[jkobflys]

Hello everyone,
i'm in the process of instaling aftermarket pipes on the rc,
i've been looking everyware for something nice and diferent but no luck,
the coolest pipes i found where from ARC Fabrication they are undertail pipes, but i cant see myself loosing the precius tunk space of this bike, so i have decided to fab my own set,
I found a cool pipe from comp werkes i will use these pipes and fab the the rest of the conections. i'n not savy when it comes to tunning a motor and i have some concern maybe you can help.
1. will the bike run shitti with just the header to the pipe ?
will i loose back pesure?
2.other than the noise what other drawbacks do you guys see?

apreciate the input.

[saf]

I make my own and customers slip on systems for custom one off stuff, they arent so hard (nothing like headers), but keep in mind that no engine wants or needs backpressure. Will you loose some, maybe, and that is only a good thing.

[m0nSteR TwIn]

Originally Posted by saf

I make my own and customers slip on systems for custom one off stuff, they arent so hard (nothing like headers), but keep in mind that no engine wants or needs backpressure. Will you loose some, maybe, and that is only a good thing.

I have stopped counting the number of times I've had the "Back Pressure" discussion.

There is no such thing as backpressure. PERIOD! That is a word made up by people who didn't know what they were talking about to describe a phenomenon in fluid mechanics that they did not understand.

A properly tuned exhaust system requires the correct length and size of tubing to create favorable harmonics, such that, at the desired engine speed, exhaust pulses resonate in such a way as to assist the evacuation of the next exhausted air/fuel charge. This is scavenging and it is the desired effect of any exhaust system. An overly large tube diameter will allow the gas to expand too quickly as it leaves the exhaust valve area, depleating it's energy and slowing it's flow throught the system, which in turn destrys the scavenging effect. An excessively short pipe will cause exhaust pulses to collide, destroying harmonics, which in turn destroys the scavenging effect. Too long and the pulses will slow down too much before reaching the exit and the pulse following will have to actually push the one in front out of the system...again very bad. A completely free flowing exhaust system offers the least resistance to flow, but creates almost no scavenging effect and is thus poor except for very large displacement engines operating at wide open throttle all the time...i.e. Drag racers.

As far as building your own. If you've got patience and a dyno and lots of pipe and welding supplies you could definitely do it. Trial and error...that's all you can do.

[m0nSteR TwIn]

This is a post on the subject from a few years ago on another forum on the topic. I"m Wytishop on that board.

MX-3.com :: View topic - Backpreasure myth

[jkobflys]

thanks for the info,
there hapens to be a local shop with a dyno as fa as the welding i'll do my self and go from there, i'll defenetly do my reaserch on the subject and try to start off with something close.

[saf]

Originally Posted by m0nSteR TwIn

I have stopped counting the number of times I've had the "Back Pressure" discussion.

There is no such thing as backpressure. PERIOD! That is a word made up by people who didn't know what they were talking about to describe a phenomenon in fluid mechanics that they did not understand.

I agree with you, on the fact that backpressure is not wanted but scavenging is. People confuse this all too often, and if thats what you mean, then i agree about backpressure. However, there IS a thing as backpressure. However its not wanted in a performance application.

Backpressure is any force applied to the piston to move a gas out of the chamber, and down the pipe. As you may know by now, as a gas leaves the head and runs thru the header it has already started a massive cooling process, this in itself isnt good, and EGT's can fall hundreds of degrees in only a foot of exhaust pipe.

Heat loss is another massive issue that is helped along by too large a exhaust where gas velocity falls, is cooled, becomes denser, slows even more, cools, becom .... you get the drift till at some stage the gas in the header is pusing the midpipe and the midpipe is pushing the tail end gas. This is a flow restriction, or backpressure.

Manufacturers run reflection plate type mufflers in cars, to send sound waves back down towards the exhaust valve, making backpressure, and keeping the system quiet. The stock 954 muffler, from pipe to large chamber, to small pipe. This is a physical trap for good flowing EGT's to cool and force backpressure into the system.

Anyways, its late, but its there. NOT THAT ITS WANTED in a perf engine, or a engine that should perform that doesnt conform to DOT EPA noise DBA regs. Long story short for those that dont know, if a engine for eg fires on 1324 firing order, when the exhaust pulse reaches the collector from cyl 1, cyl 3 will be saying "get out" and the draft in the collector will aid in this scavenging. Cyl 3 will help cyl 2 and so on.

But again this is dependent on engine rpm, degrees of ign retardation or advance and what EGT's it runs, hence the velocity, and even if your riding in the wet or dry. The header will work differently when your riding on a 20c deg day in the dry and a 20c deg day in the wet and the headers are being cooled with spray off the front wheel. The exhaust gas is cooled and will move your "power" of the header around in the RPM scale.

