BACKPRESSURE MYTH

crawl · Feb 12, 2014

This needs to be posted here because too many of you are misinformed...

chris4x4's post #24
http://www.tacomaworld.com/forum/2nd-gen-tacomas/196978-straight-pipe-question-2.html#post4322847

chris4x4 said: ↑

http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm
Quote:

Destroying a myth.

Some say that "an engine needs backpressure to work correctly." Is this true?
No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.
The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.
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 because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then 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.
__________________
Click to expand...

AND from my Prelude forum...
http://www.preludepower.com/forums/showthread.php?t=266540&highlight=exhaust

poet said: ↑

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.

Click to expand...

At the very least, please read it and do your own research if you are still not convinced.
Greg

tooter · Feb 12, 2014

nah nah nah nah nah I can't hear you I can't hear you...

Seriously, thanks for posting those very informative articles. This is the part that's most relevant to me...

Backpressure and velocity.
Click to expand...

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.
Click to expand...

I agree that a larger exhaust system favors top end performance, but with a slow turning high torque low end "industrial" engine like the 2.7, the larger the exhaust pipe diameter, the more you lose the beneficial torque enhancing exhaust gas velocity at low rpms.

So my approach was to run a free flowing long tube header that keeps the exhaust pulses separate from each other for a longer distance (longer time) so that when they recombine, the frequency of the scavenging effect favors low rpms.

Then I ran a very large long straight through 2 1/2 inch core muffler with no low rpm restriction at all. But it feeds into a smooth uniform diameter mandrel bent tailpipe which is just slightly (1/8th of an inch) smaller than the stock exhaust diameter to retain good exhaust gas velocity at low rpms.

I'm not interested in engine performance above 3,000 rpm because that's 90 miles per hour in 5th gear which is faster than I ever need to go. I am, however, very interested in getting the most torque at very low rpms because that is exactly what makes a heavily loaded work truck more pleasant to drive.

The combination of an intake manifold spacer making the individual intake runners 2 inches longer than stock, and the long tube header making the individual exhaust runners 5 inches longer than stock. Lowered the torque peak by 900 rpms, while spreading the increased amount of torque over a wider rpm range.

Greg

crawl · Feb 13, 2014

tooter said: ↑

nah nah nah nah nah I can't hear you I can't hear you...
Click to expand...

Ha.... you crack me up.

I got more to say but running late for appt. will finish my response later

crawl · Feb 14, 2014

tooter said: ↑

...I agree that a larger exhaust system favors top end performance, but with a slow turning high torque low end "industrial" engine like the 2.7, the larger the exhaust pipe diameter, the more you lose the beneficial torque enhancing exhaust gas velocity at low rpms.
Click to expand...

I never thought or said the 2.7 Tacoma's exhaust tubing was too small. I think the size is perfect and even suitable for extra power. It's way larger than my factory honda exhaust, with smoother bends and transitions. I think its good for even low boost. My issue is with the muffler...the factory muffler MUST have restrictions in it because that's what makes it quiet. The same principals of air flow apply to the muffler... highest velocity, lowest restrictions.

So my approach was to run a free flowing long tube header that keeps the exhaust pulses separate from each other for a longer distance (longer time) so that when they recombine, the frequency of the scavenging effect favors low rpms.
Click to expand...

yes.. sounds good.

Then I ran a very large long straight through 2 1/2 inch core muffler with no low rpm restriction at all. But it feeds into a smooth uniform diameter mandrel bent tailpipe which is just slightly (1/8th of an inch) smaller than the stock exhaust diameter to retain good exhaust gas velocity at low rpms.
Click to expand...

This sounds good. I assume the factory muffler is smaller than 2.5" and way more restrictive. So you definitely have opened up the system at the muffler over the stock, regardless of that slight neck down.

But just for information...

Where ever you "neck" down the system, whether at the cat or at the muffler, that's the size of the entire length of the tube. I've hand my hands on many mufflers and they are not created equal. Every manufacturer handles transitions differently. The first 3" Dynomax muffler I bought had a pitched inlet that reduced the opening to almost 2.5" ID. That was completely unacceptable and I returned it. I've seen all my friends mufflers in addition to the several I went through...and they are ALL completely different.

I'm not interested in engine performance above 3,000 rpm because that's 90 miles per hour in 5th gear which is faster than I ever need to go. I am, however, very interested in getting the most torque at very low rpms because that is exactly what makes a heavily loaded work truck more pleasant to drive.
Click to expand...

Very specific performance goals.. much different than most of us non-work truck drivers.

The combination of an intake manifold spacer making the individual intake runners 2 inches longer than stock, and the long tube header making the individual exhaust runners 5 inches longer than stock. Lowered the torque peak by 900 rpms, while spreading the increased amount of torque over a wider rpm range.
Click to expand...

Proof is in the dyno graphs.. all the mods you've done have worked well for your goals. That's pretty kool to see your hard work pay off. Congrats man

tooter · Feb 16, 2014

crawl[OP] said: ↑

I assume the factory muffler is smaller than 2.5" and way more restrictive.
Click to expand...

The stock muffler core is exactly the same inside diameter as the 2 3/8" OD exhaust tubing except it has internal chambers and baffles to cancel out the sound waves. And it actually does that quite effectively. I like a little sound so I got a 2 1/2 inch core MagnaFlow muffler just like this...

except that it's longer and larger in diameter. It's positively huge and about twice as heavy as the stock muffler.

So you definitely have opened up the system at the muffler over the stock, regardless of that slight neck down.
Click to expand...

Yes. There's definitely no restriction in the muffler. I could see from one end to the other. The muffler is center/offset, so the perforated mandrel bent core is smoothly curved between inlet and outlet. It's a simple beautifully executed design that sounds low and clear with no drone.

