4.2 FI vs. 4.0 H.O. - impressions?

Sunsport

Sunsport

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Good afternoon all.

I am looking for some opinions from folks who have experience with both the 4.2 and 4.0 or a good idea anyway.

I presently have a 89 Jeep YJ with the 4.2. 98k on it. The carb is long gone - I have the Mopar MPFI kit installed (basically all the fuel injection/intake/computer/distributor/ignition) from a 94 YJ 4.0L).

I find this engine has great low end power but above 60mph is very flat and you need to ring the guts out of it to maintain 70 going up a hill on the highway dropping into 4th (lots of hills where I live). Most of my driving is less than 60mph (100kph) however I avoid highways because of how hard I feel it needs to work. I'd like to be able to do some longer distance touring at highway speeds if needed in the future.

Back story of how I use my Jeep - no real off road (sorry die hard Jeep guys) - mainly use it for cruising back and forth to the beach (I mean it is a Pacific Blue Islander - kind of the idea!), summer cruises, towing a Sea-Doo, errands, back and forth to work. It is like a practical convertible.

No lift - tires are only slightly larger than original (235-70-15), stock gearing, 5spd.

I'm thinking of swapping to a 4.0L from a later YJ. The engine I'm looking at has similar mileage - no known issues. I'm thinking where I already have the Mopar fuel injection kit, the long block will be a straight drop in with no mods needed - maybe exhaust manifold. This seems like it would be much easier than trying to get more top end out of my 4.2L. I should note my 4.2 also should have valve stem seals done at some point (basically the same price as buying the 4.0L long block).

I've read lots of threads comparing the two, but most people are talking about off-road/slow speed and the 4.2 low end torque vs the 4.0L with fuel injection reliability. In my case, I have the FI reliability already with my 4.2, but wondering if the 4.0L would make a big difference in usability for what I use the Jeep for.

So what is a non-modified 4.0L 5spd YJ like on the highway? On paper it looks like it would be light years ahead of the 4.2 based on the much higher HP and similar torque but is that actually the case? Is a 4.0L YJ more highway friendly and maybe even have some passing power? Can it cruise at highway speed without downshifting and wringing it out up hills? Or where I've already converted to MPFI, am I not going to see much difference or negligible?

I wish I could just go drive one, but there are basically none left where I live (Nova Scotia Canada) as they all rusted out years ago. Rare to see a YJ. Rare to even see a TJ now. I had to import my YJ from the southern US.

Also, I'd never sell my YJ and buy a later 4.0L so no one say go buy one. A couple reasons - to much sentimental value to this particular Jeep, condition of it, and also 89 Pacific Blue Islander trim is my ideal jeep and you can't get that in 91 or later. Navajo Turquoise is nice, but for me, Pacific Blue is the ideal Islander color.

I guess the other part to this post - is the 4.0L a quick swap (as I believe) at this point since I've already got the factory 4.0L FI system installed?

Thanks for any feedback folks!
Nick

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Surely you'll get plenty of opinions on this one. Nice YJ BTW. One question would be what gears? My last CJ, a Jamboree edition, had 2.73 gears and was a 3 speed auto. Ive had 5 XJ's with the 4.0, 4 CJ's with the 258, 2YJ's with the 4.0 and 2 TJ's with a 4 and 6 and a 4.0 MJ, to compare. Will you get blistering passing abilities with a 4.0? Id say no. 258 with the 4.0 head? others can answer that one. Is it worth the work of a swap which I believe will require motor mounts etc and do you have a BA10 trans which IIRC changed to the AX mid year 89? Curious of a little distributor/ timing curve adjusting would do anything for you??
 
The 4.2 only had 110 HP or so. The 4.0 was 180 HP. So, 65% more HP.

My 92 4.0 Islander 5 spd would do 100 MPH back when it was on 225/75-15s, 3.55 gears. Only did that once.

It's just on 30x9.5s now, but will run 75 MPH all day on the interstate with the AC on.

You're probably going to need to ditch the Peugeot 5 spd for an Aisin AX-15 as well.
 
JKP said:
The 4.2 only had 110 HP or so. The 4.0 was 180 HP. So, 65% more HP.

My 92 4.0 Islander 5 spd would do 100 MPH back when it was on 225/75-15s, 3.55 gears. Only did that once.

It's just on 30x9.5s now, but will run 75 MPH all day on the interstate with the AC on.

You're probably going to need to ditch the Peugeot 5 spd for an Aisin AX-15 as well.
Click to expand...

He has multi point FI kit so HP is probably higher than stock, and in 89 about half yj came with AX15.
 
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I can’t speak for many stock 4.0s, I have only driven one TJ briefly and it had 3.07s and 33’s so it was pretty terrible to drive. My own 4.0 is a Jasper rebuild and is supposed to be stock. And it may be, I just don’t know how similar to stock it is exactly since I have no comparison.

