Exhaust "back pressure"....Thoughts?

[dwil4]

been doing some research and have saw mixed reviews. Mainly interested in 4 stroke's. Lots of people say a bigger moreopen pipe will = more power. Some guys say no that's not true because u lose "back pressure". I am sure that obviously u can go too big and that would harm performance but I know that u usually will have to re-jet if u open things up. Obviously we know a less restricted pipe will help with some power as is the reason guys went with the powroll or now xr's only pipe as they were a larger diameter which ='s more air flow. Im sure it prob affects different ranges of the bikes performance also. Hopefully u fellas with direct experience in this process will have some better info. If u are not gonna be serious and are gonna post some smart azz remark then please spare us and keep it to yourself. Thanks Guys.

[Wild Alaskan]

Here is what you want for maximum performance exhaust.on a four stroke. In case you don't understand more exhaust velocity generally equals better scavenging and more power.

 

1. You want minimum back-pressure

2. you want maximum exhaust velocity.

 

So a smaller pipe will give you better velocity, but will cause back-pressure at higher rpms/loads

 

A larger pipe will result in poor velocity at low rpms/loads, and better velocities and minimal back pressure at high rpms/loads

 

A restrictive muffler that causes back pressure will only ever hurt preformance.

[BushPig]

Nothing to add. Well summed up.

[Fatt_Tones]

been doing some research and have saw mixed reviews. Mainly interested in 4 stroke's. Lots of people say a bigger moreopen pipe will = more power. Some guys say no that's not true because u lose "back pressure". I am sure that obviously u can go too big and that would harm performance but I know that u usually will have to re-jet if u open things up. Obviously we know a less restricted pipe will help with some power as is the reason guys went with the powroll or now xr's only pipe as they were a larger diameter which ='s more air flow. Im sure it prob affects different ranges of the bikes performance also. Hopefully u fellas with direct experience in this process will have some better info. If u are not gonna be serious and are gonna post some smart azz remark then please spare us and keep it to yourself. Thanks Guys.

 

 

Can't attest to four strokes, but it's my understanding that the more restrictive an exhaust becomes the more power loss you will experience.  My son ran a screen type spark arrestor on his kx-125 this past summer and it plugged up a few times.  When this happened the bike had noticeable power loss.  It behaved as though it was rich.  On that same note I have had exhaust leaks which I think means more flow, and it also results in a noticeable loss of power.  So it could be that the change in exhaust flow causes the jetting to be wrong and thus causes power loss in any case where the jetting isn't adjusted to compensate for the change. 

 

So here's what I am getting at.  Most riders couldn't jet their way out of a wet paper bag.  Changing an exhaust normally requires a jetting change to reap the benefits of the performance exhaust.  Therefore, the reason you may have so many conflicting opinions is because so many people just slap on a new exhaust without giving any thought to jetting. 

 

But generally speaking I believe more flow = more power, but only up to a point...remove the exhaust altogether = max flow, but power like a turd and no amount of jetting will help....

[dwil4]

Thanks Wild Alaskan. That is what my limited experiences have shown me as well on the couple of bikes I have changed exaust on.

[dwil4]

Can't attest to four strokes, but it's my understanding that the more restrictive an exhaust becomes the more power loss you will experience.  My son ran a screen type spark arrestor on his kx-125 this past summer and it plugged up a few times.  When this happened the bike had noticeable power loss.  It behaved as though it was rich.  On that same note I have had exhaust leaks which I think means more flow, and it also results in a noticeable loss of power.  So it could be that the change in exhaust flow causes the jetting to be wrong and thus causes power loss in any case where the jetting isn't adjusted to compensate for the change. 

 

So here's what I am getting at.  Most riders couldn't jet their way out of a wet paper bag.  Changing an exhaust normally requires a jetting change to reap the benefits of the performance exhaust.  Therefore, the reason you may have so many conflicting opinions is because so many people just slap on a new exhaust without giving any thought to jetting. 

