Exhaust Backpressure Faq

[ScooterBovine]

Originally posted by chunky on http://www.hondavision.com

Backpressure: The myth and why it's wrong.

I. Introduction.

One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

II. Some basic exhaust theory.

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.

III. Backpressure and velocity

Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.

The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good.
The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.

Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.

IV. So how did this myth come to be?

I often wonder how the myth "Mazdas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all of his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.

V. So why is exhaust velocity so important?

The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).

VI. Conclusion.

SO it turns out that Mazdas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

[georgechicken]

I fear the post that this generates.....


"Whoa... so velocity is speed, right? so like, what kind of mods do i need to make my exhaust faster? how much exhaust horsepower does 1" piping give you? Do they have stickers for that?"



good article.. written well, too..

I always figured that one day, I'd take the turbine from a vacuum cleaner and just put it in my exhaust flow and create the ultimate exhaust system.. then i figured that if i hate working on my own car so much, that adding something that needed more maintenace would be stupid.. and just imagine the looks on the faces of people at auto zone when you go in asking for vacuum bags for a MX-3... everyone knows you go to Sears for those..

Daniel

[Tunes67]

LOL I just rewrote this as well and posted it in the Forum Recommendations forum LOL Excellent article too

Tunes67

[jschrauwen]

Originally posted by chunky on http://www.hondavision.com

VI. Conclusion.

SO it turns out that Mazdas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

OK........now give us the magic numbers wrt engine displacement, ie - 1", .95", .85", .75" exhaust diameter per every litre displacement??
I'm very familiar with backpressure/freeflow applications wrt motorcycles and your theoretical appears sound. It may be worth mentioning that when or if people decide on after market headers, that the tube lengths are identical from each bank of cylinders so as to maintain that harmonic pulse at the collector. FWIW

[kiwi_MX3]

My 2 Cents,

There are good books out there on this subject,

Four Stroke Performance tunning by
A. Graham Bell

Buy the book read it cover to cover, its worth it...
as its pactical experince backed by hours of dyno time

gas flow velicity in a tube is controlled by the bondary layer
at the walls of the tube, bigger the boundry layer the slower
the gas will flow...

[bmwm3guy]

OK........now give us the magic numbers wrt engine displacement, ie - 1", .95", .85", .75" exhaust diameter per every litre displacement??

I would also be interested to know what someone could calculate for this.. Any Engineering students out there?

[hgallegos915]

I mean I understand the velocity concept.. but I see some people out there that get thicker picpes or dual exhausts without knowing its velocity what they need and not a big pipe..

[bmwm3guy]

well you're partially right... if you have a 5.7V8 supercharge... a 1 inch exhaust will give you LOTS... no no LOTS of velocity.. but cause a lot of backpressure.. what you need is a balance.

I was considering going with 2.5" pipe crush bent.. so the loss in the bends will avereage it out to about 2 1/4" or so... should do.

[kiwi_MX3]

the book i mentioned in the above post cover's all the math's
as well as variations for differnt setup's

[Yoda]

Originally posted by chunky on http://www.hondavision.com

VI. Conclusion.

SO it turns out that Mazdas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

OK........now give us the magic numbers wrt engine displacement, ie - 1", .95", .85", .75" exhaust diameter per every litre displacement??
I'm very familiar with backpressure/freeflow applications wrt motorcycles and your theoretical appears sound. It may be worth mentioning that when or if people decide on after market headers, that the tube lengths are identical from each bank of cylinders so as to maintain that harmonic pulse at the collector. FWIW

The general rule of thumb for a N/A engine is 1" per litre of displacement.

[Yoda]

the book i mentioned in the above post cover's all the math's
as well as variations for differnt setup's

Another good book to have is a Text book on fluid dynamics. I have also found a lot of good info from HVAC reference books. There is a lot of info out there especially when applying ideas and technology from other industries. With this in mind when allpied to hot gases it is also possible to flow more CFM through a given pipe diameter by quickly reducing it's diameter by as much as 1/2 it diamerter if the velocty is correct. Straight flow chambers mufflers also help to extract more gas flow than a muffler that flows straight through at a continuous diameter. If set up correctly you will also have a positive affect the intake side.

[georgechicken]

But folks, DO keep in mind, that as with EVERY other part of the car, therer are only about a million variables that play into this... sizing alone doesnt constitute the velocity or backpressure of the gasses. Nor does the type of bend (crush vs. mandrel, etc)... Anyone with a basic knowledge of physical science or physics will know that temperature of the gasses has a LOT to do with how it flows. This is why we have ceramic coated exhaust headers and exhaust insulation. Not only does it keep the engine bay cooler, but it keeps the exhaust gas HOT. Ceramics and other insulators keep the gas from transferring its heat (well, much of it) to the base metal of the system, therefore cooling off. We all know that as gasses heat up, their molecules move faster and increase their volume (area it occupies). A hotter exhaust gas should therefore have a higher velocity when exiting the motor because of its added volume it occupies.

Anyone better knowledged care to verify this? Or shoot it down if I'm incorrect? I'm always up for some new learning

Daniel
... i have to go change the exhaust vacuum bags on my car now...

[Nd4SpdSe]

But folks, DO keep in mind, that as with EVERY other part of the car, therer are only about a million variables that play into this... sizing alone doesnt constitute the velocity or backpressure of the gasses. Nor does the type of bend (crush vs. mandrel, etc)... Anyone with a basic knowledge of physical science or physics will know that temperature of the gasses has a LOT to do with how it flows. This is why we have ceramic coated exhaust headers and exhaust insulation. Not only does it keep the engine bay cooler, but it keeps the exhaust gas HOT. Ceramics and other insulators keep the gas from transferring its heat (well, much of it) to the base metal of the system, therefore cooling off. We all know that as gasses heat up, their molecules move faster and increase their volume (area it occupies). A hotter exhaust gas should therefore have a higher velocity when exiting the motor because of its added volume it occupies.

Anyone better knowledged care to verify this? Or shoot it down if I'm incorrect? I'm always up for some new learning

Daniel
... i have to go change the exhaust vacuum bags on my car now...

I think your very right...I read on that too. Typically, as gases expand, they cool down, as they cool down they slow down. It was something I was reading about hollowed out catalytic converters vs straight pipes.
Keeping the gases hot adds to the exaust velocity rule, which also would say that large/dual exaust are bad for performance.

[bigtime]

The general rule of thumb for a N/A engine is 1" per litre of displacement.

So a 5 litre V8 about 5" ?

[Nd4SpdSe]

I'm sure you've seen dual exausts....right?