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Great topic.

Backpressure caused by the exhaust system (consisting of the exhaust manifold, catalytic converter, muffler and connecting pipes) of an automotive four-stroke engine has a negative effect on engine efficiency resulting in a decrease of power output that must be compensated by increasing fuel consumption.

On a car race motor open headers do not make the most usable power thru out the rpm sweep. A short header extension pipe increases horse power. We found after many dyno test was to use a crayon on the header extension pipe, where the crayon stopped melting that was the length you wanted. Here's an excerpt from a good article about motorcycle exhaust.

A common belief is that by reducing back pressure in the exhaust system you will automatically gain horsepower. Unfortunately this is not the case. With the proper fuel management system for a fuel injected engine or re-jetting in the case of a carbureted engine however the maximum horsepower will improve. Without proper tuning the more free flowing aftermarket exhaust will generally cause your engine to run lean and promptly lose 5-10% horsepower.

There is however a difference between usable power and maximum horsepower. The maximum horsepower of two engines may be similar, but the horsepower torque curves may be different. The area under the horsepower and torque curves defines the "power" the engine produces. The more area that is under the curve, the more power your engine produces.

Here the link:
https://www.tabperformance.com/Reduce-Back-Pressure-Exhaust-s/203.htm
 

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with out baffles in my roadburners the torque curve got shorter and moved further up the rpm range which took the fun out of riding around town. on a cruiser you want torque down low not up high
 

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'Backpressure' is one term I've really started to dislike because it's thrown around by people who don't understand basic exhaust system concepts. So far this is the best presentation I've seen that helps explain the basic concepts.
 

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'Backpressure' is one term I've really started to dislike because it's thrown around by people who don't understand basic exhaust system concepts. So far this is the best presentation I've seen that helps explain the basic concepts.
https://www.youtube.com/watch?v=jjPeP_Nn2B4
Unfortunately this guy left out many details. And oversimplified this.

The exhaust valve starts to open prior to the piston reaching bottom dead center. This means the exhaust gases and left burning gases are being released early resulting in less push down on the piston, this is especially important during lower RPM's. At lower RPM's less back pressure equals less torque. Most V Twin motorcycles run in the lower RPM range and are known for torque not horsepower.

Max horsepower usually occurs at a much higher RPM range, at higher RPM's back pressure is now a bad thing because at that RPM releasing the exhaust gasses early is a good thing because of how fast the pistons are moving and we want less resistance as possible. This is why high revving race car engines have almost no exhaust because back pressure hurts horsepower.

If you look at any dyno chart results you will find on lower RPM engine back pressure is a good thing for torque, and on higher RPM engines a bad thing for horsepower.

On many of the newer high performance crotch rocket bikes, there is a variable solenoid controlled (By the ECU) butterfly valve that actually regulates back pressure in the exhaust for best performance in all power bands.

Here is a dyno run with a slip on (back pressure) compared to full open exhaust (less back pressure)


 

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Back pressure does not mean the pressure in the exhaust has a backwards direction. Pressure is not a vector, it has no direction. A pressure wave has direction, and as the video describes, the wave flows out from the cylinder head thru the exhaust, and bounces back on any change in flow restriction.

I think the term is referring to the back-side of the engine, the exhaust pressure (vs the intake pressure).

something that is equally important, I have read about mods on several bikes where someone took the stock air box and filter off, and put one of those shorty little air filters right on the carb intake. Dyno tests indicate that even with rejetting, the bike lost HP. It looks cool, but its not functional. When I see a Honda 750/4 or another vintage bike set up like that, I cringe.
 

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Completely off topic but couldn't help think about it when I saw this thread. I had a "self proclaimed" mechanic working for me once. When I first bought my bike I had it at work going over it. I had it running and he walks up behind it, pulls out a dollar bill and holds it over the exhaust. As the bill is flapping in the breeze he proclaims, "well you don't have any back pressure. That's a good thing". I guess he was expecting the bill to suck towards the pipe to show there was back pressure. Moron!
 

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A long long time ago I was experimenting with exhausts and made a couple of reed valves to install at the end of my exhausts just to see what the effects would be. I didn't notice anything different performance-wise but the bike sounded like Kenny G honking out a tune so I removed the valves and gave up pursuing that path.
 

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^ that would be interesting if you could use a flexible material, that would be like pulling the neck of a balloon while letting out air

if it was connected to a lever..... Bleee ♪ Bleee Bleee♫ BLEEEE ♫
 
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