Help with selecting the right spark plug

[Wild One]

Hello all,

Here's my situation, I have a 1998 Jeep Cherokee 4.0 4WD that was suffering from a blown head gasket. My solution was to not only repair the failed head gasket but to improve the efficiancy of the engine at the same time (the side effect of a more efficiant engine is more power, more power is always good!). After considerable research for the most cost effective ways to reach this goal I came up with the following: 1) use a later casting 0331 head to take advantage of the better port shapes and port velocities that these heads offer.

See the following posts for additional info : http://www.jeepstrokers.com/forum/viewt ... p?=5&t=226
http://www.jeepstrokers.com/forum/viewt ... ?f=5&t=480

2) use a 1999 and later style intake manifold for its advanced design and optimized flow characteristics
3) use an aftermarket tubular header that meets my local SMOG requirements, I choose a GIBSON header.
4) use an aftermarket cat back exhaust system, I choose the DynoMax brand due to past good expierances with
their product, that coupled with a total purchase price of $15.00 for the system thanks to their $75.00 cash back promotion and an online discount at the time of purchase didn't hurt my discission any either!
5) "repair" the factory kink in the header down pipe, the was done with my oxy accetylene torch and various prying devices.

Now, my question is this, should I use a spark plug for the factory 0630 head or the newer 0331 head? Using NGK's web site, the factory replacement plug is a FR51 for the 0630 head or a ZFR5N for the 0331 head. The only difference is the porcelain and electrode on the ZFR5N plug for the newer 0331 head equipped engine is LONGER than the porcelain and electrode on the FR51 plug for the factory equipped 0630 head. All other parameters are the same, heat range, thread diameter and length, etc... I am worried about the plug making contact with the piston, detonation, emissions related garbage, and anything else that my good friend Mr. Murphy can possibly throw at me!

The newer head is already installed with everything hooked up and ready to run, this is my last hurdle to clear before I twist the key and hear my Jeep roar to life again. Some extra info - the bottom end of the engine is 100% factory, therefore I am running the factory dished type pistons that have the number 4 embossed on top along with the factory cam, lifters, etc.
Hopefully I can get the answer I need and bring the old girl back to life.

If you've made it this far, I congratulate you! If you have any helpful input you have my everlasting gratitude!

[IH 392]

The Autolite catalog shows the 985 for all 4.0, so I wouldn't worry about the tip style, unless you've increased the compression, then use the shorter tip and a colder heat range.

If you didn't increase the compression ratio I would use the long ones, increased compression requires a colder plug, I have a set of melted pistons to prove we learned this the hard way!, the general rule of thumb for spark plugs is "use the coldest one that won't foul", my Yamaha is perfect example for this, if I use the std heat range it's fine for trail riding, but if I race it it starts to detonate, if I put the next colder plug in it, it'll race fine but when I go to trail riding it starts to foul.

[Wild One]

Just as a follow up on this subject, I figured I would post my results to help anyone who might perform this swap. I wound up trying both spark plugs and while the jeep ran fine with both plugs styles it did in fact run smoother at idle with the newer longer style plugs installed. Seeing as I live in the greater bay area in the land of fruits and nuts, I have to have all of my vehicles pass the "enhanced" smog process. This requires strapping the vehicle down to a roller dyno to introduce a load on the engine at two speeds - this is to produce a "real world" driving emission sample.

My particular setup passed with the following readings : 15mph sample CO2% 14.9, O2% 0.2, HC (ppm) MAX allowed:53 AVERAGE:8 MEASURED:6, CO% MAX allowed: 0.67 AVERAGE: 0.02 MEASURED 0.03, NOX (ppm) MAX allowed: 522 AVERAGE:82 MEASURED:152 RESULTS: PASSED!
25mph sample CO2% 14.9, O2% 0.1, HC (ppm) MAX allowed:37 AVERAGE:6 MEASURED:4, CO2% MAX allowed:0.81 AVERAGE:0.02 MEASURED:0.01, NOX (ppm) MAX allowed:775 AVERAGE:74 MEASURED:157 RESULTS: PASSED!

The NOX #'s were a little higher than I would have liked, but given the vehicles age (1998), milage (almost 140,000 miles) and the fact that the vehicle had stale gas in the tank that was at least 8 months old this was acceptable. In the mean time the jeep is currently undergoing the rest of its "improvements" and will be back on the road full time in the not too distant future.