is exhaust back pressure a myth?

[hex999]

What do you guys think?

 

[desmond0318]

Why? Might consider as math for engineer..

 

[4gbanger]

maybe.. its a flow dynamic thing..

 

[YYC]

Some one from automotive engineering please shed some light. I'm also perplexed.

 

[D7zul]

Back pressure is good 4 low rpm power.. Eg town driving

But 4 racecar like NASCAR they eliminate d back pressure.. Coz their cars aren't 4 town driving

 

[ixeo]

Back pressure is good 4 low rpm power.. Eg town driving

But 4 racecar like NASCAR they eliminate d back pressure.. Coz their cars aren't 4 town driving

Exhaust pulse is not back pressure, and back pressure is not exhaust pulse. Look it up.

Backpressure is not a myth, its science. Reduce backpressure on an Evo by changing the cat-back, you will see higher hp OR higher boost OR faster spool time.

 

[YYC]

Could you explain the science of the back pressure?

 

[hex999]

So is it possible to achieve good low end torque with a straight pipe and a straight flow muffler but with a standard exhaust diameter? least restricted setup. Or "back pressure" is required, thus using a S flow muffler but sacrificing high end powerband?

any engineers can explain? :D

 

[Waiora_ProTuner]

like ixeo said, exhaust back pressure and exhaust pulse is different..

2-stroke exhaust is pulse...

back pressure is not a myth...
back pressure needed for power is a myth..

i'm not an engineer in automotive btw...

 

[hex999]

What about backpressure needed for low end torque? is that a myth?

 

[cvkit17]

Back pressure is friction and it is rubbish and not needed. However, flow rate is another thing to discuss as it is governed by the size of the pipe and exhaust air temperature. Back pressure is created when there is something in the way of the exhaust air discharge thus it will negatively affect the flow rate and also scavenging. Too large the pipe and the temperature will drop faster and that would affect the flow rate to drop. The cylinders are not doing the same stroke at a time. Hence the "pulse" is important to keep the system in a harmonic state as the energy from one cylinder helps the motion of another. Exhaust exit too fast, pulse is disrupted thus lower efficiency as scavenging drops. High revving cars do not need this scavenging as much as normal cars do because their pulse is very well optimized for its high rpm range. Kinda complicated thing actually....the most important thing is to get the term right for its purpose. Laymen usually say "back pressure" because they can only understand this. It's like wax and polish.. Some thought that polish and wax are the same thing.

 

[Waiora_ProTuner]

'pulse' all managed at the header (manifold)...after that, its nothing...only to silence the noise and remove poisonous gas as far as possible...

 

[ixeo]

So is it possible to achieve good low end torque with a straight pipe and a straight flow muffler but with a standard exhaust diameter? least restricted setup. Or "back pressure" is required, thus using a S flow muffler but sacrificing high end powerband?

any engineers can explain? :D

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.

 

[cvkit17]

'pulse' all managed at the header (manifold)...after that, its nothing...only to silence the noise and remove poisonous gas as far as possible...

Pulse is like a sync between the cylinders to work as a system. Too short the pipe at low rpm, you will lose scavenging which will lower the engine efficiency.

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.

Flow is volumetric rate lorh...I guess u mean flow and velocity? With a big straw, flow rate is higher but discharge at a lower pressure and vice versa. Bigger diameter, if the volume of air doesn't fill up the area of the pipe there is an expansion and it is bad if it is exhaust air because any expansion of air means lower pressure (the air itself) thus lower temperature hence lower energy to "move" or "travel".

Erm..back pressure is bad, but a bigger diameter pipe doesn't mean it is good even though it reduces back pressure. Pressure is built up when the friction (or static pressure, in tech term) is high. Let's say air reaches the silencer. The friction caused by the silencer is higher than the friction in the horizontal path. This will cause the air that reach the silencer to slow down but there is air keep on coming thus it pushes the air at the silencer and compress it. That is why pressure builds up. Just like you r in the bus and u r standing at the back, when there are more people coming in you are pushed and sardined. It's bad.

When you have a bigger diameter pipe, other than expansion of air causing the temperature to drop, the amount of air "exposed" to the surface of the pipe or extra space in the pipe will also cause the temp to drop. The conductivity of heat per inch pipe is larger.

 

[ixeo]

Which is why I say its not an accurate example.. If I was that good I won't be here talking cock la.. I making rockets already...

cabutz.