Exhaust Sizes

[[NVIOUS]]

Has anyone run a 3" single exhaust on a TJ? I would run the usual 2.5" like most, but I have some shiny 3" stainless Magnaflow mufflers & Magnaflow hi-flow cats sitting under my drag car. Seeing as the drag car is gonna get a complete new system soon I was wondering how they'd sound on the TJ, that way I'd only have to pay for piping on the TJ. Reckon 3" is too big?

Recently purchased an old chev ute for tow duties, so was planning on splitting the drag cars old twin 3" system for a single 3" for the TJ & Chev haha. If its gonna be too droney or lose all low-down torque I'll just leave the TJs system be for now

Also did some research and most seem to say the newer TJ exhaust manifold isn't worth changing for aftermarket headers if its not leaking?

05' TJ Auto, engine dead stock, if it matters

[tjnick]

i have 2.5 on mine n it sounds good.
but my brother has 3" dumped under the driver seat on his and it's louder than a v8 it's crazy. toughest thing i'v heard.
if you have a 3" chillin around. go that

[bally12]

I thnk with a 3" zorst you would lose to much back pressure causing a loss in torque. Someone may be able to confirm this

[bigredtj]

back pressure is a myth - exhaust = out

Is it? Why do cars lose power and torque on a dyno then??

[Jeeps]

I think there is some truth to the backpressure issue but mostly relating to how power is distributed through the rev range. When i upgraded to my 2.5 system from my stuffed cat & muffler it felt like i had a whole new engine, especially in the upper rev range where the TJ engine usually doesn't like to reside.

I also noticed that where the 2.5" pipe meets the factory pipes in front of the cat, they are the same size all the way up to the headers. So i've got factory 2.5" piping from the header junctions to the cat LOL

I reckon do the 3" from the cat back and see how it goes. If there's too much drone i've heard that reducing the dump pipe behind the muffler on TJ's to a smaller size knocks out the drone.

I've posted it before but here's my TJ with my $550 2.5" cat back system just after installation. And yes i can't believe i waited 8 years to do this! It's a little bit quieter than it was when new:

http://s421.photobucket.com/albums/pp300/jeeps_photo/?action=view&current=Exhaust01.mp4

cheers

[TERRA Operative]

Got a heads up about this thread from sMashed... :)



The requirement for back pressure in exhaust systems is a complete myth. The presence of back pressure is a direct indicator of a restriction in the exhaust (or no pressure will build up) and any restriction in the exhaust (or intake) will hurt performance.
The very idea of and everything that's done to an engine to produce more power involves increasing the amount of fuel and air going in and maximising the extraction of the burnt gasses to be replaced with more fuel. Back pressure prevents this from happening and as such, restricts power.

The confusion comes from the misuse of the term 'back pressure' when actually describing 'flow velocity'.

Imagine a river. Through the rapids, the flow is fast just like flow through a small pipe. But when there's a flood (you plant your foot) the rapids can't handle the amount of water and break their banks. In a pipe, this related to a severe restriction and is true back pressure.
Now, imagine after the rapids when the river is big and wide. It flows slowly, which in a pipe isn't strictly ideal, but is better than trying to cram too much down a small pipe. This lack of flow velocity is what people mistake for back pressure, which it is not.
When the flood comes (your foot hits the floor) the river can handle the flow due to it's increased size.

Try blowing water through a small straw. No matter what you try, you won't get much because of back pressure. Now do the same with a garden hose. See the difference?


But how does this relate to power delivery in our cars?

What happens with a small pipe is flow velocity is optimum at low RPM so power seems good. Punchy at the lights etc. When we approach redline, the pipe becomes restrictive and power drops off.
So we go and fit a larger pipe. But wait, the low RPM isn't as good.... Why? Oh, no back pressure.

WRONG!!

It's the loss of FLOW VELOCITY, like the big river. However, when we reach redline, the pipe will be able to handle the flow and will not restrict the power.
It's a bit of a trade off (like almost everything when modifying cars). Big pipe, slightly less low end power, but a far bigger improvement in top end power.

WE HAVE NOT LOST POWER. Peak power will only improve with less exhaust restriction. If it does drop, you have problems elsewhere that need to be addressed. (Inadequate fuel delivery, intake restriction, increased flow blowing out the spark etc etc)

What we want is NOT back pressure but FLOW VELOCITY. The magic number is about 240 feet per second or so IIRC. any more and you get restriction, any less and the pipe is unnecessarily large and laminar flow will not occur.

