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Back pressure and you!

 
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Old 08-28-2011, 03:48 PM
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Back pressure and you!

There seems to be a few misconceptions about back pressure that I would like to clear up so all future members have this information at their disposal and do not create useless threads anymore.

First, let's look at what back pressure really is:

Back pressure is NOT solely an automotive term. Many people seem to think it explicitly relates to exhaust gases, but this is untrue. As you will read in the following paragraph, back pressure is a back yard term that relates to resistance in a fluid system. Simply put, back pressure means resistance.

Back pressure is defined as the resistance to 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.

In the automotive world, back pressure is created by the bends and curves of an exhaust system combined with the confinement of the piping itself. The back pressure makes it more difficult for exhaust gases to exit the fluid system.

Misconception: A 4 stroke engine needs back pressure to work properly.

WRONG! This is untrue. Back pressure is NOT necessary for a 4 stroke engine. A better way to say this would be, "a stock engine that cannot adjust fuel delivery needs back pressure to operate normally." In reality, back pressure is unwanted and does create restrictions in the performance of a 4 stroke vehicle. The reason most people believe less back pressure means less torque is because the engine cannot compensate for the little restriction of exhaust flow efficiently; therefore, the fuel mixture is not right and the engine will suffer in performance. If the engine can be adjusted to supply the proper amount of fuel and air, no loss of torque will appear. Funny cars, dragsters, NASCAR cars. All of these extremely high performance vehicles have one thing in common: nearly no exhaust. This is because the engines are tuned to accept and even perform better with little to no back pressure.

Bottom line: Back pressure is bad for performance if the engine can compensate.

Misconception: Little back pressure causes the valves to burn.

WRONG! This issue is directly related to the previous paragraph. The burned valves are a result of the fuel mixture. Because the engine tries its best to compensate, it can lean out the fuel ratio. The engine will try to add more air because it is capable of expelling more air. The fuel delivery system cannot handle this as easily as the air intake system can. The leaned out ratio will cause the combustion process to be a much more heat intensive process than the engine was designed to handle. The result of this heat? Burned valves. This leaner mix will create progressively less power (torque loss) and can eventually fry the engine.

Bottom line: With the right A/F ratio, valves will not burn due to loss of back pressure.

So, from what you've read, you should know by now that back pressure is restrictive and unwanted. It should also be noted that your engine is a complete system and will not benefit much from upgrading one component. If power is what you desire, you need more FUEL, more AIR, a better COMBUSTION PROCESS, and a better route to expel the EXHAUST.

A 3" exhaust will show more performance over any smaller size on a dyno. The thing is, just bolting on a 3" can possibly burn valves, and cause a lean condition. This usually isn't an issue for a Honda because they run rich from the factory. For the exhaust question, you want the gases to escape the fluid system as fast as possible and the engine to understand and be capable of making up for the lost gases.

It is also true that heat wrapping your exhaust can provide better flow. This is because the wrap causes the exhaust gases to stay hot longer; therefore, causing them to move faster. Colder air is more dense and more difficult to expel from the system. Although, the cooling is inevitable and gains are minimal.

I hope this thread helps out anyone who didn't understand how back pressure and exhaust gases work!

DISCLAIMER! I am not responsible for any actions you may or may not take due to the information that I have provided!

Thanks for reading.
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Last edited by RandallSharp; 08-28-2011 at 03:53 PM.
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Old 08-28-2011, 10:28 PM
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Re: Back pressure and you!

Nice, what made you think this up? I scour these forums, no back pressure threads.

So how does Back pressure play into exhaust scavaging? From my understanding a car can produce power on the "out" stroke, but without the back pressure, would the engine lose power due to the lack of restriction? Perfect example is staying in gear while the car coasts to a stop. Fuel consumption is reduced since the TB is shut and no additional fuel is being introduced= better mpg. Bendy exhaust piping isn't back pressure, that's flow restriction. On a NA car, you need some back pressure if the cams rely on turbulence (restriction) to add torque to the baseline of power. Or else everyone would be rocking 4" headers and exhaust.

