MadCityMike

I just picked up a 91 XJ which I know is supposed to have the 4.0 MPI HO system in it, but the previous owner swapped in an late 80's RENIX motor into it. How can I tell if they converted the older motor to run with the MPI system, or if they converted everything over to the older RENIX system? Are there any mechanical performance differences between the RENIX and MPI blocks or heads? If they did as I suspect and made the older block work with the newer but already existing MPI system would the block gain the 10 or so horsepower that the MPI system offered over the RENIX, or are there other factors such as compression or porting that would keep the older motor from benefiting from the better system? Everything works fine, I'm just curious which system I have.

-Mike

91 XJ Sport 4.0 5spd

winston427

The engine block is the same but the heads, intake, fuel injection system, and engine control system are all different.

winston427

If he used the Renix head with the H.O. intake manifold the it will not perform as well. The ports on the heads are different and the do not flow efficiently when mismatched.

Abovetimberline

If they used the h.o controls the cps will have a square plug and the dist will have a ckp,cam position sensor. Besides things Winston pointed out the h.o also had a high profile cam.mine is just the opposite,a 91 with 89 controls,I am sure there is a lack of power. The other thing is the injectors on the 91 do not work as well with the renix motors due to the head that does not flow as well. Check the casting number on the head just past the 4th head bolt on the drivers side if it says 7120 it is the h.o head.

mike albo

I know the Renix dosn't have as much power,isn't it more reliable though and easier to work on,diagnos problems on?

Abovetimberline

In my opinion they are both reliable and easy to dignose and work on,The only thing on the chrysler system is the cam position sensor that can go bad,just another thing to go wrong,but they also got rid of the old cooling system,the differences are actually not alot,just my two bits.