The Mighty 258 > The Engine

Is there a difference

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SS eagle:
I'm planning on doing a 4.0 head and cam swap and I was wondering would there ba difference between a H.O. head and a normal one?? And I was also wondering I have a 2barrel carb manifold fir a 258 would that fit on 4.0 head??

tougeagle:
there would be a noticible difference between the 87-90 renix head and the 91-95 7120 head. The 7120 head has higher intake ports to allow a softer angle into the cylinders so a/f mixture can remain at a higher entry velocity.

If you're going to go as far as swapping to a 4.0 head and cam, you should go efi as well, as you could just grab that right off the donor vehicle you'd be getting everything else from. otherwise, you'll likely end up with worse gas mileage, and 50+ horsepower sitting on the table. The 4.0 high output engine was high output because of the design of the EFI system, head and cam all working together. If you're missing one of those components, it's not high output and you may not see the gains you want. The "computerized stuff" seems complicated, but it's really not.... But, to answer your question, yes a 258 intake manifold does fit on a 4.0 head, though you'll have to do some careful machining where the alignment holes are, in order for it to sit on the dowel pins properly.

SS eagle:
Ok so how much work would be involved in a efi swap??

BenM:
There are a lot of writeups out there on the EFI swap, It's involved as you need the wiring harness and computer from a donor jeep, and to either modify your transmission bellhousing or spend $ on a front crank sensor.

The aluminum intake is a direct fit, you just need to grind the bottom of the tabs to align on the dowels for the higher intake runners.

If you're going to a 4.0 head, I would suggest the newer HO head over the earlier Renix head unless you happen to have one sitting around for free. The HO package added 6-7% to the Renix 4.0 and much of the improvement is in the head and the cam. The 258 cam is very conservative, and the 258 heads don't flow well, even ported and polished.

If you're not up for the electrical work now, you can run the carb and you'll see a noticeable improvement. There is also the option of running a GM TBI injection system on your existing intake, and I've seen reports of only a few % difference in economy and power once the TBI computer is flashed with 258 information. It's a less sophisticated system but AMC actually really designed the heck out of the aluminum intake on the 258.

So I wouldn't let EFI make or brake the 4.0 head swap, an intake change down the road isn't major and the front sensor while not cheap is easy to do.

It's all about where you want to draw the line right now.

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