I was having a discussion with a friend about why car engines need back pressure a from a point of view of physics (I did physics at college not mechanics lol) it did not make any sense to my as the idea I would have thought is that you need to get rid of exhaust gasses as quick as possible

 

Anyway I did a google and found this article. This seems to make perfect sense from a physics point of view but my mate is now saying he never heard of such a thing and all the car forums he has ever been on say you need back pressure to make the engine run efficiently so you must need it (without being able to explain to me why)

 

So some experts here can surely read this and settle the argument

 

Cheers

 

 

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.

 

 

written by HCF

Edited September 27, 201312 yr by dicky96
speeling

Oh well, let's kick this off. Every forum I've been on where "everyone says" is wrong. Urban myths spread like wildfire and sometimes people who say the Emperor hasn't got new clothes are shouted down by "everyone knows".

 

So, first step: let's have your mate give one link to one forum post thread where someone says backpressure is needed for efficiency.

 

2nd, I too did physics at college and efficiency of any engine is governed by the thermal differential between the heat energy supplied in, and the heat expelled (and don't forget to take off the frictional losses of the things in the engine whizzing round and up and down). You get a lot of heat energy in by burning fuel/air mix and when it leaves the engine, it's at a lower energy state (been burnt up), the energy released expands the gases which forced the piston down. That's it. eff all to do with "backpressure".

http://en.wikipedia.org/wiki/Heat_engine

http://en.wikipedia.org/wiki/Engine_efficiency

 

3rd, even my physics courses covered the 4-stroke engine and the 2-stroke engine. A little bit of standing wave low pressure arriving back at the valve while it's still open on the exhaust stroke could well help scavenge dirty gases out of the cylinder which is a Good Thing even for 4-strokes. And for 2-strokes it's a very good thing, as is the back pressure holding in the charge until the piston moving closed the port. Aided by reed valves in some engines.

 

But "back pressure making engines run efficiently" is one of those million monkey / typewriter bits of nonsense.

Perhaps we should put it on the list of cosmic "truths":

• There's a cheque in the post
  
• I promise I'll respect you in the morning
  
• Of course I won't **** in your mouth
  
• I can get 600 hp with boost jets and a remap
  
• My last run gave me 500 miles on a tank and
  
• dump valves are a good idea.
  

Hahahaha can you tell I've had a highpressure day at t'office?