I would need 2, my cats are just off the exhaust manifold. I searched around but didnt see any made specifically for my engine, but I assume and 2.5" high flow cat would work right? I'm pretty sure my exhaust is the same diameter as you guys' with the V-8's.

 

So I am in a slightly similar situation, but I have it even worse. My dad, who had an 01 quadcab 4x4 with the 3.9 and 3.92's, got a new truck, and didn't want to get rid of his old one, so he gave it to me... I have decided to keep it for now (gets better mileage than my durango), but this thing is beyond slow... 175 hp in a truck that heavy should be illegal. I did a 0-60 run and it took 13.4 seconds. I want to spend a little bit of coin on it (up to $1000 preferably less) to at least make it remotely acceptable. What can I do?
Also... my two biggest issues:
-The torque really goes off a cliff at about 3500 rpms
-The engine makes more noise than power above 4000...
So even if the overall power doesn't increase, is there a way I can at least get a flatter power band?

 

For less than $1000, start with an intake, exhaust, and take off the clutch fan.

I bought my intake off Ebay for about $50. It's cheap, but works great.

I bought my muffler from NAPA (Dynomax Super Turbo) for about $85. I didn't bring it to a shop to weld it together (I probably should have) I used clamps and that cost another $15-20 for an extra piece of exhaust pipe and the clamps. Still around $100.

Taking off the clutch fan is free you might need to add an e-fan if your truck doesn't already have one, I don't know about the 3.9L trucks. A lot of guys go to the junkyard and use a fan from a Taurus...might require some rigging/ wiring, but shouldn't be too expensive. Less than $100 I'm sure.

Also get new spark plugs. Autolite 3923's gapped properly will make some difference too. You can probably get a new throttle body for less than $200, I think the throttle bodies from the 5.2 or 5.9 might fit on your truck I think, not sure if it would require any modifications.

Intake, exhaust/muffler, removed clutch fan, new plugs, clean the fuel injectors should net you another 20-25 hp I would say, and you will definitely notice a difference. I did.

All that for less than $400. You can also get a high flow cat, not sure how much that would run, but from what I hear around $100. That might give you another 5hp or so.

That is an old pushrod engine and isn't designed to rev high, but that should let it breathe easier and will free up a few hp and some torque.

 

Thanks for the quick reply I will look into all of those things. If I could manage to find a 5.2 or a 4.7 or a 5.9 from another truck, how hard would it be to transplant? And if I could, which one of these would be the best? Also what is the redline on the 3.9 magnum engines? I have never seen this thing go over 4500... and when is peak torque and hp?

 

well imo i would go with a 5.9 so many things you can get out of those motors.

 

about the cats i have no idea if the 3.9 is 2.5" or not but i am 85% sure it is

 

If you are going to swap motors definately get the 360 (5.9) they are tried and tested. You will get terrible mpg but you can easily get 600+ hp out of it if you go ***** to the wall. IT wont be for under $1000 though.

 

Supposedly max hp on that motor is at 4800 rpm which means it shouldnt go much over 5000 rpm. No need for revs if you arent making power anymore. If your motor doesn't make power above 4500 then something is wrong, fuel injection cleaning, intake, exhaust, that type of stuff should help alot. Buy some seafoam and do the seafoam treatment, pour half the bottle into the motor and half into the gas tank. There are instructions online on how to do it, I've never had the need to do it but it's relatively straight forward. Burn off some of the carbon deposits that have built up over the years.

Do the seafoam, get a cheap intake off ebay, take off the clutch fan, and get a new muffler and see where your at. I bet you will be very surprised.

 

HOLY CRAP! 600 hp? A dakota with one of those must go like nobody's business! As much as that is appealing, I think I will just stick with what it came with unless I can root around my local scrapyard and find something. As far as high rpms, it certainly is making more power up until 4800, but it is just a really underwhelming experience... and it sounds asthmatic when it revs (It sounds like its gonna fall apart, so I avoid revving it which makes it even slower)...
So like when I wanted to pass someone on the highway (they were doing 90 km/h, I like doing more like 110) on a moderate hill, I floored it, it dropped down to second, it revved quite high (upper 4000's) and it pretty much didn't do anything. Then it shifted into 3rd and I actually lost speed :/ This thing just needs more pulling power period. It was pretty embarrassing that I couldn't pass a honda civic in this thing, but my friend who was with me thought it was funny.
I know this thing is underpowered to begin with, but based on that experience, it feels like it is making more in the neighborhood of 130 hp...

 

I might see what I can do as far as exhaust goes tomorrow because I have the day off. What would give me the best combo of power and fuel economy (and a little sound, but this is no V8, so that isn't my main concern) without failing emissions?