It wasn't long after buying my Jeep that "mod fever" got the better of me. An output of "only 193hp" from the 4.0 engine wasn't enough despite the fact that it delivered strong performance. My initial aim was to break the 200hp barrier but later on I realised that even more horsepower/torque could easily be wrung out of the engine. Here's a summary of the modifications that I carried out and the results:

1. K&N air filter (February 1996, 48k miles)

The K&N air filter element #33-2018 replaces the stock paper element and drops straight into the airbox.

2. Modified S-pipe (March 1996, 50k miles)

The stock S-pipe from the exhaust manifold to the "cat" is very restrictive. It should be 2.25" in diameter but it narrows down to 1.75" at the bends. I had a section of my S-pipe cut out before the first bend after the manifold and after the last bend before the "cat". It was replaced by a section of 2.25" pipe with smoother bends and welded in place.
What a difference! The exhaust immediately sounded throatier and the response was amazing for such a simple mod.

3. Splitfire Spark Plugs (August 1996, 56k miles)

I replaced my stock Champion copper core plugs with regular Splitfire plugs. They reputedly provide a thicker spark that results in more complete combustion of the air/fuel mixture. I didn't notice any gains in performance nor gas mileage but they lasted 40k miles so longevity was the best reason for installing them.

4. Splitfire Ignition Wires (November 1996, 61k miles)

These 8.0mm dual core ignition wires are thicker than the stock 7.5mm ignition wires and provide a stronger spark. There was no performance gain after installing them but they're very durable and I had them in my Jeep for over 150k miles.

5. Flowmaster 40 series 2.25" muffler (January 1997, 63k miles)

This thing is LOUD! I replaced my stock muffler with this unit expecting better sound and performance. Ultimately, there was only an increase in noise that became annoying at certain rpm.
A few days later, I had a small section of my tailpipe cut off and a Pacesetter Monza 3" chrome tip with resonator welded in place. I was pleased with the result. Not only did I have a cool-looking tailpipe but some of the annoying drone from the muffler was drowned out.

6. Borla Header (November 1997, 76k miles)

I replaced the stock exhaust manifold and my modified S-pipe with a Borla stainless steel header and 2.25" mandrel-bent S-pipe. This is a very high quality piece and it was installed easily (4 hours). 
I was expecting a big gain in performance but the result wasn't as good as I expected. This was probably because my old modified S-pipe was so good! The sound from the Borla header was really sweet though and it undoubtedly mellowed out the gruffness of the Flowmaster muffler.
The only downside was that the engine now ran hotter. Coolant temperatures often crept above 210*F (100*C) during normal driving, a possible sign that the engine was running lean.

7. JET Stage 2 PCM (August 1998, 89k miles)

What a disappointment! JET Performance claimed a 14hp gain from this mod but if anything, there seemed to be a slight loss of midrange performance. At least the engine ran cooler after installing the recommended JET 180* thermostat.

8. Splitfire Triple-Platinum Plugs (August 1999, 104k miles)

The old regular Splitfires lasted 40k miles but these proved to be even better, lasting 106k miles.

9. Modified Insulated Ram Airbox (December 1999, 109k miles)

Details of this modification are on the "Ram Air" page.
I removed the plastic flap in front of the airbox opening, enlarged the opening by 50%, removed the venturi from the tube exiting the airbox lid, and insulated the airbox with foiled windshield sunshade.

10. Flowkooler High-flow Water Pump (April 2000, 116k miles)

The original stock pump proved to be very durable but it ultimately started to leak from the "weep hole". Over the course of two months, the leak gradually increased until the engine could no longer hold the coolant.
The Flowkooler water pump was installed in 3 hours. It reduced coolant temperatures at highway speeds by 15-20*F (about 10*C) so I was definitely happy about that!

11. Electric fan conversion (June 2000, 119k miles)

Cherokee dual electric fan conversion

12. Poweraid Throttle Body Spacer (September 2000, 123k miles)

This 1" thick billet aluminium spacer fits between the throttle body and the intake manifold. It increases the effective intake runner length thus producing a small increase in low/medium rev torque. The helix bore is just a marketing gimmick.
It wasn't as effective as it was hyped up to be by advertisements but the engine was slightly more responsive, and there was a gain of 0.8mpg from this mod, so I wasn't complaining!

13. Relocated IAT sensor to cold airbox (April 2001, 135k miles)

IAT sensor relocation

Rear wheel torque was increased by 4lbft at 1500rpm, 6lbft at 2000rpm, 7lbft at 2500rpm and 7lbft at 3000rpm. Pinging under load above 3000rpm was reduced but the engine was still running lean. Gas mileage was unchanged at 20.5mpg.

14. Ford Racing 24lb/hr injectors (October 2001, 143k miles)

The stock injectors flow only 21.0lb/hr at a factory-preset fuel pressure of 39psi and cannot provide enough fuel for 220hp. The Ford injectors flow 24.0lb/hr at 39psi and satisfy the engine's demand for additional fuel. When I installed them the pinging was eliminated and performance slightly improved. Unfortunately gas mileage took a dive from 20.5mpg to a miserable 17.8mpg and the engine now ran too rich! Since the engine no longer needed the extra fuel enrichment from the relocated IAT sensor, I plugged the wiring harness back into the stock sensor.