I completely agree on the adaptive qualities of a motor play a major part, but you can get a lean condition by just having a clogged pcv valve. I'm researching this, hopefully I can add some pics or junk.

In terms of turbo/superchargers.... All of what you're saying is 1000 correct. That needs be as straight pipe as possible. Any restriction will wind up out the waste gate.
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Old 08-28-2011, 10:34 PM
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Re: Back pressure and you!

why did i read this
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Old 08-28-2011, 11:52 PM
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Re: Back pressure and you!

Quote:
Originally Posted by Sideswipe SI View Post
Nice, what made you think this up? I scour these forums, no back pressure threads.

So how does Back pressure play into exhaust scavaging? From my understanding a car can produce power on the "out" stroke, but without the back pressure, would the engine lose power due to the lack of restriction? Perfect example is staying in gear while the car coasts to a stop. Fuel consumption is reduced since the TB is shut and no additional fuel is being introduced= better mpg. Bendy exhaust piping isn't back pressure, that's flow restriction. On a NA car, you need some back pressure if the cams rely on turbulence (restriction) to add torque to the baseline of power. Or else everyone would be rocking 4" headers and exhaust.

I completely agree on the adaptive qualities of a motor play a major part, but you can get a lean condition by just having a clogged pcv valve. I'm researching this, hopefully I can add some pics or junk.

In terms of turbo/superchargers.... All of what you're saying is 1000 correct. That needs be as straight pipe as possible. Any restriction will wind up out the waste gate.
I was just having an interesting conversation with my girlfriend's dad and it seemed that people had a few false beliefs on back pressure, so I decided to make a thread. Plus, I haven't been too helpful to the forum and I am trying to be. I'm not a professional on this stuff, but from my understanding, this is how it works. If I'm wrong, please correct me so I don't seem like an idiot. From my knowledge on the subject, a low pressure system is created and helps "pull" the exhaust gases from the head. Is this what you mean by scavenging?

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why did i read this
No one forced you.
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Old 08-29-2011, 08:54 AM
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Re: Back pressure and you!

I know this is onlu in close relation to what you're saying, but having short exhaust piping or just running headers and a cut-out can cause the valves and even the head to cool faster than the rest of the block and warp. True or untrue? I dont know, but i've heard of it happening. So if you are trying to minimize backpressure, dont minimize exhaust piping. Minimize the bends in the piping
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Old 08-29-2011, 02:37 PM
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Icon1 SH DRIVER the professor strengthens things out

First of all, air pressure is just another way of measuring stored potential energy. A good example of what I mean, is when you blow up a balloon to let us say to 10psi and then let it go. The stored energy in the gas is released and the balloon experiences a build up of acceleration,velocity ,and interia.By constricting the passages of the intake and the exhaust ,you create an accumulation of this type of potential energy. In the case of an engine this potential energy measured in air pressure is taken from the earths built up air pressure in the the case of the intake and the engine in the from of a slight pumping loss at the exhaust.As this potential energy is dissipated, it acts to increase the air going through the engines rate of acceleration,velocity and interia which is just what the engine needs at low and mid rpms for good throttle response and proper gas swirl in the cylinder for properer mixing of the gas and fuel. The idea of trading a little hp for enhanced gas flow through the engine should not be a foreign idea to most of you seeing that people who use turbos and superchargers do it all the time. As the engine RPMS go above about 5k the air flow is good and not in need of the ever increasing back pressure induced boost so the constricted intake and exhaust becomes a performance negative.The VTEC Preludes by opening and closing certain valves on the intake gives the user the benefits of back pressure below around 4900 rpms and a free flowing intake above 4900 rpm. Some exotic cars have valves in their exhaust that perform a similar function.In the past most engines did not rev much past 5500 rpms so it was not to hard for an engine builder to create an exhaust and intake that was a good compromise between low and high rpms.With todays engines revving as high as 9k rpms ,with out some kind of valve tricky, real compromises have to be made.VTEC serves that purpose just fine.This also explains why some race cars don't need VTEC because their cars are geared and used in such a way that they don't really care about how an engine runs below 5k rpm just that it does.
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Old 08-29-2011, 03:55 PM
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Re: Back pressure and you!

i swear SHDRIVER is a rogue A.I that the government couldn't hold onto...
good post OP, nice information
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Old 08-29-2011, 10:25 PM
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Re: Back pressure and you!