 

But generally speaking I believe more flow = more power, but only up to a point...remove the exhaust altogether = max flow, but power like a turd and no amount of jetting will help....

I believe u hit the nail on the head with that about un-knowledgable jetting. Yea no pipe would = no scavenging affect.

[highmarker]

With the available head pipes and mufflers being too big and velocity isn't going to be an issue. Also, sadly with fourstrokes louder is usually going to make power. You can almost "hear" how much more main jet it'll need on a pipe swap just by the increase in sound.

[CalKirley]

*Edit - didn't read closely enough. Disregard the following.

 

 

Referring to exhaust velocity - why is it that you don't want higher exhaust velocity as opposed to a greater mass flow? The kinetic energy of a gas is described as 1/2MV^2, where V is the velocity of the gas and M is the mass. With a higher kinetic energy, wouldn't there be more of a scavenging effect as opposed to the larger mass flow?

 

Also, is there an equation for calculating backpressure in turbulent flows such as exhaust gasses?

Edited December 17, 2014 by baseballpro3112

[ScootMcGoot]

Sounds like a dirt bike exhaust could benefit from one of those adjusting nozzles like you find on the back of a fighter jet.  Linked to the rpm of the engine it opens as the bike revs higher then you have the best of both worlds, high velocity at low RPMs but minimal backpressure in the high RPM range.

[kx450f63]

been doing some research and have saw mixed reviews. Mainly interested in 4 stroke's. Lots of people say a bigger moreopen pipe will = more power. Some guys say no that's not true because u lose "back pressure". I am sure that obviously u can go too big and that would harm performance but I know that u usually will have to re-jet if u open things up. Obviously we know a less restricted pipe will help with some power as is the reason guys went with the powroll or now xr's only pipe as they were a larger diameter which ='s more air flow. Im sure it prob affects different ranges of the bikes performance also. Hopefully u fellas with direct experience in this process will have some better info. If u are not gonna be serious and are gonna post some smart azz remark then please spare us and keep it to yourself. Thanks Guys.

 

What research have you been doing and can we see it?

 

I say try this one... dyno a current mx 4 stroke bike with the exhaust in tact then remove the exhaust entirely, repeat test run, then try it with just the head pipe, and compare results.  I'm guessing you will have your answer.

 

The problem is, the option that makes the most power is not socially accepted. 

[dwil4]

let me clarify research... my research was internet reading. lol. If u google "back pressure in a 4 stroke" u can also see the research. Haha

[EnglertRacing]

back pressure was made up by some idiot named dumbo who had the assumption bigger was better then found out this wasn't the case perplexed and cross eyed he came up with the word back pressure and figured an engine needed it.  dumbo never grasped the idea of velocity

 

you size an exhaust for optimal velocity at the desired rpm.  the diameter hopefully is working in conjunction with a tuned length also aimed at a desired rpm.  working together they create a depression in the combustion chamber as low as -7psig. 

so header A. on a crf450 thats 2.5" diameter and has a length of 26" tuned for 8000rpm produces a theoretical bs scavenging signal of -2psi

while header B. thats 1.625" diameter and has a tuned length of 26" tuned for 8000rpm produces a theoretical bs scavenging signal of -6psi

 

 

so which one is it thats actually got the back pressure here.

spit that word out and stomp on it.

quit spreading it.

Edited December 17, 2014 by EnglertRacing

[EnglertRacing]

What research have you been doing and can we see it?

 

I say try this one... dyno a current mx 4 stroke bike with the exhaust in tact then remove the exhaust entirely, repeat test run, then try it with just the head pipe, and compare results.  I'm guessing you will have your answer.

 

The problem is, the option that makes the most power is not socially accepted. 