I find that I need to drop it to 3rd for a good highway pass. Otherwise, acceleration is decent. It feels torquey down low and doesn’t have much up top, but I wind it out anyways because staying in the lower gear is more advantageous than lugging it in a taller gear. It’s hard to quantify but I’d imagine your swap wouldn’t really make things much better. But if you need the valve stem seals anyways, then maybe it’s worth it in your case.

I bought a used engine that I planned on swapping mine with because the block was disgusting on the inside in the cooling channels. The ‘new’ engine ended up being worse than my Jasper by a long shot (bad in other ways, mostly valves not sealing), and so I plan to eventually rebuild that one to new stock specs, and then swap it in. I debated a stroker but it just seems like a rabbit hole of tweaking that I really don’t want to deal with. Stock + low gears does the job for me.
 
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Thanks for the feedback so far. I do have a late 89 build with an AX-15 (which I verified).

I do believe the HP to be higher on my 4.2 with the MPFI (4.0L injectors, intake, distributor, computer, etc.). Still, it is lacking on the highway. Otherwise, great power around town. I'm not looking for a speed demon, but downshifting just to hold 60mph up a slight hill is annoying.

Gearing is stock and I did check what it was years ago. Can't recall of the top of my head but was the standard high gearing. I don't want to change this. I don't want to drive the revs a way up plus the 4.2 really has no steam in the higher revs anyway.

4.2/4l head frakenmotor - not really a can of worms I want. Looking to leave as is or a simple drop in solution.

Didn't realize the 4.0 used different motor mounts than a 4.2? I thought it was essentially the same block.

Thanks for the feedback so far on the experience driving. If you can cruise all day at 70mph with a 4.0L, there is a significant difference.
 
3.07 was the standard gearing if I remember correctly. Mine had the optional 3.55. It should be on the diff tags.

There is a reference doc somewhere that tells what is interchangeable on the various years. I don't know if the motor mounts are different.

You might check Novak or Advance Adapters to see if they have any info. The AX15 should bolt right up to a 4.0.
 
Needing to downshift at 60 on a minor hill is pretty par for the course on any of the jeep engines with 3.07 gears. If that’s what you have, no motor besides maybe a stroker will feel good. The higher revs don’t necessarily help much but the low end torque from the lower gearing helps anything. Not saying you need to go change gearing since you don’t want to but what you are experiencing is quite normal and I don’t think a 4.0 will help there, because they don’t like 3.07s either.
 
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You can do the 4.0 head swap, install a good RV or aggressive camshaft, add a 390-600 cfm carb/ stay FI or go EFI and have gobs of power. There is literally no need to swap from a 258 to a 242... none!, and don't listen to that stroker bullshit because most of those are only in the 270/300 range and your 258 can do that all day with bolt-on's and slight modifications.

If you're looking for something far more radical 6,7,800 plus hp/tq, then yeah, source a 242 but there are very few people who can build that engine and none of them have come from the shade-tree crowd that I've ever seen.

The Edelbrock head is the best head out there... stock its going to out flow any of the cast options, modified forget about it...none of the cast heads come close, the aluminum head also weighs HALF of what the cast offerings do and dissipates heat far better AND! you don't have to modify it to install it...just bolt it on. Now by going with that head you'll want the best aftermarket header and that's the Banks revolver header... stock manifolds crack as do all the other cheap headers...don't waste time and money, just get the Banks header and call that done. Clifford performance offers a 4 barrel intake manifold for carb, EFI or you can use the later 242 intake manifold and go with fuel injection. You may as well spring for a FlowKooler water pump as cylinders 3/4 - 5/6 struggle to get adequate cooling as it is and those pumps circulate 30-40% more coolant faster, especially at idle speed, upgrade to an aluminum 3 row radiator (Griffin is great) and aluminum V-belt pulleys from CVF Racing...they're lighter and again will cool faster... ditch the clutch fan for an electric fan on a relay switch... couple more HP not spinning that fan. Roller rockers are an easy 10-15hp gain just bolting them on that ready to run Edelbrock head. Needless to say but, you'll gain the most performance from the cylinder head swap and camshaft.

Newcomer Racing has the hardware to bolt up the intake and header, nice copper gasket too and a billet aluminum water neck that corrects the geometry, eliminates the need for a valve cover spacer and makes installing those hoses a hell of a lot easier and again aluminum...cools faster...lighter weight which also saves gas once you add up the weight of all the heavy parts you took off and replaced, but gained cooler temps and more performance. BrownDog Super mounts/ engine mounts secure everything nicely.

