Exhaust End Caps

[MXFastGuy]

Some of the aftermarket four stroke exhaust makers offer different end cap sizes. This makes sense, as you can tune the power characteristics with them.

What doesn't seem right is that some of these end caps (or inserts) are nothing more than a flat plate. It seems like having an abrupt "wall" at the end of the muffler would not be so good. They sure give the exhaust note a funny sound. It almost sounds louder with one of these inserts than without. It seems like a tapered insert which gradually reduces the opening would be better than a flat plate.

I am speaking specifically about the BBR exhaust for my TT-R 125, but I know some other exhuast makers do the same thing.

So, what's the deal with these plates that serve to reduce the exhaust opening? They make the bike sound funny, and it just doesn't seem right. Please enlighten me.

 

[jarredtaylor]

They increase system backpressure... low end torque...

Vacuum also plays a part in exhaust systems as well. Here's an example:

On stock cars they route the exhaust pipe in front of the rear wheel.. it is flush with the side of the car.. why???

As air passes by at high speeds.. it actually creates vacuum which aids exhaust flow and "sucks" the exhaust out of the car...

Not quite the same principle but endcaps change the flow of exhaust and vacuum carachteristics producing power in different places in the powerband. Systems with no backpressure make less torque.

By the same token.. airbox breathability also plays an important on vacuum and exhaust velocity.. i.e. torque. A carb with too big a venturi causes unatomized fuel and loss of fuel velocity into the combustion chamber.. thus robbing you of torque. Too small a venturi causes a lean condition and not much power.

Hope this helps.

 

[snaggleXR4]

Hey,
I have an XR's Only pipe that is a Supertrapp design. It has a solid end cap as you speak of, but there are circular discs in front of it. This makes the exhuast come out in radial waves, rather than right out the back. You can add and remove discs to tune for loudness/airflow. Other than that, I can't tell you what is better or worse about a Supertrapp. However, MXfastguy is correct about exhaust pipes in general. For an ideal exhaust system, the exhaust would be 100% vacuumed out of the engine. When the engine has to push out the exhaust, you loose power. Furthermore, the vacuum pressure in removing the exhaust, also helps to suck in the air/fuel intake charge for the next fire. Interesting stuff, it is a total science all it's own.

 

[jarredtaylor]

Yea i was talkin to my dad the other day about this. He drags chevy's and builds motors. He makes a tube from the crankcase breather and makes a t_fitting on the exhaust collector. He told me that the exhaust pushing past the vent tube hose creates negative vacuum on the engine and helps pull the piston downwards and suck in air/fuel better too. He says you have to know what size tubing to use and how far back to put the t fitting on the exhaust tho because different engines breathe differently according to the manifold that your using etc...

Kinda makes me wonder just how much more they can push the envelope with the motorcycle stuff.. after all a motor is a motor.

Do they knife cranks on 4 strokes... in theory this cuts thru oil better and gives a 1-2 extra ponies. What about cams for these bikes...?? I had a set of hollow camshafts for my VW... they were made by a austrian company that also makes the camshafts for Formula 1 cars.... any of these trick parts available (no i dont race supercross).

JT