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Originally Posted by vandal138 View Post
I know this is onlu in close relation to what you're saying, but having short exhaust piping or just running headers and a cut-out can cause the valves and even the head to cool faster than the rest of the block and warp. True or untrue? I dont know, but i've heard of it happening. So if you are trying to minimize backpressure, dont minimize exhaust piping. Minimize the bends in the piping
Theoretically, I would imagine the cooling principal is true. Realistically, I can't see it being a big enough factor to make a difference. I would call it implausible to assume the very minimal loss in temperature could warp the head or valves because they dissipate the heat naturally anyways. I mean, it's possible, but not something I would worry over or even consider an issue.

Quote:
Originally Posted by SHDRIVER View Post
First of all, air pressure is just another way of measuring stored potential energy. A good example of what I mean, is when you blow up a balloon to let us say to 10psi and then let it go. The stored energy in the gas is released and the balloon experiences a build up of acceleration,velocity ,and interia.By constricting the passages of the intake and the exhaust ,you create an accumulation of this type of potential energy. In the case of an engine this potential energy measured in air pressure is taken from the earths built up air pressure in the the case of the intake and the engine in the from of a slight pumping loss at the exhaust.As this potential energy is dissipated, it acts to increase the air going through the engines rate of acceleration,velocity and interia which is just what the engine needs at low and mid rpms for good throttle response and proper gas swirl in the cylinder for properer mixing of the gas and fuel. The idea of trading a little hp for enhanced gas flow through the engine should not be a foreign idea to most of you seeing that people who use turbos and superchargers do it all the time. As the engine RPMS go above about 5k the air flow is good and not in need of the ever increasing back pressure induced boost so the constricted intake and exhaust becomes a performance negative.The VTEC Preludes by opening and closing certain valves on the intake gives the user the benefits of back pressure below around 4900 rpms and a free flowing intake above 4900 rpm. Some exotic cars have valves in their exhaust that perform a similar function.In the past most engines did not rev much past 5500 rpms so it was not to hard for an engine builder to create an exhaust and intake that was a good compromise between low and high rpms.With todays engines revving as high as 9k rpms ,with out some kind of valve tricky, real compromises have to be made.VTEC serves that purpose just fine.This also explains why some race cars don't need VTEC because their cars are geared and used in such a way that they don't really care about how an engine runs below 5k rpm just that it does.
Dude, I generally try to be as intelligent and comprehensive as possible, but WTF? First off, it annoys readers if you do not "separate" your writing. No one would read a book if it was one giant paragraph. Secondly, you need to dumb it down and make more sense. I'm college educated and what/how you write material confuses even me.

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i swear SHDRIVER is a rogue A.I that the government couldn't hold onto...
good post OP, nice information
Thanks man. I'm trying to give back a little.

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Old 08-30-2011, 09:55 PM
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Originally Posted by SHDRIVER View Post
First of all, air pressure is just another way of measuring stored potential energy.
Yes....

Quote:
By constricting the passages of the intake and the exhaust ,you create an accumulation of this type of potential energy....
Err...what? The thermodynamics police have a warrant out on you for violating the Second Law.

It sounds like what you're talking about is pulse tuning, which seeks to sequence exhaust gas pulses coming out of individual exhausts (the header) into the collector (the exhaust pipe) so that the lower pressure behind each pulse helps draw the one following down the pipe. That's the basic purpose behind a tuned exhaust, but it's seeking to minimize the effects of back pressure, not produce it. In fact, exhaust design tries to maximize the kinetic energy of the exhaust -- keep average exhaust velocity in the pipe as high as possible, which means the back pressure, a static measurement, is as low as possible. Back pressure is no one's friend.

