With my header and carb setup I've had NO problems keeping up with 4.0L's. I
still have an HEI that I will install.


"Mike Romain" <romainm@sympatico.ca> wrote in message
news:433FE516.9486B4F2@sympatico.ca...
> If you are willing to trust some glue to hold all the coolant in, then
> it might be a good thing. You can use the 4.2 intake, but the exhaust
> manifold has to be 4.0.
>
> All these guys that do any mods to the 4.2 like that head job or the FI
> job all claim this big HP increase. Well... you get the same increase
> they are crowing about for free by just killing the emissions computer
> on the 82's to 91's, which by the way they have to do anyway for the
> mods... LOL!
>
> The rpm 'red line' goes from about 3300 rpm to 4400 and you get at least
> a 10 mph increase in each gear.
>
> My 86 CJ7 will just light up my 33's off the start if I punch it, it
> then hits about 52 mph in 2nd at 4400 rpm, 3rd pulls hard up to 75 mph,
> 4th buries the speedo and I have no urge to find out how fast 5th will
> go...
>
> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's
>
> Steve G wrote:
>>
>> Ive been reading about the 4.0 head swap for 4.2 motors. Is this really
>> as
>> good as it sounds or are there more headaches to be found?
>>
>> What all has to be replaced besides the head?
>>
>> Have to be fuel injected then?
>>
>> 40hp gain? really?

 

With my header and carb setup I've had NO problems keeping up with 4.0L's. I
still have an HEI that I will install.


"Mike Romain" <romainm@sympatico.ca> wrote in message
news:433FE516.9486B4F2@sympatico.ca...
> If you are willing to trust some glue to hold all the coolant in, then
> it might be a good thing. You can use the 4.2 intake, but the exhaust
> manifold has to be 4.0.
>
> All these guys that do any mods to the 4.2 like that head job or the FI
> job all claim this big HP increase. Well... you get the same increase
> they are crowing about for free by just killing the emissions computer
> on the 82's to 91's, which by the way they have to do anyway for the
> mods... LOL!
>
> The rpm 'red line' goes from about 3300 rpm to 4400 and you get at least
> a 10 mph increase in each gear.
>
> My 86 CJ7 will just light up my 33's off the start if I punch it, it
> then hits about 52 mph in 2nd at 4400 rpm, 3rd pulls hard up to 75 mph,
> 4th buries the speedo and I have no urge to find out how fast 5th will
> go...
>
> Mike
> 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
> 88 Cherokee 235 BFG AT's
>
> Steve G wrote:
>>
>> Ive been reading about the 4.0 head swap for 4.2 motors. Is this really
>> as
>> good as it sounds or are there more headaches to be found?
>>
>> What all has to be replaced besides the head?
>>
>> Have to be fuel injected then?
>>
>> 40hp gain? really?

 

Yes, it's as good as it sounds.

You might need to change the manifolds, but I'm not sure. If your 258 is an
old one, like mine, it would be stupid to put on a 4.0 head and NOT change
both manifolds. My motor had the old style manifolds that had 6 into 1
collectors, these are very inefficient. You want the intake manifold that
has a large block under the carb/throttle body with long tubes running to
each intake port; the long tube that runs the length of the motor with short
tubes to the various intake ports doesn't work nearly as well. The exhaust
manifold has the same breathing problems and the intake manifold, but going
the other way; you want change from the 6 into 1 manifold to one that is 6
into 2 into 1, or some variation that makes tubes that can hold the entire
contents of the cylinders before coming to a restriction.

The 4.0 head doesn't have to be fuel injected, but if you use fuel injection
or a carb is important when selecting the intake manifold to use.

Before you bolt the 4.0 head down, you need to look at how the water galleys
line up between the head and the block. There are (I think) two water
galleys on the head that are exposed to the outside, and these need to be
blocked off with JB Weld, or something like that. This is an easy job, and
should NOT cause you to pass on doing the upgrade.





"Steve G" <stevncin@charter.net> wrote in message
news:xeo%e.11022$uD2.686@fe05.lga...
> Ive been reading about the 4.0 head swap for 4.2 motors. Is this really as
> good as it sounds or are there more headaches to be found?
>
> What all has to be replaced besides the head?
>
> Have to be fuel injected then?
>
> 40hp gain? really?
>
>
>

 

Yes, it's as good as it sounds.

You might need to change the manifolds, but I'm not sure. If your 258 is an
old one, like mine, it would be stupid to put on a 4.0 head and NOT change
both manifolds. My motor had the old style manifolds that had 6 into 1
collectors, these are very inefficient. You want the intake manifold that
has a large block under the carb/throttle body with long tubes running to
each intake port; the long tube that runs the length of the motor with short
tubes to the various intake ports doesn't work nearly as well. The exhaust
manifold has the same breathing problems and the intake manifold, but going
the other way; you want change from the 6 into 1 manifold to one that is 6
into 2 into 1, or some variation that makes tubes that can hold the entire
contents of the cylinders before coming to a restriction.

The 4.0 head doesn't have to be fuel injected, but if you use fuel injection
or a carb is important when selecting the intake manifold to use.

Before you bolt the 4.0 head down, you need to look at how the water galleys
line up between the head and the block. There are (I think) two water
galleys on the head that are exposed to the outside, and these need to be
blocked off with JB Weld, or something like that. This is an easy job, and
should NOT cause you to pass on doing the upgrade.





"Steve G" <stevncin@charter.net> wrote in message
news:xeo%e.11022$uD2.686@fe05.lga...
> Ive been reading about the 4.0 head swap for 4.2 motors. Is this really as
> good as it sounds or are there more headaches to be found?
>
> What all has to be replaced besides the head?
>
> Have to be fuel injected then?
>
> 40hp gain? really?
>
>
>

 

Yes, it's as good as it sounds.

You might need to change the manifolds, but I'm not sure. If your 258 is an
old one, like mine, it would be stupid to put on a 4.0 head and NOT change
both manifolds. My motor had the old style manifolds that had 6 into 1
collectors, these are very inefficient. You want the intake manifold that
has a large block under the carb/throttle body with long tubes running to
each intake port; the long tube that runs the length of the motor with short
tubes to the various intake ports doesn't work nearly as well. The exhaust
manifold has the same breathing problems and the intake manifold, but going
the other way; you want change from the 6 into 1 manifold to one that is 6
into 2 into 1, or some variation that makes tubes that can hold the entire
contents of the cylinders before coming to a restriction.

The 4.0 head doesn't have to be fuel injected, but if you use fuel injection
or a carb is important when selecting the intake manifold to use.

Before you bolt the 4.0 head down, you need to look at how the water galleys
line up between the head and the block. There are (I think) two water
galleys on the head that are exposed to the outside, and these need to be
blocked off with JB Weld, or something like that. This is an easy job, and
should NOT cause you to pass on doing the upgrade.





"Steve G" <stevncin@charter.net> wrote in message
news:xeo%e.11022$uD2.686@fe05.lga...
> Ive been reading about the 4.0 head swap for 4.2 motors. Is this really as
> good as it sounds or are there more headaches to be found?
>
> What all has to be replaced besides the head?
>
> Have to be fuel injected then?
>
> 40hp gain? really?
>
>
>