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CrazyMotorcycleGuy
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back pressure?
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Question back pressure? - March 5, 2008, 07:29 PM

so im building a streetfighter, and im trying to do something different with my exhaust. the question i have is how much back pressure do i actually need. basically i want to get rid of the can entirely and run headers and a little mid pipe w/ a tip. how much would that effect low end??? would i just need to tune it w/ a power commander? thanks


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March 5, 2008, 08:28 PM

I'm pretty sure you could over come the issue with a power commander.


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March 6, 2008, 05:19 AM

How much back pressure do you need? None! Search for my earlier posts on this topic. If you keep the header and collectors the same, its very likely you will have little if any change in low end exhaust velocity, and your high end will likely improve...which may result in needing a p.c.

The reason why people think they need backprsssure is an exhaust optimized for a certain flow rate, exhibits increased backprsssure at every other flow rate...so a low-end tune means more backprsssure at high flows...which is bad! The ideal exhaust exhibits zero backprsssure at all flow rates (which is impossible)


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March 6, 2008, 06:18 AM

You're building a streetfighter. Don't worry about back pressure.
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March 6, 2008, 07:19 AM

Quote:
Originally Posted by nunyadambusiness
You're building a streetfighter. Don't worry about back pressure.
Um are you being serious? I have been doing research on pipes as of late (because I too am going to make my own pipes) and back pressure is necessary. Especially for a street-fighter. If I remember correctly "back pressure" = torque. A streetfighter without any torque is well... not much of a streetfighter.

You can buy plenty of universal baffles from places like DennisKirk.com (signing up for their free catalog is the best way to view them all) that you should be able to use. Baffles, again from what I have gathered in research, slows the exit of exhaust air thus creating "back-pressure" thus creating torque. {nevermind that they also help reduce the sound to a legal, or somewhat legal, level ===== which I could never understand how those cruiser guys get away with their pipes because they are in general much much MUCH louder than our sportbike exhaust}

Let me state though I am by NO MEANS WHATSOEVER an expert in this subject. I am just passing on what I have gotten from others pertaining to the subject. So I may I have mis-interpreted what I was told but that's just my two cents. If I am wrong, please correct me.


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March 6, 2008, 07:59 AM

Put in a few turns and be done with it. Back pressure is for losers. Back protectors are for winners:

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March 6, 2008, 08:35 AM

Quote:
Originally Posted by Husker4Life
Um are you being serious? I have been doing research on pipes as of late (because I too am going to make my own pipes) and back pressure is necessary. Especially for a street-fighter. If I remember correctly "back pressure" = torque. A streetfighter without any torque is well... not much of a streetfighter.

You can buy plenty of universal baffles from places like DennisKirk.com (signing up for their free catalog is the best way to view them all) that you should be able to use. Baffles, again from what I have gathered in research, slows the exit of exhaust air thus creating "back-pressure" thus creating torque. {nevermind that they also help reduce the sound to a legal, or somewhat legal, level ===== which I could never understand how those cruiser guys get away with their pipes because they are in general much much MUCH louder than our sportbike exhaust}
Absolutely, and totally wrong. I mean, look at it at the simplest form. The exhaust must leave the engine during the exhaust stroke (piston rising in cylinder). Backpressure makes this more difficult, the engine has to expend effort to push the exhaust out--REDUCING torque!

For more details see my post in this thread:
Making a short exhaust can. Streetfighter type mod question.

I swear, one of these days I'm gonna find all these "experts", chain them to a fence and throw stuff at them.


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Last edited by birdman; March 6, 2008 at 10:17 AM..
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March 6, 2008, 08:40 AM

I, too, will be there... throwing things at them... not because I'm pissed at thier misinformation, but because I am mean spirited.


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March 6, 2008, 04:35 PM

i see........ i didnt know if the lack of back pressure would cause me to foul a plug and i know w/ a car its horrible on the valves. so the overall assumption is that i can run headers and about 8 inches of mid pipe and be OK?


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March 6, 2008, 05:25 PM

Quote:
Originally Posted by saki-bomb
i see........ i didnt know if the lack of back pressure would cause me to foul a plug and i know w/ a car its horrible on the valves. so the overall assumption is that i can run headers and about 8 inches of mid pipe and be OK?
huh?! fouling plugs due to lack of backpressure? no. How would breathing easier result in rich running...unless you mean low backpressure (and thus high efficiency) at one rpm, that you are never using and therefore MORE backpressure at the rpm you ARE using which would make your efficiency crap at that point = less breathing = more rich = fouled plugs.

horrible on valves? um...well, you might have an engine that breathes easier, which would make you run lean, so that could be bad...but its not the lack of backpressure, its the lack of putting more fuel for your newfound increased breathing. And this is contrary to the other point.

Also, more backpressure = higher EGT = bad on valves.

For god's sake...BACKPRESSURE IS ALWAYS BAD, ALWAYS, FOREVER AND EVER AND EVER AND EVER AND EVER. If you could have an exhaust that would have zero backpressure (or negative) at all flow, it would be the ideal exhaust.

Seriously, where are you reading this stuff? because I have a bridge in NYC for sale...cheap, and I might use the readers as a mailing list to find suckers...err, buyers


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March 6, 2008, 05:53 PM

I totally hear Handels Messiah "FOREVER... AND EVER... AND EVER..."


