Spark Plug Choice and Gap w/ current mods

Been doing plenty of research but the results are so varied I don't have a solid basis to run on here for brand or gap...

Brief synopsis:

K&N intake, TB spacer (useless I know), Banks header, 2.25" pipe, hi-flo cat, 2.5" to dump. No real further plans other than a 62mm TB.

Currently running Taylor 8.2mm wires with Champion Truck plugs (438's I believe), gapped to 0.040". I know stock calls for 0.035", but I figured the extra 5 thousandths wouldn't hurt, and possibly be beneficial with the larger wires. Still running stock coil and ignition system.

Power down low is still the same, however inclines and freeway speeds are a bit more anemic now. Used to be able to take a 2% grade or so in 5th and only drop a few mph during the climb. Now I'm finding a downshift to 4th is required, and sometimes that's just barely adequate. Would this be due to plug gap? Plug choice? Haven't had bad results with Champ's T-plugs in the past on the same rig in question here. Mileage is pretty decent, managing about 250-260mi/tank, but thats mostly rural driving, ie: 45-55mph. Freeway speeds, mileage isn't quite as good as it used to be.

Gotta make a road trip down to California here in a couple weeks, so I'd like to get this sorted out before hitting the road.

Ideas? Close the gap to stock? Try NGK V-power plugs? I'm used to having bad around town mileage and decent on the freeway, even with stock gears (3.27 in the 8.8), AX-15, and 33's (proper pressures). Any suggestions and help appreciate.

Thanks peeps!

- M@

Stick with the stock champion copper cores (truck plugs are fine, same crap different box), and put your gap back to stock. the wider gap is making it more difficult for the spark to jump across the electrodes, which with your stock coil is making for a weak spark. I'd guess that would make enough of a difference for you to feel.

The only time you need to widen your plug gap is if you are running a higher than stock voltage coil, and at that point I would upgrade to platinum plugs to keep the hotter spark from burning the plugs up prematurely.

Sweet deal, appreciate it man. Figure l probably through a new O2 sensor in there too. I haven't replaced it in the four years I've owned the thing... Definitely past due.

On that note, do you know of anyone that offers a hotter coil? I know Accel offers one but it's the same voltage (42000v), and the Davis coil is only 45k.

Honestly unless you run big fat injectors and more than stock displacement I wouldn't bother. The 4.0 is a workhorse not a racer - it's designed to make low end torque and it does that very well in stock form.

I just got a set of Truck Plugs for my Wagoneer, and compared them with the coppers. The electrode looks a little bigger, but other then that they look the same. definitely a little more HD though.

How much did those wires set you back?!? With a stock coil high end wires are a real waste of money IMO.
I have had very good luck with the red DENSO wires, which I get from RockAuto for 12 bucks. (on my 94, not my '00 obviously!)

For 18 bucks you can get the primo wire set from NAPA with a lifetime warranty, part number 700444. 7mm wires but they are shielded and silicone jacketed, and should be plenty enough to handle the demands of a stock based ignition system.

The Champion truck plugs have 20% larger electrodes and glazed insulators that are supposed to be stronger than standard plugs, this would keep the ceramic portion of the plug from breaking during vibration but I have yet to see that happen :dunno: I think the main benefit of them is the larger electrodes, this will make them last much longer under harsh conditions such as WOT under heavy load, like flooring it with your wheels locked up against a rock. Their website touts a 3 year unlimited mileage warranty on the truck plugs as well, perhaps another vote in the favor of the truck plugs vs the standard plugs.

No change in compression, so there is no reason to change your plugs to something else. Stock gap too. Champion truck plugs or NGK are good.

And get rid of the Taylor wires. I've read that they can cause problems more than any other. http://speedtalk.com/forum/viewtopic.php?f=1&t=25712&sid=edb5ba2ddce2e72362d0cdc62c4c7c15

Luckily they were only $40. Haven't had any issues with them in the past, but a few friends running them on higher horsepower, drag V8's and such have had the issues discussed in that link.

I'll close up the plug gap first thing in the morning and see how it works out. Guess I'm ultimately running out of bolt-ons to get power, and it's finally time to re-gear. Sigh.

gearing to correct for tire size will make a bigger difference than just about anything you can do to the motor as far as bolt ons go. you have to remember that the XJ's 4.0 is an inline 6, not a v8. She's built for torque not top end HP

Yeah, there's been more than one occasion where I've forgotten the power orientation of the 4.0, heh. Been so busy building up the 357ci for my S10 that I'll cross reference knowledge and information :-/

I appreciate the help guys, you've been immensely helpful, as always.