Quote:
As the engine RPMS go above about 5k the air flow is good and not in need of the ever increasing back pressure induced boost so the constricted intake and exhaust becomes a performance negative...In the past most engines did not rev much past 5500 rpms so it was not to hard for an engine builder to create an exhaust and intake that was a good compromise between low and high rpms.
Nooo! As engine speed changes, the time the valves stay open changes -- the faster the engine speed (rpms), the faster everything happens. Cam design controls that by changing the duration -- the amount of the cam's rotation that the lobe pushes the valve open. The penalty for a single cam shape is that it works best at a particular design speed, and everything else is a compromise. A cam optimized for low-rpm response, such as in a road engine, doesn't keep the exhaust valve open long enough to expel the burnt charge in the cylinder at high rpms...conversely, a long-duration cam optimized for high rpm stays open too long at lower rpms and sucks exhaust gases back into the cylinder. Racing engines -- designed for sustained high revs -- used cams designed for that purpose. Late '50s racing engines ran around 10,000 - 11,000 rpm. Road sports car engines of the same vintage routinely redlined around 6500 rpm, primarily because running greater revs demanded a cam that performed poorly at lower engine speeds. VTEC (and later variable geometry cam mechanisms) changed that by providing a means to run multiple cam profiles, which opened up the path to higher-revving road engines without compromising low-speed response.

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i swear SHDRIVER is a rogue A.I that the government couldn't hold onto...
Only if A.I. stands for Absolutely Insane. :)
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Old 08-31-2011, 04:28 AM
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Re: Back pressure and you!

Whoa! What did I miss? Jamie hit it right on the head. As a plus side to the original plan of VTEC, car owners also had a completely civil automobile, with some punch when civility isn't neccessary. I must stress anyone can drive a vtec civic or a NSX, and with little difficulty. But enough of this.

Believe it or not, the new I-VTEC system does away with the back pressure and even uses it in the active cylinder management. Torque all day, free flowing hp after you're moving. With most fuel efficiency. 3/4/6 cylinder selections, whoa. Heavy duty.

Most Big Block gurus also fight day and night on cams are best for the cars. Most muscle cars don't see 7500rpm but the debate when power comes in is alive and well. Of course some want the powuh before 5500rpm, some want the build up to 3500 then all Hell brakes loose for civility in city driving. But if the car isn't turbo, the biggest headers are 3 3/4 piping, 3 7/8 for boost. , with a three inch system from cat-back, 4" for turbos.

Unfortunately, people do not go overboard finding ways to improve flow because you need to have the same amount going out as coming in, just to keep vacuum pressure good.

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Old 08-31-2011, 04:40 AM
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Re: Back pressure and you!

Wow I gave up halfway...BUT to me backpressure happened when I couldn't pass smog because an exhaust leak caused air to go in and mix with the exhaust. Failed NOx. But now that it's fixed my car has no more "back pressure" and runs 10x better
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Old 09-14-2011, 02:49 PM
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Re: Back pressure and you!

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Originally Posted by RandallSharp View Post
Theoretically, I would imagine the cooling principal is true. Realistically, I can't see it being a big enough factor to make a difference. I would call it implausible to assume the very minimal loss in temperature could warp the head or valves because they dissipate the heat naturally anyways. I mean, it's possible, but not something I would worry over or even consider an issue.
This was a big arguement with the V8 guys on the trans am forum i was on. Some people swore by running open cutouts at the headers and others called it negligent engineslaughter. I never knew, i just threw it in there in case someone could shed some light on the situation if it held some truth. Thanks for not bashing!
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Old 09-15-2011, 04:50 PM
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Re: Back pressure and you!

poets, "Backpressure: destroying a myth," thread. - Honda Prelude Forum : Honda Prelude Forums


old thread - but there's some more for discussion....
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