4.2L exhaust ideas needed
 

Hello everyone,
I have a 2003 F150 standard cab shortbed with 4.2L and 5 spd manual trans. I developed an exhaust leak that has progressively gotten worse and found it is a quarter size hole rusted through the driver side catalytic converter, there is no fixing it as it is literally turning into swiss cheese. Replacing the catalysts is not an option due to cost ($700-800) and its a beater farm registered truck which does not require them. I have access to a friends exhaust shop to build my own exhaust. However, I'm used to doing exhaust systems on hot rods, not a stock v6. What size exhaust would you recommend to maintain the lower end torque/acceleration while adding a little more power and a fairly decent sound (I know it will not sound like a V8)? It would be easier for me to do true duals as this is what I'm used to doing but I fear that may be too much for this little engine. One option would be to reduce the stock exhaust down from 2 1/2" to 2" for each side for a true dual system. I think the 2" duals would provide sufficient backpressure to maintain the lower end torque while adding some performance and sound gain. The other option would be do a single 2 1/2" inlet and dual 2 1/4 inch out but would require me to fabricate a custom 2 in to 1 Y pipe. What do you guys think? The muffler(s) will either be cherry bomb or welded 2 chamber. Thanks in advance.

My setup is a SI/DO glasspack from the stock 2 1/2" Y-pipe to 2 1/4" out, exiting in the stock location behind the rear wheels. 2 1/4" dual tailpipes are the right size, if it's easier for you to do true duals, use a muffler that has an internal X-pipe (or see below). This will preserve the low end torque.

These are LOUD with no cats:


You could use a separate X-pipe with a standard DI/DO muffler or 2 separate mufflers:

If this is relegated to Farm use, there are several things your over concerned with.
A. Back pressure is never a desirable attribute in any exhaust system, hot rod or otherwise Get rid of that myth because back pressure leaves the cylinder with exhaust gas that takes up combustion chamber space from the new charge and does not burn the second time, reducing power
B. You will not see any improved low end torque at low RPM you use on the Farm no matter what you do with the exhaust. the larger cross section you make it thinking it will improve flow, only reduces the flow speed and causes 'less' evacuation of the cylinders. You need to keep flow rate (gas speed up) because the exhaust has mass and causes a sleight vacuum behind each pulse to help fill the cylinder on the next intake cycle. So get rid of the back pressure myth..
C. Yes your noise level will go up on your planned alterations because there are only 3 cylinders to dampen the exhaust pulses and noise vs 4 from a v8.
D. For a Farm truck, you will have two codes set most of the time because the rear Ox Sensors no longer function their intended use. It won't affect the motor opertion in any way for Farm use.
E. The truck is not hot rod high RPM material so don't over think in that direction for it's use you put it to.. It won't help.
Good luck.

I would just cut all the rotten pieces out, and weld in new pipe and get a Walker muffler and call it a day. Hopefully you can get away with it for ~$150 or less.

Even though it's a farm truck, having a noisy truck is annoying. It could also scare animals like horses.

if you have the skills to weld together an entire exhaust you should be able to weld up that cat
I've done it and I'm no pro welder
just cut back to good metal and weld on a patch, the cat is pretty thick steel to begin with

4.2 isn't going to sound good with an opened up/dual exhaust anyway

That's a matter of opinion.

Thanks guys for the advice. I ended up cutting out the rusted out sections of the exhaust and essentially created my own offroad y pipe re-using the good sections of factory exhaust. Having access to an exhaust shop with a nice tubing bender helped. Everyone was right, this thing is very loud even with a muffler without the cats. I'm thinking of adding a cherry bomb in addition to the muffler to help quiet it down just a tad. Other than that the exhaust install went in great!!!

What did you use for a muffler and tailpipe?

I used a Thrush welded muffler (part number 17650) and a single tailpipe same as factory. I like the welded mufflers over the crimped thinner steel mufflers but I'm sure they are not as quiet. The muffler was around $22 from Advance Auto Parts with discount codes online and award rebate.