does BACK-PRESSURE increase MPG vs straight thru...

[MadMax]

This was written for Hondas but I think the same principle applies...

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 "Hondas 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 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 Hondas don't need backpressure, they need as high a flow velocity as possible with as little backpressure as possible.

(Source)

[GeneW]

Originally Posted by MadMax

This was written for Hondas but I think the same principle applies...

First thing I object to....

Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently.

Wrong. The aim of the exhaust system is to allow the catalytic converter and emission system to effectively process exhaust gas to reduce emissions. Efficiency is a secondary factor, otherwise damage to the converter will occur.


Come along with me, visualizing the exhaust system and motor....

1. Gas velocity through the exhaust pipe is not continuous. The person who wrote your guide seems to treat exhaust gas as a continuous flow, which is not so. They paid lip service to the idea of pulses but then modeled their explanation in terms of pipe diameter and not geometry of the system, which is a big factor in "tuning" it so that pulses do not interfere with one another or form resonances that MAY cause increases in backpressure.

Exhaust gas "pulses". A pulse will contain a mixture of sine waves and thus different frequencies and thus will interfere with each freqs and pulses from other cylinders (and probably from previous pulses from the same cylinder - depending upon the transit time from the exhaust port to tail pipe.

The pulses will have SOME inference with one another. They may act like waves going down a canal, either reinforcing or canceling one other to some degree. This is where the frequencies present in the pulses matter, some will interfere, others will not. The shape of the exhaust, both in terms of the length of pathways but also a given diameter of pipe, matter. In a continuous flow bigger is better, but that may not be the case with pulsatile waves.

There will be turbulent flow through the interior of the pipe that will effect some viscous drag, as well as boundary layer conditions. This is dependent upon the diameter of the pipe though boundary layer conditions close to the side of the pipe are influenced by gas temperature and heat transfer, how much I'm uncertain.

There be transference of heat between the gas pulses themselves.

There is also mechanical transfer of vibration from the exhaust pipe to the gas and vice versa. Don't know how much of an effect that this will have on pressures and hence back pressure and flow rates.

The rate of gas pulsation is dependent upon the RPM of the motor. My intuition is that there will be an optimal rate of pulsation for a given exhaust system, one that causes the least amount of back pressure. Might even lie in the motor's normal sphere of operation.

We would have to model these effects to come up with a exhaust system shape and diameter of exhaust pipe.

The plot thickens.....

You have a catalytic converter which will alter the chemistry and thus the molality of the exhaust. That will alter back pressure to some degree. To make things even more interesting, you have two oxygen sensors which feedback into the ECU, which will then alter the chemistry of the exhaust, and this matters because fuel efficiency is a measure of average fuel consumption and not any particular singular combustion event. Chemistry is a big deal for fuel efficiency. There may other emission controls like EGR, which will be influenced by the ECU.

We have to view the exhaust as a coupled system, starting at the exhaust port, with influence from the other three exhaust ports, the effects through the exhaust manifold, through the catalytic converter, resonators if any and the topology of the exhaust system itself. PLUS the ECU!!!

I cannot assign weights to some of the effects, especially the catalytic converter, but I think it's safe to assume the follow effects go in this order of precedence... (however one must gather data rather than guess...)

1. RPM, which will be a factor with along displacement to govern total gas volume. We can view at this first approximation a simple "volume of displacement times the frequency that the piston compresses on the exhaust stroke divided by 2 (four stroke engine, every turn a piston rise and fall). You will probably never get any more gas into the system than the motor can deliver - though that Catalytic converter could create pressure through its chemical action. Changes the enthalpy of the gas either more or less to some unknown degree.

The frequency of the gas pulsations and their chemistry will affect resonant frequencies of the system and the interference of the gas pulsations.

2. The ECU will govern the chemistry of the exhaust and its temperature. I think the ECU is where anyone ought to start modifications, if it's legal in your locale.

3. The Yaris's VVT will have some effect upon valve timing, and hence the rate of dwell for the exhaust valve. Since the VVT is under the control of the ECU its effects will be fedback from the exhaust gas itself.

Back pressure from the catalytic converter, which will be governed in part by its temperature and the chemistry of the mixture and changes to the mixture.

I've just described backpressure, by the way, and not mileage. Mileage is a function of quantity of fuel per mile, which is dependent upon the efficiency of the motor, which will be moderated by the ECU.

Where backpressure, inside of the combustion chamber, comes into play is how it influences the combustion process, and indirectly influences the following combustion (since the exhaust gas carries away heat from the combustion chamber and thus influences how the next mixture will behave and influence the ECU through the O2 Sensor). So there is a thermal history effect at play here.

Unless you got a lab, lots of time and professionals to set it all up, I'd leave this problem to the Pros.

Gene