an engine needs backpressure to work correctly

[CALCXX]

Destroying a myth.

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. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

- Adapted from Thomas V.

[cbrxxquad]

Needless to say I have some problems with that.

[tomek]

Well,the world is full of self proclaimed internet experts.

[cbrxxquad]

Well, I think I am.

[cbrxxquad]

http://www.google.com/search?sourceid=chro...Is+this+true%3F

[Mekanix]

All I'm gonna say is all those top fuel dragsters don't have mufflers on them. Maybe backpressure is necessary, maybe it isn't. I don't see how it can be avoided while making an engine quiet enough to be tolerated and also installed in the tight confines of an engine bay. With those two considerations, it seems the desire should be to minimize any that you can.

[CALCXX]

Tell ya the truth. I don't know.

I posted this to hear the discussion.

I'm all ears.

I'd like to hear what you have to say.

[tomek]

Well, I think I am.

You are not alone.

[CALCXX]

Did I hit a nerve or something? What's up?

[EVLXX]

Needless to say I have some problems with that.

I'll second that.

Well,the world is full of self proclaimed internet experts.

Pot to Kettle.... Pot to Kettle.... come in Kettle..... Kettle this is Pot, I wanted to inform you...... You're "Black".

Tell ya the truth. I don't know.

I posted this to hear the discussion.

I'm all ears.

I'd like to hear what you have to say.

Did I hit a nerve or something? What's up?

Yes and No, On both counts.

What you posted above is a sales pitch, and there is some truth in it and there is some..... embellishment of truth in it, so much so it's almost false. My guess is you got it from a BMW ad somewhere.

....

The real truth of back-pressure is, it's good and bad. Under certain running conditions it's actually needed, like a 2 stroke engine for example. The small stinger pipe on a 2 stroke pipe is designed to promote a little back-pressure to increase the density of the gas and to promote Sound Wave action for one thing. It does more but I'm not going to go there.... if you want to learn more about that, buy this.. http://www.webbikeworld.com/books/sportbike-performance.htm

As for hitting a nerve.... each to there own right?, well each of those 2 has there own opinion on how all this mechanical shit works, (and so do I), so the truly good question is, it more like asking yourself this ... "which prophet do you want to believe in ?"

Now with that analogy, this prophet would tell you... to go read the Book I stated above, the come back and see me......

[CALCXX]

Eric, I retraced the path of where I copied this and you are correct, it is BMW related.

No where along the return path did I see anything "for sale".

It seems I have stumbled upon a sensitive subject.?? :icon_surprised:

[EVLXX]

Well not so much sensitive really... as it is mysterious and dangerous.

Since exhaust flow design doesn't have easy mathematical formula for everybody to easily understand,1 of 2 things happens. People either believe or they Know. 1. Some people rather than taking the time to actually learn and apply the physics of whats going on choose the path of speculation, probablies, Itolduso's, and hearditwas's. These people aren't always the unintelligent either, but they are like most of us, and just don't really see the need in learning about gas fluid dynamics, when for the most part, OE and aftermarket manufactures already do a pretty good job at it. The really problem with this group comes when someone throws out a sales pitch like the one stated above, and maybe sales pitch is the wrong thing to call it too, maybe it's just persuasive information. In any event these people which is probably 99% of the population make there decisions upon perceived trust. Then there's the geeks who want to know, and do take the time to understand just what in the heck is going on in there. Gas Fluid dynamics, yep sounds like anybody can learn that one in a jiffy..... NOT! So these guys rather than being ostracized for there knowledge mostly don't say a word. They either are already engineers, or are maybe the geeks that should be, but maybe being an engineer just doesn't seem all that thrilling to them, but in any case they learn the physics and formulas.

So as one of those people that has learned some about this exhaust stuff, more than most, but not as much as some others, let me just say the truth in the Back-pressure thing goes more like this. BACK-pressure is bad in most cases of a 4 stroke engine, but pressure is not. Back-pressure on the other hand is Good in a 2 stroke engine, when the waves of pressure are controlled and designed to help stuff the cylinder with the next charge. So considering the proportionate size of a motor to it's exhaust. A small exhaust is good for Low rpm power, but restricts High RPM power a lot. A good midsized exhaust promotes the better of both worlds, but lacks in both still. A Large exhaust is horrible for low rpm power, but great for high rpm power. Why ? because it all has to due with FLOW.

The truly #1 most important thing with exhaust design is "Flow" characteristics.

So..... do you see how this can easily get twisted and turned into a topic that nobody, except for an expert, really wants to touch ? Hence the mysterious and dangerous comment.

[Crazy Dave]

Look at any WW11 airplane engine with the tiny little exhaust stubs and tell me no back pressure is bad.

I wonder how much more HP they could get with modern tuned headers.

[EVLXX]

Seems to me that a variable exhaust pressure would be idea then, eh?

More back pressure for low RPM and less for high?

Um.. sort of correct. The problem is, everyone keeps calling it "Back"-pressure.... when it's really not correct way of stating it, but it's what everybody automatically understands...

and yes, a variable exhaust would be ideal then, hence why over the last Decade most all Major manufacturers have been making them in some form or fashion with moving valves inside the exhaust system somewhere.

Look at any WW11 airplane engine with the tiny little exhaust stubs and tell me no back pressure is bad.

I wonder how much more HP they could get with modern tuned headers.

Those little tiny stubs work like the tiny stingers on most 2 stroke pipes...... single cylinder and high flow design.

