SuperTrapp has been doing that for years:

I know that doesn't quite answer your question Joe, but it starts it in the right direction I think.

In the performance street and roadrace motorcycle world where people spend a lot of time on the dyno and changing out exhaust systems, Super trapps aren't very popular because they don't perform as well as the more conventional performance systems.

The magazine comparison tests don't even usually bother to run Supertrapps, when they do they come in at the rear of the pack, figuratively speaking. I'm the only person I personally know who ever ran one on a street bike and I dumped it. I got better performance with a quiter conventional system

I think it would be more productive to do something to change the effective tuning (lengths) of the header and forward pipe systems. Things can be done by opening passages between header tubes or into chambers off the tubes. Or like the Yamaha EXUP system that changes the efrective pipe diameter with a servo. But IMHO, it's not worth the effort.

Dick

 

[corvettecrazy]

you are looking for something like this but it controls the exhaust pipe not a cut out.