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Hello Moses,

I’ve just acquired a 1991 XJ Cherokee with 4.0L, AX15, 3.07 axle gears.

The seller knew virtually nothing about the truck, and only had it for a couple months before deciding it was not for him. It is in remarkable mechanical shape for the 200,000+ miles on the odometer, relatively new 4” lift and brand new 31x10.50r15 tires.

I have limited driving experience with the 4.0L, except for a 1990 Renix version with automatic transmission that was quite sluggish in its stock form. 
 

This new acquisition can put you back in your seat from off idle, and has more power than I would expect.  The somewhat clean engine (little oil leaks for 31 years old), the 24lb Mopar injectors, and a few other details makes me believe the engine is not original, or a least modified.
 

My question then is how can I identify whether this engine could be a stroker without dropping the oil pan to examine the crank?  I suppose I could try to measure the extra ~1/2” of piston stoke through the spark plug hole…any other ideas that would not require significant disassembly?

Instead of a stroker, could an aftermarket cam provide this performance on a stock 4.0L?  The engine idles very smooth, and the EFI seems to be compatible with whatever cam it has.

I suspect a stroker due to your past comments of unsatisfactory performance for such axle gearing and tire combinations as I have here.  The prevailing advice would be to seek 3.72-4.10 gearing for this combination, but it performs very well as it is.

Thank You

JJH

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Hi, JJH...Congrats on the '91 4.0L XJ!  Odd that the AX15 came with 3.07 gears, but nice to have a manual transmission.  With 31x10.5r15 tires, fourth gear, let alone 5th, should feel like overdrive!  A bit rough on the clutch with this combination.  Our '99 with the AW4 automatic came stock with 3.55s.  3.73s or 4.10s would be a good fit with your tires.  I installed 4.10s in our XJ for use with 33" tires.  Given your performance assessment, 3.73s would be my pick for the 31" tires and AX15.

I agree that something's cooking with the head snapping performance and 24 lb. injectors.  That's very characteristic of a 4.6L.  Your idea about measuring the stroke through the spark plug hole is sensible.  This is the only practical way beyond verifying the crankshaft casting number (either a 4.2L crank or a SCAT stroker crank).  You would need to drop the oil pan to see the crankshaft casting number but could also measure piston travel from the bottom side.  Short of that, a softer material for measuring the piston depth at B.D.C. (through the spark plug hole) would be my choice.  Perhaps a clothing tape measure or a pipe cleaner? 

You could also do this with a liquid, though a far more elaborate method:  Move the piston to B.D.C. and fill the cylinder through the spark plug hole to the height of the head gasket.  You would pour the liquid from a graduated cylinder (think science project).  When the liquid reaches the lower edge of the head gasket, that's the volume of the bore/stroke combination.  You would have a "cc" amount that could be converted to cylinder volume then compared to either a 4.0L stroke/bore or the 4.6L stroke/bore.  (Multiply this single cylinder's volume by 6 to make the 4.0L or 4.6L comparison.)  The liquid in the cylinder could be evacuated through the spark plug hole with a wet vacuum, a hand pump or an extractor.  For a liquid, you could use something harmless like SeaFoam® Motor Treatment.  Any residue would burn off in the cylinder.

It's doubtful that any camshaft that delivers head snapping performance would idle smoothly or be EFI compatible.  I ran the CompCams 252 grind for years, this was EFI compatible until the late 4.0L C.O.P. (coil on plug) engines.  The cam had good lift and mild duration for high idle vacuum.  While camshafts like this do provide strong low to mid-range power, they do not require 24-lb injectors.  That's the clue.

Moses

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Thank you Moses.  I should have thought of the volume measurement myself!  I'll attempt the pipe cleaner through the sparkplug hole, then the liquid method if in doubt. I will, however, be needing a new throw-out bearing very soon, so dropping the oil pan will not add all that much effort when the time comes.

Ironically, I am in the process of building a stroker for a CJ (that you advised my on in another forum).  Not having driven a stroker yet, I am enthused by what I am experiencing.  Even more so if this turns out to be a 4.0L and I have more power to look forward too in a 4.6L

This is the first XJ I have owned and am loving it right away.  One of my favorite vehicles is a 2nd Gen Toyota 4Runner with 22RE and manual transmission.  The XJ is very reminiscent of the Toyota, but thankfully more simple (and cheap!).  I have owned several CJ's and now wish I had tried the XJ out sooner.  I do have a 252H cam purchased years ago before production stopped.  At some point I expect I'll get into this engine and use the cam if it does not have a performance cam already.

Thank you again.

 

JJH

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JJH...Some comments below...Pleased you appreciate the XJ:

48 minutes ago, JJH said:

Thank you Moses.  I should have thought of the volume measurement myself!  I'll attempt the pipe cleaner through the sparkplug hole, then the liquid method if in doubt. I will, however, be needing a new throw-out bearing very soon, so dropping the oil pan will not add all that much effort when the time comes.

A one-piece oil pan is very handy, the best one is Felpro, easy to install overhead as it comes with threaded pegs to hold the gasket in place and align it.  I did a rear main seal change out on the XJ 4.0L.  Easiest two-piece seal change-out to date!

48 minutes ago, JJH said:

Ironically, I am in the process of building a stroker for a CJ (that you advised my on in another forum).  Not having driven a stroker yet, I am enthused by what I am experiencing.  Even more so if this turns out to be a 4.0L and I have more power to look forward too in a 4.6L

Looking forward to your findings on this engine with the 24 lb. injectors...

This is the first XJ I have owned and am loving it right away.  One of my favorite vehicles is a 2nd Gen Toyota 4Runner with 22RE and manual transmission.  The XJ is very reminiscent of the Toyota, but thankfully more simple (and cheap!).  I have owned several CJ's and now wish I had tried the XJ out sooner.  I do have a 252H cam purchased years ago before production stopped.  At some point I expect I'll get into this engine and use the cam if it does not have a performance cam already.

All good plans!  Keep us posted.  Yes, the XJ Cherokee is far cheaper to maintain and upgrade than a Toyota truck or SUV!

Thank you again.

 

JJH

 

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