exhaust diameter for an auto???

[steven]

which diameter would you do? I don't know that much on the subject so I though I would ask and get some opinons.

[joph09]

most people go 2-1/4" Inner diameter piping, whether an auto or standard

[Slapshotman7]

Well I believe that the largest you would want to go is 2.25-2.5" diameter, The reason is that the 4cyl relies on back pressure for power and any larger it would take away from the pressure and lose power. If you've got a turbo than you may want to go slightly bigger in diameter.

Thats atleast what I have been led to believe, anyone else care to add?

[evan938]

dont go 2.5". its too much.

[joph09]

why don't you try going for the evo fusion exhaust group buy if everything works out fine you'll have 2-1/4" stainlesssteel direct bold on exhaust no more worries

[steven]

Well because I lost my job so money is tight for me right and I'm not sure when I'm going to have the money to throw around ;) but its always nice to plan ahead that it is.

[Jonny666]

Well I believe that the largest you would want to go is 2.25-2.5" diameter, The reason is that the 4cyl relies on back pressure for power and any larger it would take away from the pressure and lose power. If you've got a turbo than you may want to go slightly bigger in diameter.

Thats atleast what I have been led to believe, anyone else care to add?

NOOOOOOOOO, this is the biggest lie anyone has ever told anyone!!!!! please stop the circle of lies!!!! BACK PRESSURE IS THE ENEMY! it does not help your vehicle in any way! this myth started because someone put on an exhaust pipe that was too large, saw a decrease in performance and assumed that it was because there was "not enough back pressure" THIS IS WRONG!!!!! the truth is; flow of gas and liquid can be divided into 2 types. Laminar, and turbulent. Lamilar is strait flow through a pipe with lower velocity on the walls and increasingly more velocity towwards the middle of the pipe. Turbulent flow happens when you have too little flow in too much pipe. the flow is no longer "guided by the pipe" and it start's to swirl and move in a random fasion. its like a 20 lane highway with no lines on it and grandma is zigzaging back and forth holdin up traffic except everyone is grandma and exhaust doesnt need to pass a drving liscence to be allowed in your pipe. Turbulent flow can be more restrictive than laminar flow. this is why your exhaust piping must be sized for the application AND for the RPM you wish for it to be effective.
As your motor increases in rpm the volume of exhaust gas increases. the least restrictive point of flow is right at the start of laminar flow. from there it get's more restrictive with smaller or larger pipe. you need to size the piping so that the "sweet spot" matches the rest of your mod's and kicks in right where you want it. an exhaust that is WAY too large will be turbulent all the way through the powerband because not enough exhaust flow is created to reach laminar flow. This causes more backpressure than a properly sized pipe. pipe fitter's / plumbers have known about this for ages, they call it something else though, oversized pipe loss or something along those lines. It is however the same concept. Please i beg of you. dont ever tell anyone ever again that backpressure is required for performance. each time i see it on the forum's, i die a little inside. Thank you that is all.

[zx2uner]

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.

^^^^^
There ya go!

[steven]

Alright thanks I think I'm going to go with 2-1/4" anything bigger would seem like a waste to me anyways ;)

[Slapshotman7]

Well ****, guess I was lied to haha. Thanks for clearing it up:)

[PSUsouthpaw]

Alright thanks I think I'm going to go with 2-1/4" anything bigger would seem like a waste to me anyways ;)

I have a 2-1/4 on mine, and I like it. It has a nice sound and it has helped out fairly well. Muffler choice is a big part of how the car will look, soiund and feel. I personally recommend a dynomax super turbo muffler.

Sucks about your job...good luck finding something else.

[steven]

thanks yea I think I'm going to go with the sharkracing 4-2-1 headers then add a new cat then have the 2-1/4 piping with this muffler I'm planing on getting from ford ;) well as soon as I can take the trip to the bank or to my friends house 2 bad my back tire is torn to hell and I don't trust that dounut all the way there and back.

http://www.elantraxd.com/forums/showthread.php?t=20778

my friend has the same kind on his car and its sounds really good ;) a nice rumble and if its to loud there's ways to tone it down :)

[cclngthr]

I would NOT go with any larger than 2 inches because you end up losing more power and the ecu isn't capable of adding enough fuel to compensate. The XD ECU compensates somewhat, but not enough as a GM ECU does (the mapping window is much narrower).

Also the cam profile is designed so the valves will open at times before the piston hits bottom. Here, the piston is has not hit bottom when the exhaust valve opens. The pressure that pushes the piston down escapes out the exhaust system,. which reduces the power.

You do need enough backpressure in the system to have the engine producing power. Too little pressure, not enough power. Too much backpressure, not enough flow, which also reduces power.

When I had my XD, I had a 2.25 inch catback and found the ecu couldn't compensate enough to increase power. I discovered through experimenting that a 1.75 inch catback produces more power do to the above conditions.

[only1db]

i believe it was dyno tested by ford that a three chamber muffler was better than a straight through design...with the same diameter...2.25...

the engine does NEED backpressure to maintain...like mentioned above...

isnt the stock exhaust 1.75?

[Cypher]

stock exhaust is 1 5/8" or something like that. i forget.

[cclngthr]

i believe it was dyno tested by ford that a three chamber muffler was better than a straight through design...with the same diameter...2.25...

the engine does NEED backpressure to maintain...like mentioned above...

isnt the stock exhaust 1.75?

I was told the outside diameter of the inlet pipe of the stock muffler is 1.8 inches. Outside diameter of the outlet pipe is 1.78 inches. About 1 3/4-7/8 inches.