 

without a header on the engine it would run like DOG S....T

look up tuned length

bolt this on

itll make 5hp on a pc or fmf

and it makes about 10hp more than a fmf or pc on over rev

 

 

megaphones with the contest,

they have no defied tuned length

this one looks to run well from 5-10000 rpm

with a megaphone you have a header thats 20" long

and 21, and 22, and 23 . and so on till about 36"

every point along the diffuser is a reflection point

they create a lower amplitude longer duration scavenging signal that works over a wide rpm range without overscavenging

 

 

while a traditional header with no taper proveds a high amplitude short duration scavenging reflection

so basically  the header only syncs up to the motor at 1 rpm like 6820rpm and works slightly okay for a like 200 hundred rpm before and after this number

 

 

hooray for megaphones

heres johnny campbells baja 1000 winner

thats a pro circiut

strange that ones not what they sell off the shelf??!!??

also look at the diameter of the core

 

 

and if you wanna bring up reverse cones as proof for backpressure,

no they speed up the flow before the exit and act as a barrier to air trying to enter and come towards the precious low pressure reflection the megaphone generates

 

 

 

my opinions come from studying / building exhausts for 15 years now

starting with two strokes got my formulas form blair, bell, and jennings

then stock cars

and now 4 stroke singles

her is the best pipe ive done so far

we went through 8 pipe designs on the car

a dozen end cap angles and diameters

this pipe is actually has a 1/4" smaller diameter header than anyone elses thats 70+hp through a 1 5/8" od header into antireversion chamber and 1 3/4" tube.

results,

complete demolition of the field,

by my 15 year old brother weighing 215lbs

vs adults some as light as 140lbs

Edited December 18, 2014 by EnglertRacing

[dwil4]

lots of info and thoughts here. thanks for all the replies fellas

[kx450f63]

Are there any published reports of tests of power output without an exhaust other than the port and runner? 

 

I assume there would be a combination that does not benefit from any external restriction other than the atmosphere.

 

Tuned length refers to the fact you are working within a specific set of parameters.  Such as, I could use a little extra scavenging effect around 6000 rpm to help with exiting a turn and I am willing to give up some at 4000 or 8000rpm to make that happen.

 

How do you think the exhaust system in pic #1 would handle coming up short on a double?  My point is, there are limitations in design due to certain disciplines.  Do you think you could make that work on an mx bike and meet the regulations?

 

I agree that system in pic #1 most likely works best for its discipline.  Could it make more peak power without any restrictions and if you are willing give up a little in certain areas?

 

My experience is only in trial and error and have not studied the subject much. 

Why do you think the most powerful internal combustion engines in the world use just enough pipe at the end of the exhaust port to get the nasty stuff away from the engine?

[KJ790]

Are there any published reports of tests of power output without an exhaust other than the port and runner? 

 

I assume there would be a combination that does not benefit from any external restriction other than the atmosphere.

 

Tuned length refers to the fact you are working within a specific set of parameters.  Such as, I could use a little extra scavenging effect around 6000 rpm to help with exiting a turn and I am willing to give up some at 4000 or 8000rpm to make that happen.

 

How do you think the exhaust system in pic #1 would handle coming up short on a double?  My point is, there are limitations in design due to certain disciplines.  Do you think you could make that work on an mx bike and meet the regulations?

 

I agree that system in pic #1 most likely works best for its discipline.  Could it make more peak power without any restrictions and if you are willing give up a little in certain areas?

 

My experience is only in trial and error and have not studied the subject much. 

Why do you think the most powerful internal combustion engines in the world use just enough pipe at the end of the exhaust port to get the nasty stuff away from the engine?

 

If you could make power with just an exhaust port then Formula 1 and top fuel dragsters would be running this setup. The fact is that to make the most power (especially at high RPM's) you need more length to the exhaust tract than just the exhaust port. The exhaust pulse still needs to be in the exhaust pipe when the exhaust valve opens for the next event to create a scavenging effect, which increases power. If the exhaust is too short then each pulse will exit the end of the exhaust before the next exhaust event and you will not have scavenging.

[grayracer513]

If you could make power with just an exhaust port then Formula 1 and top fuel dragsters would be running this setup. The fact is that to make the most power (especially at high RPM's) you need more length to the exhaust tract than just the exhaust port. The exhaust pulse still needs to be in the exhaust pipe when the exhaust valve opens for the next event to create a scavenging effect, which increases power. If the exhaust is too short then each pulse will exit the end of the exhaust before the next exhaust event and you will not have scavenging.

