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the 'lude makes me poor.
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Discussion Starter #1 (Edited)
Back pressure, Exhaust velocity and scavenging.
The myth: “engines need some backpressure.”

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 its consequences are. I'm sure many of you have heard or read the phrase "engines need some backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.

How the myth came about:

It is easy to see how this misunderstanding arises. Lets’ say that Max puts a 3-inch system on his normally aspirated car. He soon realizes that he has lost power right through the power band. The connection is made in his throbbing brain….

Put on 3" pipe = loss of backpressure = loss of power.

Max erroneously concludes that you need backpressure to retain performance. He has ignored the need for exhaust gas velocity to get that scavenge effect.

The other myth: “engines can get burned valves from not enough backpressure”


How this myth came about:

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust for his.

Once these vehicles received performance mods that reduced backpressure, they tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Some basic exhaust theory

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

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 its 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 power band is located 2-3000 RPM you'd want a narrower pipe than if your power band 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.

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).

Conclusion.

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


Cited from various diffrent website. If anyone notices anything i got wrong or did not include please send me a messege and i'll take care of it. -Poet.
 

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the 'lude makes me poor.
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Discussion Starter #3 (Edited)
Thanks a lot for this. It was informative and cleared up a lot for me.

I do have one question though. How do you prevent valve-burning? Through a tune?
well yes it would be through a tune, but, in a PFI car such as the prelude, or pretty much any modern car for that matter, the stock ecu will be able to more then adjust for pretty much any exhaust mod.
 

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the 'lude makes me poor.
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Discussion Starter #7 (Edited)
way to call someone else's hard work and knowledge your own.

http://preludepower.com/forums/showthread.php?t=211254
when did i say that it was my own? i actually didn't copy it from that guys post (witch he didn't write either.) though i copied it, 3 part's of it from various diffrent sites (& changed one part of it, manily because most of them didn't exsplain about the valve burning myth) each of of witch said they had copied it from diffrent sites, and those sites from others. wich is why i was only able to put that i had cited it from various websites and couldn't list the actual authors names.
 

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the 'lude makes me poor.
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Discussion Starter #9
Does this theory work the same with intakes? Would they badass AEM V2 im looking at really be muhc better than my current AEM CAI?
kinda, the scavaging affect (at least i don't think) would applie as much, but the velocity part would, although this would matter much more with the intake manifold & runner desighn, i don't know how much of an affect this would have with an intake. and how much better "any" could be then anouther.
 

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2ludes 1s2000
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Glad to have this sticky. It's almost a common sense thing...

Back pressure is the good friend of emissions.

Emissions are the enemy of power.

The aftermarket sells products that offer freer flow. Why would we be paying so much more for larger less obstructed outlets to improve performance if this whole backpressure thing had any merit.

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REST IN PEACE
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im not going to bother reading this novel

but i am going to say that your ECU needs to be tuned in order for "no back pressure" to work in your favor. just lopping off you exhaust isnt going to give you m4d crazE p0nies
 

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the 'lude makes me poor.
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Discussion Starter #14
im not going to bother reading this novel

but i am going to say that your ECU needs to be tuned in order for "no back pressure" to work in your favor. just lopping off you exhaust isnt going to give you m4d crazE p0nies
thanks for the "fuck what you guy's have to say; but here's my opinon." post.
 

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lol... so technically backpressure isn't back pressure... its actually a combination of rpm and exhaust diameter to create the best flow... not the lack of exhaust flow to create more torque like many may have thought of it as

so my lude in other words is losing alot of power with the stock exhaust?
and 2.25 would prolly be better then 3 inch
 

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well yes it would be through a tune, but, in a PFI car such as the prelude, or pretty much any modern car for that matter, the stock ecu will be able to more then adjust for pretty much any exhaust mod.

Is this any different for forced induction?
 

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:Rock::Rock::Rock:Once again...Poet ownz all:Rock::Rock::Rock:
Oh and for u asses who gonna hait, he didnt say HE WROTE THIS, he simply did a shit load of research to help US people(and u haiters), what exhaust mods do. he didnt claim it, but after all the research he did, he deserves to be able t post it as his. Good job bro! thanks, exept now i gotta lower my exhaust pipe by 1/4.
 

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New title is new
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Pipe sizing is based on horsepower.

Though with a turbo setup you lose all the pulse tuning past the turbo, so it generally is simply bigger = better. Not always though.

On another note, turbos cause between 1 and 3 psi of backpressure per psi of boost. 1psi being a properly sized turbo, 3psi being a poorly sized turbo.
 

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Wow good post. Just read it and it was more info than just clearing up a rumor.

So what would be an sufficent pipe sizing for the H22 with all bolt ons?

Id go with 2.5.
 
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