15. Ported throttle body and spacer (February 2002, 147k miles)

The engine now needed more air to correct the rich condition. The stock TB has a bore of 60mm that tapers down to 55mm below the throttle butterfly. I had the taper bored out to 60mm so that the bore was 60mm along the whole length of the TB. The bored 60mm TB flows ~505cfm @ 1.5inHg. I also had my Poweraid TB spacer bored out to 62mm, matching the bore of the gasket and intake manifold plenum opening. These mods really sharpened throttle response. Performance across the rpm range improved slightly but gas mileage remained unchanged and the engine still ran rich.

16. Homebrew Powerstack FIPK (March 2002, 148k miles)

Details of this modification are on the "Homebrew FIPK" page.

17. Bored 62mm throttle body (June 2002, 151k miles)

My engine still demanded more air. I increased the bore of the TB to 62mm, matching the TB spacer and the intake manifold inlet. I also fabricated a 62mm oversize throttle butterfly to match the larger bore. It's slightly thinner than the stock butterfly. This needed a lot of care to get it absolutely right and to get the butterfly to sit at the correct angle (~8deg) when closed. The throttle shaft did not need to be modified. Once I screwed the butterfly onto the shaft, I cut the protruding ends of the screws flush with the shaft to smooth things off.
The 62mm TB flows ~550cfm of air @ 1.5inHg. Throttle response and performance felt even sharper. The engine pulled smoothly with no hesitation or flat spots. The whistle at light throttle was reduced, probably because there was no longer any steps between the TB, TB spacer and manifold to cause turbulent airflow. The only downside was that the idle speed increased to 1000rpm initially but after a few hundred miles it returned back to the normal 750rpm. Gas mileage improved further to 19.5mpg. Another step in the right direction but still 1.2mpg less than when the engine was stock.

18. Intake manifold heatshield (June 2002, 151k miles)

Thermotec intake manifold heatshield

The result was a rear wheel torque gain of 4lbft at 1000rpm, 2.5lbft at 1500rpm, 1.5lbft at 2000rpm, and 1lbft at 2500rpm. Not bad for such a cheap and easy mod. Calculated output 220hp/262lbft.

*This completes the Stage 1 tuning of the 4.0 that comprises only external bolt-on performance mods. The next stage involves internal modifications to the engine for further HP/TQ gains.*

19. Oil temperature gauge (October 2002, 155k miles)

Engine oil temperature gauge installation

20. Coolant temperature sensor (CTS) resistor trick (March 2003, 161k miles)

CTS resistor trick

21. DIY ported cylinder head (June 2003, 163k miles)

Details about this modification are on the "Ported Head" page. The result was a 7lbft gain at 4000rpm and a 15lbft gain at 5000rpm so I was very happy with that. Gas mileage increased only marginally to 19.7mpg so the engine was still running slightly rich. Calculated output 235hp/269lbft.

22. MAP sensor voltage adjuster (September 2003, 166k miles)

MAP adjuster

The stock MAP sensor input voltage was 5.05v. I reduced the voltage in small steps in order to shorten the injector duty cycle and correct the rich condition. I finally settled at a MAP input voltage of 4.25v. At that voltage, the air/fuel ratios at idle and under light throttle cruise conditions were 15.1-15.9:1, right at the optimum level for best gas mileage. Not surprisingly, average gas mileage improved to 20.8mpg and I'm very happy about that. This was a very cheap mod and it works beautifully.

23. More vehicle weight reduction (January 2004, 170k miles)

My XJ was 83lb or 2.4% lighter than stock, weighing in at 3607lb with half a tank of fuel and driver, and ran the 1/4 mile in 15.39 @ 91.1 (almost a full second faster than stock).

24. Larger diameter exhaust (February 2004, 171k miles)

Since the engine is producing 235hp with the ported/milled head, a 2.5" exhaust is needed for maximum performance. I replaced the Flowmaster with a Magnaflow 2.5" stainless steel muffler and a Mike Leach 2.5" tailpipe. These "straight-through" mufflers are reputed to have the highest airflow. It was quieter than the Flowmaster at idle, below 2200rpm, and above 3500rpm. In the 2200-3500rpm range, there was an annoyingly loud drone. I cured it by using the previously removed Flowmaster muffler to replace the factory glasspack that was in front of the Magnaflow. Now the exhaust sounds much better!

25. Low-buck 4.6L stroker engine (June 2004, 174k miles)

By now, I knew that 235hp/269lbft was as much as I was going to get out of 4.0 liters without tearing into the short block. I chose instead to go for more displacement, so I built a 4.6L stroker long block in my spare time and had it swapped into my Jeep as soon as it was finished. It's the same as the "poor man's" recipe on the "stroker" page except for the Crane 753905 camshaft. The details are on my Junker to Stroker site.
This is the engine the Jeep should have had in the first place! It's a torque monster and the performance is outstanding, leaving the old 4.0L engine in the shade.