But just for information...

Where ever you "neck" down the system, whether at the cat or at the muffler, that's the size of the entire length of the tube.
Click to expand...

Yes. I understand. My restriction is right at the end... the 2 1/4 OD tailpipe. I had already tried a smaller 18" long 2 1/2" core straight through MagnaFlow with the larger 2 3/8" OD stock tailpipe and gave up too much low end.

I've hand my hands on many mufflers and they are not created equal. Every manufacturer handles transitions differently. The first 3" Dynomax muffler I bought had a pitched inlet that reduced the opening to almost 2.5" ID. That was completely unacceptable and I returned it. I've seen all my friends mufflers in addition to the several I went through...and they are ALL completely different.

Very specific performance goals.. much different than most of us non-work truck drivers.

Proof is in the dyno graphs.. all the mods you've done have worked well for your goals. That's pretty kool to see your hard work pay off. Congrats man
Click to expand...

Thanks, man.

Greg

crawl · Feb 16, 2014

Great minds.......we have the exact same Magnaflow muffler except mine is 3"

1moonshine2 · Feb 16, 2014

I'm glad somebody posted the back pressure myth articles. I was too lazy to look it up, and post links. Nobody is going to pay any attention to it anyway.

A muffler alone isn't going to do much of anything except change noise level.

After market mufflers are designed to do one thing: get money from your pocket to the the pocket of the seller...

tooter · Feb 16, 2014

1moonshine2 said: ↑

After market mufflers are designed to do one thing: get money from your pocket to the the pocket of the seller...
Click to expand...

...but they sound really neato.

RdRunr · Feb 19, 2014

1moonshine2 said: ↑

I'm glad somebody posted the back pressure myth articles. I was too lazy to look it up, and post links. Nobody is going to pay any attention to it anyway.

A muffler alone isn't going to do much of anything except change noise level.

After market mufflers are designed to do one thing: get money from your pocket to the the pocket of the seller...
Click to expand...

I think a muffler can make a difference. See my posts at:http://www.tacomaworld.com/forum/4-cylinder/306689-tacoma-noob-exhaust-question.html#post7877866

I drew the black line between the points to get an average MPG before and after changing the muffler. I see about a 1 MPG improvement. This was on a '98 F-150 with 4.2L engine and 5-spd.

crawl · Feb 20, 2014

RdRunr said: ↑

I think a muffler can make a difference. See my posts at:http://www.tacomaworld.com/forum/4-cylinder/306689-tacoma-noob-exhaust-question.html#post7877866

I drew the black line between the points to get an average MPG before and after changing the muffler. I see about a 1 MPG improvement. This was on a '98 F-150 with 4.2L engine and 5-spd.
Click to expand...

^^^

RdRunr said: ↑

Old truck MPG plotted below. I did 10 before and 10 after the new muffler. The red line is where I changed the muffler. The black lines are roughly centered around the MPG, sorta like an average. The first point after the new muffler is low because I was just about at the end of the tank so it is really the pre-muffler MPG. The 2nd point after the muffler is low cuz I was really revving the engine and shifting way late; enjoying my new muffler! Once I got back into my routine driving, kinda hard to say there wasn't an increase in MPG. Don't know if the factory muffler was real bad, or the new one is much better. Probably a combination of the two?

Click to expand...

I totally agree!!!!!!

If the muffler is a restriction in the exhaust system, changing it out for a less restrictive better flowing muffler will increase performance and MPG.
Thanks for posting your experience RdRunr.

BlazingTaco · Aug 31, 2016

I just had the a custom made cat back installed on my '16 this last Friday. I did it because I hated hearing so much sewing machine noise from the injector cleaning cycle on the motor and the stock exhaust was already louder than a stock exhaust should be, but the tone was ugly. We used a Magnaflow 2.5" core even though Magnaflow themselves use a 3" setup on their retail kit. The result was a near silent top gear experience and improved fuel economy on the road trip back over the weekend. I would say I gained at least 1 mpg or more just by opening up the exhaust. Backpressure is not the key here, it's pulse scavenging that makes the difference and when you uncork an exhaust too much that scavenging is affected by loss of velocity, it's all about balancing scavenging and velocity.

Also, worth noting weight savings. The factory trash can muffler is huge because of all the baffles and chambers inside, it weights a ton compared to a similar straight through or baffled aftermarket unit. You can save 20-30 lbs depending on the application being replaced. Every little bit counts when you're toting tools, gear, or a boat.

tooter · Aug 31, 2016

I totally agree. It's all about reflected pulse timing. The longer you can make the "plumbing" on both sides of your engine (intake and exhaust) the more torque you'll get at lower rpms because you lengthen the elapsed time of the reflected pulses by adding more distance for them to travel. Longer distance, more time, lower rpms affected.

I originally went from stock to Magnaflow and then from Magnaflow to Flowmaster. When I dyno tested all three mufflers head to head, Flowmaster won. As you can see from the chart, they're actually all pretty close.

Red: stock.
Blue: Magnaflow
Green: Flowmaster

The Flowmaster Series 50 Delta Flow muffler lowered the peak torque by 100 rpms from 2,900 to 2,800 rpms (stock peak is 3,800 rpms) and was slightly better overall at lower rpms.

Greg

BlazingTaco · Aug 31, 2016

Flowmaster droned like crazy on the last vehicle I used one on though.

tooter · Aug 31, 2016

That depends on which Flowmaster you used and whether or not your tailpipe runs to the rear of your vehicle. The 50 series Delta Flow has two Helmholz resonators built into it to cancel sound waves of different frequencies.

This is how it sounds...

https://www.youtube.com/watch?v=cMv-imsBZLI

This is how it sounds in normal driving...

https://www.youtube.com/watch?v=FBwx6WIfQTg

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