Now, I know its everyones idea to get more power by freeing up the exhaust, but with our bikes, a lack of back pressure wrecks havoc on tuning the carbs...

Soo, instead of having a muffler thats loud and dont clean the air...

why not slap on an automotive high flow cat in place of the y-pipe under the bike, then run it straight piped to the side?

Itd have the correct back pressure, the cat'll quiet it down a lot, itd be a lot cleaner, and idk, seems like a decent idea?

Its no more than 2 feet from the engine, so it should reach temp like it should.

thoughts, or am I crazy?

The Myth of Backpressure

no myth, w/o the proper backpressure, the power band will be out of useable range for these motors.

Ya, you get more power, at 13000rpm, right before the pictons turn to glass and shatter

Quote from: xunedeinx on October 24, 2011, 04:35:35 AM
no myth, w/o the proper backpressure, the power band will be out of useable range for these motors.

Ya, you get more power, at 13000rpm, right before the pictons turn to glass and shatter

Not really a myth, just a miss-understanding/ missuse of the word. The effects are there, but the "backpressure" reasoning is off.

And what exactly about high rpm would cause the pistons to turn to glass?

Back on topic though - Sounds like it would work, although it wont be as quiet as you think (at least not as much as a muffler). Also the longer the pipe after the muffler (or cat in this case) the louder it tends to be.

That being said - try it

Dont know, but im always told not to rev the piss out of this too much or itll start shattering parts...

Maybe balancing or weak conn rods?

Yes, you'll shatter the valves as they float away from the cams and contact the piston.

Nothing about a cat would improve performance, but it would create power loss and massive heat issues (they get quite hot.) Leave cat convertors to the OEMs.

Back pressure is a myth, the ideal situation is a scavenging effect from the exhaust, you want the flow of the hot gasses to pull on the gas in the chamber and help it vent. There is no benefit to back pressure, its a old mechanics tale. Strait pipes always make the most power. The biggest thing is having a X or H pipe to make it a linked system so that the exhaust from cylinder one can pull on the exhaust from cylinder two. I think of it like traffic, if there is nothing blocking the flow of traffic then everyone moves along and never slows down, if something blocks off a lane of traffic everything starts to back up and congest and not flow well. Back pressure is like a granny in the fast lane.

BUT on the other hand you don't want your exhaust to be a huge 4 inch pipe like ricers throw on their Hondas. Exhaust needs to be kept hot and fast moving. any big expansion points in the pipe causes problems with flow and allows the exhaust to depressurize and cool off making it lose power. Strangely a slightly shrinking exhaust would be ideal it would keep the pressure stable as it cools and becomes denser, as a side note "cool" doesn't mean its cold just that its not 2000 degrees, if anyone has put their hand behind the exhaust of a car or bike knows, its not insanely hot.


Revs are fine for a bike or any engine, Nothing in our bikes will shatter, BUT! if you over rev it you can break a valve spring, throw a valve into the piston and bust a piston up. Our Conrods and pistons should be forged or Hypereutectic. 

I beleive the stock exhaust at least on the newer bikes have a form of "H" pipe built in.

Quote from: noiseguy on October 24, 2011, 05:55:07 AM
Nothing about a cat would improve performance, but it would create power loss and massive heat issues (they get quite hot.) Leave cat convertors to the OEMs.

yeah, I could see that being a problem depending on the cat - I fogot about the fact that we're air cooled.

Quote from: spoolin6er on October 24, 2011, 06:12:06 AM

Quote from: noiseguy on October 24, 2011, 05:55:07 AM
Nothing about a cat would improve performance, but it would create power loss and massive heat issues (they get quite hot.) Leave cat convertors to the OEMs.

yeah, I could see that being a problem depending on the cat - I fogot about the fact that we're air cooled.

Mines water cooled.

Just havent figurerd out where to put the coolant yet....

I run a LN2 cat, super efficient...

Quote from: crzydood17 on October 24, 2011, 07:12:53 AM
I run a LN2 cat, super efficient...

I think the chinese last night as cat. Crazy awesome. Mmmm, kittymsg