26. Mustang 65mm throttle body (July 2004, 175k miles, 1100 miles on stroker)

Mustang 65mm TB upgrade

A stroker engine has more demand for air than a 4.0L so I made it breathe through a bigger straw. The result was sharper throttle response and a little more torque. The part throttle whistling noise disappeared altogether.

27. Mobil 1 0W-40 synthetic engine oil (October 2004, 177k miles, 3000 miles on stroker)

I had been using 20W-50 dino oil for the break in period so it was time to change over to synthetic. A dyno comparison of various synthetic oils to dino oil showed that Mobil 1 0W-40 produced the biggest HP gain so I wanted to try it out for myself.

28. DIY oil catch can (December 2004, 179k miles, 4500 miles on stroker)

DIY Oil Catch Can

29. Custom rear traction bar (May 2005, 184k miles, 10k miles on stroker)

Custom traction bar

30. Upgraded oiling system (April 2006, 200k miles, 25k miles on stroker)

Melling High Volume Oil Pump and Baffled Oil Pan

31. Custom 3.0" S-pipe (June 2006, 201k miles, 27k miles on stroker)

The result was a richer exhaust note and stronger acceleration at higher rpm. Calculated output 256hp/316lbft (21hp & 47lbft more than the modified 4.0 engine).

Custom Stroker Exhaust

32. Upgraded valvetrain (November 2006, 208k miles, 34k miles on stroker)

Yella Terra 1.6 ratio roller rocker arms and Crower chromemoly hardened pushrods

Unfortunately I found two damaged lifters and one rounded off cam lobe on the Crane 753905 camshaft that I used in the original stroker build, so I had to swap my old stock camshaft and stock valve springs with new lifters back in. With the MAP adjuster set at 5.10v, the JET Stage 2 PCM removed, and mild revisions to my homebrew cold air intake, the Jeep put down 200rwhp @ 4750rpm and 254rwtq @ 3500rpm on a Dastek load-type chassis dyno. The engine pulled strongly with a razor sharp throttle response and the valvetrain was much quieter. The loss of performance was insignificant after reinstalling the stock camshaft.

33. '01 XJ intake manifold swap (May 2009, 228k miles, 54k miles on stroker)

I swapped the new style curved-runner intake manifold with power steering pump, PS pump mounting bracket, and tensioner pulley from a junkyard 2001 Jeep Cherokee into my Jeep. Installation was straightforward. The only modifications required to complete the job were lengthened wires to the IAT sensor, slight modification to the routing of vacuum lines, and a longer drivebelt. On the initial test drive the engine ran slightly lean but that was easily corrected by raising the MAP voltage to 5.30v. Now the engine feels even stronger than it did with the original log-type intake manifold and that impression was borne out on the dyno. The Jeep put down 200rwhp @ 4700rpm and 258rwtq @ 3500rpm (246hp/311lbft at the flywheel) with a torque gain of up to 5lbft from off idle to 4750rpm. Fuel economy also improved by nearly 1mpg.

Very reluctantly sold Jeep  on 13 December 2016 after 21+ years of ownership: 260.5k miles, 86k miles on stroker.
It's been an absolute peach and is forever etched into my heart!

Tuning Stages:

Stage 1 (220hp @ 5000rpm, 262lbft @ 3750rpm) No internal engine mods.

1. Induction: Cold air intake, bored 62mm TB, bored 62mm TB spacer.
2. Exhaust: 2.25" exhaust system, header, high-flow cat and muffler.
3. Replace stock clutch fan with electric fan.
4. Ford Racing 24lb/hr injectors.
5. Relocate IAT sensor to cold air intake.
6. MAP adjuster or adjustable fuel pressure regulator.

Stage 2a (240hp @ 5100rpm, 284lbft @ 3500rpm) Low/medium rpm torquer engine

7. Ported stock cylinder head.
8. Raise CR to 9.1:1
9. CompCams 192/200 degree cam #68-115-4.
10. 1.7 ratio roller rockers.

Stage 2b (257hp @ 5300rpm, 290lbft @ 4000rpm)

7. Big valve ported head.
8. Raise CR to 9.5:1.
9. CompCams 206/214 degree cam #68-231-4.
10. Raise rev-limiter to 5600rpm.
11. 2.5" Exhaust system.
12. Strengthen bottom end: ARP 112-6001 rod bolt set, magnaflux and shot-peen connecting rods, balance rotating assembly.
13. Upgrade oiling system: high volume oil pump, oil cooler.
14. Upgrade cooling system: aluminium radiator, high-flow water pump.
15. 3.55 axle gears + 28" tires with automatic and manual transmission.

Stage 3

4.6L-5.0L stroker engines: 242-360hp, 303-391lbft depending on build specifications.
Kenne-Bell 5.5psi intercooled supercharger kit: 45% more hp, 53% more torque.
505 Performance 6.0psi turbocharger kit: 44% more hp, 47% more torque.
Nitrous Express (75hp) N2O injection kit: 76hp/117lbft gain.