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This space for rent
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I hear this stuff about how a motor requires back pressure to make power all the time, and no one has ever been able to explain why. Probably because it's a myth that's been around for a long time.
Since no one can explain it, but they always seem to want to argue with me about it, here's some outside experts.

Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth.
Tom Vago's Chrysler Website - Cloud Car Performance Exhaust FAQ


EXHAUST SYSTEMS - Be aware that exhaust systems do not "make HP." A more efficient system merely relieves excess restriction or back pressure if it exists.
Note: If someone ever tells you that "all exhaust systems need some backpressure," you are talking to the wrong guy.
http://www.kennebell.net/techinfo/dodge-jeep-chrysler/ptcruiser.pdf


Or do your own Google search for "exhaust restriction increases power?". I couldn't find anyone in the first five pages of search results that agreed with that statement.


And racers run mufflers because they are required to by the organizers regulations. And those were brought on by people complaining about the noise from racetracks. I raced motocross in the US and roadraced in the UK back in the late 60's and early seventies and no bike ran a muffler at the track, it was straight pipes, megaphones, and expansion chambers all the way.
 

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GHOSTRIDER
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I hear this stuff about how a motor requires back pressure to make power all the time, and no one has ever been able to explain why. Probably because it's a myth that's been around for a long time.
Since no one can explain it, but they always seem to want to argue with me about it, here's some outside experts.



Tom Vago's Chrysler Website - Cloud Car Performance Exhaust FAQ




http://www.kennebell.net/techinfo/dodge-jeep-chrysler/ptcruiser.pdf


Or do your own Google search for "exhaust restriction increases power?". I couldn't find anyone in the first five pages of search results that agreed with that statement.


And racers run mufflers because they are required to by the organizers regulations. And those were brought on by people complaining about the noise from racetracks. I raced motocross in the US and roadraced in the UK back in the late 60's and early seventies and no bike ran a muffler at the track, it was straight pipes, megaphones, and expansion chambers all the way.
+1 Thank you! Couldnt have said it better! Look at top fuel engines!
 

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Discussion Starter · #8 ·
I hear this stuff about how a motor requires back pressure to make power all the time, and no one has ever been able to explain why. Probably because it's a myth that's been around for a long time.
Since no one can explain it, but they always seem to want to argue with me about it, here's some outside experts.



Tom Vago's Chrysler Website - Cloud Car Performance Exhaust FAQ




http://www.kennebell.net/techinfo/dodge-jeep-chrysler/ptcruiser.pdf


Or do your own Google search for "exhaust restriction increases power?". I couldn't find anyone in the first five pages of search results that agreed with that statement.


And racers run mufflers because they are required to by the organizers regulations. And those were brought on by people complaining about the noise from racetracks. I raced motocross in the US and roadraced in the UK back in the late 60's and early seventies and no bike ran a muffler at the track, it was straight pipes, megaphones, and expansion chambers all the way.
I guss what I still do not get it that on this web page Dynojet Jet Kits - Offering Motorcycle Jet Kits & ATV Jet Kits
thay talk stage 1-7 jet kits . After stage 1 thay require aftermarket pipe .
Would Stright Pipe count as a good after market ?
So how would a stage 2 not work on stock pipes if they are truly not required .
 

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Navy Vet Search & Rescue
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Stage kits are just different sizes of jets for your carb. The higher the stage # the bigger the jets are. Your engine needs a balanced ratio of fuel to air. While the exhaust system is restricting the flow some, you are still only going to get a certain amount of flow increase even if you completely remove the exhaust. In order to increase flow any farther you will need to do things like modify the intake, port and polish the head, etc. Going to a larger stage kit without making other mods is just going to screw up your fuel to air ratio by putting in more fuel without increasing the air and that is just going to cause it to run too rich. You have to keep the fuel/air ratio balanced as you increase flow into and out of the engine.
 

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GHOSTRIDER
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Heres the "skinney" Karl, Welcome to the forum! Now, these aluminium piston,aluminium head bike engines are very heat sensitive and will melt down if theyre ran too lean and too hot too long! On the other hand, if theyre ran too rich, they foul plugs and begin building up carbon on pistons,valves and ports causing other problems Im not going to get into now but the idea is to find the "sweet spot" where the air fuel mixture gets completely burned without too much air(Lean) or too much fuel(rich). With that understood, whenever you add an "aftermarket" exhaust to your bike, that new pipe usually opens the exhaust up and gives less restriction all the way to the intake thus the engine is now drawing or trying to draw more air. This creates a "lean" condition that can best be addressed by dumping more fuel into the mix via "Bigger jets!" now then, thats two of the three changes that can be made, the third being necessary only if you've opted for the stage 3 "jumbo jets" and thats open up or do away with the restrictive air box and its filter and installing Pod filters allowing more air to mix with the extra fuel to be nearly completely burnt and pushed into the non-restricted exhaust...Wheew, all that just for a meaner sounding bike? He11 yeah! Farout! You need to learn how to "read" the plugs and get used to pulling those carbs at least a dozen times before you get it all right! And hey, keep checking ebay for some desent pipes because they will sound better than straight pipes! Get a service manual for your bike and prepare to check your valve clearances because thats critical routine maintainance if you want the money you dumped into the bike to pay off in perfomance! Keep that paperwork you got with the dynajet jet kit because its gospel! Good Luck! I Love an inline 4 screaming 9000rpms through an open header! But Im old and deaf...lol!
 

