is exhaust back pressure a myth?

[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..

 

[DeaconFrost]

keyword is: scavenging effect

I agree that's the keyword.

Here's from wiki:

For example, fast flowing heads and a tunnel ram intake combined with a poorly planned camshaft and exhaust system will cause the air to "slow down" and "speed up" throughout its journey, thus reducing its scavenging potential. So, to increase scavenging potential, the air must maintain a positive linear acceleration curve.

Air "slow down" and "speed up = reduced scavenging
Air positive linear acceleration curve = increased scavenging.

For me that's a very simple concept: you want to kick air (exhaust) out from the piping as fast as you can.

So imagine if there's no piping at all?

It's as if trying to kick out something, that is already out.

---------- Post added at 03:35 PM ---------- 6 hour anti-bump limit - Previous post was at 03:26 PM ----------

even F1 have manifold but nothing after that...

That's why I said we need to define the problem. Is it:

1. Performance problem? (performance loss)
2. Aesthetics problem? (dirty engine etc.)
3. Practicality problem? (noise, fumes, pollution, tikus & lipas masuk block etc.)
4. Law/legality problem? (JPJ, F1 rules, etc.)

Imagine one person who decides to focus only on performance and performance alone, he will do without exhaust piping at all. B-U-T as I said of course dirt & cockroach will enter the block, engine will have to be rebuilt after every run, engine will be dirty, loud unbearable noise, fumes and F1 will say no you cannot enter the race.

 

[cvkit17]

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.

2" pipe sounds abit big to me...my hunch. Coz my old junk sentra with SOHC was on 2" initially when I men-ah-beng-kan the car. 4-2-1, silencer, S flow muffler all in 2" pipe. Power seems ok but FC increases. And it is noticeable that the power band at 4-5k rpm is better. But all by butt-dyno la. Then I changed to 1.8" and FC reduced. Lower band power is better.

I guess you can re-tune your auto car to suit the decreased back pressure for a better performance. But since you like to play with sticks...:rolleyes:

 

[DeaconFrost]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

 

[ixeo]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

2.
Because that's what Formula 1 uses.

 

[DeaconFrost]

2.
Because that's what Formula 1 uses.

I bet F1 does that because of F1 rules & practicality. Not performance.

F1 Rule: Must put manifold.
Practicality: Put trumpets only and exhaust gas will go out faster but wind can also blow exhaust gas back in.

 

[ixeo]

I haven't seen any engine with individual exhaust runners/trumpets. Got sample to share?

 

[DeaconFrost]

I haven't seen any engine with individual exhaust runners/trumpets. Got sample to share?

I haven't seen one. I bet it will be very impractical on the road, but it will yield extra hp on dyno.

Meanwhile:

Also shown here are the exhaust tailpipes (red arrow, centre left), which must extend at least 170 to 185mm behind the rear axle line (and have a single exit).

Formula 1® - The Official F1® Website

---------- Post added at 04:44 PM ---------- 6 hour anti-bump limit - Previous post was at 04:34 PM ----------

 

[^pomen_GTR^]

I agree that's the keyword.

Here's from wiki:


Air "slow down" and "speed up = reduced scavenging
Air positive linear acceleration curve = increased scavenging.

For me that's a very simple concept: you want to kick air (exhaust) out from the piping as fast as you can.

So imagine if there's no piping at all?

It's as if trying to kick out something, that is already out.

---------- Post added at 03:35 PM ---------- 6 hour anti-bump limit - Previous post was at 03:26 PM ----------



That's why I said we need to define the problem. Is it:

1. Performance problem? (performance loss)
2. Aesthetics problem? (dirty engine etc.)
3. Practicality problem? (noise, fumes, pollution, tikus & lipas masuk block etc.)
4. Law/legality problem? (JPJ, F1 rules, etc.)

Imagine one person who decides to focus only on performance and performance alone, he will do without exhaust piping at all. B-U-T as I said of course dirt & cockroach will enter the block, engine will have to be rebuilt after every run, engine will be dirty, loud unbearable noise, fumes and F1 will say no you cannot enter the race.

