4WD Mechanix Q&A Vlog: Why Build a Jeep 4.6L Stroker Motor?

[@harvestblades]

WTching this on the side of I29 after just losing my 4.0. Would love a 4.6, but would be just happy with a fresh 4.0 in general I can treat right & relia ly get 300k+ Mike's out of. I've always wanted a 4.6 stroker even if it just lands in my XJ for now. Best of both worlds as he describes it better performance on the street where my Jeep spends 99% of the time & especially on the Interstate, & still awesome off road. I'm sold now just to get my XJ home & figure out how to pay for this stroker build!

[@RoadReadywithMosesLudel]

Sorry about the I-29 abrupt exit! The 4.6L build is not much more cost wise than thoroughly rebuilding a stock 4.0L. Additional things needed are a 4.2L crankshaft (or a new Scat unit) and the right rod/piston combination. The rest would be the same concerns you have with a stock rebuild: new camshaft/lifters, bearings, block machining, timing chain and sprockets, cylinder head rebuilding and making sure the pushrod lengths are correct after the head and block deck machining. Good shops are familiar with this "4.6L" stroker build. I like a "torque/RV" camshaft (more valve lift and mild valve opening duration) with compression around 8.7:1 for a wider range of fuel octane choices. This is a low-end torque and good fuel mileage build. Higher compression and a performance camshaft require 318/5.2L V8 Mopar or 302 Ford V8 injectors. The 302 Ford V8 injectors are common for many 4.6L builds. (See 4WD Mechanix Magazine for more details.)

[@marcusharley3064]

Bought a 2002 TJ 4.0, planning on going stroker 4.6. What would be the longevity of the 4.6? Years and or mileage?

[@RoadReadywithMosesLudel]

You raise a good question. The largest consideration is that the stroke length has increased. That means more piston travel per mile. Offsetting this is the increase in torque, which reduces overall load on the engine. (I would measure this as higher manifold vacuum than the 4.0L at any rpm. The 4.0L is notorious for a mediocre torque and power curve.) Cylinder wear can be dramatically reduced by using a quality synthetic motor oil, which can offset the wear factor from more piston travel per mile. Keeping final compression ratio around 9:1 or less will help longevity. A mild camshaft (stock or "RV" grind) with modest valve overlap will decrease wasted fuel and fuel washing the cylinders, a considerable wear factor. For mileage and longevity, a target of 4,500 peak rpm is more than enough, stock and RV camshafts will run out by 4,500-5,000 rpm. The reciprocating and rebuild parts (crankshaft assembly with quality bearings, an upgrade double-roller timing chain set, a high volume oil pump and such) should hold up very well...In terms of lifespan "mileage", while a well-maintained and not abused 4.0L is good for 200-225,000 miles, I would expect the same from a mildly built 4.6L if driven like a traditional utility truck and not a race car. Some examples of longevity inline truck sixes that were "factory" strokers would be the Ford 300 and GM 292. These engines could easily run 200,000 miles on mediocre, conventional motor oil.

[@marcusharley3064]

Thank you for the extended reply. Here's my situation, I been welding for 32 years, and I have owned a few trucks to carry my welding gear. Now that I'm 58, getting on and off a pick up bed has gotten tough. So, I made a small trailer, to carry my 345lbs engine drive welder. I was pulling it with a 2012 Jeep JK, with the V6 engine. But the V6 keeps having expensive repair issues. So, I bought a 2002 Jeep TJ a month ago. It has the 4.0 engine, with automatic trans. With 200k miles on it, It lacks power. I mentioned it to the Jeep specialized shop in my area, they said the engine is fine, that I just need to get use to the characteristics of the 4.0. They mentioned if I went with a stroker motor, for me not to expect more than a couple of years with it. The 4.0 with 200k miles on it now. At take off, its responsive for about an 1/8th of a mile, then I have it almost pedal to the metal and it takes about a mile to get to 55 miles per hour. To me, it does not feel right. I have tried high octane gas, with no difference. The injectors are new, and new tune up. So, I'm thinking about starting with a new Long block. What are your thoughts? Suggestions? Ideas? Advice? Guidance? @@RoadReadywithMosesLudel

[@RoadReadywithMosesLudel]

The Pentastar 3.6L V6 had issues in the first production year(s). You likely inherited them with the 2012 JK. As for the 2002 TJ automatic, this is the last year for the 32RH transmission. A three-speed Chrysler unit with electronic shift interface and a lock-up converter, the 32RH evolved from the traditional 999/904 3-speed. You do not have overdrive, it's a straight 3-speed (1:1 in 3rd gear). The axle ratios with the automatic models can be quite tall, like 3.07:1 or 3.55:1. Even with a stock tire diameter, this is tall gearing, especially the 3.07s. See which axle ratios you have front and rear. Does the Jeep have oversized tires and stock axle gearing? This could be your performance lag. As for overall engine condition, I use a "four basics" approach to engine condition/troubleshooting (discussed in my Bentley Publishers Jeep books). You need normal oil pressure, normal and uniform compression, correct valve lift and normal valve timing. Oil pressure and no knocks is an easy one. Valve lift is about camshaft lobe wear and lifter condition. Valve timing is about the timing chain and sprockets. A standard compression test is helpful as a quick check, but I would run a cylinder leakdown test on a 200K mile 4.0L. That can be telling with regard to cylinder seal and wear points. I would also want to know the amount of timing chain wear. A simple manifold vacuum test is revealing here. Rotating the crankshaft back and forth while watching for camshaft position sensor movement is an indication of timing chain and sprocket wear...Make sure there are no engine codes around the camshaft position (sensor). Late valve timing could be part of the performance lag. On that note, the C-O-P (coil on plug) engines like your 2002 will not work well with the 252 camshaft. In fact, you need to narrow down the CompCams offerings that are compatible with a C-O-P 4.0L (2000-2006). They have grinds specifically for your engine. I like the mild specs with good lift on the camshaft you have in mind, however, you need to confirm that this is a grind compatible with the late 4.0L. The wrong camshaft will throw codes. Chrysler's OEM camshaft syncs the camshaft position sensor with the crankshaft position in a certain way. CompCams is thoroughly familiar with this issue. While you still want a mild camshaft with good lift, the valve timing must align with the late engine's sensors and PCM programming.