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The Philosopher King
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1,345 Posts
Discussion Starter · #1 ·
Okay tech guru’s I have an interesting question for you…how much back pressure does a motorcycle really need??

I will clarify I have a 1995 ZX6c (Ninja 600R) and for a while I have noticed that my exhaust is far louder than most and that my exhaust can is never very hot, I started thinking that maybe I had a hole in my pipe so I checked it out…sure enough there was a rather large hole in the end can right were the pipe meets the can. I started to think about whether this “performance wise” was a good thing or a bad thing. Obviously bike motors need some back pressure but how much is enough?? Is having a hole in the end can actually beneficial (I am thinking to the old 70’s bike they use to straight pipe for performance and sound). I am about to put in a jet kit and k&n air pods and a 4 degree timing advancer but I am wondering if I am going to have to fix the exhaust pipe first to actually get the better performance out of it…but what do the pros think??

Any and all help is greatly appreciated
 

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Premium Member
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11,196 Posts
Back pressure helps with the lower end of the RPM scale to make the bike pull harder. But on the top end near redline you want as little resistance to flow as possible.

This is what the Yamaha EXUP valve does for the bikes that use it. It's a control valve in the exhaust system that closes it down a lot at idle and for the first few thousand revs then opens progressively as the revs climb until probably about 5000 to 6000.

So technically your leaking system will be making the engine run leaner and at the same time making your pull aways from stops more boggy.

And yes you really should fix the system before you do all that engine work. If you don't then you'll need to fine tune the idle mix and needle height all over again when you finally get around to fixing the exhaust.
 

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builder of bikes
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4,802 Posts
Formula 1 cars run with as little back pressure as possible. Why? Back pressure is not needed to make power. Proper scavenging of exhaust gases is. Why doesn't F1 use exhaust valves? Rules. But there is more to it. Look at the intricate piping for exhaust:



For street and motorcycle use, the exup or SET valve (suzuki) is used to modulate scavenging on exhaust that are limited by space to simple design. They don't create backpressure, they shorten header length with the sonic feedback in the flowing gases. These valves also help in cutting down noise and passing strict EU noise laws.

Back pressure does not create power. Period. It moves the torque curve around. Yes, this can be optimized to a specific engine's needs, but the scavenging aspect of a properly designed exhaust is FAR more important.

Now remember power is torque per second. So if you make the same torque but just in a different spot, you make no extra power. Unless some magical valve (impossible) can push the peak torque to 14000 rpm. :)

So an engine with no exhaust valve has a static torque curve that must be optimized with exhaust cam timing. This CAN affect scavenging and must be carefully designed to maximize both with no hinderences. If you have a valve, you can free the camshaft to exploit the scavenging to it's fullest.

Watch CART, NASCAR, F1, Moto Gp and if you hear of a retirement from a broken exhaust header it's due to no power from no scavenging and exiting hot gases most likely somewhere it will cause fire.

Scavenging?

It empties the exhaust chamber as much as possibly to increase the vacuum for more intake charge. It runs cooler so denser more powerful air can be exploded in the compression stroke. This is needed since most engines have a small exhaust valve to balance exhaust gas speed and reducing temps from the pressure drop across the valve.
 
G

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double check the holes in the can to be sure they are not there to allow moisture to escape. the stock can on my old 750 had a hole on either end to let water drip out.
 
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