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

I haven't seen one. I bet it will be very impractical on the road, but it will yield extra hp on dyno.

Meanwhile:


---------- Post added at 04:44 PM ---------- 6 hour anti-bump limit - Previous post was at 04:34 PM ----------

i would say no exhaust would be the lowest of all....

its not like kicking out something.....but more like pulling something up...u climb a hill alone using rope is slow...but what is 1person already up there help u by pulling u up....and u help person below u the same way and continuous....so more ppl can go up faster and easier than one by one climbing alon....that is what scavenging effect helps in efficiency......

______________________________________________________________________

and if u take example of dragster car using alcohol fuel....their engine already at 2000-4000whp and if u look at their valve and port size...there is no way to make it bigger to allow higher C.E.F (coefficiency of flow) hence the just dump the exhaust without any piping since if they put the piping, it would reduce C.E.F (when maximum port size already being used but still not enough CEF)

and another reason they use very short piping because their engine was forced induction (supercharged)...hence shorter dumping is better.........if u take any turbocharged engine...and remove the downpipe....i would say the boost came earlier...higher hp would be very possible.....higher boost also.....but their turbo manifold still have to follow scavenging effect design very much like n.a tuned length exhaust manifold......


try find a picture of v8 F1 engine and compare with your dragster v8 engine...see closely their design...u'll see the differences...

 

[DeaconFrost]

its not like kicking out something.....but more like pulling something up...

I humbly beg to differ taikor.

I think it's pushing (blowing through a straw) rather than pulling (sipping through a straw).

since if they put the piping, it would reduce C.E.F (when maximum port size already being used but still not enough CEF)

I think putting piping on a dragster will simply cause more restriction. More restriction = bad, more 'backpressure' = similar, also bad.

 

[cvkit17]

Let's imagine this:

Engine: 4 cylinder inline
Position: Hotside (exhaust side) facing backwards

1. Exhaust manifold (2.25 inch collector) installed + full piping (2.25 inch)
2. Exhaust manifold (2.25 inch collector) installed with no piping afterwards (infinite inch=atmosphere)
3. No exhaust manifold installed (infinite inch=atmosphere). Replaced with 4 trumpets similar to throttle trumpets.

If these 3 setups are dyno-ed, which one will yield the most horsepower/response? I think it will be no. 3.

That's why I think, the older Honda B16/B18 have their hotside facing forward, while the newer Honda K20 have their hotside facing backward.

It's the ultimate testament to the 'get exhaust gas out ASAP' concept.

My bet is on no.3 if I am a dragster. But if I am not, I will go for no.2 set up. It has more advantages than the little power it can give using no.3

I bet F1 does that because of F1 rules & practicality. Not performance.

F1 Rule: Must put manifold.
Practicality: Put trumpets only and exhaust gas will go out faster but wind can also blow exhaust gas back in.

It's not the rule. If you look closely at an F1 car, they have this 2 huge exhaust pipe coming out of the body and terminated before the end of the car. Here's why I prefer no.2 if I am not a dragster. The exhaust air amount from an F1 car is not little. This hot air, blowing out from the exhaust like a jet actually helps in giving more downforce to the car as it hits the rear wing.

I humbly beg to differ taikor.

I think it's pushing (blowing through a straw) rather than pulling (sipping through a straw).

I think putting piping on a dragster will simply cause more restriction. More restriction = bad, more 'backpressure' = similar, also bad.

Actually, pushing or pulling, it is the same if the pressure differential value is the same. When the exhaust gas escapes from the cylinder, it is not only because it is pushed but it is also pulled. The pressure inside the combustion chamber is higher than the atmosphere pressure thus the gases find its way to travel. Of course, when the cylinder moves up, the pressure inside gets higher and the exhaust gases flows with better "push".

Now scavenging is a different thing. It is the effect of using the exhaust air to pull fresh air in. Because the exhaust air moves forward, it actually creates a low pressure point, a vacuum point, that help draw fresh air in. But scavenging is great to improve efficiency at low rpm because the duration between pulse is high. If the engine is revving high, scavenging does not really help much until a point that it is negligible in terms of giving more powahhhhhhh