exhaust tube diameter?

[BuffDragon]

I've got a mild 302. nothing fancy. no smog crap, small 4 barrel on the top.
existing setup is dual exhaust. stock manifolds thru 2" pipe to super quiet mufflers, then out the back 2" pipe. pipes/mufflers run on either side of the driveshaft, then straight out the back on either side of the rear tank.
its too quiet, as it was set up by PO as a hunting rig.
I'd like a little rumble since its now mostly a sunny day cruiser and ranch toy.

buddy of mine hooked me up with a pair of stainless magnaflows. single 2.5" in/out.

what's the best way to do this?

I'd kinda like to run either 2.25" or 2.5" pipe all the way from the manifolds, but I have concerns about that:
1: what do I need to hook up to the manifolds (since existing is 2" pipe)
2: is there enough backpressure running 2.25/2.5" the whole way or should I stick with the dual 2"
3: would I be better off crossing the driver's side over to the passenger's side and running both mufflers under/on that side to get away from the fuel lines/etc
4: if I put downturned dumps on the mufflers, will I get fumed in the cab with the top off?

thanks for the input yall.
this isnt a high rpm racer and I'm running a np435, so I've got low power (gearing) when I need it off road.
I remember seeing someone that did true duals all under the passenger side, but cant seem to find the pics and the thread about it.

 

[blubuckaroo]

The factory head pipes are 1-3/4”. If you run them to duals, you really can’t get much better flow than a single 2-1/2” since the duals are limited by the head pipes.
If you use πr^2 :
[1/3-4”] 1.750/2 = .875 X .875 =.766 x 3.1416=2.41 X 2 because of duals = 4.81 sq. inches.
[2.1/2”] 2.50/2 = 1.25 X 1.25 = 1.56 X 3.1416 = 4.91 sq. inches.
However, if you’re like me and would rather have the sound of true duals, you’ll do what you have to.

 

[BuffDragon]

so I guess that begs the question:
is there much point in going from 2" (existing) to either 2.25" or 2.5"?
or simply just just put the mufflers in where they are now, leave the 2" pipe as is and just use an adapter from the 2" to 2.5" (so muffs go on).


(I hear ya on the "dual exhuast" sound. been driving a diesel for a few years and miss the rumble of an opened up v8)

 

[blubuckaroo]

If you still have the factory size head pipes, then it's not goung to matter.

 

[BuffDragon]

If you still have the factory size head pipes, then it's not goung to matter.

thank you for the input!

then I guess for the time being, I just have the magnaflows stuck in place of the quiet ones and retain the 2" pipe untill down the road when I do headers. and replace the pipe then.

so, should I continue with the pipes run all the way our the rear, or dump it directly behind the muffler? or what?

thanks again, yall!

 

[broncnaz]

If running duals I'd go no bigger than 2 1/4". 2.5in is to big for a stock bronco. As for the head pipes its up to you. If its already 2in inner dia. pipe then i would probably leave it and just adapt the mufflers then run bigger pipe to the exit. Run the pipes all the way out and side exit. if you dump it underneath the vehicle you run the risk of getting more gas fumes inside the cab not a good idea.

 

[BuffDragon]

Run the pipes all the way out and side exit. if you dump it underneath the vehicle you run the risk of getting more gas fumes inside the cab not a good idea.

maybe this is a stupid question, but is it that big of a deal when the top will never be on it? (its also a half cab, "if" I ever put it back on)

 

[DirtDonk]

Still noticeable, but nowhere near as rancid as when the top is on.

Paul

 

[keith.bronco67]

i have 2.5'' true duals i also welded in an ''h'' pipe hooked up to long tube headers and exiting out of 3'' tips i have no problems with engine performance in the way of back presure. every one seems to think u need back presure but when u ask them why they have no reasone or proof but i dont know everything

 

[blubuckaroo]

i have 2.5'' true duals i also welded in an ''h'' pipe hooked up to long tube headers and exiting out of 3'' tips i have no problems with engine performance in the way of back presure. every one seems to think u need back presure but when u ask them why they have no reasone or proof but i dont know everything

I think the need for backpressure thing is a myth too. Came from people who modified their exhaust without following it up with a jet change.

 

[Master Chief]

This is about the easiest explanation I can find, it came from http://en.wikipedia.org/wiki/Manifold_(automotive):

"Great care must be used when selecting the length and diameter of the primary tubes. Tubes that are too large will cause the exhaust gas to expand and slow down, decreasing the scavenging effect. Tubes that are too small will create backpressure against which the engine must work to expel the exhaust gas from the chamber, reducing power and leaving exhaust in the chamber to dilute the incoming intake charge. Since engines produce more exhaust gas at higher speeds, the header(s) are tuned to a particular engine speed range according to the intended application. Typically, wide primary tubes offer the best gains in power and torque at higher engine speeds, while narrow tubes offer the best gains at lower speeds."

Years ago when I was stationed in Maryland I used to drag race with a home built altered running a stock 1968 396 chevy engine with a quadrajet and open headers, the thing turned better times when collectors were attached to the headers. My only explanantion for the better times was a slight increase in exhaust backpressure.

On my Bronco I run Ultra-Flow headers from JBG which are 1 1/2" tubes into 2" pipes with an "h" pipe through flowmaster 50 mufflers then out the sides behind the rear wheels.

Hope this helps...

 

[blubuckaroo]

Oh I agree that long, smaller pripmary tubes are more efficient at low speeds, however after the header, you can do whatever you want as long as you adjust your jetting for the setup. Some people just don't want to go to the trouble. Then they will complain about the carb, headers, ignition ...
The only shortcut to getting it right is to transplant a FI and avoid the tuning altogether. I enjoy the tuning part too much to go there.

 

[Master Chief]

as long as you adjust your jetting for the setup. I enjoy the tuning part too much to go there.

I certainly agree with this...proper tuning is part of the fun and enjoyment, I went two rods richer on my Edelbrock carb to get it right.

 

[broncnaz]

X2 on blubuckaroos post its about jetting, timing ect. so many people swap exhuasts and just think they have lost of gained something not always true. Seat of the pants feel or louder sound doesnt mean it makes more power.
I've run both 2.25 and 2.5in duals on my bronco I always felt that the 2.5in setup did run better at least at the higher rpms in my setup it may have. Currently running 2.25 duals and I seemed to have gained a little lowend when I swapped back. While I think there are benefits to back pressure and some back pressure is going to be presant in any system unless you have no bends. even H pipe setups increase back pressure at that point which actual helps scavenge from one side to the other as pressure rises and falls.
The main point is big huge pipes dont help the scavenge effect. Most of our engines are built for low RPM's and those big pipes dont help much.