258 4.0 Head Swap Results

258 4.0 Head Swap Results

73CJ

Crazy Sr. Respected Jeeper
Posts
3,530
Thanks
8
Location
Detroit Lakes, MN
Vehicle(s)
73 CJ5 4.0 head on a 258, HEI, Holley Projection w/ 4.56, Detroit lockers Narrowed D44 front and full float D44 rear T14 w/ D20 4 wheel discs, 38.5 Boggers
Can we try this one again?

Post your thoughts about your AMC 258 i6 / 4.2l 4.0 Head Swap Results .
 
Can we try this one again?

Post your thoughts about your AMC 258 i6 / 4.2l 4.0 Head Swap Results .

Thank you for starting this valuable thread again! :D

My Swap

1978 AMC 258 i6 / 4.2l Block that takes 1/2" Head Bolts(No spacers required)

1995 Cherokee #7120 4.0 Head Casting with all stock parts(Rebuilt)

1978 Stock Push Rods(Correct Length for my swap)

1995 Tubular Exhaust Manifold(New)

1980's Aluminum Intake Manifold

1960's or early 1970's Motorcraft 2100 Carburetor(Rebuilt)

1995 Cherokee 4.0 Valve Cover(Baffle restrictor plates removed!)

I had to seriously remove a lot of metal, Steel and Aluminum from my power steering bracket to get it to clear the front exhaust manifold tube. I filled the extra water jacket ports using bio degradable packing peanuts to hold JB Weld in place while it hardened. No leaks so far. Home made throttle cable bracket, and I shortened the cable quite a bit so it's not wrapped around. The rest was pretty much plug and play. It all bolted together like the parts were made for each other.

I gained a huge amoung of horse power. I don't need to down shift nearly as often as I use to. I no longer lose a lot of speed on long highway up hills, I almost never have to put my foot on the floor like was almost always the case on the highway before, and my gas mileage went up quite a bit as well! I now get roughly 4 days of driving to work on a tank of gas instead of three! It's the cheapest increase in horse power that I've ever had on any engine I ever built. I just wish I'd done this swap last year when I first built the engine.
 
2 thumbs up. I used a Clearwater ready to install 7120 head with welded jackets. As mentioned the PS pump bracket takes a little modification but nothing major.
My opinion is it's job #1 for the AMC 258 i6 / 4.2l .:chug:
 
Did either of you guys do any other changes at the same time as you did the head swap?:cool:
like a new carb?
 
Did either of you guys do any other changes at the same time as you did the head swap?:cool:
like a new carb?
I had to swap carbs because my original cast iron 1978-1979 intake was for a 1 barrel Carter YF carb. The 1980's intake I had to switch to to make the swap work was a two barrel. That shouldn't have affected HP or MPG much during normal operation as long as both were jetted correctly. My old Carter YF was rebuilt and working perfectly. After the head swap I'm barely giving any gas when maintaining 65mph or better on highway up hills, where before the swap I had my foot on the floor and watched my speed drop rapidly!

It's my opinion that anyone with a stock AMC 258 i6 / 4.2l that has the mechanical know how to pull this off should do it asap. They will be thrilled with the results. :)
 
I replaced the 1 BBL with a 2 BBL and got a major improvement, I replaced/updated the ignition system and got a major improvement, I replaced the AMC 258 i6 / 4.2l head with a 4.0 head and got some improvement but not major amounts of horsepower and torque.
I think the 4.0 head swap is a good idea and if you have a cracked AMC 258 i6 / 4.2l head it would be the best replacement. Without an upgrade in cam shaft, especially from a 1 BBL cam, I have my doubts as to the stellar claims made of this mod.
I read something in one of the "I'm not spam" threads about a dyno test on the head upgrade producing 3-5 HP above the AMC 258 i6 / 4.2l . I would like to produce this for you but have been unable to find it again. So we can say my information is flawed or we can say the information is lacking to determine this one way or the other.
As "seat of the pants" testing goes, if you are happy with the results, I am happy for you. Until someone finds a power comparison done on a dyno my first priority will be to upgrade the ignition. :cool:
 
I replaced the 1 BBL with a 2 BBL and got a major improvement, I replaced/updated the ignition system and got a major improvement, I replaced the AMC 258 i6 / 4.2l head with a 4.0 head and got some improvement but not major amounts of horsepower and torque.
I think the 4.0 head swap is a good idea and if you have a cracked AMC 258 i6 / 4.2l head it would be the best replacement. Without an upgrade in cam shaft, especially from a 1 BBL cam, I have my doubts as to the stellar claims made of this mod.
I read something in one of the "I'm not spam" threads about a dyno test on the head upgrade producing 3-5 HP above the AMC 258 i6 / 4.2l . I would like to produce this for you but have been unable to find it again. So we can say my information is flawed or we can say the information is lacking to determine this one way or the other.
As "seat of the pants" testing goes, if you are happy with the results, I am happy for you. Until someone finds a power comparison done on a dyno my first priority will be to upgrade the ignition. :cool:

I had already upgraded my ignition with the great choice of an HEI when I first built my engine last year, so that stayed the same. I also stuck with the stock 1 barrel cam, which I believe is the exact same cam they put in 2 barrel engines from the factory. Someone please correct me with the actual lift and duration numbers if I'm wrong about that. I do know that the HP rating between the 1979 1 and 2 barrel engines were about the same at 110 hp. One other reason I don't believe the carb made any real difference is that on long uphills at highway speed with the 1 barrel carb before the head swap, it didn't really make any difference if I was at 3/4 throttle or full throttel, which tells me the engine just didn't have enough in it to use all of the 1 barrel. I'm noticing that even more with the 2 barrel with which I never floor it.

With a 4.0 head you get bigger valves, bigger better flowing ports, much higher compression(9.2 to 1 instead of 8.0 to 1) and a much better flowing exhaust manifold. All of those things other than changing cams are exactly what you would do to build a race engine, so there's no way you shouldn't see a serious horse power gain. Even with the same small cam, the engine will flow a whole lot better with the larger ports, and the bigger valves opened at the same lift and duration.

My seat of the pants comparison is more than just a feeling that it has more power and better MPG. It's a simple fact that before, I could not maintain 65 mph on hills and now I can do it easily, and it's a fact that before, I could only drive to work for three days on a tank of gas and now I can make about four trips on the same exact roads. This is an apples to apples comparison. The only other change I made was the carb, and I wasn't even using the whole CFMs of the old one barrel Carter because the engine couldn't make use of it.

It's the head that did it. Plain and simple.

I'm with you that I would love to see a before and after Dyno comparison. Most sites claim a 40 hp gain from the head. I haven't found a site that backs that up with actual dyno charts, but based on my results, I totally believe I gained that much. It's like night and day. :)

****EDIT****

I've swapped a whole lot of carbs in my life on a lot of different engines and vehicles 2 barrel to 4 barrel, quadrajet to Holley etc...) and I've never come close to seeing this kind of HP gain unless the old carb was not functioning correctly, and my old Carter was freshly rebuilt and working fine.
 
Last edited:
Did either of you guys do any other changes at the same time as you did the head swap?:cool:
like a new carb?

I did several things at one time so have little idea of what each on their own was responcable for but the improvement the head, new exhaust and Mopar MPI yeilded was very significant to say the least.
I still have the 1bbl cam and no plans to replace it, the Jeep runs great as is.
 
Well I wasn't sure if this post would survive or not, but it looks stable for now.

Did either of you guys do any other changes at the same time as you did the head swap?:cool:
like a new carb?

I did many changes at the same time:

Reman 7120 head w/ JB Weld
.030 OS pistons - stock
Comp fuel injection cam
Holley 1D fuel injection
Clifford Intake
Clifford exhaust
HEI

dist.png
exh2.png
exh.jpg
Not sure what each item contributed. . . .

It pulls much harder. Doesn't cough (bog) at the top of hills and almost though a long mud hole.

Would I do it again? H*ll Ya! :D
 
Last edited by a moderator:
I would too, no question, but when we do multiple changes it is not possible to attribute the performance to any one thing.

I don't have fancy test equipment but I do have a hill. It's about 1/8 mile 20 degrees and paved. I start at the bottom in 2nd gear and time the distance with a stop watch. As long as I am doing one thing at a time I can get a pretty good idea how much bang I got for my buck. We need to establish a personal constant to measure against.

I am not saying the head swap is a bad idea, it is a good one. I had a cracked AMC 258 i6 / 4.2l head and this was the very best solution. I am saying there is a lot of mist and smoke blowing around this and a lot of other modifications because we can not document the results in measurable quantity.:cool:
 
Well I wasn't sure if this post would survive or not, but it looks stable for now.



I did many changes at the same time:

Reman 7120 head w/ JB Weld
.030 OS pistons - stock
Comp fuel injection cam
Holley 1D fuel injection
Clifford Intake
Clifford exhaust
HEI

Not sure what each item contributed. . . .

It pulls much harder. Doesn't cough (bog) at the top of hills and almost though a long mud hole.

Would I do it again? H*ll Ya! :D

Nice setup! How much did the Holley FI run you?

And now for the big questions. How many miles have you put on it since you built it? How's the JB weld holding up? Any leaks?

