I had a mechanic say I should swap out my 3.8L V6 with either a 4.2L or 4.6l truck motor, I can't remember which one he said but he said it would plug into my wire harness and fit with no issues. I am wondering exactly what size I can go, from what year, make and model I need to swap with and not have to change everything except the motor.

 

Well, he's wrong. Have him explain to you how; a harness and ECU for SIX (6) cylinders,
with a coil pack and wires for a SIX (6) cylinders, is going to work with an EIGHT (8) cylinder.

You need the ECU, and the V8's wiring harness, and all eight coil packs.

THAT mechanic, is a moron...

The same car, as a GT, comes with a 4.6, not a 4.2 motor, that is the truck
motor, 202 HP; your 3.8 is 190. You do not want a 4.2L motor in your mustang,
you would still probably lose to another V6 due to the extra weight, and every V8
mustang.

Again, that mechanic is a moron.

 

If it was the 4.2L engine that the mechanic was talking about, then THAT one's a V6 and would have the proper number of injectors. It stands a much better chance of being able to re-use your original wiring harness than a 4.6 would. But I'm just not familiar enough with this swap to confirm whether it's that simple of a plug-n-play. You might have to change the oil pan or something in order to make it fit.

 

I believe it was the 4.2L truck motor. I'm not interested in racing so why is the truck motor a bad idea? I'm not trying to install a v8

 

You're only gaining 12 HP for the weight, is all, but if you must:

The 4.2 heads are identical to the 99+ Mustang 3.8 heads. The newer ones with the temp
sensor in the head is screwed into a hole...

The 4.2 cam is better than the 3.8 cam

Get yourself a 4.2 short block that runs, put your top end on it, and call it a day. That's the
cheapest thing for you to do. This WILL retain all your original wiring... This is another
common swap, but the power gains are small.

You are going to need a 01+ 3.8 oil pan, the harmonic balancer and flywheel/flexplate for the 01+,
or the ones that come on the 4.2, and an 01+ 3.8 oil pick-up tube and screen. Everything else swaps
over from your 3.8.

Here is the swap page:
https://www.v6mustang.com/threads/th...ap-post.92629/

 

Let's take a giant leap back to ask a question. What is your purpose or objective with replacing the motor or why are you asking? If we can understand that, we can better answer why it may or may not be a good idea.

A 4.2L will fit as it's essentially a 3.8L engine with a longer stroke (there are some caveats though so it's not just a drop-in replacement in some cases). A 4.6L will also fit but it's not a simple swap because it's a V8.
https://mustangforums.com/forum/v6-1...-an-3-8-a.html

Just swapping to a 4.2L for the sake of swapping is a waste of time and money as pointed out since there is no real power or design advantage.

Now, if you go for a 4.2L Stroker kit and install it in your 3.8, that's a different story.

http://www.moranav6racing.com/catego...?CategoryID=16

This will give you a great foundation for forced induction or nitrous as well as include a nice bump in base power. If you're going to stick with a V6. Start there.