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Discussion Starter · #1 ·
I would like to dispel a certain myth that I continually see popping up on this forum. Please note, this is for informative purposes only, it is not a taunt or a challenge.

BACK PRESSURE

Many people believe they lose power due to a loss of back pressure. This is impossible with a 4 stroke engine. To break it down to the basics, an engine is an air pump, the more air it can move, the more power it will make. If this weren't the case, why do you see high horsepower race cars running open pipes of large diameter? Or better yet, turn on some NHRA action and check out the zoomie pipes on a Top Fuel car. ZERO BACK PRESSURE...and they make North of 6000hp.

There are one main ingredient, as well a mathematical equation needed in order to create a proper flow of air through the system from intake to exhaust. Mainly you need to know the cubic feet per minute (CFM) of both the intake (whether Cold Air Intake (CAI) or stock) as well as CFM of your (stock or custom tubing and muffler) exhaust.

To expand more on the cfm required for the intake side. CAIs with a high flowing filter seem to be very prevalent also. One thing to keep in mind, the gains you are netting from these CAI's are mainly at the top end, and realistically you are seeing the gains based on smoother airflow vs. more air flow to the engine. Take a look at the factory air intake tubing, "With its extra "mufflers", does that seem like a very good way to pull air?" Think of it as a straw with more passages to the sides, wouldn't be as easy to take a drink of your Coke as it would have to fill those passages first.

A 5.3l comes out to be around 323 cubic inches. A street engine has a volumetric efficiency of about 85%, so at 6000rpm the 5.3l only needs 477cfm of intake charge. A CAI that flows 900+ CFM is not necessary. A smoother air path however, will cut down on turbulence and make a more consistent fuel mixture that flows better through the intake plenum and valve. On the same note, a 6.0l at 6krpm only needs 537cfm of intake flow.

An easier and not as exact way to do it is based on 2 multipliers when calculating CFM needed for each side, this is the "fast and loose" way. 1.5 x HP for the intake side and 2.2 x HP for the exhaust side, when figuring CFM required. So a 325hp 5.3l needs 487cfm on the intake side and 715cfm on the exhaust side. This is figuring at wide open throttle (WOT) and 85% efficiency. Keep in mind these numbers are for a street vehicle that doesn't live at the rev limiter.

With this being said, your quality paper filter that comes from the General is more than efficient in feeding your pickup. To maximize your intake and exhaust, using a 5.3l engine as an example. You only need an exhaust system that will flow 715cfm based on the above numbers.

So most of your 3.0"+ diameter mufflers are going to be adequate for your DD. If the 3" muffler doesn't flow enough in the brand/model you want, go up in size and adapt it to your 3" pipe. Side note: Mandrel bends are the only way to go, it may be more money, but the end result is much better.

One more thing to note about the numbers from above. All of these are based on maximum rpm, not the low and mid range where your vehicles live everyday. At 2500rpm, that same 5.3l only needs 199cfm on the intake side and 307cfm on the exhaust side.

The below read is borrowed from : ecomodder "Exhaust - beating a dead horse"
This actually has the hard figures on how to find the numbers. This gets way more technical and uses EGT to factor the true number.

Naturally Aspirated Intake Airflow in CFM

cubic inches x rpm
________________ x volumetric efficiency* = airflow in cfm
3456

* Most engine are 80-85 percent efficient on average. At lower rpms, they are generally at the higher end, the good one getting to 90 percent and sometimes more, at peak torque.

Example using a 2.3L (140ci) eng-

140 x 3000
_________ x .90 = 109.37 cfm
3456

To get exhaust flow, you need to know the exhaust temp at the speed and engine load you want to tune for, which is highly variable. I have a lot of data on diesels (max economy in a diesel usually occurs at 550 F EGT, for example) and could make an average "guesstimate" but not much on gassers. The only way to know is to install a pyrometer on your engine and find out. I'll use 600 degrees as an average for a gasser at a moderate load. The "460" and "540" are constants and I don't have time to go into what they are.

exhaust temp in Fahrenheit + 460
___________________________ x intake airflow cfm = exhaust flow
540

Example for our 2.3L at 600 degree EGT (at the previously determined 3000 rpm)

600 + 460
________ x 109.37 = 214.69 cfm exhaust flow
540

You can find the exhaust flow of many performance mufflers at various manufacturers sites and sometimes in performance magazines. OEM parts... you'll have to test yourself. I have done a bit of that and some OEM systems are surprisingly good... some abysmal. Most are OK for low rpm use. As to pipe size, the flow will vary according to the number of bends but see below for some average number for straight pipe. At a given diameter, number of bends and airflow, a mandrel bent system will have 27 percent less back-pressure than a typical crimped system. This I have seen personally on a flow bench.

Straight Pipe Flow By Diameter

5-inch: 2200 cfm +
4-inch: 1800 cfm
3.5-inch: 1400 cfm
3-inch: 1200 cfm
2.5-inch: 900 cfm
2.25-inch: 600 cfm

I have heard people comment about how they lost low end torque after putting on a free flowing exhaust setup. This technically is not the case, what you have done is put on a setup with too much capacity and slowed the velocity of the exhaust. The faster the exhaust pulses can flow to the end of the tailpipe the more they will scavenge the remaining exhaust. There is a lot of science that can be put into an "ideal" exhaust setup. That should also be read as a lot of MONEY.

