4-2-1? 4-1? Extractors broken down!

Zeroed

1,500 RPM
Senior Member
Nov 14, 2003
1,705
45
3,148
Hi guys. I would like to take the opportunity to discuss a little about "backpressure".

"Backpressure" is a huge misconception that has spread to a lot of Malaysians. Im not an expert, but have studied many technical articles that has made this apparent.

"Backpressure", aka reverse pressure / congestion in the exhaust system, is *always* bad. When most people say "backpressure" they are actually referring to a more restrictive exhaust system, i.e. smaller diameter pipe. What this creates is *exhaust velocity*.

There are two aspects to exhaust flow, velocity and volume. A big pipe will flow more volume, but at a slower speed (less velocity). A small pipe will flow less volume but at a higher exit speed / velocity. Remember our physics lesson where if you squeeze the water hose, the water shoots out further? Thats because velocity has increased at the small opening.

The common misconception comes, I guess, from the exhaust in the smaller pipe having a higher air pressure. Technically however higher pressure is not "backpressure".

A higher velocity in the exhaust flow will allow the exhaust system to make full use of the scavenging effect, i.e. the exhaust pulse from one cylinder actually creates a vacuum which "pulls" out the exhaust gasses from the other cylinders, reducing engine pumping losses (parasitic loss). This then improves torque.

However, there is a limit as to how fast the exhaust can flow, and when the engine hits this limit, the smaller pipe will be restricting the exhaust flow, causing "backpressure". This is bad because the engine is then pushing out the exhaust gasses, whereas a good exhaust system will actually help the engine pull out the gasses.

It is very important then, that someone who tunes his exhaust chooses the correct piping sizes. Like extractors, there is no 100% good or bad size, there is only compromise. On a 4G93 for example, a 2" will create more torque at lower rpms, but lose horsepower at higher rpms, while a 2.3" will lose torque at low rpms but gain horsepower at higher rpms. A smaller diameter pipe will also always benefit cruising, since your engine will not be pushing large amounts of exhaust gasses at this time and would greatly benefit from exhaust scavenging (reducing pumping losses).

Keep in mind however that you have to weigh the benefits against the sacrifices. On a stock 4G93 for example, a 2.3" system will give you very little benefits, while sacrificing a lot, when compared to a 2" (meaning you gain very little high end power across a small range, but lose a lot of low end power across a wide range).

The above is however only applicable on N/A engines, turbocharged engines have a whole different physics lesson to their exhaust systems.


My 2 cents from my understanding of exhaust systems, do correct me if Im wrong. :)
 
Last edited:

zmays

Known Member
Senior Member
Feb 20, 2006
271
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1,528
Very good info bro Iszo, Zeroed

Reading your writings remind me about what I have learnt few years ago. If not mistaken the science formula that consist fluid pressure, velocity, temperature is as follow (maybe incorrect but this is what I recall);

PV=RT

P = fluid pressure
V = fluid velocity
R = fluid constant (I guess)
T = fluid temperature

As mentioned by bro Zeroed, the small exhaust pipe will have higher velocity and low pressure. The equation is below:

P = RT/V

From the equation, if the exhaust temperature is same for whole rpm range, then the faster the exhaust speed will create lower pressure in the exhaust system. Therefore I was wondering is it possible to cool down the exhaust temperature? Because due to the formula, high velocity, low temperature exhaust system can create less pressure thus can pull out more exhaust gasses from engine cylinder. Is my assumption correct?
 

drexchan

2,000 RPM
Senior Member
Nov 23, 2004
2,908
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Zeroed has got it exactly right about the size, scavenging effect and what 'back-pressure' is.

---------- Post added at 03:27 PM ---------- 6 hour anti-bump limit - Previous post was at 03:13 PM ----------

PV=RT

P = fluid pressure
V = fluid velocity
R = fluid constant (I guess)
T = fluid temperature
V is volume. This is the Gas Law for for gas thermodynamics, not fluid dynamic.

The correct relation is PV=nRT, with n being number of mol (1 mol = 6.02x10^22 particles).

---------- Post added at 03:36 PM ---------- 6 hour anti-bump limit - Previous post was at 03:27 PM ----------

Therefore I was wondering is it possible to cool down the exhaust temperature? Because due to the formula, high velocity, low temperature exhaust system can create less pressure thus can pull out more exhaust gasses from engine cylinder. Is my assumption correct?
You started with using the wrong formula so.....

Anyway, you don't want to cool it down because that will only increase the density, and denser gas has higher inertia like a heavier pulley is harder to spin. You want to keep it hot.

Then there's a consideration on the piping size too as bigger pipe has larger external surface for heat dissipation. Like what Zeroed has pointed out on how the size affects velocity and scavenging, it affects the temperature (and then the density, then the pumping lost) too.
 
Last edited:

monsh

Known Member
Mar 27, 2011
157
18
518
Kuala Lumpur
well, to my believe, there's a reason the engineer created the pipe path and also size they used for each engine .
probably the best option u have for ur system .
unless u go for performance engine or turbocharge ur engine, then its already a different story.

i think there should be a follow up on this .
probably the effect of s-flow, straight, twin exhaust .

and also the effect of straight pipe, orignal path pipe .

btw, after a few months of research, i think its best to stick to the original route + original pipe size.

probably just change to 4-2-1 header,catcon, mid bullet,back bullet, straight muffler.

well to me, i dont want to compensate the fuel consumption just for power. its not worth the modification then i believe .
 

drexchan

2,000 RPM
Senior Member
Nov 23, 2004
2,908
100
3,163
Bukit Jalil
drexchan.fotopic.net
S-flow, straight flow, mid bullet, back bullet, mid box, back box, muffler, resonator.. arrggg.......

I am going to have a long whine about these terms.
 

monsh

Known Member
Mar 27, 2011
157
18
518
Kuala Lumpur
these questions need deeper thoughts .

and the terms used is almost similar .

i think bullet/box/resonator is similar thing .
 

monsh

Known Member
Mar 27, 2011
157
18
518
Kuala Lumpur
yeap, thumbs up for the great article but then again, there's a lot of factor to the performance and its always the "lose some gain some" condition when u play around these things .
 

kumbang

Junior Member
Senior Member
Jan 3, 2011
17
4
503
Selangor
nice write as always.

Yup, wish theres more explaination on pipe size, bullets, s-flow n etc

can i like mix the pipe size, ex : 1.6 till mid then smaller size?
 

drexchan

2,000 RPM
Senior Member
Nov 23, 2004
2,908
100
3,163
Bukit Jalil
drexchan.fotopic.net
nice write as always.

Yup, wish theres more explaination on pipe size, bullets, s-flow n etc

can i like mix the pipe size, ex : 1.6 till mid then smaller size?
Piping and such should be discussed in another topic. I am working with izso to get a condensed and easy to understand article on it.
 

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