Now, the problem we face with an exhaust is that flow is never constant, instead being a function of RPM. More air in, more air out.
We are aiming for the optimum flow velocity at maximum power output. On many (properly setup) N/A cars, this will be at redline.
So how big do we want our pipe? Use the following as a rule of thumb:


A pipe will flow 115cfm per square inch of cross-sectional area. You need 2.2cfm of flow capacity per horsepower.


Plus a bit for bends, mufflers and cat.


Choose a pipe size (rounded to the nearest pipe size, preferably up) to suit your application and there you go.
The above rule is for straight pipe, bends and mufflers will drop the flow rate slightly, so use mandrel bends and straight through mufflers. But you were going to do that anyway, right? :)

Remember, ANY pipe beyond the last merge in a set of properly designed headers (not those POS stock manifolds) is a restriction. Once the exhaust pulses have reached the merge, created the vacuum back up the pipe (the scavenging effect) and left the merge, there is no further need for pipes or the like. Anything beyond there is just inhibiting the flow of new exhaust gas into the pipe.
Unfortunately we can't drive cars that sound like funny cars, so we have to compromise with a full exhaust.



Remember, the requirement for back pressure is a myth, we want flow velocity. For a naturally aspirated 4L engine, a 2.5" mandrel bent, straight through system will see you out for anything you'll do to the engine.





Just sayin' :)

[bigredtj]

Cool so back pressure is the wrong term, so my question would be what characteristics do you seek from a jeep engine - its certainly not redline valve bouncing top end, its mid range and torque. Peak output is pretty pointless.

So i would assume from what you have said there is definitly no point in having a huge exhaust in a wrangler or the like as you will loose power everywere except at the top were you dont need it?

Which gets back to the whole point 3 inch is overkill for a wrangler and you will probably loose power were you need it to gain a barely noticeable amount at the top end were you dont.

[Jeeps]

So 'back pressure' does exist, but it should be called 'flow velocity' ;)


cheers

[[NVIOUS]]

Well i decided to slide under the jeep and actually measure my stock exhaust, and was suprised that my factory '05 is already 2.5" after the two cats merge into single pipe. So surely 3" wouldnt be too bad if its only 1/2" bigger than stock? Currently the Jeep has bugger all highway overtaking power with the 33s so figured it may help slightly together with a snorkel & filter? Dont want to re-gear for the 33s just yet

My factory piping has a big dent in it just after the front cat, guessing thats not factory but not sure how it would get there, theres nothing it could foul on. See below:

http://i6.photobucket.com/albums/y236/VNAero/My%20Jeep%20Wrangler/DSC00791.jpg

If I was to install my magnaflow 3" cat, reckon I could just merge both header collectors (keeping top O2 sensors in stock location) into a single 3" down to the cat, and simply extend the bottom O2 sensors and put them side by side in the same pipe after the cat? This would keep all 4 O2 sensors and hopefully keep the ECU happy. Any ideas if I havent confused eveyone? Or am I better off keeping those stock cats, fixing that dent and running 3" cats back?

Might just to do it, got a 3" oval & 3" barrel magnaflow mufflers, if its too loud i'll try n quiet it down later...

[Macca2801]

Nvious...mate with that crush in the tube your system is about a 1.5" at best! You may find just sorting that and getting a more laminar flow in the exhaust will improve performance across most of the rev range (not just peak)!

Matt.

[TERRA Operative]

So 'back pressure' does exist, but it should be called 'flow velocity' ;)


cheers

Two different things. Back pressure is the pressure built up in a pipe from pushing the gas through it, flow velocity is how fast the gas moves through.





3" is too big for a jeep 6 and probably even an 8. I would stick with 2.5" straight through, with mandrel bends for a good balance.

The 1/2" difference between 2.5" and 3" equates to a pipe that is 69% larger in cross sectional area... That's getting a bit too big for a 4 liter..........

[bally12]

I know Its a bit off topic but we are all here to learn, how does a 3" system suit something like a wrx, a 2L 4cyl turbo and not a 4L 6cyl jeep engine?? I know the turbo has something to do with it but what and how?

[TERRA Operative]

A turbocharged engine running 14.7PSI boost (double atmospheric) is breathing twice the air of an equivalent engine of the same capacity. More or less will breathe in proportion.

Ie, my 1.6 liter corolla on full boost breathes the same air as a 3.2 liter, hence the 2.5" exhaust on that car.
That's where the extra power comes from. More air/fuel in = more power = more exhaust gas out.

Also the WRX will have a much higher redline, (My corolla is good for 8000 RPM on stock internals) so at redline, more air is being pumped through than the Jeep at redline (assuming the WRX breathes the same amount as the Jeep at the same RPM).

