is exhaust back pressure a myth?

[esthapo]

Its engineering not myth laa bro...

I know some of the guys feeling of maintaining 'back pressure & my car will accelerate like rocket'

 

[DanzEterna]

Its engineering not myth laa bro...

I know some of the guys feeling of maintaining 'back pressure & my car will accelerate like rocket'

i was just about to say that haha..

 

[cvkit17]

Its engineering not myth laa bro...

I know some of the guys feeling of maintaining 'back pressure & my car will accelerate like rocket'

That sounds like farting prettt preeett pruuuuhhh

 

[YYC]

Performance Exhaust - More Horsepower - YouTube

This video explains it pretty well, just like some of cvkits and ixeo said.
So basically back pressure is bad, but compromise got to be made to maintain optimum between flow restriction and pressure. Right?

 

[Waiora_ProTuner]

i would go into entire lengthy discussion, but i need to go to work tomorrow..

best exhaust is NO exhaust..


-credit to AutoSpeed -

The idea that an exhaust has to provide a certain amount of back-pressure if best engine performance is to occur is a very strange. In fact, back-pressure will only increase pumping losses, leading to higher fuel consumption and lower performance.

The myth has come about because fitting a large exhaust on some cars has resulted in a decrease in power. But that decrease is nearly always because the engine is no longer appropriately tuned! For example, after being fitted with a big exhaust, MAP-sensed engine management systems may run leaner than optimal. Ignition timing is also likely to be no longer best for performance.

It seems obvious, but if you change the efficiency of an engine – eg by a major exhaust change – the original parameters for which the engine was tuned no longer apply. Therefore, if you want to be assured of getting the best results, you should factor-in an engine management retune at the same time as you get your big exhaust.

Having said that, in 20 years of modifying cars – including turbo and naturally aspirated - I’ve never seen power fall with a big exhaust.

And if you’re still a believer in back-pressure, just put a potato up your exhaust tip and see if performance improves...

AutoSpeed - Automotive Myths

 

[D7zul]

exhaust setup on race car..

definitely not for road use

 

[DeaconFrost]

i would go into entire lengthy discussion, but i need to go to work tomorrow..

best exhaust is NO exhaust..


-credit to AutoSpeed -

The idea that an exhaust has to provide a certain amount of back-pressure if best engine performance is to occur is a very strange. In fact, back-pressure will only increase pumping losses, leading to higher fuel consumption and lower performance.

The myth has come about because fitting a large exhaust on some cars has resulted in a decrease in power. But that decrease is nearly always because the engine is no longer appropriately tuned! For example, after being fitted with a big exhaust, MAP-sensed engine management systems may run leaner than optimal. Ignition timing is also likely to be no longer best for performance.

It seems obvious, but if you change the efficiency of an engine – eg by a major exhaust change – the original parameters for which the engine was tuned no longer apply. Therefore, if you want to be assured of getting the best results, you should factor-in an engine management retune at the same time as you get your big exhaust.

Having said that, in 20 years of modifying cars – including turbo and naturally aspirated - I’ve never seen power fall with a big exhaust.

And if you’re still a believer in back-pressure, just put a potato up your exhaust tip and see if performance improves...

AutoSpeed - Automotive Myths

Most people when replacing their stock cat with a 'bullet', they'll just discard the stock exhaust/oxygen sensor (which is connected to the stock cat). This is the first thing that they'll do.

How then the stock ECU is going to determine whether the car is running rich or lean?

So when performance is affected, they'll say "definitely because the diameter of the piping is too big".

What the f**k.

 

[hex999]

So if backpressure is irrelevant. I wonder why do many people keep saying that s flow mufler is better for automatic transmission and low end torque? I personally think this is not true, what i think if im not mistaken is the engine powerband is determined by the exhaust diameter and length. If they sticked with their original piping size and use a straight flow mufler would improve power slightly due to less restrictions.
I personally decat my car, no mid box straight flow 1 in 2 out mufler but standard piping diameter. I did not lose low end torque but gain more power base on my butt dyno. But my fc decreased, gain up to 50km more mileage due to lighter foot. Only bad thing is when i let off the throttle. Rpm drops fast.

So s flow mufler actually compesate those who use oversized piping? What is the real purpose of s flow mufler? Bad method of sound proofing? Any idea guys?