[cbrxxquad]

Let me add a piece of information that will move this to another level. If you don't open the exhaust valve around half way down the expansion stroke so the pressure starts going out the exhaust pipe, and the exhaust pipe is long enough to hold the column of air intact long enough that it will pull on the cylinder when empty and draw some of the intake through the cylinder during overlap, not only will it stall the engine due to excess pressure on the exhaust stroke, that overcomes the rotational forces built up in the power stroke, but the free radicals left over will pre-detonate the next power stroke.

That is the physics behind the lean condition.

[EVLXX]

Wao, wao wao bub..... are you sure we should move this to a new level ?

Now you might have to explain those free radicals to others...... otherwise someone might get the notion that there's some micro-mini crazy guys from the backwoods running around inside there pipes.

Or the effects of having a lot of valve overlap and less lobe separation. (which was the norm, but now is not)

[speakeasy]

Seems to me that a variable exhaust pressure would be idea then, eh?

More back pressure for low RPM and less for high?

Isn't that the issue Yamahahaha and Honduh addressed with the VTEC technology?

[CALCXX]

Damn, this is a deep subject? Way over my head.

It's got me searching for exhaust scavenging, cam lobe centers, collector length etc,ect,ect,

[EVLXX]

Okay you condescending twat waffles, keep it rolling.

Take it to the next level, it's Friday and I'll be drinking a beer later tonight needing something to respond poorly too, so it might as well be this thread.

I think both of you are examples of free radicals, btw.

Thank you.

Seems to me that a variable exhaust pressure would be idea then, eh?

More back pressure for low RPM and less for high?

Isn't that the issue Yamahahaha and Honduh addressed with the VTEC technology?

.... well the answer depends on if you were referring to the exhaust or the Cams.

Damn, this is a deep subject? Way over my head.

It's got me searching for exhaust scavenging, cam lobe centers, collector length etc,ect,ect,

No... it's not way over your head.... buy the Book I linked you too.... then you'll get a lot closer to where I'm at, and I won't have to try and screw up a perfectly good explanation.

Friday night..beer flowing, nothing to read here..

I'm a sad panda..

Sorry, No time to write more..... to much work, and no time to play. Lately I've just been popping in here with just enough time to catch up and maybe throw in a word or two.

Plus I'm not sure I want to .... give up my crazy interpretations just yet. I'm currently trying to save some moneys to build another........ ummmmmm, test.

[tomek]

Look at any WW11 airplane engine with the tiny little exhaust stubs and tell me no back pressure is bad.

I wonder how much more HP they could get with modern tuned headers.

All of them with the exception for turbocharged P38,P47 and some US bombers were supercharged via centrifugal blowers.Engines like that does not really depend on exhaust to start intake process.

Second,aero engine work in fairly narrow rpm range plus there was simply no space for merging exhaust header.

Now,those little stubs provided additional trust,it is estimated that in typical V12 powwered fighter it was equal to about 60-70 hp at 400 mph.

[blackhawkxx]

Now,those little stubs provided additional trust,it is estimated that in typical V12 powwered fighter it was equal to about 60-70 hp at 400 mph.

I believe that. At a NHRA race not too long ago, a funny car broke a header at the block and the force pushing down almost turned the car over. Never would have thought.

[EVLXX]

Look at any WW11 airplane engine with the tiny little exhaust stubs and tell me no back pressure is bad.

I wonder how much more HP they could get with modern tuned headers.

Those little tiny stubs work like the tiny stingers on most 2 stroke pipes...... single cylinder and high flow design.

Eerrrrrrch... wait, back up. After reading Tomeks post above I was confused, so I had to go and look for myself, and I found out that when I wrote the above, I was thinking of WW1 planes and not WW2 planes.

WW2 plane engines were like dragster engines,,,, plus they didn't have the need, nor the space for exhaust system.

...

Back on topic.... the only 2 advantages of an exhaust system are on Normally aspirated engines (to control the gas flow) and noise suppression. As far as controlling the gas charge it can help, and it can be of little help, it really depends on what your trying to achieve. Take most cars for example, they don't car about making more power with the exhaust, they just want to get the gas and noise away from the occupants inside the car.

Now on the other hand we could look at 2 stroke GP motorcycles, in which case it's all about making more power.

Everything in between is a balance.

[tomek]

Now,those little stubs provided additional trust,it is estimated that in typical V12 powwered fighter it was equal to about 60-70 hp at 400 mph.

I believe that. At a NHRA race not too long ago, a funny car broke a header at the block and the force pushing down almost turned the car over. Never would have thought.

That is pretty much why Senna crashed and consequently died at Imola,at least according to Schumacher who was right behind him when it happened.

Back in that era exhaust on F1 car was part of downforce generating package.His car got slightly bend out of shape and Senna lifted the throttle causing further loss of traction.

[cbrxxquad]

Now,those little stubs provided additional trust,it is estimated that in typical V12 powwered fighter it was equal to about 60-70 hp at 400 mph.

I believe that. At a NHRA race not too long ago, a funny car broke a header at the block and the force pushing down almost turned the car over. Never would have thought.

That is pretty much why Senna crashed and consequently died at Imola,at least according to Schumacher who was right behind him when it happened.

Back in that era exhaust on F1 car was part of downforce generating package.His car got slightly bend out of shape and Senna lifted the throttle causing further loss of traction.

I thought his steering wheel came off...

[blackhawkxx]

I never heard that about Sena. You should couldn't tell what happened from watching the race.