Myth or Reality, low backpressure=low torque

[whamo]

Excuse me if your not a motorhead, but I thought those of you who are might be intereseted in the following. Its often been repeated that if you dont have enough backpressure the torque output of your engine will suffer. I have even repeated this statement. However I have never heard any proof of this "fact". I read an exhaust system comparison in one of the automotive magazines where they dyno tested on a V8 engine, back to back 2.5,3.0 and 3.5" exhaust systems. Torque and horsepower increased with each increase in exhaust system diameter. A quick google search revealed this article. Which I thought was quite revealing, and dispels some commonly held myths about exhaust system backpressure.

"xhaust backpressure the myth

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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 "Engines 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 "Engines 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)."

__________________

Steve Frisby

1971 Datsun 240z-under restoration(wifes car)

2004 Nissan 350z (wife thinks its her car)

1971 455 Stage 2 skylark custom

[Arne]

No fair, Steve. That's not enough choices. How about a choice that says, "It depends on many other inter-related factors."

[whamo]

Sorry Arne, I remember you and I talking about the exhaust system diameter affecting torque output. I guess it is more accurate to say that exhaust velocity will affect power output. The summary of the article is that backpressure robs horsepower. It doesnt follow that a larger exhaust system automatically produced more horspower, because exhaust velocity may suffer. In summary, a well designed exhaust system that reduces backpressure while keeping exhaust velocity high will increase horsepower and torque output.

[hls30.com]

So since we know the volume and valve timing of the pulses, and the velocity of exploding gasoline, we could fairly easily compute the proper diameter for a straight exhaust...not that bends, losses to transitions, and god fobid the resistance of a turbocharger...

Will

[Walter Moore]

I always wondered if people were confusing "back pressure" in a 4 cycle engine (which is bad) with the standing wave formed in a properly tuned exhaust on a 2 cycle engine (which is essential)

Not many people today have a lot of experience with 2 stroke engines, but back when I was in high school and college several of my friends had 2 cycle motorcycles. When those things hit the RPM where the standing wave forms (what they called "on the pipe") the horse power output went WAY up. I had a friend who dropped his Kawasaki 550 three cylinder bike repeatedly because he would forget about that and absent minded-ly accelerate gently down the road, only to have the thing fly out from under him when he hit the magic speed. fortunately for everyone involved he sold the thing before it killed him.

If the exhaust on a 2 stroke isn't literally tuned to produce that standing wave, the performance really suffers. (The legend is that Yamaha originally only made musical instruments, but got into motorcycles because they were involved with tuning the exhaust pipes on other manufacturer's products.)

[JonnyRock]

Point V is describing the "Venturi Effect" where a low pressure zone is found after some kind of air restriction and eddies are created.

[Oiluj]

OK... So what's the optimum pipe size for a Z? I have 2-1/2" and think it's to big. I'm thinking of going to 2".

[bpilati]

When designing systems for flow, it's always good to reduce losses.

[doradox]

Too much or too little exhaust gas velocity is bad. However, the thing that really matters is pressure wave tuning. Header pipe diameter and length as well as collector configuration,size, and length.

This article

http://www.burnsstainless.com/TechArticles/Theory/theory.html

explains things fairly well without resorting to garden hose analogies.

Steve

[ozconnection]

Too much or too little exhaust gas velocity is bad. However, the thing that really matters is pressure wave tuning. Header pipe diameter and length as well as collector configuration,size, and length.

This article

http://www.burnsstainless.com/TechArticles/Theory/theory.html

explains things fairly well without resorting to garden hose analogies.

Steve

I agree with this.

I substituted my original Nissan factory exhaust for a modified system a while ago now. I have absolutely no regrets. I wanted more low speed power and torque and that's exactly what I got.

On another thread, "the great exhaust debate", I posted a dyno graph of the results of my new and improved exhaust system. Even then, I had a couple of people doubt the value of a well designed system. These people were well educated but simply refused to accept the truth in what was being shown to them. :stupid:Look at the graph, there are no 'dips' anywhere in the low end of the rev range, (dyno) proof to me that low backpressure really is the key to improving torque.

Cheers.

[John Coffey]

As I've always said, "There's no such thing as back pressure." Its just pressure. Funny thing is, the pressure will always be highest right at the exhaust port if the exhaust system is properly designed.

[whamo]

As I've always said, "There's no such thing as back pressure." Its just pressure. Funny thing is, the pressure will always be highest right at the exhaust port if the exhaust system is properly designed.

That is a good point. I believe mostly we are dealing with semantics in the discussion.