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Exhaust Help


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:lol: Ain't it great living in the "Dirty South"? My CEL has been on for over two years now. Mine is throwing the O2 and the emissions charcoal canister code. I wish I could give you some useful input on the dimensions of my aftermarket muffler but, I picked mine off of a '96 R50 at our local scrap yard for $40 (pipe and all)! It sounds pretty nice when I throttle it without the dreaded "drone" at cruising speeds.

I just got mine so I'm not sure what's setting the CEL off, I gotta go get it read by a friend of mine but I'm hoping it's just the filler cap not sitting right since that's such a cheap and easy fix but who knows.

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... but I'm hoping it's just the filler cap not sitting right since that's such a cheap and easy fix but who knows.

 

 

HA! That was my first step. Hey, I don't know where in 'Bama you are from but, I'm heading to Talladega with a group of friends for a mud bog (not bringing any trucks) this Saturday and would gladly bring you my filler cap for free! At least I know that it's good? :shrug:

Edited by devonianwalk
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Since it's a duel in single out I'm thinking of doing a true duel all the way back and just buying two catalytic converters some more piping and then two mufflers. Anyone ever done this or have any thoughts?

If it's a VQ (and I assume VG) it already has 2 cats....

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I've had a Dynomax catback before on a Cherokee. Wasn't bad. Cheap but not much low end grunt. What dual inlet diameter? 2"? Haven't seen many/any 1.75" dual inlets.

 

So....yes, 2"? No, something different?

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HA! That was my first step. Hey, I don't know where in 'Bama you are from but, I'm heading to Talladega with a group of friends for a mud bog (not bringing any trucks) this Saturday and would gladly bring you my filler cap for free! At least I know that it's good? :shrug:

I'm all the way on the coast right now so that wouldn't work, but I really appreciate it though. Y'all have fun at that mud bog!

 

 

Sent from my iPhone using Tapatalk

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If it's a VQ (and I assume VG) it already has 2 cats....

Maybe it does, I'm not extremely familiar with exhaust systems. It has two pipes leading into one metal oval that I assumed was one cat but I guess it could just be a housing for two cats and has one exit pipe.

 

 

Sent from my iPhone using Tapatalk

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Its small, i'm thinking like 2" (maybe even 1.75" or for you Canadians 4.445 cm), i know my Xterra friend ran 2.5, from the CAT's back. Sounds REALLY good, nice and throaty, but this is also the same guy who put a stupid throttle body spacer, so he has his wins and misses, lol

 

-Kyle

 

Kyle is correct, It's just on 2". I went out & measured it.

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Not trying to hijack the thread, but this seems like a good place for this article on exhausts.............

 

"There is a common misconception that engines need backpressure in order to run properly, generate low end torque, etc. That is simply untrue. Backpressure is a bad thing. Always. Take a look at a top fuel dragster...how much backpressure do you think those zoomie headers make? Very little, and those engines produce 6500 hp. So, what is backpressure? Any fluid flowing through a pipe experiences drag on the walls of the pipe. This depends on a number of factors, including the diameter of the pipe, the smoothness of the inside of the pipe, the viscosity of the fluid, and the velocity of the fluid. This drag results in a pressure drop through the pipe. In order for the fluid to flow at all, the pressure on one end of the pipe must be higher than at the other. In an exhaust system, that pressure drop is what we refer to as backpressure. It's pretty obvious that the engine has to produce this pressure differential, so the less power it has to spend making pressure to push the exhaust out, the more power it can send to the wheels. Given that exhaust pipes are pretty smooth, and that we can't change the viscosity (thickness) of the waste gas being forced through the pipes, we are left with basically 2 parameters we can have any control over: The pipe diameter and the gas velocity. Unfortunately, the pipe diameter controls the gas velocity since the volume of gas is prescribed by the engine. So, we really only have one thing we can change. So, bigger pipes allow less pressure drop for a given volume of gas because the velocity is lower. The pressure drop (backpressure increase) is proportional the gas velocity squared, so if I double the gas velocity (by reducing the cross sectional area of the exhaust pipe by half) then I quadruple the pressure drop.

Well, there's an easy solution for that: Just make the exhaust pipe bigger. Bigger pipe, lower gas velocity, less pressure drop, so less backpressure. Wow, that was easy. After all, this is the way it's done for basically any type of commercial plumbing system. Need less pressure drop on a chilled water pipe or a natural gas line? Just make the pipe bigger. But wait, there's a problem....Having a huge exhaust pipe has killed my low end torque!!! What's different? Oh, there's no backpressure!! Therefore backpressure makes torque!

Wrong.

An exhaust system is different than just about any other plumbing situation. How? Because the flow is pulsed, and this turns out to be a big deal. Every time a pulse of exhaust gas runs through the pipe, a strange thing happens: it as it passes, it has a little area of vacuum behind it. Just like a NASCAR stocker running around the track, the pulse generates a little bit of a vacuum behind it. In NASCAR, a driver can take advantage of another driver's vacuum by getting right behind him and driving in it. The wind resistance is drastically reduced. This is called drafting. Well, how big the vacuum behind each pules is depends on the gas velocity. The higher the velocity, the bigger the vacuum the pulse has behind it. Now, this means that I can "draft" the next pulse, just like in NASCAR. In NASCAR, it's called drafting, in an exhaust system, it's called scavenging. You've probably seen this term used when talking about headers, but the same concept applies in the pipe. I get the maximum scavenging effect if the gas velocity is high, so the pipe needs to be small. By maximizing the scavenging effect, I help to pull pulses out of the combustion chamber, which means the engine doesn't have to work as hard to do that. This has the most effect when there's a bunch of time between pulses...in other words, at low rpm. As the revs rise, the pulsed flow becomes more and more like constant flow, and the scavenging effect is diminished. So, at low rpm I need a small pipe to maximize scavenging, and at high rpm I need a big pipe to minimize pressure drop. My exhaust pipe can only be one size, so it's a compromise. For a given engine, one pipe diameter will make the most overall power (i.e., have the largest area under the curve on a dyno chart).

So, the loss of torque has nothing to do with backpressure, and everything to do with gas velocity. So you need exhaust components that are not restricive (manifolds/headers, mufflers) and that are sized correctly for your application. To further dispel the "backpressure is necessary" theory, try this if you want. If you have access to a vehicle with open headers, make a block off plate that will bolt to the collector. This plate should have only a 1" hole in it for the exhaust to flow through. That will give you PLENTY of backpressure, and zero scavenging. Then you can report back on how much low end power it has.
The one exception to sizing an exhaust is for turbo cars. Since the turbo is in the exhaust stream, the gas flow spinning the impeller tends to come out of the turbo with the pulses greatly diminished. In this case, you can get away with running a larger pipe than on an equivalent HP N/A engine because you can't take as much advantage of the scavenging effect."

 

Courtesy; J Branne, DSMTuners.com

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on my 97 (3.3) the only stock piping is the tail pipe the rest has been replaced with 2-1/2" but the tail pipe is stock at 1-3/4" i had to put 2 down sizers on the outlet of my muffler because i was dropping from 2-1/2" to 1-3/4" and couldent find one that droped that far so i have a 2-1/2 to 2 than a 2 to 1-3/4 to get my tail pipe to hook up but from my cats to the muffler its 2-1/2" and i used a single in single out because the PO had it replaced and had custom pipe cut and welded to do the y ill get a pic of it today (the y)

Edited by thorpe991
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