Adjustable Flow Exhaust? a good idea?

[digitalsam]

Am I thinking correctly that the reason people don't recommend switching to any old style of exhaust tip is that different exhausts allow more or less air to exit the bike changing the pressure on the engine?

If this is the case, couldn't you create a device kind of like a camera aperature that you could open or close manually to adjust the back pressure?

Am I way off here? Honestly, correct me if I am way off. I don't know how much I understand about the stuff.

[Guy_b_g]

Check out SuperTrapp...already being done with removable discs...works quite well.

[Crazcnuk]

It works but is only really useful for racers as you can tune for changing conditions.

Most of us want bikes we don't have to tune every other day!

[81xsproject]

It could be done quite simply by utilizing intake vacuum to actuate a diaphram driven restrictor. When the manifold pressure is high (close to atmospheric) the restrictor would be open and when the vaccum is low (highest vacuum) the restrictor would be pulled closed. It seems to me that something so simple would have been done already if it was truly beneficial.

[bikerphil]

I have my Kerker set up so the baffle pipe can be dislodged from the can and be pushed forward for an open exhaust. To quiet it back down, I grab the baffle's cross bolt with a hook and pull it back into place. It can be done in about 10 seconds without removing anything from the bike.

[WMarshy]

It works but is only really useful for racers as you can tune for changing conditions.

Most of us want bikes we don't have to tune every other day!

They arnt just for racing applications and you shouldnt have to make constant changes as most people dont see that large of a change in environment that would justify a change. Once they are set up good you should be good. If I had the $ to throw down on two baffels I would replace my rusted mega phones on my stock pipes and never look back. Not only do they look nice, sound nice, they can improve the power of our bikes. If done correctly.

[petejw]

a guy up the road has a new harley,
with straight thru pipes, it was fitted
with a flap on the back of the exhaust,
but i cant remember how it was
actuated.

[fredintoon]

a guy up the road has a new harley,
with straight thru pipes, it was fitted
with a flap on the back of the exhaust,
but i cant remember how it was
actuated.

Hi Pete,
back in the 1920s when most bikes (and cars) had an exhaust cut-out as a standard fitting I believe they pulled on a wire cable to operate it.

[Ivan]

It could be done quite simply by utilizing intake vacuum to actuate a diaphram driven restrictor. When the manifold pressure is high (close to atmospheric) the restrictor would be open and when the vaccum is low (highest vacuum) the restrictor would be pulled closed. It seems to me that something so simple would have been done already if it was truly beneficial.

I believe it has, and was called EXUP by Yamaha, others have done similar. I think it was electrically actuated via servo in stead of vacuum.

[digitalsam]

What about the same sort of thing on the air intake? Switching to pods requires a rejet. What if you were able to reduce or increase the amount of air coming in to the engine?

[DiverRay]

What about the same sort of thing on the air intake? Switching to pods requires a rejet. What if you were able to reduce or increase the amount of air coming in to the engine?

I think that is called a constant velocity carb, such as the carb fitted stock to our bikes. As for the exhaust, it would be a lot of work, and trouble to keep clean and operational over 100K miles of normal owner use. the extra one to five HP probably would be easier to achieve via close tolerance machining. JMHO

[Ivan]

What about the same sort of thing on the air intake? Switching to pods requires a rejet. What if you were able to reduce or increase the amount of air coming in to the engine?

There is such a device already fitted to every scoot! It's actuated by the right grip.

Actually, in all seriousness, they have been doing what you are trying to grasp on cars for some time now. Ford calls it "Intake Manifold Runner Control". It changes between long intake runners for low end torque and short runners for high rpm horses. Yamaha calls it "V Boost". First engine I remember seeing it on was the Chrysler 3.5 they put in the Intrepid. It actually works quite well, in every application I can think of. Toss in variable cam timing and the 100 hp per liter mark is the base line instead of the goal. The 2000 Ford Focus rally car pulled some 350+ HP continuous from their 2.2(?).

[Guy_b_g]

Many of the newer sport bikes use a servo controlled variable length intake.

Basically a telescoping velocity stack at the intake of the fuel injection system.

[MeatTooth]

There is such a device already fitted to every scoot! It's actuated by the right grip.

That's awesome!...and true

[WMarshy]

There is such a device already fitted to every scoot! It's actuated by the right grip.

Actually, in all seriousness, they have been doing what you are trying to grasp on cars for some time now. Ford calls it "Intake Manifold Runner Control". It changes between long intake runners for low end torque and short runners for high rpm horses. Yamaha calls it "V Boost". First engine I remember seeing it on was the Chrysler 3.5 they put in the Intrepid. It actually works quite well, in every application I can think of. Toss in variable cam timing and the 100 hp per liter mark is the base line instead of the goal. The 2000 Ford Focus rally car pulled some 350+ HP continuous from their 2.2(?).

Many of the newer sport bikes use a servo controlled variable length intake.

Basically a telescoping velocity stack at the intake of the fuel injection system.

The same theory applies to exhaust as well as intake. Unfortunately, its harder to apply to the exhaust without having a really bulky system. Thats why it is more common to see two stages for the exhaust, full length, and cut-out opened. Buel has it and its open via servo controlled by ECM when it hits X% throttle positions... Its cool stuff but a little compicated for "back yard" mechanics to actually make effective. It would require lots of calculations and a dyno to verify that it opens at the correct RPM to actually improve performance.