I'd bore the engine .030 over, have it decked .020 ...add a new crank and balanced rotation assembly , don't forget to align hone for the crankshaft and a good hyperutectic piston, forged rods and you're well on your way to an efficient, long lasting powerful 258 that'll do well beyond 80mph uphill with a head wind, on the interstate if you need it too while also having awesome off-road ability.
 
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PCS - quite a bit of information regarding a major build there. That seems to be far over and above what I need though. I don't need it to be fast or a ton of power. Just looking at simple options (and cost effective) that would make things a little more comfortable on the Trans-Canada for short trips.

From one description above re: the 4l, seems it would probably be a good route factoring in cost and ease of install if I have a drop in ready stock engine.
 
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Thinking about it, I recall that you will probably (almost certainly) need a new PCM if you do end up swapping the motor. The PCM supplied in the Mopar kit is not from any specific 4.0 setup, probably due to the difference in displacement.

I’m curious what the results are and you’ll be able to answer your own initial question if you do the swap, but I think you might end up needing to downshift even more than you do currently. Just my guess. The upside might just be that the 4.0 is happier feeling with the revs than the 4.2, possibly.

I agree with keeping the motor stock. You’ll spend a small fortune doing all the stuff mentioned above and most of that will affect reliability. Bolt on stuff doesn’t help performance either.
 
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machoheadgames said:
I agree with keeping the motor stock. You’ll spend a small fortune doing all the stuff mentioned above and most of that will affect reliability. Bolt on stuff doesn’t help performance either.
Click to expand...
No one said it was cheap. bolt-on stuff does in-fact increase performance, The Edelbrock head, Banks Revolver header and a larger Carb or EFI 4barrel would absolutely increase performance over stock while shedding weight, dissipating heat faster... even more so with a camshaft upgrade. None of which would affect reliability. 🙃
 
Sunsport said:
PCS - quite a bit of information regarding a major build there. That seems to be far over and above what I need though. I don't need it to be fast or a ton of power. Just looking at simple options (and cost effective) that would make things a little more comfortable on the Trans-Canada for short trips.

From one description above re: the 4l, seems it would probably be a good route factoring in cost and ease of install if I have a drop in ready stock engine.
Click to expand...

Buying someone else's junk engine that was more likely than not dogged and abused is a gamble for the time and effort of installing it...plus switch the harness/brain, unless you're talking about buying a new re-man 4.0L and you'll still need a harness..
 
machoheadgames said:
Thinking about it, I recall that you will probably (almost certainly) need a new PCM if you do end up swapping the motor. The PCM supplied in the Mopar kit is not from any specific 4.0 setup, probably due to the difference in displacement.

I’m curious what the results are and you’ll be able to answer your own initial question if you do the swap, but I think you might end up needing to downshift even more than you do currently. Just my guess. The upside might just be that the 4.0 is happier feeling with the revs than the 4.2, possibly.

I agree with keeping the motor stock. You’ll spend a small fortune doing all the stuff mentioned above and most of that will affect reliability. Bolt on stuff doesn’t help performance either.
Click to expand...

You are correct - just pulled the part number on the ECU - it is specific to a 4.2 FI. I had always believed it was just a 4.0L ECU. Interesting!
 
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Sunsport said:
You are correct - just pulled the part number on the ECU - it is specific to a 4.2 FI. I had always believed it was just a 4.0L ECU. Interesting!
Click to expand...

Yeah it starts with a P or something doesn’t it? I believe indicating from the Mopar performance catalog.
 
machoheadgames said:
Did you read what he wants to do?
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Yes, I did, he asked for experience from people with both engines. Being he has a 4.2L it would be cheaper and less headache to upgrade and freshen up that engine. Most people don’t even know you can build the 4.2L and go straight to the 4.0 for the stroker. The 87-90 4.2L is the most supported I6 Jeep 258 when it comes to the aftermarket… he has the perfect candidate.

He said he needs more power, that’s the complaint, what I suggested isn’t cheap but it absolutely is MORE POWER and depending on camshaft and head modifications (port polish, bowl work, larger valves, springs and 1.7 roller rockers) he’d have more power than the average Stroker build which is around 270hp/300tq. which by today’s standards isn’t shit, let alone race car numbers.

Thank you (bow)
 
Sounds like you have gotten a lot of information about the performance differences so I won't add to that.

As far as the actual conversion, take a look at your bell housing. You'll need to see if you have the mounting provisions for the crank position sensor at about the 10 o'clock positions looking from the rear towards the front. If you have that, the AX 15 will mount right up to the 4.0. If not, you'll either need to relocate the crank position sensor (you may have already done this with the EFI system) or change the bellhousing. If you change the bell housing it will force some additional changes such as an external slave cylinder. There is a bunch of information about adaptability for the AX15 here; https://novak-adapt.com/catalog/adapters/engine-to-transmission/kit-amax15.html
Feel free to reach out if you have any questions.
 
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