Originally Posted by dustinslaton
I'm finally about to start restoring my 89 Jeep Cherokee Laredo. It's going to be a daily driver, and stay pretty much stock except for a few toys (2" lift, tire rack, etc.)
My question is about the changes that happened to the 4.0 in 1991 resulting in about 14 more HP. I read that the main change was added multipoint fuel injection. I'm not a big mechanic, so I don't know exactly what that means. Can you help with that?
Was there any other significant change to the 4.0 that year, or any year after that which would make it worth upgrading the motor/components as I'm doing the rebuild? The engine is solid, and ran fine when I parked it 6 years ago aside from oil seal leaks.
Thanks for your help.
1987-1990 was pre-OBD ("RENIX", after the control system used.)
1991-1995 was OBD-I
1996-2007 was OBD-II. OBD-II may be further divided into "early" and "late" - "early" OBD-II used the #0630 head, while "late" OBD-II used the #0331 head and distributorless ignition.
The 6-242 has always
had MPFI, there's no such thing as a carburetted 6-242 from the factory.
The primary difference between RENIX and OBD is the location of the intake ports in the cylinder head - RENIX has exhaust and intake more or less on the same line, while they were raised slightly for the 1991 Model year. This reduced the angle that the intake charge had to turn through (slightly,) which made the intake port flow more efficient. This can be seen when comparing the raw flow numbers of the RENIX and #7120 (1991-1995) head for the 6-242.
Internally, the engines are otherwise (mechanically) identical. There were some small revisions (graphite coatings on the piston skirts, addition of a main bearing cap brace) in later years, but they're not that significant on a stock engine.
While the "High Output" (OBD-I/II) engine may have had a higher peak
output, the RENIX generates its peak output (both torque and horsepower) at a slightly lower crankshaft speed - making those values more useful. People get too tied up in the peak
values (marketing tends to depend on that...) without looking at the overall output curve - and it's more important to know the overall output curve than peak power (trucks also tend to be mismarketed by horsepower ratings - it's torque
output that's important, and peak horsepower output will always be above 5200rpm anyhow, which makes it relatively too high to be useful.)
If you want more detailed information, it's been done to death - usually by me...