back pressure - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
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post #1 of 15 (permalink) Old 05-21-2018, 08:37 PM Thread Starter
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back pressure

what is it? why is it important? how does different exhaust change it?

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post #2 of 15 (permalink) Old 05-21-2018, 09:02 PM
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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/Reduc...aust-s/203.htm
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post #3 of 15 (permalink) Old 05-21-2018, 10:19 PM
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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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post #4 of 15 (permalink) Old 05-22-2018, 06:25 AM
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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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post #5 of 15 (permalink) Old 05-22-2018, 06:48 AM
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I'd explain it to you, but there's a lot of math involved.

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post #6 of 15 (permalink) Old 05-22-2018, 08:19 AM
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Quote:
Originally Posted by Diogenes415 View Post
'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)



Last edited by baron58; 05-22-2018 at 08:23 AM. Reason: add picture
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post #7 of 15 (permalink) Old 05-22-2018, 08:56 AM
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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.

Last edited by KCW; 05-22-2018 at 09:05 AM.
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post #8 of 15 (permalink) Old 05-22-2018, 01:09 PM
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I ate mexican for lunch yesterday and had spicey stir-fry for dinner last night. Let me tell you ... I HAVE SOME BACK PRESSURE!!!!
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post #9 of 15 (permalink) Old 05-22-2018, 01:37 PM
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Have you noticed less power?
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post #10 of 15 (permalink) Old 05-22-2018, 01:42 PM
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Any exhaust leakage either gaseous or liquid?
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