Oh, and how are the cam gear and push rods holding up with that HEI and the 4.0 head???:grinjeep:
 
The reason I said it was "job one" at least for a guy who does not plan on digging in deeper than head work is that really is the basis for better exhaust and intake improvements, I hate to see the guy who buys a header for his AMC 258 i6 / 4.2l head and continues to try and squese more power out of it and learns about the 4.0 head after the fact and has to toss out the AMC 258 i6 / 4.2l header.
 
Do you see my point about having something to measure??:D
 
Do you see my point about having something to measure??:D

Yes. that would be nice if we had info available that said x=5hp, y=8hp, etc. etc. but in reality there is no such info that I'm aware of so we give "seat of pants" hp ratings instead. Member Brock Grimes was very adamant that the 4.0 head produced 40 hp on the dyno and while I was the sceptical one I was in no place to argue.
My twisted way of thinking was the 4.0 mpi motor is rated at 190hp, the AMC 258 i6 / 4.2l 2bbl is 120ish? and the real diference is the head, fuel injection system and exhaust so if you adapt those parts to a AMC 258 i6 / 4.2l lower end you should be in that realm correct?
Far from the dyno but all I know is my cj went from something that was pretty underpowered for the high altitude I like to visit to a joy to drive.
 
Last edited:
It is pretty hard to measure HP but pretty easy to measure time.:D
 
The reason I said it was "job one" at least for a guy who does not plan on digging in deeper than head work is that really is the basis for better exhaust and intake improvements, I hate to see the guy who buys a header for his AMC 258 i6 / 4.2l head and continues to try and squese more power out of it and learns about the 4.0 head after the fact and has to toss out the AMC 258 i6 / 4.2l header.

Hey, I resemble that remark! :eek:

I bought a brand new exhaust manifold for my 1979 head to replace a cracked one. I then decided to do the 4.0 swap and bought another brand new manifold for the new head.

I ended up selling the 1979 manifold for half of what I paid for it last fall.

I still insist that the head made most if not all of the difference on my engine. I count the new, redesigned exhaust manifold as part of the head swap because switching manifolds is required when going to the new head.

The only thing I changed at all that could make any difference is the carb, and I've already explained why I don't believe it's possible that it could have made this huge increase.

The head on the other hand has all the improvements I listed earlier. Smaller chambers for much higher compression. Bigger intake and exhaust valves. Bigger better shaped ports. And a much better flowing exhaust manifold. There is absolutely no way that you wouldn't see big gains in HP from all of those improvements.

Swapping carbs wouldn't hold a candle to the HP the head would make.

Ask any performance engine builder and see what they think would make the most HP gain when you list all of this.
 
Hey, I resemble that remark! :eek:

I bought a brand new exhaust manifold for my 1979 head to replace a cracked one. I then decided to do the 4.0 swap and bought another brand new manifold for the new head.

I ended up selling the 1979 manifold for half of what I paid for it last fall.

I still insist that the head made most if not all of the difference on my engine. I count the new, redesigned exhaust manifold as part of the head swap because switching manifolds is required when going to the new head.

The only thing I changed at all that could make any difference is the carb, and I've already explained why I don't believe it's possible that it could have made this huge increase.

The head on the other hand has all the improvements I listed earlier. Smaller chambers for much higher compression. Bigger intake and exhaust valves. Bigger better shaped ports. And a much better flowing exhaust manifold. There is absolutely no way that you wouldn't see big gains in HP from all of those improvements.

Swapping carbs wouldn't hold a candle to the HP the head would make.

Ask any performance engine builder and see what they think would make the most HP gain when you list all of this.

Just don't ask someone that has a monetary interest in it.;)
When I was doing my research on these mods I was in contact with Hesco and was told that their aluminum head with MPI would add almost 100hp.
Hesco Forums: 4.0 head w/MPI in Ca.
 
Let me also add to the above that the Hesco alum. head is $2,000 and the MPI is $2,700 so for $4,700 plus other parts you had better get 100hp! Pretty steep figures for sure.
 
Just don't ask someone that has a monetary interest in it.;)
When I was doing my research on these mods I was in contact with Hesco and was told that their aluminum head with MPI would add almost 100hp.
Hesco Forums: 4.0 head w/MPI in Ca.
What's scary is they might be right. You'd be surprised what a really well designed and ported head can do for an engine, but at a really high cost. With the port work, I had 3 grand in my Dart heads on the Vette. And the dyno showed it! :D
 

Jeep-CJ Donation Drive

Help support Jeep-CJ.com by making a donation.

Help support Jeep-CJ.com by making a donation.
Goal
$200.00
Earned
$25.00
This donation drive ends in
Back
Top Bottom