2-1 reversion, tq vs. hp
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Road Master
Joined: Nov 2011
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From: Hahira Ga
2-1 reversion, tq vs. hp
I have so often read statements about 2-1 pipes producing the best power yet at the same time I have seen so many dyno sheets showing a sizeable dip in the torque curve due to reversion supposedly. It seems that it is hard to tune out. When those statements are made, are they referring to peak horsepower but at the expense of lower end torque. For a Harley rider, looking for daily performance, that 0-80 acceleration, wouldn't a pipe and tune focused more on higher low end torque and a smoother torque curve be more satisfying?
Road Captain
Joined: Jan 2009
Posts: 714
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Also, motor characteristics (cam) can have a say in when and where the power is delivered. Everything works together as one unit, and pipes aren't the end all of a total performance package-just one of the players...
Road Warrior
Joined: Aug 2015
Posts: 1,712
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From: Puyallup, WA
I'll try to answer a couple of your questions.
First off, reversion and collector systems (2 into 1, etc) are two separate things. Reversion, in its simplist form, is some exhaust gas traveling backwards into the engine, due to a number of factors. A collector system, scavenges exhaust; in other words as one little ball exhaust gas rolls out the pipe, it creates a low pressure area behind it that helps pull the next little ball of exhaust gas from the next cylinder. This what a tuned collector system is all about, getting header lengths tuned to help scavenging.
A stepped exhaust, is one that increases in size from the exhaust port towards the tip.
Everything in the engine world is a trade off. If you want top end horsepower, like drake racing style; go with a wide open unrestricted exhaust system. However with unrestricted system, the velocity of the exhaust slows at low rpm. This causes the engine to work harder to push out the exhaust, robbing low end torque. Think of a river going through a narrow channel, it roars through fast. Then the river widens and the flow slows, same with exhaust.
Back pressure, doesn't really create torque; it's the faster exhaust velocity that robs less power that helps with the torque. Just so happens that a more restrictive system that keeps exhaust velocity up, also has more back pressure.
Cam timing has a ton of effect on how reversion effects performance. Match your exhaust to your cam, match your cam to your style of riding/performance.
With all this said, a good matched system with a good tune will produce a nice torque curve.
First off, reversion and collector systems (2 into 1, etc) are two separate things. Reversion, in its simplist form, is some exhaust gas traveling backwards into the engine, due to a number of factors. A collector system, scavenges exhaust; in other words as one little ball exhaust gas rolls out the pipe, it creates a low pressure area behind it that helps pull the next little ball of exhaust gas from the next cylinder. This what a tuned collector system is all about, getting header lengths tuned to help scavenging.
A stepped exhaust, is one that increases in size from the exhaust port towards the tip.
Everything in the engine world is a trade off. If you want top end horsepower, like drake racing style; go with a wide open unrestricted exhaust system. However with unrestricted system, the velocity of the exhaust slows at low rpm. This causes the engine to work harder to push out the exhaust, robbing low end torque. Think of a river going through a narrow channel, it roars through fast. Then the river widens and the flow slows, same with exhaust.
Back pressure, doesn't really create torque; it's the faster exhaust velocity that robs less power that helps with the torque. Just so happens that a more restrictive system that keeps exhaust velocity up, also has more back pressure.
Cam timing has a ton of effect on how reversion effects performance. Match your exhaust to your cam, match your cam to your style of riding/performance.
With all this said, a good matched system with a good tune will produce a nice torque curve.
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pablo pelon
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Nov 7, 2007 09:06 AM
