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Unread 05-10-2012, 10:43 PM   #1
dustinslaton
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Difference between 89 and 91 4.0L?

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.

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Unread 05-10-2012, 11:21 PM   #2
USAXJ
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Multipoint fuel injection just means each cylinder has its own injector. The main differences were intake and compression I think. Jeep changed the intakes several times throughout the years. Wikipedia is a good source info about the changes.
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Unread 05-10-2012, 11:28 PM   #3
5-90
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Quote:
Originally Posted by dustinslaton View Post
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...
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Unread 05-11-2012, 06:11 AM   #4
cruiser54
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5-90 is right on the money here as usual.
The peak horsepower gains in the HO were primarily due to a 58mm throttle body versus a 52mm throttle body on the Renix and a better flowing exhaust manifold. I'm sure the intake ports helped somewhat but I'm not prepared to give the head ALL the credit.
The 14 HP figurre between the two series of engines is rather large. It's more like 8 HP from the last generation of Renix. From 1987 to 1990, ECU changes added HP fro 173 to 182 on the Renix.

FWIW, I've done the Renix to early HO swaps and they're super easy. Some things to watch out for but no big deal.
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Unread 05-11-2012, 09:28 AM   #5
dustinslaton
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So it sounds like it's not really worth the trouble, and may even be beneficial to keep the 89 setup since it's got better the HP in the lower RPM range?

I'm definitely a fan of NOT changing things unless I have to. I just wish they had a few more bolt on options for pre-91 models.

Now that I know what to look for and terminology I'll do some searching on here and web for Renix and OBD. Thanks guys!
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