 

[cvkit17]

So if backpressure is irrelevant. I wonder why do many people keep saying that s flow mufler is better for automatic transmission and low end torque? I personally think this is not true, what i think if im not mistaken is the engine powerband is determined by the exhaust diameter and length. If they sticked with their original piping size and use a straight flow mufler would improve power slightly due to less restrictions.
I personally decat my car, no mid box straight flow 1 in 2 out mufler but standard piping diameter. I did not lose low end torque but gain more power base on my butt dyno. But my fc decreased, gain up to 50km more mileage due to lighter foot. Only bad thing is when i let off the throttle. Rpm drops fast.

So s flow mufler actually compesate those who use oversized piping? What is the real purpose of s flow mufler? Bad method of sound proofing? Any idea guys?

S flow muffler can insulate noise more than straight flow ones. But because of its way of insulation, it also causes more friction. On why S Flow muffler is better for auto cars, I am always sceptical about it unless I see a proper dyno chart and an explanation behind it. Introducing more friction to an oversized set of piping does not really help. Like i mentioned, the bigger the pipe, the faster the temperature drops. Also, the pulse will be different and the engine has to be re-tuned.

 

[TitanRev]

So if backpressure is irrelevant. I wonder why do many people keep saying that s flow mufler is better for automatic transmission and low end torque? I personally think this is not true, what i think if im not mistaken is the engine powerband is determined by the exhaust diameter and length. If they sticked with their original piping size and use a straight flow mufler would improve power slightly due to less restrictions.
I personally decat my car, no mid box straight flow 1 in 2 out mufler but standard piping diameter. I did not lose low end torque but gain more power base on my butt dyno. But my fc decreased, gain up to 50km more mileage due to lighter foot. Only bad thing is when i let off the throttle. Rpm drops fast.

So s flow mufler actually compesate those who use oversized piping? What is the real purpose of s flow mufler? Bad method of sound proofing? Any idea guys?

Powerband is determined by both intake and exhaust design, not just the exhaust....length, diameter of these 2 items will change or move the powerband.

All these is about calculation of flow in and out....you increase the flow out of the engine, then you need to also improve the flow into the engine....engine is essentially a pump....there's in and out....if your incoming is not good, how good you design the out going is also equal 0.

 

[DeaconFrost]

This is my opinion - please feel free to correct it.

My opinion is the shorter the exhaust, the better. If no exhaust piping at all, or not using extractor at all - the better.

B-U-T, of course, if exhaust fumes come out from the 4 big holes on the block, oncoming wind will blow the fumes and our engines will be dirty as shit. Well, that is an aesthetic problem and will affect practicality.

Next problem is if there are 4 big holes on the block, sand, dirt, tikus, lipas all will come in. Still, that's not a performance problem, the same way as the open four throttle trumpet is not a performance problem. The problem is on day-to-day practicality.

Day-to-day practicality is what determines the law. That's the reason why we bother to put full exhaust piping across the length of our car - the law.

Now imagine if we remove 3" diameter exhaust piping so that there will be no exhaust piping at all. That will be a change from 3 inch to Infinite inch - because no piping means you're dumping the exhaust fumes straight to the atmosphere.

Is it better or worse?

Should be better lah, from performance view. From JPJ view, that's a different story. It'll sound horrible, it'll spit fumes everywhere, it'll be ugly, people around you will be angry, it'll have zero practicality but it'll also have zero restriction too.

So if no exhaust is better, why is oversized piping bad for performance?

 

[^pomen_GTR^]

ok lets separate things up....



rule of thumb:

backpressure must be kept at lowest level so the exhaust flow at highest efficiency.....but the lowest back pressure exhaust type is without any silencer that would make the sound level so high and droning sound effect....hence manufacturer use silencer and various type of muffler to reduce the sound...of course it had to be bigger in size to counter the back the backpressure flow.....




but exhaust pulse must be arranged properly (ie with tuned manifold length and collector type 4-1 short, 4-2-1 short, 4-1 medium, 4-2-1 medium, 4-1 long, 4-2-1 long) in order to fully utilize the exhaust scavenging affect to help pump out the exhaust gas and at the same time sucking the fresh air+fuel mixture for next engine cycle....

keyword is: scavenging effect

---------- Post added at 10:29 AM ---------- 6 hour anti-bump limit - Previous post was at 10:27 AM ----------

This is my opinion - please feel free to correct it.

My opinion is the shorter the exhaust, the better. If no exhaust piping at all, or not using extractor at all - the better.

