Stock Exhaust Setup

tyhuck · November 2, 2011

I am currently running a Magnaflow muffler with a 3" single tailpipe. (The truck came with a Flowmaster and a turndown under the cab so I changed it just to get rid of the drone. I have no idea how this truck performed with the factory muffler on it.) and while I like the sound, I wish it were louder. Initially, I went with a long bodied muffler to keep the sound down when I went into the woods during hunting season, but I think that its still too loud to sneak into the woods undetected with. So, I want to have a spare factory muffler to swap out during hunting season, and then put my loud muffler on the rest of the year. Then I figured that since I have a quiet muffler for hunting, I could get some serious sound when Im not hunting. So that got me to wonder what would happen if I just ran a muffler delete pipe on the factory system.

Since joining the forum Ive seen hundreds, if not thousands, of threads on exhaust and mufflers. Ive come to the conclusion that if you are running a stock motor, then a full flow, staight pipe exhaust will actually hurt your performance more than help it. However, I was talking to my local muffler guy, and he said that unless you remove the cats, you will not remove any restriction from the system. And just like the majority of the threads Ive read on here, he agreed that on a stock engine, a performance muffler just makes noise.

So I was wondering, just what part of the exhaust system is the most restrictive. I know that factory cats on newer vehicles are fairly "high-flow", but can they outflow the muffler? If I left the cats on my truck, would it have enough backpreassure to keep some low-end power, or is a muffler a "must have" for decent low-end performance? I know that I can get really nice sound out of a shorter muffler, but if I use a piece of exhaust tubing rather than a new muffler, I can save some coin. But, I dont want cheap out if I cause worse performance on the low-end for towing and such.

spankrz · November 2, 2011

i am like you, i cannot tell left from right on this issue. nobody agrees, and i have never seen anyone post a dyno sheet to prove any of it. i can see how less back pressure would hurt torque, which is what i care about most in my truck, but at the same time nothing beats the sound of long tubes and true duals. (i prefer a flowmaster 40 series, but let's not make this thread into another muffler war ).

Doug_Scott · November 2, 2011

The most restrictive part is the size and location (relative to combustion chamber) of the exhaust valve, and the camshaft specs (lift, duration and overlap).

The tone and the direction of the exhaust is important when hunting as well, deep tones are omni-directional ( they go everywhere), whereas high pitched noise is directional.

tyhuck · November 2, 2011

But, how does the valvetrain affect backpreassure? I was under the impression that a lack of backpreassure would damage the valvetrain, so it made sence to me that the exhaust system on the exit side of the valves was what produced the restriction producing backpressure. But, Im definately not a mechanic so if you can explain how it works to me, Ide really be greatful. I like learning new stuff.

11Silvy · November 2, 2011

But, how does the valvetrain affect backpreassure? I was under the impression that a lack of backpreassure would damage the valvetrain, so it made sence to me that the exhaust system on the exit side of the valves was what produced the restriction producing backpressure. But, Im definately not a mechanic so if you can explain how it works to me, Ide really be greatful. I like learning new stuff.

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 For example, "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?

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.

dlewis1111 · November 2, 2011

Wow, you went completely geek about exhaust. Sorry, didn't make it past the second paragraph.

11Silvy · November 2, 2011

Wow, you went completely geek about exhaust. Sorry, didn't make it past the second paragraph.

I just copied/pasted it...it makes some sense.

redjewel2ss · November 2, 2011

Chris's post hit backpressure spot on!

the most restrictive part of the exhaust is the cats. replacing the stock ones with low profile, high flow cats will be the best thing to improve the trucks performance. also the stock y-pipe's bends are restrictive. replacing the stock y-pipe with one of the same diameter, but with mandrel bends will be less restrictive. the stock muffler is not as restrictive as some may think. but i think your current magnaflow muffler is as least restrictive as you can get without changing up its diameter. if you want quiet during hunting and loud on the streets, Corsa is probably the best exhaust you can choose. i may be biased since i have their exhaust, but it truely works. i don't here it at all at low RPMs and cruising, but when i accelerate hard the deep sound is there. i got my corsa db series single exit exhaust for only $520 shipped.

changing up the exhaust with a new y-pipe and cats can throw off the tune of the engine though. it is important to have it custom tuned if you decide to replace the stock cats. the cat-back portion of the exhaust is pretty straight, that is why you won't notice any difference when changing that from stock

Easytide · November 2, 2011

Wow, you went completely geek about exhaust. Sorry, didn't make it past the second paragraph.

I just copied/pasted it...it makes some sense.

Actually that was a good and acurate article. Thanks for taking the time. From what i,ve learned over the years , all the talk about BP/scavaging...ect was derived from ultra high HP racing engines,where rpm ranges in 9-12 g,s are normal. Most folks dont realize certain race mods not only dont apply to street engines,they dont work as well either . I have dyno.d engines that actualy rpoduced more HP and torque by dumping the headers and big pipes and reinstalling manifolds and a more common semse duel system. I also have never seen a properly functioning Cat Conv effect HP one iota...and we,ve tested quite a few.

tyhuck · November 2, 2011

Im not really interested in making my truck a race car. Its a 4wd for goodness sake. I take it to the woods and occasionally tow a trailer with it. I just want it to perform like it was designed, or maybe slightly better than stock. If I left all of the tubing in front of the muffler untouched from the factory and just replaced the muffler and back, would the 3" tailpipe be too big to create effective exhaust velocity?

Also, if I did go with the factory muffler for hunting situations, would removing the muffler entirely and putting a piece of tubing in its place cause any problems, other than being stupidly loud? I guess I could get a cheap glasspack and put int there when its not hunting season.

TimmyG · January 3, 2012

I am no expert but i would buy an electric cut out, and run your cherry bomb or what ever u want on one side and maybe a stocker out the other so you can pick from your seat

SRMN8R · January 3, 2012

I am no expert but i would buy an electric cut out, and run your cherry bomb or what ever u want on one side and maybe a stocker out the other so you can pick from your seat

There you go. If you don't want to drop the $$ on the electric cut out, you can get manual ones from Summit Racing or Jeg's (I think). A simple y-pipe with a block off plate that's held on with 3 wing nuts. My buddy runs them on his Camaro drag car. Easy as pie.

uh6tdrvr · January 4, 2012

Here is a project, take a water hose and a bucket. Time how long it takes to fill the bucket with the spicket valve open the same amount with you thumb over the end spraying then time if with out your thumb over the end, which fills faster?