back pressure myth

[frd906]

does the i6 require back pressure from the exhuast or not, cheers matt

[BHDOGS]

Yes it does however the size of the exhaust matters my own opinions are 2 and ahalf inch on mildly modded and stockish cars and 3inch on majorly worked n/a and ofcourse turbo or supercharged I6's

[Stav]

The less backpressure you have the faster your car will go and more power it will make.

[Falcon4L]

I agree with stav, I have a 3" on mine and it goes faster than the smaller systems I ran.

[T3man]

No engine needs backpressure.

[frd906]

you have to have some back pressure and not have none at all

[The Monty]

3 inch is good, 2.5 inch is good. Its up to you.

[Stav]

You reduce pumping lossess by openning the exhaust up regardless of the countless exhaust shop experts advice. The car gains power because the piston has less restriction to push against.

[The Monty]

Why do you want the car to have backpressure?
That means there is a restriction. We aim to have no restriction.
Thats why cars at the drags, either drop the exhaust before the cat, dump it out from the collectors behind the front gaurd, or if really crazy have some Zoomies

[NC1183]

Originally Posted by frd906

you have to have some back pressure and not have none at all

Is that a question or a statement? If its a statement, a-bit badly written, then why bother asking the question?

[frd906]

i said dosent the engine have to have some back pressure, alright, then no back pressure at all meaning no cat or muffler

[frd906]

but the muffler and the cat is the restriction, if you had a high flow cat and muffler you still have back pressure but very little, at all

[The Monty]

We cant run no exhausts on the street, hence why we have "Hi-flow" cats and mufflers.

[Stav]

Originally Posted by frd906

but the muffler and the cat is the restriction, if you had a high flow cat and muffler you still have back pressure but very little, at all

Thats right .But for a street registered car you need these parts. If the car is an offroad car then its fine

[frd906]

but in saying that, you will always have pressure, unless you are a idiot and just have straight pipe all the way through, like the bommbadores

[Stav]

Originally Posted by frd906

but in saying that, you will always have pressure, unless you are a idiot and just have straight pipe all the way through, like the bommbadores

Have you measured the pressure ? What were your readings?

[frd906]

i have to replace my cat as all of you no i had that accident , so im replacing the cat tonight or tommoro, the going to spend 88 dollars to get the car on a machine to make sure she is running spot on and will let you no,

[JC]

You're always going to get some backpressure - In an I6, you have the exhaust gasses from 6 cylinders all bombing down their individual pipes to a collection point somewhere in the system (unless you run 6 exhaust pipes). When these gasses meet, most of it go down the exhaust, but some goes back up creating "back" pressure. Restriction of the system is not the same thing, and opening up the pipes past the collector point will help reduce back pressure, but it will not be eliminated.

Having answered your own questions about 4 posts after you posed it, I'm not even sure what your intention is with this thread.

[frd906]

i was just asking on peoples thoughts

[frd906]

and as to see if it was true or not

[PepeLePew]

I was reading up on this subject the other day and found this article....

The below is quoted from:
http://www.miata.net/garage/KnowYourCar/S4_Back.html

The general idea being excess backpressure is a no-no, but no backpressure can equal compromised exhaust gas velocitys....not a Ford but totally relevant.

<SNIP>
Urban Myth Number 42: "MX-5s need backpressure"


It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.



If Max had chosen a 2 1/4" pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.

As a general rule, a normally aspirated MX-5 will get better high RPM performance with a 2 1/4" exhaust system (2 1/2" or above is just too wide to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" system is for mid to high RPM petrol heads. Your mechanic should be able to advise you what exhaust system will best suit you driving style and needs.



Forced induction (turbo or supercharged) MX-5s perform better with the high volume pipes (2 1/2" to 3"), but that’s another story. The choice of a 4 into 2 into1 or a 4 into 1 header to exhaust set is yet
</SNIP>

[Stav]

Originally Posted by PepeLePew

I was reading up on this subject the other day and found this article....

The below is quoted from:
http://www.miata.net/garage/KnowYourCar/S4_Back.html

The general idea being excess backpressure is a no-no, but no backpressure can equal compromised exhaust gas velocitys....not a Ford but totally relevant.

<SNIP>
Urban Myth Number 42: "MX-5s need backpressure"


It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realises that he has lost power right through the power band. The connection is made in his throbbing brain…. put on 3" pipe = loss of backpressure = loss of power. Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.



If Max had chosen a 2 1/4" pipe he would have achieved better performance in the mid- to high-RPM power band. You need the combination of the least positive (close to zero) backpressure possible with the highest gas velocity achievable to create performance. The diameter of the pipe (and smoothness of internal finish and bends) will strongly influence if your exhaust change is going to create performance or lose power.