 

The fact is that fuel dragsters very nearly do use just the port.  With the blower in place at the boost levels being run on those engines, acoustic tuning of the exhaust is both meaningless and actually impossible.  The pipes you see them run now are as big in diameter as practicable, and only long enough to get the exhaust clear of the car, and/or blow the smoke off the rear tires.  The idea is just to let it go away with as little impedance as possible.

 

But without a supercharger, the way it works is that the exhaust valve opens, and a pressure wave moves down the length of the pipe.  On reaching the end, the abrupt expansion of the gasses into the open air causes a "reflected" negative pressure wave (partial vacuum) to travel back up the pipe toward the engine.  The length of the pipe is such that this wave of low pressure gets back to the exhaust valve just as the intake valve begins to open, so that it helps to remove the last bit of exhaust from the chamber and also aids the start of the intake flow.

 

Megaphones modify this effect somewhat.  If you want to really dig into the matter on a scientific level, you should look for the works of Dr. Gordan Blair on the subject. 

[kx450f63]

Oh ya forgot to mention... cool vid.  I'm not taking anything away from your bro. or your engine but it looks like the competition could use some driving lessons!

 

...and your engine looks like it needs valve guides or seals at the least.

 

Do you think exhaust design has anything to do with cam specs.?

 

Ya Gray, guess I forgot about the blowers on the fuelers.

[EnglertRacing]

Oh ya forgot to mention... cool vid. I'm not taking anything away from your bro. or your engine but it looks like the competition could use some driving lessons!

...and your engine looks like it needs valve guides or seals at the least.

Do you think exhaust design has anything to do with cam specs.?

Ya Gray, guess I forgot about the blowers on the fuelers.

It's on its last overbore its a two ring piston the only ring pack avaliable at the time was a std top ring and napier second... No 3 piece oil rail. Smoked like crazy used a ton of oil... Dirty chamber.

The piston it has now has a 3 piece oil ring not a single puff of smoke doesn't use a drop of oil.

Several of the drivers actually drive real Sprint cars....

The white 7 car's driver owns the track, builds the cars, builds Sprint cars (777 sprint cars) and won a few 410 championships at victorville speedway.

But that night my brother destroyed them.

And yes cam timing has a ton to do with length and diameter. EVO, IVO, and EVC all change the pipe

As well as compression ratio, valve diameter, port cross section, even the intensity of the cam (number of degrees from .050" lift to 0.200"lift.)

And also intake size, intake restricted engines have different requirements than ones with plenty of intake port.

[EnglertRacing]

The fact is that fuel dragsters very nearly do use just the port.  With the blower in place at the boost levels being run on those engines, acoustic tuning of the exhaust is both meaningless and actually impossible.  The pipes you see them run now are as big in diameter as practicable, and only long enough to get the exhaust clear of the car, and/or blow the smoke off the rear tires.  The idea is just to let it go away with as little impedance as possible.

 

But without a supercharger, the way it works is that the exhaust valve opens, and a pressure wave moves down the length of the pipe.  On reaching the end, the abrupt expansion of the gasses into the open air causes a "reflected" negative pressure wave (partial vacuum) to travel back up the pipe toward the engine.  The length of the pipe is such that this wave of low pressure gets back to the exhaust valve just as the intake valve begins to open, so that it helps to remove the last bit of exhaust from the chamber and also aids the start of the intake flow.

 

Megaphones modify this effect somewhat.  If you want to really dig into the matter on a scientific level, you should look for the works of Dr. Gordan Blair on the subject. 

 

 

top fuel is a different animal

they cant open the exhaust valves when they want to, the pressure is so high that it bends the pushrods when evo is too early! they have to open them far later than they would like to

i've heard numbers like 800lbs of down force from the exhaust.

and that when they drop a cylinder or two on one side that the car actually turns from lack of exhaust force on one side.

but yes their exhausts are basically just to get it out