[Windex]

Just a bit more reading for those that are interested in the subject

http://www.hondacivicforum.com/forum/showthread.php?t=3064

[stroked]

i see hondacivic in there and now im not even gonna bother.

[Doctor W]

I know Its a bit off topic but we are all here to learn, how does a 3" system suit something like a wrx, a 2L 4cyl turbo and not a 4L 6cyl jeep engine?? I know the turbo has something to do with it but what and how?

I dunno.......I don't know how Jap cars (or Korean, Chinese or whatever) work!;).....

...........at least that's what I always say!:D

[Doctor W]

A lot of info on here already ......so here's another 2 cent's worth:

.....FIX/or replace that restriction first off!!!!!

3" is TOO big for a Jeep 4.0, especially stock like you've got!......2 1/2" is plenty for everything but a totally over the top 4.7 stroker that'll never be driven like a regular Jeep.

Have you considered using that neat Magnaflow Cat and muffler with 2 1/2" pipes, using adapters to connect them up???? I mean you've already got them why waste them? ......and as you say, you could use the other side of it for the Chev.....I used to have a Chevy Blazer 4WD with a 400 SB and I had a 2 1/2" system in that - it worked very well thank you!

You really need to regear if, with 33's you reckon you're too far down on power.

[Tassie Jeeper]

i am making a 3" stainless steel exhaust for mine and buying a 4" high flow sports cat which i will flair the exhaust from 3 to 4 for cat and back to 3 again for the rest of the exhaust as the cat is the most restrictive part of an exhaust system. then ill run an oval hotdog with an s bend in the perfirated pipe just to reduce the bark:mrgreen:

[CRUSHU]

I have a stock TJ driveline, and need to replace the exhaust from the headers back.
I will be getting a press bent system, thinking 2.5", with a few good mufflers, to quieten down the horrible sound 6 cylinder engines make. I want it as quiet as stock, or more. I will also be fitting a new cat.
I will be regearing to 4.56's with 33" tyres.

Does that sound about right?

Also, whats the size relationship for the air inlet? I'm thinking of making a stainless snorkle for it, and am not sure whether 3" (mandrel bent) will be enough.
Would there be any gain on replacing the ducting from the air box to the throttlebody with larger stainless too?

[Jeepin It]

my understanding of back pressure is that you need it.. big time...... I just think its called the wrong thing.. Its not relating to the amount of flow, but lets go back to basics here. Big bang happens in cylinder, valve opens, that spent gas needs to go somewhere now the quicker that can get out of the cylinder the better. A massive exhaust dusnt help this because it reduces vacuum in the manifold because it is more free flowing. their needs to be some restriction so that when it is exiting the exhaust is it still pulling a vacuum so it pulls the gas out the next cylinder

[sssboi]

What size is the stock exhaust on the TJ

[Jeeps]

I have a stock TJ driveline, and need to replace the exhaust from the headers back.
I will be getting a press bent system, thinking 2.5", with a few good mufflers, to quieten down the horrible sound 6 cylinder engines make. I want it as quiet as stock, or more. I will also be fitting a new cat.
I will be regearing to 4.56's with 33" tyres.

Does that sound about right?

Also, whats the size relationship for the air inlet? I'm thinking of making a stainless snorkle for it, and am not sure whether 3" (mandrel bent) will be enough.
Would there be any gain on replacing the ducting from the air box to the throttlebody with larger stainless too?


How old is your system and how many klm has it done? I'd be replacing the oxygen sensors too ;) I wouldn't say that it's a terrible noise from the TJ. Did you look at my video earlier in this thread? I will admit though that as i'm getting older quiter things please me more LOL ;)





What size is the stock exhaust on the TJ

Mine (2002) is 2.5" factory from the headers to the cat and then from the cat it was 2.25" down to the tailpipe. Weird i know. !? I suspect it was to cut down on exhaust noise.


cheers

[CRUSHU]

An efficient exhaust will have smaller tailpipes than the rest of the system. As the gas cools, it contracts.
Thats why ricers and their Milo tin mufflers make me laugh....

[CRUSHU]

Mine has 200'ks, 97 model

What indications would faulty sensors give?

[sssboi]

What would be a good way to get a bit of a deeper note from the exhaust?

:rolleyes: I don't mean making it sound a honda civic with a 3" muffler (no offence)

[CRUSHU]

I've never heard a nice sounding 6, they are best quiet!

[Tassie Jeeper]

I've never heard a nice sounding 6, they are best quiet!

mine has a rather meaty note and thats just standard with a shortened muffler its deep and rumbly without being loud (soon to change)