Hey All - A friend and I are considering doing the above mentioned to his 1989 YJ. He is most likely going to get a used MPI kit that does not include the wiring harness or computer. I am guessing he'll get the stuff cheap. Below are some general questions you guys might be able to help answere.....

1. Where's the best place to pick up a wiring harness and computer (new or used)?

2. What major problems, or challenges, should we be prepared for? What are some tips and tricks that will make the job easier and not a PITA?

3. We have heard the 4.0 head swap helps increase power because of improved porting. Is the compression ratio altered too? Will it cause any problems with the MPI. I had read somewhere that certain water jacket ports will need to be plugged before installing on a 4.2 block. (I think my numbers were right back there..........)

3A. It seems you could reduce the compression enough to use regular gas (via thicker headgasket) with the 4.0 head and MPI. Does that seem reasonable?

4. I recall reading somewhere that certain MPI sensors can be adjusted using washers as spacers for improved performance/tuning..... Does this ring any bells with anybody.... Vague I know, sorry about that.

Feel free to send any info my way, I will greatly appreciate input from this community!

This message has been edited. Last edited by: wheeldude,

"3. We have heard the 4.0 head swap helps increase power because of improved porting. Is the compression ratio altered too? Will it cause any problems with the MPI. I had read somewhere that certain water jacket ports will need to be plugged before installing on a 4.2 block. (I think my numbers were right back there..........)"

True, compression is higher with the headswap. There are 11 water jackets that need to be plugged. they can be welded (nickel cast) or JB weld works well also.

Cant comment on the MPI. When i did the swap, i used a 500cfm Edelbrock and the power difference was amazeing.....makes our 4.0 YJ look weak. I also used flat pistons and a 260* cam along with TFI ignition. for exhaust i went with an XJ header, glasspack, and tucked the exhaustline up above the framerail.