As a general rule, a normally aspirated MX-5 will get better high RPM performance with a 2 1/4" exhaust system (2 1/2" or above is just too wide to retain exhaust gas velocity for street driving). The general consensus is that a 2 1/4" system is for mid to high RPM petrol heads. Your mechanic should be able to advise you what exhaust system will best suit you driving style and needs.



Forced induction (turbo or supercharged) MX-5s perform better with the high volume pipes (2 1/2" to 3"), but that’s another story. The choice of a 4 into 2 into1 or a 4 into 1 header to exhaust set is yet
</SNIP>

Hmm those experts are at it again lol. My real life testing has shown that a car with a bigger exhaust needs a tune to complement it.Without a tune their test results are useless

[T3man]

Originally Posted by frd906

you have to have some back pressure and not have none at all

I wish I knew what you were trying to say here.

In your first post it was quite clear what you asked "does an I6 engine need back pressure". The simple answer is categorically NO.

This is not my thoughts on the subject - it is a stated fact. It has been proven so many times that it amazes me how aften it still comes up - an urban myth that won't go away.

It is simple physics that a high school kid can teach you. The best exhaust is NONE - straight out from the exhaust valve/port to the atmosphere - this allows for the engine to be tuned for it's potential maximum power. As you add varying lengths of piping and restrictors like cats and mufflers then the tune of the engine has to be compensated and power is lost - like I said, it is simple physics and is undisputed fact.

[Jack91]

Hate to say I told you so. That apology can be made via email, msn or personal message.

[PepeLePew]

"He has ignored the need for exhaust gas velocity to get that scavenge effect"

If the scavenge effect isnt relevant, we might as well all bin our headers

I've thrown a PM to Thunder in the hope he can help out with some real knowledge here...

[T3man]

Maximum exhaust gas velocity is achieved when the gas can leave the combustion chamber totally unimpeded. Again, it is simple physics and there is no room for argument.

Exhaust "scavenging" only becomes relevant once you add piping to the exhaust passage; like on motor cars. And it is true that the exhaust system has to be properly designed and the engine appropriately tuned to get the maximum benefit. But the most power an engine can possibly produce will always be with the lowest possible backpressure. On a street driven vehicle that maximum power may not be at appropriate revs for maximum driveability so quite often an exhaust system will be used that promotes better driving response at the cost of the loss of some potential power.

[PepeLePew]

Im prepared to sound stupid as Im just guessing here and forget most of what I was taught over the years....

So exhaust scavenging is used to help reduce the effects of having a limited escape path for the gases by effectively REDUCING the backpressure that would be normal in a system of 'x' size (well for each cylinder that is being 'scavenged'? So scavenging does NOT increase the velocity of escaping gases above that that would be experienced by venting to atmosphere?

Im making no suggestions above, just asking questions!

[BHDOGS]

basically who gives a about that stuff no the engine doesnt need back pressure but if u want to drive it on the road it needs an exhaust so will have back pressure if you like it or not the end jesus

[paule11]

Originally Posted by T3man

I wish I knew what you were trying to say here.

In your first post it was quite clear what you asked "does an I6 engine need back pressure". The simple answer is categorically NO.

This is not my thoughts on the subject - it is a stated fact. It has been proven so many times that it amazes me how aften it still comes up - an urban myth that won't go away.

It is simple physics that a high school kid can teach you. The best exhaust is NONE - straight out from the exhaust valve/port to the atmosphere - this allows for the engine to be tuned for it's potential maximum power. As you add varying lengths of piping and restrictors like cats and mufflers then the tune of the engine has to be compensated and power is lost - like I said, it is simple physics and is undisputed fact.

Ive been in a few cars without exhausts no extractors on and they went a lot worse than when you bolt up extractors on to them didnt sound too good either . Never seen anyone race a car without an exhaust.
Pepe Le Pew is on the right track

[T3man]

Originally Posted by PepeLePew

So scavenging does NOT increase the velocity of escaping gases above that that would be experienced by venting to atmosphere?

Im making no suggestions above, just asking questions!

That is correct - scavenging is used to MINIMIZE THE LOSSES due to exhaust slowing down once it hits a pipe. The exhaust gas cannot accelerate after it leaves the combustion chamber. Scavenging works only over a small rev range - on race engines it is designed to work best at close to max revs/power. On road cars it is naturally in the lower and mid range to promote driveability.

paule11> I don't doubt that - BUT the simple fact remains that the engine was tuned with an exhaust system in place - you remove it and the tune will be shot - you re-tune it for NO exhaust and it will go harder that it ever did before. Race engines run the absolute minimum of exhaust piping - think about it - would they do this if there was a benefit to be had by running an exhaust pipe twice as long?