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Author Topic: Exhaust Back Pressure  (Read 409 times)

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Offline BuRP

Exhaust Back Pressure
« on: November 14, 2021, 06:53:46 am »
Often touted as 'one needs it' or 'crucial', any exhaust will build up at least some of this pressure.
I found a vid that shows how much of it you need....





.... as little as possible!
QED


Oh sure, the "test" of the vid is full of holes but let's not go there lol, or... by all means do?
I'll get the popcorn out already  :thumleft:
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Offline VaalBaas

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Re: Exhaust Back Pressure
« Reply #1 on: November 14, 2021, 09:01:01 am »
Ook niskierig

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Offline Gene

Re: Exhaust Back Pressure
« Reply #2 on: November 14, 2021, 09:45:14 am »
Most effective back pressure is for two strokes, not 4 stroke engines
 

Offline Cracker

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Re: Exhaust Back Pressure
« Reply #3 on: November 14, 2021, 10:00:18 am »
Without back pressure, my ride would choke ...................
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Offline 2StrokeDan

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Re: Exhaust Back Pressure
« Reply #4 on: November 14, 2021, 10:34:44 am »
It depends on the state of tune of an engine.

My friend's 350 Goldstar is so tuned that he needs to put his boot tip over the exhaust to create backpressure until the revs climb.

Think about that.
 

Online Dux

Re: Exhaust Back Pressure
« Reply #5 on: November 14, 2021, 11:04:34 am »
It depends on the state of tune of an engine.

My friend's 350 Goldstar is so tuned that he needs to put his boot tip over the exhaust to create backpressure until the revs climb.

Think about that.

That is not due to back pressure but tuned length , if he runs an exhaust with a different tuned length it will be fine at lower rpm but will then lose power at higher rpm . Unfortunately those old engines had crude combustion chamber designs which made them more sensitive to things like exhaust design .
The important thing to remember is that back pressure is a load of bullshit , the efficiency of an exhaust is dependant on diameter , length and changes in diameter from exhaust port to the silencer . As the exhaust valve opens it releases an energy pulse , at each change in diameter or section there is a reflected energy pulse sent back down the exhaust , when timed correctly these pulses can be utilised to remove more exhaust gasses from the combustion chamber while also helping to draw in more air fuel mixture to the combustion chamber . This naturally helps with good power deliver , but the reverse is also possible where the reflected pulses are so out of sync that they can either push exhaust gasses back into the combustion chamber or even worse push air fuel mixture back down the inlet port again , both of these cases lead to reduced combustion efficiency and reduced power , usually felt as a flat spot . By putting the shoe over the Goldstars silencer they have artificially changed the tuned length which improves the running at very low rpm .
Essentially 4 stroke exhausts work on similar principles to 2:stroke expansion chambers , so not unusual to see tapered headers or even tapered sections further down the exhaust system , even reverse tapers in some sections . Exhaust design is not as simple as people tend to think .
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Offline 2StrokeDan

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Re: Exhaust Back Pressure
« Reply #6 on: November 14, 2021, 12:10:22 pm »
It depends on the state of tune of an engine.

My friend's 350 Goldstar is so tuned that he needs to put his boot tip over the exhaust to create backpressure until the revs climb.

Think about that.

That is not due to back pressure but tuned length , if he runs an exhaust with a different tuned length it will be fine at lower rpm but will then lose power at higher rpm . Unfortunately those old engines had crude combustion chamber designs which made them more sensitive to things like exhaust design .
The important thing to remember is that back pressure is a load of bullshit , the efficiency of an exhaust is dependant on diameter , length and changes in diameter from exhaust port to the silencer . As the exhaust valve opens it releases an energy pulse , at each change in diameter or section there is a reflected energy pulse sent back down the exhaust , when timed correctly these pulses can be utilised to remove more exhaust gasses from the combustion chamber while also helping to draw in more air fuel mixture to the combustion chamber . This naturally helps with good power deliver , but the reverse is also possible where the reflected pulses are so out of sync that they can either push exhaust gasses back into the combustion chamber or even worse push air fuel mixture back down the inlet port again , both of these cases lead to reduced combustion efficiency and reduced power , usually felt as a flat spot . By putting the shoe over the Goldstars silencer they have artificially changed the tuned length which improves the running at very low rpm .
Essentially 4 stroke exhausts work on similar principles to 2:stroke expansion chambers , so not unusual to see tapered headers or even tapered sections further down the exhaust system , even reverse tapers in some sections . Exhaust design is not as simple as people tend to think .



Dux, tuned length is simply backpressure provided at a certain range in an engine's revs. My friend can tune the length of the pipe to have decent midrange, but his engine won't then rev out.

It's like 2stroke pipes, revvers and grunters.

Best example of "backpressure" working is a 2stroke in it's powerband.  4strokes are the same, just much less so.

Another example is Yamaha's "EXUP" valve, simply avariable restriction in the exhaust to create more back pressure in the midrange, for an engine with lots of valve overlap.
« Last Edit: November 14, 2021, 12:11:50 pm by 2StrokeDan »
 

Online Dux

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Offline 2StrokeDan

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Re: Exhaust Back Pressure
« Reply #8 on: November 14, 2021, 01:29:46 pm »
https://www.cycleworld.com/how-do-motorcycle-exhaust-pipes-work/


I think we are on the same page, my "back pressure" is your "positive return wave". :thumleft:
 
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Offline jaybiker

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Re: Exhaust Back Pressure
« Reply #9 on: November 14, 2021, 04:51:35 pm »
It needs to be mentioned that the same pressure wave resonance applies in induction systems too and for optimum power both inlet and exhaust need to be harmonised.

As has been said it's a very complex business and up until the 60's/70's it was more a dark art than a science with a few inspired geniuses who did get it something like right by their uncanny instinct.

It became a science through the work of a professor of engineering at Queens University in Belfast, Dr. Gordon Blair. He started using computers to perform the almost infinite number of calculations required which would otherwise occupy an army of human mathematicians.

Yamaha took an interest in his work and funded his research programs. They benefitted more than somewhat for their generosity!   :3some:
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Offline BuRP

Re: Exhaust Back Pressure
« Reply #10 on: November 15, 2021, 10:25:37 am »
The important thing to remember is that back pressure is a load of bullshit

Best remark thus far!  :thumleft: :thumleft: :thumleft:
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