The important things to remember are:

1. Too small of a diameter on the exhaust tubing is bad (2" pipes are not enough)
2. Too large of a diameter on the exhaust tubing is bad (dual 3" exhaust on your 325hp 5.3l is overkill and counter productive)
3. Scavenging and Velocity are important
4. When buying a muffler or exhaust system, find out how many cubic feet per minute (CFM) it flows....this is key!
- For Example - If you put a 750cfm carb on a 383ci engine, why would you put a muffler that only flows 350cfm?
A good mandrel bent single pipe 3" cat-back exhaust system is adequate for most 5.3l pickups. You 6.0l guys might want a 3.5", especially with some head work or a cam.

When you have a few spare days, take some time on Google and read about the mythological creature "back pressure".

Extended reading:

Great article on exhaust and the development of mufflers:
Exhaust Science Demystified

Another good read:
Dynomax Exhaust System - CHP Insider

One more that has another chart for CFM flow:
How to Calculate Muffler Size and Exhaust Pipe Diameter

This was done many years ago, and is floating around on a lot of the forums. Gives you a good snapshot and some of the mufflers have the accompanying part number:

OEM...............................................................................225
Gibson SuperFlow 788200.............................................311
TTS Bullet Cat Converter..............................................324
Flowmaster 40 series 42540...........................................352
Flowmaster 50 Series.....................................................362
Flowmaster 40 series 43040...........................................392
Dynomax Super Turbo...................................................410
Flowmaster 62631..........................................................435
Carsound Cat Converter.................................................436
FLP Cat Converter.........................................................440
Edelbrock Victor 5535...................................................562
Flowmaster 40 series 435409.........................................576
Edelbrock RPM series 5511...........................................579
Flowmaster Delta Force Racing 54040-10.....................634
Edelbrock 304 series 5560.............................................640
Edelbrock Gen App 5505..............................................708
Dynomax Race Magnum Welded 17216......................711
Borla XR-1 40600.........................................................836
Borla XR-1 40615.........................................................854
Dynomax UltraFlow SS 17263...................................1000
Dynomax Race Magnum Welded 17218....................1000
Borla XR-1 40575.......................................................1100
Dynomax UltraFlow....................................................1133
Edelbrock Victor 5537.................................................1400
Borla XR-1 40450........................................................1400
Borla XR-1 40741........................................................1450
Dynomax UltraFlow SS 17296, 17268........................2200
Dynomax Race Magnum Welded 17220, 17224..............................2200
Dynomax Race Magnum Welded 17225.....................................2600

I appreciate the opportunity to clear up some unknowns in the community!

Have a great day,
Marc
 
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I also agree... but your entire post is nothing new to me

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Discussion Starter · #4 ·
The information isn't new. Lately, I have seen 2 to 3 posts a week about back pressure. Just trying to help the people being misinformed about the topic. It's not directed at those "in the know". It will also benefit those who take the time to search.
 

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Informative post! Would you say true dual 2.5in pipe is too big for my '13 5.3?
 

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Unless this is made a sticky.... I doubt many people will see it once we quit posting here making it an active topic :/. But you're right there are a lot of backpressure questions and lots of people believe a lot of the myths.

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Discussion Starter · #7 ·
If set up correctly, no it won't be too big. The system will need a balance pipe of some sort whether it be an x-pipe or an h-pipe. You will also have to tune out your rear O2 sensors. True duals aren't exactly easy or inexpensive on these pickups. There is more to than just the pipes themselves.
 

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gmcultr said:
Unless this is made a sticky.... I doubt many people will see it once we quit posting here making it an active topic :/. But you're right there are a lot of backpressure questions and lots of people believe a lot of the myths.

sent from my Verizon Wireless Galaxy SIII
Agreed.
 

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what do you mean tune out rear o2 sensors? My truck (06 6.0L)came with dual all the way back with the exception of 2 in 2 out magnaflow muffler in the middle. I would have to measure pipe diameter but im guessing its 2.5"

Good thread and info though. I was never concerned about back pressure as much as i was with sounding and looking good. haha.
 

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If you keep your cats you dont have to have the rear o2s turned off...

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Good stuff!
 

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I agree with OP. Here is an example of the same thing.

When I purchased a 2013 Freightliner Columbia semi truck, we put in a factory rebuilt DDEC IV 12.7L Series 60 engine. We put on a ported/polished/coated exhaust manifold, replaced the waste gate turbo with a larger Borg Warner 171702 non waste gated turbo and put a PTP turbo blanket on it. We also put Walker Megaflow mufflers on the stacks up the back of the cab. All in an effort to increase exhaust flow.

