Thinking of going stroked

on my 76 CJ5. Plan on buying a 94 4.0, I already have the preferred crank and rods in my 258. This isn't about I6 vs V8, sticking with the I6. Not too many posts about going stroker in something as old as my 76. Found the stroker forum and been lurking there last couple of days. Would like some feed back from those who have done the swap in something as old as mine. The alternative is modding my current 258 with injection. But I like the idea of the added torque from a 4.0. I have upgraded the both axles to d44 to support the 35's. The jeep has been sitting for 15 years (indoors the whole time) and the engine has corrosion from anti freeze, white fuzz all over the front. So while the motor had a fresh rebuilt (less than 5K on it) it needs to be gone through and see what's going on inside. While I enjoyed the stock 258 for many years prior, time to upgrade. My budget has increased, so not worried about what was spent 20 years ago. What I'm worried about is all the wiring and electronics going into my old jeep. I look at the newer motor and wonder where all this junk goes. Am I in for a headache?

 

Got a friend who did it with a 4.0L from a TJ. Since he got the engine, he never been able to make it run good, always problem.

I'll suggest to go with an injection system. You will gain some power and you won't have anymore problem with carb.

M2c

 

The only sensible way to do this is to use the multiport injection from the 4.0L HO donor, IMO. The Mopar MPI that comes with these engines is an excellent system.

I would put the 4.0L in and sort out the MPI first, then when I had all that sorted to my satisfaction, I'd pull the 4.0L and build it into a 4.6L. The MPI should work as-is as long as you get all the wiring and plumbing right. And pulling the engine of a CJ is pretty easy...

I don't think you'll find the 'seat of the pants' feel that different with the 4.0L (242) than the 258. In fact, the 242 should feel a little less strong off the line, since its torque curve peaks at so much higher RPM than the 258. The peak torque is only 10 ft-lbs different. I'd expect the most difference on the highway, since the 242 makes considerably more HP than the 258.

 

I'd love to build a stoker motor as well, I have all the parts laying around. There is a guy over on the jeep-cj.com forums that sells / modifies 4.0 OBD-I wiring harnesses. There has been a few people over there that used them in a 4.0 swap. Should be no different for a stroker, at most the only thing that would change is different fuel injectors. I've been lurking for quite some time over on the Jeep strokers forum. If you stick to a simple stroker build, you should be fine. It seems like where all the issues pop up is people start doing all kinds of custom head / valve work, custom camshafts, springs, etc..

 

I'd do the 4.0 head (cam too maybe?) and FI system on the 258 bottom end. That seems like the easiest way to get the higher displacement and the superior 4.0 head all in one package without having to even take the block out of the jeep.

However, a stroker build would be cool.

 

My thinking about this is a little different. I see two central things that change between the 4.2L (258) and the 4.0L (242).

First is bore/stroke ratio. The 242 has a shorter stroke and a larger bore than the 258. The shorter the stroke, the higher in RPM the torque peak will be. You can see that in the HP and torque numbers for the two engines. The peak torque between the two engines isn't vastly difference, but the HP is - 242: 180@4750 and 258: 110@3500, a 63% increase. The main difference is that the 242 torque peaks at 4000 RPM while the 258 peaks at half that, 2000. Now, horsepower is basically torque times RPM, so the same torque at a higher RPM makes proportionally more horsepower. Indeed, given the torque curves, you would expect double the power from the 242 versus the 258, so the 242 seems to be detuned compared to the 258. This may explain why you can make such impressive gains in HP over the 242 for the stroker motor - if the cid increase were to blame, you would expect a 15% increase proportional to the volume increase, rather than the 40% increase claimed for the 'poor man's stroker.'

Second, I think that the 242 has an effectively higher compression ratio than the 258. The static compression ratios don't tell you a lot; the 242 is 8.8, and the early 258 is 8.0. This combined with more aggressive cam timing should make the dynamic compression ratio of the 242 higher. I've read claims that the main advantage of the 4.0L HO head is not in breathing but in the increased compression resulting. Supposedly you can get most of the performance improvement by using the earlier 4.0L head (not HO) which boosts the compression similarly. I believe the factory 258 manifolds bolt up to the early 4.0L head. You could even stroke the early 4.0L, though you would not get the benefit of the excellent Mopar MPI that comes with the HO engines. I expect you could run the earlier Renix (Renault-Bendix) injection parts with the Megasquirt 3.0, which supports sequential multiport injection. That would be something different ...

Recall the old school performance formula, the 4 Cs. Camshaft, Carburetion, Compression, Cubic Inches. Making the engine a 281 instead of a 258 only supplies one element of that formula. I would guess that you can gain a lot by using readily available and cheap junkyard parts, without tearing the engine apart completely. There are a lot of junkyard parts for these engines, and parts from other engines that could be adapted.