Backpressure - Harley Davidson Forums: Harley Davidson Motorcycle Forum
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post #1 of 76 (permalink) Old 11-02-2012, 08:35 PM Thread Starter
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Before I was a bike guy I was a car guy. One thing I learned over the years is that ANY backpressure is a bad thing. I truely believe that many people get it confused with a syphoning effect.

Backpressure is pressure that is working against the flow. Anything that is against the flow will hurt performance. Sort of like the stupid stock air deflecter on the new bikes.

I understand I'm a new guy on here and all but I keep reading on here about how backpressure is a good thing. IT'S NOT A GOOD THING!

Syphoning of exhaust is a good thing. It's where the pulse of the last stroke pulls the air from the combustion chamber as it flows down the pipe. BACKPRESSURE would slow that flow.

OK. I'm off my rant.
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post #2 of 76 (permalink) Old 11-02-2012, 09:20 PM
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Bases on what the engine guys on here have said to me via PM's you my friend are correct.

From what I understand though, SLIGHT back pressure can help scavenging and yield more power.

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post #3 of 76 (permalink) Old 11-02-2012, 10:14 PM
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You'e always going to have some back pressure unless you're in outer space. Atmospheric pressure at sea level is 14.7 psi and that will be there no matter what sort of exhaust system you use. Of course, as you go up in altitude, that will decrease, but so will the available atmospheric oxygen and thus the horsepower you develop unless you supercharge.

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post #4 of 76 (permalink) Old 11-02-2012, 11:48 PM
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How come horsepower actually went up when adding more restriction in my Thunderheaders?

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post #5 of 76 (permalink) Old 11-03-2012, 01:08 AM
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Any backpressure? Really?

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post #6 of 76 (permalink) Old 11-03-2012, 03:20 AM
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Here is a quote from RB racing.
Our testing has shown that the Harley V-Twin is extremely sensitive to back pressure which is why 2-1's that use small or restrictive collectors don't breathe well, and why restrictive baffles shut the motor down. Disc type baffles are good for spark arrestors on dirt bikes but they have no place on a big inch V-Twin. You don't make power by adding restrictions to your exhaust system whether it's a bunch of stainless discs or some damn piece of aluminum billet machined into a Harley butt plug! Ever wonder why they had to put a hole up the center of the discs? Well, one reason was the discs are so damn restrictive you have to have several pounds of them to get enough flow through their waffle shaped passages. Good mufflers, but they have nothing to do with performance and they sure as hell do not create vacuum as has been claimed.

When you get confused by all the bullshit simply ask the following question.." Do they run them in NASCAR or in Formula One?". Nope, they run straight pipe collector systems without any stupid discs or aluminum butt plugs. It's always funny how, when the money is on the line, all the little things like discs, billet caps, anti-reversion flaps, reverse megaphones and other such nonsense somehow don't make the field. Black Hole technology does not involve stupid eye candy or outdated, or just pain dumb, ginger bread.
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post #7 of 76 (permalink) Old 11-03-2012, 05:00 AM
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Baffles, plugs or discs are a cheaper way to create back pressure for what is normally an UNTUNED exhaust system.

What RB Racing fails to mention, is that although race car exhaust may be straight through, the diameter, the number of bends, and the degree of those bends are all scientifically calculated to create a tuned exhaust, sans muffler.

Street vehicles are REQUIRED to have a muffler, by Federal and local laws.

"Backpressure" is a mechainical force that slows EXHAUST VELOCITY. By maintaining a 300 ft/min exhaust velocity, you maintain optimal power and torque via proper exhaust flow.

Lower that, ans the numbers lower with it. You may gain a small amount of HPs in the top end, but not torque.

Raise it, and the numbers lower again.

There are gimmicks on the market and there are things that are real, and people that know what works and what doesn't. HD uses 1.75" diameter exhaust tubing for a reason.

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post #8 of 76 (permalink) Old 11-03-2012, 09:23 AM
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I've fooled arround with a few pipes and exhaust systems on differant vehicles and the effects they have. Adding a baffle to slow the exhaust flow a bit can help in low RPM torque but will usually hurt high end output a bit if the pipe is in the proper tune for your motor. Adding a bit of back pressure is a fix for a pipe out of tune. Pipes are tuned for a torque/HP curve for a purpose so choose a pipe that gives you torque where you want it. 2 into 1s generally run smoother and have more peak torque/HP. You can have loads of bottom end torque and give up some top end or you can loose some bottom end and pick up the top end. It's a trade off that you have to decide.

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post #9 of 76 (permalink) Old 11-03-2012, 09:48 AM
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Back pressure for race & every day driving differ form the most part.

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post #10 of 76 (permalink) Old 11-03-2012, 10:34 AM
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I want it all. I want lowend torque and topend HP. So..what do I do??? Punt???
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