OK I am sick of not having enough power in my stock 258 4.2L (w/weber). (See my CJ specs below) I have thought of the 4.0 head conversion with Mopar kit but the final cost is close to $2800 to 3k. I have thought of swapping to a GM engine but what cubic inch I'm not sure which one or a 1997 or newer 4.0. I would like to end up with at least 200-210HP. What GM engines are the most popular swap? Will newer 4.0 OBDII engines directly bolt to the same bell housing as a 4.2? Any other suggestions on swaps or ? along this same subject.

The problem we have is thinking about the Hp ratings. These ratings are at near or maximum rpm. At maximum rpm the engine you have now will have plenty of horsepower..
Your problem is the gears not the engine. If you keep the gears and put another engine in there you might have a small gain. If you replace the gears with some lower ones you will have an instant huge gain in usable hp. With 35 in tires I would want about 2750 rpm at 60 mph.
What are your rpm at 60 now?
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

Well two factors here; 1. The tach is not working and is getting replaced soon. 2. I can't get to 60 mph. 50-53 mph is tops then it just runs out of power.
On a slight incline up a hill, I have to shift down to third gear just to maintain enough power to go about 35-40 mph.
Another issue is the carb setup, while the weber is better than the stock carb it still starts hard at high elevation (I drive in elevation from sea level to 9,000 above SL), will die on steep hills, ( I don't want to constatnly be rejetting) plus it won't Calif smog.

Well I do not know what Calif requires for a 78 model but here All we get is a sight inspection for a vehicle that old..
Any amc should bolt right up to that tranny I would think. It ain't going to be like putting on a tail light. You will bave many modifications to make and you will prolly spend MONTHS getting it running again..

Then when you are finished and it will only run 45 mph you will take my advice and put GEARS in there..
Big JIm

Big Jim, after your first email I thought about what you said and the lower gearing certainly makes sense, I'm thinking maybe 4.56 gears might do the job, what do you think?. Maybe if I combine this with the Howell TBI conversion I can smog legal and get the power I need. Are you familiar with the Howell TBI conversion? Does this system automatically adjust the fuel air mixture and timing when the vehicle is at high altitudes/elevations 7,000 and above?
California smogging is tough. 1976 and older vehicles are exempt. CA had a law in place that a vehicle 30 years old would become exempt so in 2008 a 1978 vehicle would become exempt. Well thanks to Gov Terminator that was repealled and changed to stop this year and now will remain that 1976 vehicles and older will be exempt and thats it for the future.

I doubt that is enough gear.. You will need 4:88's or lower.. I would investigate 5:13's ( I think that is the number) More rpm means more horsepower.
Yes a TB install is a fast great modification for that ride. It is still a mod and there will be ox sensor and cat and other things to do to get it correctly installed.
You made a large step with the big tires and so you will have to make a large increase in rpm to get back and a little ahead of stock to get the driveability you expect.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />

You could get some smaller tires and get the same results as lower gears but I'm sure you don't want to do that. 4.10 gears are stock on many 4cyl Jeeps for street tires and barely low enough for 33's on a strong six. I would check out the engines condition very well before I spent hundreds of dollars on a EFI or other MPI upgrade. Do a compression test at least. I believe the 4.0 will bolt up to a 258 bell housing but there is no place on the 258 BH for a CPS. there are kits to put the CPS elsewhere I think. 4.3 CHevy V6's are popular to swap in as are 305 and 350 V8s. There's tons of info online about all of them if you look for it. Ford 302's and 351's are also popular with folks like you that already have a Ford transmission. I think I would look for a 302 If I had your running gear. <img src="/forums/images/graemlins/cheers.gif" alt="" />