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Gents:

I used to be involved with all sorts of ICE efficiency tests. One of the efficiencies is "volumteric efficiency" and this is the work done to suck air and fuel into the combustion chamber and the work done to blow out the spent gases. The harder it is to blow out the spent gases, the more work is done and the volumteric efficiency goes down and so does the crank horsepower. It is right there in the math, but there are not many thermodynamicists left on the planet these days. That written, there can be some resonances that will actually produce a slight negative pressure wave at the exhaust port, and briefly "pull" at the exhaust port, but this is ususally at lower power and an 10 or 20 rpm band, which is of no use. Interestingly, did you know that the loud crackle of the exhaust is a sonic boom?
 

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Discussion Starter · #12 ·
Heres the "skinney" Karl, Welcome to the forum! Now, these aluminium piston,aluminium head bike engines are very heat sensitive and will melt down if theyre ran too lean and too hot too long! On the other hand, if theyre ran too rich, they foul plugs and begin building up carbon on pistons,valves and ports causing other problems Im not going to get into now but the idea is to find the "sweet spot" where the air fuel mixture gets completely burned without too much air(Lean) or too much fuel(rich). With that understood, whenever you add an "aftermarket" exhaust to your bike, that new pipe usually opens the exhaust up and gives less restriction all the way to the intake thus the engine is now drawing or trying to draw more air. This creates a "lean" condition that can best be addressed by dumping more fuel into the mix via "Bigger jets!" now then, thats two of the three changes that can be made, the third being necessary only if you've opted for the stage 3 "jumbo jets" and thats open up or do away with the restrictive air box and its filter and installing Pod filters allowing more air to mix with the extra fuel to be nearly completely burnt and pushed into the non-restricted exhaust...Wheew, all that just for a meaner sounding bike? He11 yeah! Farout! You need to learn how to "read" the plugs and get used to pulling those carbs at least a dozen times before you get it all right! And hey, keep checking ebay for some desent pipes because they will sound better than straight pipes! Get a service manual for your bike and prepare to check your valve clearances because thats critical routine maintainance if you want the money you dumped into the bike to pay off in perfomance! Keep that paperwork you got with the dynajet jet kit because its gospel! Good Luck! I Love an inline 4 screaming 9000rpms through an open header! But Im old and deaf...lol!
Thx good info gave me a little more confidance when I try to pull this off .
Ill keep reading up and visiting here ... I like this sight .
 

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Obfuscation Engineer
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Steell .

Do we really want to get into the the effects of rod length to stroke ratio, cam profiles and timing , static and dynamic compression ratio , brake specific fuel consumption , intake and exhaust design and the effect all these things have on cylinder scavenging and fluid resonance within an internal combustion engine ? :D

Backpressure is a term overused by those that may know but can't explain the resonances present in a 4 stroke ICE . I'll just start and end with the engine as an air pump fouled or aided by the addition of intake and exhaust plumbing . Less than 100 % volumetric efficiency is common . 100% volumetric efficiency is obtainable for reasonable amounts of money and effort . 100% volumetric efficiency and greater over significant portions of RPM is the holy grail of high performance engine development .

This is not a question that is going to be answered in a post , a thread , or single book . I've been doing this for near 30 years and still get surprised . I recommend that you retain the muffler until you find the improbable benefit of removing it or a more likely suitable replacement .

~kop
 

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Yes. Let's get into it.

Especially the stroke and bore ratio debate.

Here is the funadmental thermodynamic question. The engineering units of power are Joule per second, and the unit is obtained by multiplying volume by pressure change per second, and volume is defined by bore and stroke. So theoretically, any shape of combustion camber (tall and skinny, or short and broad) should make the same power. But theory and experiment differ. Why?
 

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itching to ride
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Well for one the shorter stroke allows for greater piston speeds. And the larger bore that goes along with it means more valve area and less shrouding. Greater valve area means you can flow more air/fuel through that air pump we call an engine.

Not to be disagreeable with anyone but straight pipes do not make power. In almost all applications they will in fact cost power.
 

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BACK ON TWO WHEELS
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youre 100% right, straight pipes will ONLY gain power at MAX volumetric flow, with no need for backpressure.....do you ever see a dragster driven at 1/2 throttle....no its WOT (Wide open Throttle) or waiting for the green at idle...
 

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BACK ON TWO WHEELS
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and before a huge BS debate starts...if you dont have a degree in engines engineering or physics...please dont pull bs out of you A$$
 

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Four-stroke racing engines usually use megaphones which will create negative pressure at the exhaust valve at the proper moment and "supercharge" the engine. Two-stroke engines use "expansion chambers" which are just two megaphones welded together nose to tail to achieve the same effect. "Straight Pipes" on a four-stroke create a LOT of noise (the main reason for cutting off the mufflers) and a very sharp and narrow power band depending ON THE LENGTH OF THE PIPE...
 
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