[m0nSteR TwIn]

Originally Posted by saf

I agree with you, on the fact that backpressure is not wanted but scavenging is. People confuse this all too often, and if thats what you mean, then i agree about backpressure. However, there IS a thing as backpressure. However its not wanted in a performance application.

Backpressure is any force applied to the piston to move a gas out of the chamber, and down the pipe. As you may know by now, as a gas leaves the head and runs thru the header it has already started a massive cooling process, this in itself isnt good, and EGT's can fall hundreds of degrees in only a foot of exhaust pipe.

Heat loss is another massive issue that is helped along by too large a exhaust where gas velocity falls, is cooled, becomes denser, slows even more, cools, becom .... you get the drift till at some stage the gas in the header is pusing the midpipe and the midpipe is pushing the tail end gas. This is a flow restriction, or backpressure.

Manufacturers run reflection plate type mufflers in cars, to send sound waves back down towards the exhaust valve, making backpressure, and keeping the system quiet. The stock 954 muffler, from pipe to large chamber, to small pipe. This is a physical trap for good flowing EGT's to cool and force backpressure into the system.

Anyways, its late, but its there. NOT THAT ITS WANTED in a perf engine, or a engine that should perform that doesnt conform to DOT EPA noise DBA regs. Long story short for those that dont know, if a engine for eg fires on 1324 firing order, when the exhaust pulse reaches the collector from cyl 1, cyl 3 will be saying "get out" and the draft in the collector will aid in this scavenging. Cyl 3 will help cyl 2 and so on.

But again this is dependent on engine rpm, degrees of ign retardation or advance and what EGT's it runs, hence the velocity, and even if your riding in the wet or dry. The header will work differently when your riding on a 20c deg day in the dry and a 20c deg day in the wet and the headers are being cooled with spray off the front wheel. The exhaust gas is cooled and will move your "power" of the header around in the RPM scale.

My point is, and has been for years....IT'S NOT BACKPRESSURE!!! The word "backpressure" has no meaning in this application. "Flow restriction" is a much more descriptive term and you used them both in the same sentence. But they aren't the same thing. When the muffler reflects the pressure wave back to interfere with the next wave coming down the line, it's called "Destructive Wave Interference" not backpressure. When the gas pulses slow downa nd bunch up in the tail end of the exhaust, that's not backpressure either, that's a poorly designed pipe with bad flow characteristics, but the gas is still ALL moving away from the engine.

If you insist on calling it backpressure I will agree that too much it's bad. Some flwo control is necessary to create the desired harmonics in the header, but too much kills the scavenging effect. The word backpressure though, from a Thermodynamic and Fluid Mechanic perspective, has no meaning and should not be used.

[CBR929RE]

Originally Posted by saf

I

Backpressure is any force applied to the piston to move a gas out of the chamber, and down the pipe.

that sentence contradicts itself. its true back pressure is a force applied to the piston but it doesn't move gas out of the chamber down the pipe. it does the opposite. Back pressure in an internal combustion engine occurs when exhaust gas reenters the combustion chamber. this can be caused by the downward suction of the piston if the pressure in the exhaust pipe isn't great enough to keep the exhaust gases going in the intended direction. its more of a problem in poor performance, low rpm engines.

and yes it is bad because reburning those gases doesn't give as much power as fresh air/fuel.

[saf]

Originally Posted by m0nSteR TwIn

My point is, and has been for years....IT'S NOT BACKPRESSURE!!! The word "backpressure" has no meaning in this application. "Flow restriction" is a much more descriptive term and you used them both in the same sentence.

Fair enuf on that, i see what your saying and were thinking the same thing.

The only reason i use the term backpressure is im a engine builder by trade, tuner and fabricator of items such as turbo manifolds and so forth. Ive been taught from day one, that at some point the engine will outflow a OEM exhaust and this is called baskpressure.

Turbines to function correctly they need a manifold that will allow the collection of backpressure along with a correct A/R & trim to sustain backpressure for the turbine to spin and generate boost on the front side. Anytime you need to make a bike or car quiet by slipping a disk between a join infront of the muffler, your creating backpressure and a by product, quieter note. This is what ive been taught with ...

Dictionary.com
back·pres·sure [bak-presh-er] –noun /ˈbækˌprɛʃhttp://cache.lexico.com/dictionary/g...una/thinsp.pngər/ Pronunciation Key - Show Spelled Pronunciation
a resistant pressure exerted by liquid or gas against the forward motion or flow of an exhaust or pipe system: Careful control of backpressure ensures an even supply of oil from the well.
Also, back pressure.