B-U-T, of course, if exhaust fumes come out from the 4 big holes on the block, oncoming wind will blow the fumes and our engines will be dirty as shit. Well, that is an aesthetic problem and will affect practicality.

Next problem is if there are 4 big holes on the block, sand, dirt, tikus, lipas all will come in. Still, that's not a performance problem, the same way as the open four throttle trumpet is not a performance problem. The problem is on day-to-day practicality.

Day-to-day practicality is what determines the law. That's the reason why we bother to put full exhaust piping across the length of our car - the law.

Now imagine if we remove 3" diameter exhaust piping so that there will be no exhaust piping at all. That will be a change from 3 inch to Infinite inch - because no piping means you're dumping the exhaust fumes straight to the atmosphere.

Is it better or worse?

Should be better lah, from performance view. From JPJ view, that's a different story. It'll sound horrible, it'll spit fumes everywhere, it'll be ugly, people around you will be angry, it'll have zero practicality but it'll also have zero restriction too.

So if no exhaust is better, why is oversized piping bad for performance?

no exhaust doesnt means better....

because we loose scavenging effect a.k.a pumping effect to help increase the engine efficiency as a pump.... (engine is a type of pump that produce energy from burning another type of energy a.k.a fuel)

 

[Izso]

I think the current engineering marvel is the Mazda Skyactiv engines, that comes with optimized exhaust header/manifolds. Very good low end torque for a naturally aspirated engine, maybe you can google it up and read more about it? Lots of papers on it.

The easiest way to explain..is imagine your exhaust system is a straw
use a big straw, like those for bubble milk tea, and a small straw.. like McDonald's.

take a mouth full of water, and try to blow the water through the straw with full power, you will find that
1. with the big straw, the water will flow slower, i.e. shoot nearer but ALL the water release from your mouth very fast. in other words, the amount of time it takes for all the water to come out from your mouth is shorter

2. with the small straw, the water will flow faster, i.e. shoot further, but ALL the water in your mouth will release slower. In this case, the amount of time it takes for all the water to come out from your mouth is longer.

So 2 factors at play, flow & volume. you can say with small straw there's "backpressure" and with the big straw you lose "pulse or power". This is NOT a very accurate example, it is only meant to illustrate the difference between big & small exhaust diameter.

Cool! I was actually thinking of that straw example until I read what you wrote. Great minds.. great minds.

S flow muffler can insulate noise more than straight flow ones. But because of its way of insulation, it also causes more friction. On why S Flow muffler is better for auto cars, I am always sceptical about it unless I see a proper dyno chart and an explanation behind it. Introducing more friction to an oversized set of piping does not really help. Like i mentioned, the bigger the pipe, the faster the temperature drops. Also, the pulse will be different and the engine has to be re-tuned.

I know shit about exhausts and only know what everyone knows. My own car has a 4-2-1 header, flexible joint, a large straight resonator and a large rear true-sflow baffled muffler. All 2" piping and connections. Tested with straight flow rear mufflers and other designs like twin loop and all.

Conclusion is what I have now is the best setup for me. Wish I had a highflow catcon instead of that resonator but no one really can afford a magnaflow highflow catcon in m'sia. Stupidly high priced.

And I've tested straight flow with my ex-3-speed auto before. For some reason the gearbox needs the back pressure or whatever you call it to shift. With almost no back pressure from the straight flow setup the bloody thing shifted at much higher RPMs. I couldn't even use the foot liftoff trick to shift 'manually'. Stupid thing increases FC and moves the shift point to be much higher.

So if you ask me, I think autos need the back pressure to shift properly, not because of low end torque or whatever.

And straight flow for my manual does have some loss in low end torque. SOHC has so many downsides. Sigh.

 

[desmond0318]

So if no exhaust is better, why is oversized piping bad for performance?

I terkejut la bro, even F1 engine have a exhaust like those old days ppl plays vr4 turbo but the exhaust came out at the bottom of the side door

Regards about bro Ixeo said about mazda's exhaust, no kidding le.. tuned length

Image copycat from google

 

[Waiora_ProTuner]

don't confuse exhaust manifold and the rest of it (cat con, silencer, bullet, muffler, piping)..

tuning, sizing, length, diameter, whatsoever, all happening in manifold...after that it's nothing...

even F1 have manifold but nothing after that...

individual exhaust like hot rod,monster truck, thats bad, no scavenging..