So I have fallen on a few really good deals in the past couple months....

Powder Coated Nerf Bars- $50
Billet Grill- $60
Flowmater 50 series single in dual out (1 yr old)- Free- Gave to friend
Set of Chrome Shorty Headers (in the box)- $125
K&N CAI (in the box)- $40
2 Flowmaster 40 series single in single out- $40
Hopefully getting a set of 33x12.5x15 Wrangler MTR's for pretty cheap too

Now I just need need a mild cam and free weekend now that I have a serious pile of parts laying around. Going to redo the entire exhuast and just cut out the cats (plugged anyway) and run and X-pipe with it dumped at the axle.

Hopefully with the cam, new exhuast and no more plugged cats I should see/feel some serious gains. Then maybe I will throw one of those custom tunes at it and see what happens.

So no real point to this thread, just a update on the project. Hopefully I can get it all done in the next month or so. Right now I am just way to busy.

 

I'll give you $10 for the muffler

 

I am using both the 40's and I gave away the 50 to a friend.

 

You may want to put some kind of restricter on the front of the exhaust to keep some of the back pressure. The 5.3's like a little bit of gas pressure.

 

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 "Engines 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 "Engines 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).

 

Thanks for that info! I knew some of that but was still not sure how it all worked and what the balance was with the back pressure and this. I guess I havent hit that class yet lol. Must be covered in fluid dynamics. Makes total sense tho!

Correct me if I am wrong but the engine basically works as an air pump. Get as much air in and out as smooth and as quick as possible. With the CAI and new exhuast this should go way up. I might even try and rig up some kind of ram air setup while I am messing with everything... well see.

THANKS AGAIN! Great info!

 

Correct about the air pump. The faster you can get the air in and out the better.

A simple way to look at it is view your exhaust pipe(s) as a straw and your exhaust gasses as water, and the air in the engine as a mouth full of water. If you start with a tiny cocktail straw and try to blow the water through it, it's going to take a long time and a lot of effort. If you move up to a regular drinking straw, it's much easier, less time, less effort. Get a piece of 1" PVC and it would be even easier. But eventually you're going to get material that's TOO big. Picture trying to blow a mouth full of water through 3" PVC. You won't be able to blow it all out, there will be residual in the pipe laying on the bottom because you don't have the pressure to blow it through. Same with an engine. I run 4" exhaust because the truck has the power to blow it out. Put 4'' exhaust on yours and the exhaust gasses are going to cool, slow down, and sit in the pipe keeping the other incoming exhaust gas from exiting like it should and negatively affecting performance.

 

This is the exact reason to NOT put a completely unrestrictive exhaust on the vehicle. You want a continuous exhaust flow to help "suck" or keep the flow at a constant rate during a specific range.

I already understood this. That is why I suggested a limited amount of back pressure. Mainly due to the LOWER power band range of these motors. By putting a smaller area of pipe in one area you essentially cause an pressure chamber (Helmholtz resonance) that will want to push all of the air out and then when for that small amount of time inbetween the exhaust cycles, there will be a negative pressure in the chamber area due to the velocity of the exhaust moving out. Therfore, you get a better flowing exhaust due to this back pressure of the chamber that is very UNRESTICTIVE yet causes a little back pressure when the exhaust is being pushed out.

So that is that. Basically the same thing that is said above however, many other principles prove that it is benificial to have a restrictive area to cause this extra velocity to help the exhaust exscape the chambers while allowing the most amount of flow. Hence the cat-back exhaust systems. keeping the catylitic converter in place not only for the environmental benifits but to help keep the unresticted flow yet keeping the back pressure that it would cause.

Something to keep in mind, a Y pipe theoretically cause the same thing as the Helmholtz resonance suggests. It basically causes the exhuast to be sucked out using the pulsing exhaust pressures. That is why older cars use them and can make their exhausts so much more effiecient.

Just something to consider when talking about back pressures.

To read about the Helmholtz Resonance: http://en.wikipedia.org/wiki/Helmholtz_resonance

 

Now, DamnYankee's truck has a turbo which acts like a pressure chamber which keeps the pressure in the manafold area and allows the turbo to push the intake air in the motor (Big air pump). That is why his exhaust doesn't need to have a restrictive area of the exhaust pipes. He already has it in the exhaust manafolds.

Once again a restrictive area.

 

I think we're getting at the same thing.

Typically when people say they want backpressure they aren't aware of exactly what's involved (same with people saying they want 4" exhaust). I've seen people literally beat exhaust pipe in saying they were trying to increase back pressure thinking a whole bunch was a good thing...

Just trying to provide some educational reading.