4.0l vs 4.2l how much better

[Seagulls n Eagle]

Any opinions on how much better a 4.0l is than the 4.2l
In this case it's going to be carborated but Fuel injected possible in the future.

I'm wondering how much better:

 in general, drivability

Better fuel economy?

Durability

Anything else?

Curious to hear people's thoughts

[carnuck]

No difference if carbed. They are basically the same engine but the 4.0L comes alive with EFI. The 4.2 wasn't really built for EFI so it doesn't do quite as well.

[1985amceagle]

I've got an 87 Comanche with a 4.0 with around 176000 miles. the fuel injection seems to give it more power, and the 4.0 likes higher RPMs better than a 258 due to the short stroke. they used a better timing chain design in 87 compared to 85. If I was in an engine swap, would most likely go with the 91 and later Chrysler fuel injected 4.0 as the renix has some little quirks in it.

[mudkicker715]

I stroked my 2 4.0l they are carbed. They go crazy with power over the 4.2l. Gas is the same.

[carnuck]

Stroked motor is fine with carb or EFI but they REALLY come alive with LPG!

As for the EFI, '91-95 is simplest to work with (all under the hood except trans controller) The 4.0L bottom end is the same '87 to '96 pretty much. They changed the pistons with graphite impregnation in the skirts to quiet piston slap in '97. The head changed in '91 and while you can use the earlier Renix or carb intake on them with some mods, the HO intake or Clifford carb intake for HO works best. (HO intake from '99+ for brake booster clearance)

[Seagulls n Eagle]

Great info. Now I don't have to feel I should have swapped in the 4.0. On with the tranny project.

I'm curious to learn more about the LPG gas conversion.

Quick question on that, can the LPG conversion be done as a duel fuel? Gas/LPG
I have grandiose ideas of a hydrogen hybrid, but then again that would have to be after I have a running
Car.

Ill still be intrested to learn more, I would be happy to get on the road for now.

[eaglefreek]

I have to disagree a bit. A engine is an air pump and doesn't know whether it has fuel injection or a carb. If the carb is tuned properly and a good intake, it will have about the same power as EFI. EFI will do it more efficiently, usually. The 4.0 with a carb will be much better than a stock 258 in my opinion.

[carnuck]

The problem is a center mounted carb on an inline six floods the center 2 cyls and leans out the end 2 no matter what you do. LPG is vapor so it works better in a manifold system. LPG wants more compression to make power though. It's about 105-110 octane.

[Nightpath]

To answer your questions :

Driveability : More power and it's noticeable. Car doesn't struggle as it used to.

Economy : About the same, maybe a bit worse, but that's more because I can't tell right now due to a buggered fuel gauge. Going a different gear than 3.55 or larger tire would amend this. My old car used to get solid mileage with a properly tuned 4.2 and MC2100, I think it was close to 23MPG. I'd go out on a limb to say I'd getting less than that now by a few MPG.

Durability : The engines are about the same, but the newer 4.0 gets more oil to the 6th cylinder, has a better flowing head as well. Both excellent engines. I'd have 0 worries about either motor to be honest, they are known to take a severe beating and last well into the 3-400 000 mile range.


Having run both though, the 4.0 wins over the 4.2. I did carb mine for shits and giggles but it wasn't tuned properly. Ran fine though, so if you decide to just swap the motor and keep the stock drivetrain it's an option and DOES increase the power. A solid option for you would be a head swap.

You also have to look at your transmission, transfer case, axle gearing as well. Are you sticking with the stock 998, going 727, AX15 or AW4? NP231, NP242, or the stock NP12x?

If I had the cash, I'd have my 4.0L stroked, run a AX15 (or maybe still the AW4) with the NP242 and keep it fuel injected and run the 3.55 gears I have now.

As it is now, the 4.0 / AW4 / NP242 / 3.55 combo is pretty good. More than enough to keep that old bird flying and definitely a difference that can be noticed. It can actually crawl through this deep crap snow without a problem, the old drivetrain I had to slam the pedal down.

I'm still debating swapping the gears out for a shorter gear, something in the lower 3.xx range. This car isn't too much for offroading and sees more on road travel.

In the end, a properly tuned 4.2L with a solid ignition system and a good carb will provide more than enough performance and reliability. The only reason I changed all mine out is because I'm retarded. They drive very similarily. The stock 4x4 on the Eagle is sickeningly capable as well, moreso than a lot of people realize. It *is* a true 4x4, albeit no low range, and I've had it rolling through 3 feet of water, with 1/3 of that mud at the bottom, 3-4 feet of snow, along roads with torrential downpours, and so forth. And it ran like a boss.

Only thing I hated was the carb in the cold, and I live in Canada...that ended up being a huge selling point for me.