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Discussion Starter #1
how come race bikes that run only on the track still use mufflers? wouldn't they gain more power from running straight pipes if they were jetted properly? would my 91 zx-6 gain power if i switched to a straight pipe rather than my vance&hines slip on muffler and it was jetted properly? any input appreciated.
 

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You would think that the least amount of flow resistance would make the most horsepower, and in some engine designs it does, but racers don't simply need the highest horsepower figure possible. When one is accelerating out of corners a smoother, more predictable powerband with a healthy midrange is infinitely preferable to a pipey, on/off-switch. Straight pipes typically shift the meat of the powerband toward the red-line like some temperamental two-stroke.
By managing the backpressure, they're able to spread that horsepower hit and share it with the lower rpm, making easier on both the rider and the tires.
A drag-bike could possibly benefit from a "straight-pipe", but not a roadracing bike, and certainly not a rider on public roads with neighbors.
-CCinC
 

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RIP Deron Harden :-(
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backpressure

chris explained it quite well. it all comes down to backpressure. almost all engines want to see some kind of backpressure. unless you are running a turbo or blower, you really want some there. for the same reason, you don't see someone with a V6 car running 4" exhaust tubing. now, on my turbo diesel, i run 4" and will most likely be changing it part-way to 5" stacks, because it is turbo it wants the most unrestrictive exhaust flow possible, within reason. i am guessing that just having 10 feet of pipe on a turbo motor gives it the backpressure it wants to see. now, take that for what its worth, i am no mechanic, i only know what i have learned from people that do know what they are doing.
 

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backpressure doesent always come from the length or unrestriction of the exahst, it has alot too do with the way the exahst is shaped. if its shaped right the flow off the exahst should suck out the exahst..sometimes the muffler give the bike more backpressure.. its a very tricky thing, not enough and it will run bad and lose time, too much and you can lose timeing. your zx-6 redlines around 14,000rpm right? so if you got a exahst pipe that was the same diameter as the one you have now, but only an inch shorter and with a megaphone on the back of it it would benefit alot probably. but it would be too loud, i have a 4 stroke 50 cc honda z50 with a megaphone exahst that i built and you can here that thing coming from like 2 miles away.

cheers
 

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AZ's Official Mechanic
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G3SSB said:
its a very tricky thing, not enough and it will run bad and lose time, too much and you can lose timeing.
exhuast will NOT change your timing in any manner BUT it will change the combustion chamber scavenging characteristics. :wink:
i agree it has to do with the backpressure, to much and its strangled, too little and it doesnt pull the old exhaust out of the chamber wll enough to pull in a nice fresh mixture charge. thats why the engineers spend $$$$$$$ on R&D so that you dont have too. :roll:
 

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Discussion Starter #6
kool. i guess u would want a more predictable power range on a motorcycle, especially since bikes like the ones in moto gp have already reached such high peak hp levels. i think i will take off my slip on muffler next weekend though, jus for shits an giggles to see how loud it is ;)
 
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