Results? On the hardest of pulls with gross weights of up to 80,000 lb, the exhaust gas temps never exceed 900F. Most of normal cruising, EGT's stay between 600 and 700 on pulls. Fuel mileage is consistently 20% or more better than the national commercial trucking average. I can put this 12.7 Detroit up against 15L Cummins and Cats and, at a minimum, walk the hill with them. Many times, I can walk past them. All at the same time getting better fuel economy.

OP has the idea correct. You want to get that exhaust moving. There is a balance between size and heat of the exhaust. You want EGT's coming out of the heads to be lower, but you do not want to lose those temps very much in the exhaust, simply because too cool of an exhaust downstream will create a stagnation in exhaust flow just like too small of a restriction. It gets really technical, but suffice to say, the OP is darn close to dead on in his comments.

Good Post!!
 

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I agree with everything in the original post & I am not trying to start anything, but I have drag raced since 1987.
First car was a 1987 Corvette with a 383ci about 450hp & over the past 15 years of trying every combo on the planet, I found that the car would run faster with the exhaust on than with it off every time I tried it. So why would the car run faster with an extremely high flowing exhaust than open headers?

Now present day I have a 1969 Camaro with an 800hp engine & it runs faster with open headers than with any combination of exhaust.
 

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K so i have an 07 4.8 n for some time ive wanted to do a cat back flowmaster i want the dual so wat size pipe .ive steered away from the ideawen someone put an eexhauston there truck n blamed back pressure for what ever happened.
 

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I just couldn't resist "stealing" this:

"Back pressure is a very misunderstood entity. The assumption is that larger pipes = lower back pressure and smaller ones = more back pressure. That is a simplistic view of things... but it is within that paradigm that the idea of "back pressure for torque, free flow for top end power" emerges.

It is not accurate.

At any given RPM, what is desired is the maximum flow. You want the gases to be moving out of the cylinders, through the exhaust system, and out the tailpipe as efficiently as possible. According to Bernoulli's law, we know that as pressure increases, flow decreases, and as flow increases, pressure decreases. So we want the lowest possible back pressure at any given RPM.

If you want an engine with stump-pulling torque, with peak power at 2000 RPM, you want an exhaust system that provides the least possible back pressure at 2000 RPM. Such an exhaust system is necessarily going to be smaller in diameter than one which is tuned for maximum power at 5000 RPM on an engine of the same displacement. That smaller diameter does not mean it "has" more back pressure than a wider diameter one! Back pressure is not a static measurement of a given exhaust system like tube diameter or wall thickness. It varies with RPM and engine load, and a tube that provides the least back pressure and the most flow at one RPM won't have the least back pressure or flow at other RPMs.

So, the exhaust that gives the stump-pulling torque has LESS back pressure than the wider diameter exhaust at low RPM. In the larger exhaust at low RPM, there is flow stagnation and turbulence, and the result is that there is more resistance to flow (back pressure) than the smaller exhaust... at that RPM.

Rev the engine toward the red line and the back pressure inside the small-diameter tube will increase, and flow will be restricted. However, in the wider tube, the previously stagnant flow will increase and become more laminar, and the back pressure will decrease.

So each of the two exhausts above has the least back pressure within the RPM range for which it is designed. "Back pressure" as a single number simply does not tell which exhaust is most suitable for any given application. It's all about flow... pressure is secondary to that. If you think in terms of matching the exhaust flow to the flow potential to the rest of the components in the engine, the confusion over back pressure becomes irrelevant.

So, go for the exhaust system that has its peak flow potential in the RPM range desired. If your engine is built for torque, you would want a smaller exhaust system; a revver will want a bigger one.

Also, flow stagnation sounds like drone! Many times you will hear about someone who had too wide an exhaust on his car, and it boomed like there was no tomorrow. Put in a narrower, more appropriate exhaust for the RPM (and displacement... talking NA vehicles here, as power adders act like bigger engines in terms of exhaust flow), and it quiets down noticeably. I had that happen with my Ford Ranger when the exhaust guy inexplicably decided to custom build me a 3" cat back on my 2.9 when I had asked for the Dynomax cat back. The noise was horrendous, and the low end was gone. I went back and got what I had actually wanted, and the bottom end was back, and it was near silent-- drone free, with the same type of muffler (Super Turbo). "

Words from ASCARIS on 01/08/2014.
 

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So if I got an Dynomax 17225 muffler with 3 in exhaust on my 2012 sierra that will
Be about the best flow I could get without overkill?
 

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Discussion Starter · #19 ·
save yourself some money and just get a 3" muffler in the same design, you won't be giving anything up.
 

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69434 said:
I agree with everything in the original post & I am not trying to start anything, but I have drag raced since 1987.
First car was a 1987 Corvette with a 383ci about 450hp & over the past 15 years of trying every combo on the planet, I found that the car would run faster with the exhaust on than with it off every time I tried it. So why would the car run faster with an extremely high flowing exhaust than open headers?

Now present day I have a 1969 Camaro with an 800hp engine & it runs faster with open headers than with any combination of exhaust.
Picture or video of the camaro required.

Sent autoguide.com app via HP Touchpad Cyanogenmod
 
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