Great Write-Up on Exhaust Performance
 

Written for Honda specifically but the general idea applies. Looking at the care with which the stock manifold was made, my guess is AMG designed our systems very well for the stock motor.

Originally Posted by honda-acura.net / clubel.ca

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 4cylinder 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.


written by HCF

Great article
 

I always wondered about how an exhaust system built power, especially on our cars. The car was on the hoist the other day, and looking at the exhaust, it is impressive. I remember on my 1997 C36 thinking the same thing. Mandrel bends, quality tubing, free-flow muffler with nice tips.

I had a 1992 500E, and the car was one of the best I have ever driven. Pig on gas though. Well I changed the exhaust on the car and it hurt the performance. I removed the stock resonator and muffler to install a single 3" pipe leading into a muffler (high flow Dynamax). After the muffler (replace the resonator) it lead into another muffler with a single 3" outlet. The car was not the same after. With the article above, I can see changing the diameter and the performance mufflers changed the whole "dynamics" of the system. The exhaust has to be built from the heads back and work together as a "system" Then you take it another level and go from the intake (filter, TB etc), internals (cam etc). heads to the exhaust. Bottom line, I regret doing it. It created some sound, that's all.

Now looking at the exhaust on our car (W210 AMG E55), I see little benefit from the X pipe (isn't the resonator performing this function?:nix:) This can save some weight though. From reading above, I can see headers helping the cause though. Starting the exhaust gas velocity right at the head to increase power makes sense..as long as the rest of the exhaust has the correct dimensions to take the added velocity?

Good article though. thanks for sharing....

Shaun

P.S. The AMG engineers are obviously very good at their craft:y

I've had the same experiences. Over the past 25 years I've spent thousands and thousands on messing with NA and turbo exhausts. It really did seem like a bit of voodoo and with velocity and flowrate needing to be balanced and targeted, I now understand why some systems worked great and other were worse.

I kind of agree with you regarding the weight of the resonator. I might be wrong but unless you cut open the resonator, I don't think they meet.

x-pipe
 

I have read other forum regarding the x-pipe on the W210 E55. The best mod by far for getting some hp on the upper range of the rpm without touching the engine is doing the x-pipe with resonator delete. The dual exhaust pipes on these engine do not meet. That's the best bang for the money without getting into headers. Changing the oem cats to high flow cats would help but mainly for sound. :D

1) The center resonaton do NOT made the X pipe function cause they are not "Cross" flow between the 2 pipes. its only a cavity to decrease resonance & perform sound. This is why the X pipe works a lot better

2) The X pipe loction is very important in the system too, you would install the X pipe as short as possible just before the exhaust manifolds to increase scavengin between the heads.

Just my 2 cents :) and hope that help

Fabio Daniel

Would be nice if our cars can offer such accomodation:D The closes to the headers are the down pipe after the first cats. I guess you will have to put the x-pipe in the place of the 2nd cats and get a 9" shortie cats for the secondary or just straight pipes.:naughty: and take the resonator out with mandrels and straight pipes in place.

As for me, I'll not gonna race my car so I'll just replace the big resonator with the x-pipe.:D

I have heard the cars get loud with the resonator removed.

You are correct my friend! best location for the x pipe is in the 2nd cats place! install some hi flow primary's and replace center resonator by a 45° mandrel bends pipes. Thats the best you can do with our exhaust system!

Fabio Daniel