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March 6, 2008, 05:54 PM

Forgive me for being skeptical here Birdman, but if what you're saying is true, that no back pressure is the best... then why run an exhaust at all? Why not just take off the headers and be done with it?
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March 6, 2008, 06:27 PM

Quote:
Originally Posted by Prizontatt
Forgive me for being skeptical here Birdman, but if what you're saying is true, that no back pressure is the best... then why run an exhaust at all? Why not just take off the headers and be done with it?

I'm glad you asked that! skepticism is always good.

The reason why is think about it this way, when the exhaust valve opens, there is a big pulse of air at high pressure, and it has to flow out of the cylinder and eventually into the air.

Since the pressure in the cylinder is >> normal air pressure, the exhaust would burp into the air with relatively little velocity (its not expanding at all, except into the air, which is an inefficient expansion).

By expanding the gas into a tube, which ideally, slowly increases in diameter, the pressure of the exhaust is converted into kinetic energy--speed, so that way, the pressure of the exhaust decreases until it gets to the end, and the velocity increases (the exhaust is cooling during this process). This way, the exhaust can leave the cylinder faster--the effective pressure decrease from inside the cylinder to the exit of the pipe is greater than if it were just an open port on the head.

This is a small effect compared to the cylinder to cylinder effect. Since the exhaust valves for each cylinder open at different times (in the rotation of the engine) what a header (4 into 1 or 2 into 1 depending on bike) does is since each exhaust pulse is followed by a low pressure (resulting from whats called a rarefraction wave that bounces back from the end of the pipe, but regardless of the technical reason, think of it as a high pressure and then a low pressure for each cylinder). By linking multiple exhaust ports together with the correct length of pipe, the low pressure pulse of one cylinder bouncing back can hit the exhaust valve of the next cylinder just as its valve opens...in effect "sucking" the exhaust out of that cylinder--so the pressure decrease from inside the cylinder to the exhaust (the greatest restriction) is the greatest.

problem is, this only works over a very narrow band of flow rates.

Basically, the closer the pulses are together in time, the shorter the header pipe needs to be, and the larger in diameter.

Which is where the whole confusion comes from. For really high flow rates, timing of the pulses is less important than just providing enough room for the exhaust to make it out--the velocity is already there. For lower flow rates, you want narrow pipes to keep the exhaust moving quickly (maintain exhaust velocity).

Thus, a narrow pipe header will provide better flow (lower backpressure) at LOWER flow rates---BUT since its a narrow pipe, it will create higher backpressure (more restriction) when you try to force more exhaust through it. Since this type of exhaust is more efficient at lower flow rates, but creates a restriction at high flow rates, it generates more efficiency "down low"--so more torque (since X hp at 2000rpm is 2x the amount of torque to make X hp at 4000rpm for example)...and high "backpressure" (at high rpm).

A larger exhaust pipe would have the opposite effect...

So...if you stated "high backpressure AT HIGH FLOW means more torque" you would be correct, assuming the backpressure was lower at LOW FLOW.

see---its the backpressure (the pressure necessary to get the exhaust out of the motor) at a SPECIFIC flow condition that needs to be lowered. since you can't have an infinite number of switchable exhaust pipes on a bike, you design an exhaust to optimize efficiency at a specific flow condition...on street bikes, this is typically at lower flows (more rideable low rpm performance) on a race bike, high flows (more power at peak power).

Exhaust power valves (EXUP, etc) allow kindof both...by effectively reducing the effective diameter of the pipe, they change both the resonance and (when set for low-rpm) increase the exhaust velocity of a small flow--more efficiency. When opened up at high flow, the pipe is effectively larger so high velocity at high flow = high efficiency.

The reason people remove the exhaust valves for racing is even in the open condition, they impart some resistance to flow at really really high flow rates, so removing them will improve peak power...at the expense of a peakier torque curve (not a big impact in racing as most of the time is spent in a narrow rpm range)

does this help?


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March 6, 2008, 06:31 PM

Quote:
Originally Posted by Prizontatt
Forgive me for being skeptical here Birdman, but if what you're saying is true, that no back pressure is the best... then why run an exhaust at all? Why not just take off the headers and be done with it?

Also, look at the extreme motors...F1 in particular...basically, they have really stubby, very large diameter headers...and thats it.

Because they want zero, or negative (due to resonance) backpressure over a narrow rpm range at VERY high flow (remember, they are making close to 300+hp/l @ 19000rpm)

Top Fuel Dragsters...stubby pipes with no pipe to pipe mating---the flow concern here is just getting the exhaust out at no backpressure since the exhaust is the restriction (nitro allows ~2:1 to 3:1 A/F ratios vs ~10-12:1 for max power with gasoline...so most of the stuff going in the intake valve is liquid, not gas, but it all comes out as gas)

very high power aircraft engines (radial engines)...stubby single tube headers just to accelerate the exhaust flow out of the head


Marc
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March 6, 2008, 06:59 PM

Quote:
Originally Posted by birdman
Absolutely, and totally wrong. I mean, look at it at the simplest form. The exhaust must leave the engine during the exhaust stroke (piston rising in cylinder). Backpressure makes this more difficult, the engine has to expend effort to push the exhaust out--REDUCING torque!

For more details see my post in this thread:
Making a short exhaust can. Streetfighter type mod question.

I swear, one of these days I'm gonna find all these "experts", chain them to a fence and throw stuff at them.
Do me a favor and read this post Certified tech will do work (page 14 post # 210) and toss the BS flag if it is indeed BS (by the way my post was number 209.


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Blessed is he who, in the name of charity and good will, shepherds the weak through the valley of the darkness.
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