American Heritage Dictionary - back·pres·sure

Residual pressure opposing the free flow of a gas or liquid, as in a pipe or an exhaust system.
(bāk'prěsh'ər)

Wikipedia :
Backpressure usually refers to the pressure exerted on a moving fluid by obstructions or tight bends in the confinement vessel along which it is moving, such as piping or air vents, against its direction of flow. For example, an automotive exhaust muffler with a particularly high number of twists, bends, turns and right angles could be described as having particularly high backpressure. Back pressure in the exhaust sense of the term, is usually termed as being a "bad thing" for performance; however, in the interest of reducing exhaust sound to levels allowable by public noise ordinances, back pressure can be regulated using systems from simple butterfly valves to fully computer controlled units sensing pressure in the exhaust pipe itself.

BUT, you can look at it the way you say it. A muffer is a flow restriction, a non mandrel bend is a flow restriction, incorrect welds that protrude into the exhaust and even a turbo is a flow restriction. I just havent looked at it like that, probably cause ive never heard it said like that.

Originally Posted by CBR929RE

that sentence contradicts itself. its true back pressure is a force applied to the piston but it doesn't move gas out of the chamber down the pipe. it does the opposite. Back pressure in an internal combustion engine occurs when exhaust gas reenters the combustion chamber. this can be caused by the downward suction of the piston if the pressure in the exhaust pipe isn't great enough to keep the exhaust gases going in the intended direction. its more of a problem in poor performance, low rpm engines.

No thats not backpressure, thats a combination of camshaft overlap and duration but that is vacuum issue (as you see you have said aswell in your first underscore) when the piston draws fresh air on inlet stroke straight after the exhaust stroke. Thats the sole reason for camshaft overlap, is so you can draw some fresh air and fuel thru the chamber and out past the exhaust to enure you have as much burnt exhaust gas out of there as possible.

But there shouldnt be exhaust gas re-entering the chamber even in a low perf engine, be it a GN-250 or a 78 Toyota Corolla. If you have burnt gas in the chamber, you have no go. And the second underscore is exactly what backpressure or flow restriction is, and is not wanted nor needed. Pressure or force in a opposite direction does not aid in flow.

If you turn on a garden hose and put your finger over the end, you have not created more flow by bumping up the pressure in the line from 0 to say 4psi , you have made only a greater velocity at the outlet or restriction (think of a venturi in a carb) so by having more pressure in the exhaust you are not going to be making this re-entry as you call it a lesser problem.

[CBR929RE]

Originally Posted by saf

No thats not backpressure, thats a combination of camshaft overlap and duration but that is vacuum issue (as you see you have said aswell in your first underscore) when the piston draws fresh air on inlet stroke straight after the exhaust stroke. Thats the sole reason for camshaft overlap, is so you can draw some fresh air and fuel thru the chamber and out past the exhaust to enure you have as much burnt exhaust gas out of there as possible.

But there shouldnt be exhaust gas re-entering the chamber even in a low perf engine, be it a GN-250 or a 78 Toyota Corolla. If you have burnt gas in the chamber, you have no go. And the second underscore is exactly what backpressure or flow restriction is, and is not wanted nor needed. Pressure or force in a opposite direction does not aid in flow.

If you turn on a garden hose and put your finger over the end, you have not created more flow by bumping up the pressure in the line from 0 to say 4psi , you have made only a greater velocity at the outlet or restriction (think of a venturi in a carb) so by having more pressure in the exhaust you are not going to be making this re-entry as you call it a lesser problem.

ok but when there's cam overlap aren't there some situations where you can suck exhaust back into combustion chamber in addition to the new air/fuel you're trying to suck in? isn't that also back pressure? generally yes the exhaust will go out but what if there's a stuck valve or one that doesn't close all the way allowing the gas to return. from what I always understood its back pressure pushing the exhaust backwards. I know the idea is to get the correct amount of overlap where the new stuff pushes the old stuff out.

[gearhead14]

Actually the purpose of overlap is to use the momentum of the exhaust gasses leaving the cylinder to initiate the pull of the air fuel mixture into the cylinder using the scavange effect. The tuning of an exhaust system to the flow characteristics of the motor uses harmonics to maximize this effect, the same as tuning an air box creates high pressure when the intake valves are open, you want the exhaust to be at low pressure when the exhaust valves are open. Remember that the cylinder should be close to evacuated of exhaust gasses when the intake starts to open, but the piston has not started downward travel yet. The intake opens several degrees before TDC.

[saf]

Originally Posted by CBR929RE

ok but when there's cam overlap aren't there some situations where you can suck exhaust back into combustion chamber in addition to the new air/fuel you're trying to suck in?

RE, No mate, what the inlet and exhaust valves do is flow the air out of the port (exhaust) as air is flowing towards the valve in the inlet , and into the valve on the exhaust , the flow keep its original path via scavenging. As the piston is at TDC, the exhaust valve is almost closed, so it wont have a chance to suck back burnt exhaust gas on the intake stroke.