Many of us think of weight as the enemy, whether it’s our beltline or our Jeeps. Weight places additional stress on all kinds of Jeep parts from the suspension to the drivetrain, makes it more difficult for them to climb obstacles, and makes them more tippy in off-camber situations. Even though most of the accessories we install (bumpers, winches, rollcages, and so on) add weight to a Jeep, smart Jeepers are always looking for ways to cut weight. There aren’t many situations where added weight is beneficial, but one of the few relates to the rotating mass of the engine.
Centerforce has been making high-inertia flywheels for select applications for many years, and recently expanded their offerings to popular Jeep applications, including the 3.8L and 3.6L engines in the JK, and the 4.0L used in YJs, XJs, and TJs. An offshoot of their complete line of billet steel flywheels that are SFI-certified, the company’s High-Inertia Flywheels have added weight compared to a standard flywheel. The extra heft means more inertia, and that has several benefits for off-roaders. First, there’s more resistance to stalling at idle and right off idle, so there’s no need to feather the gas pulling away from a stoplight and less likelihood of killing the engine when starting off on a hill. The ability to keep the engine running at lower rpm has all kinds of other benefits off the pavement. At higher rpm the engine is less likely to lug in mountain passes, so you can go longer before needing to downshift. This can even lead to better fuel economy, all without any discernable difference in how the engine revs.
The guinea pig for the swap was our ’84 CJ-7, which received a 4.0L transplant in a previous article. In terms of the engine and transmission, the Jeep is identical to any manual-equipped 4.0l-powered Jeep with an external slave cylinder. The earlier internal-slave manuals are also basically the same as far as the clutch and flywheel are concerned. Though our application is technically a mishmash of parts, the High Inertia Flywheel works with all 4.0L engines from 1991 to 2006. As with many procedures, the first step is to disconnect the battery.
We went straight to the source to find out the details on Centerforce’s newest High-Inertia flywheel for 4.0L engines, traveling to the company’s headquarters to install one of the first production flywheels to come off the assembly line. While we were at it, we installed a new Centerforce II clutch because you don’t go that deep into a swap without replacing wear parts. The procedure is no different than a normal clutch R&R, though we did run into a few surprises on our installation that you might want to be aware of when doing your own clutch and flywheel change. Also note that you can order the inertia flywheel by itself, or Centerforce offers a complete package that includes the flywheel, clutch, pressure plate, throwout bearing, pilot bearing, bolts, and alignment tool all in one.
The Centerforce High Intertia flywheel looks similar to a factory iron flywheel but is very different. It’s machined out of billet steel and is SFI-certified in case you want to go drag racing. Most of the added mass is on the backside of the flywheel, so it’s compatible with stock clutches and components. We’ve used the Centerforce II clutch on several different vehicles and have always been pleased with their performance and longevity, so we took the opportunity to install a new clutch at the same time.
We weighed the Centerforce flywheel and clutch, then compared it to the stock clutch and flywheel assembly once it was removed. The stock flywheel weighed in at 26.3 pounds, while the Centerforce flywheel weighed 40.1 pounds. That difference in weight is significant, especially when you take into account the added inertia with engine rpm. The total weight of the Centerforce flywheel and clutch assembly was 60.5 pounds, compared to 43.4 pounds with the stock flywheel and clutch.
Clutch gurus Will Baty and KC Payne handled the wrench-spinning duties on this project. The Jeep is equipped with a 4.0L from a ’99 Cherokee and a transmission from a ’98 TJ, both of which were junkyard specials. The tranny is adapted to the original Dana 300 transfer case. Our AX-15 has an external slave cylinder, but the procedure and most of the parts are basically the same for earlier AX-15s in YJs and XJs. Yanking the transmission involved disconnecting the shifters and both driveshafts, removing the belly pan, and disconnecting a few wires. The process is easier with a lift and a transmission jack, but with the proper equipment you can do this at home.
The Jeep was equipped with a fairly new stock replacement clutch that was purchased at a local parts store. It worked fine for our mostly stock application, but engagement was a little harsh, and it took quite a bit of pedal effort to disengage the clutch. The Jeep could use slightly lower gearing than the 4.10s in the differentials, and we were feathering the clutch off-road more than we would have liked. We had also developed a strange vibration on startup when the transmission was in Neutral with the clutch engaged.
We found the source of the vibration. When the tranny was separated from the engine, the pilot bearing hit the ground! Unbeknownst to us, there are two different pilot bushings used with the 4.0L: one large and one small. Our engine and tranny came from different vehicles, and the engine was originally in front of an automatic. We ordered what we thought was the correct bearing for the application, but we were wrong. The ID is the same, but the OD is different and determines the depth of the bearing in the crankshaft.
It makes more sense when you look at the back of the crankshaft. There are two machined surfaces in the back of the crank, and they will accept both large and small diameter pilot bearings. The smaller of the two is deeper in the crankshaft by nearly 3/4 inch, and this is the one we installed. As a result the transmission input shaft was just barely engaging the pilot bearing, and the vibration caused the bearing to eventually walk its way out of the crank. Fortunately, this was caught before any real damage occurred to the tranny.
The Centerforce guys performed a series of measurements to determine the amount of stickout on the transmission input shaft and cranskshaft to verify which pilot bearing we needed. It turned out to be the larger of the two. The moral of the story: if your donor engine and transmission came from two different vehicles, do the math to determine exactly which pilot bearing you need or risk major transmission damage.
The Centerforce flywheel installs just like the factory one and includes new ARP flywheel bolts. It’s very important to torque them to proper specifications, which for a 4.0L is 105 lb-ft. With 40 pounds of rotating mass, you don’t want it to come loose. Note that the bolt spacing is such that the flywheel only lines up with the crank bolt holes one way. Also don’t forget the dust shield the goes on the back of the block.
It’s always a good idea to check-fit the clutch disc on the transmission input shaft before preparing to install the clutch assembly on the flywheel. Note that the clutch disc only installs one way and is marked to indicate which side should be toward the flywheel. You’ll need a clutch alignment tool or an old transmission input shaft to line up the clutch disc properly.
Once the clutch disc is aligned, install the pressure plate. Centerforce uses several tricks to improve the engagement and holding power of the clutch, but one of the more visual is the weight on each clutch finger. Those weights are on a spring-loaded ring, and as engine rpm increases, the weights move outward on the fingers and increase clamping force. This means more holding power at higher rpm. There are multiple internal tweaks they make to pressure plate that improves pedal feel, engagement, and the all-around characteristics of a quality clutch.
One of the last pieces of the puzzle before completion was the installation of the throwout bearing and clutch fork. We strongly recommend replacing the throwout bearing as part of any clutch replacement because it’s a wear item and safe insurance against problems. Be sure to grease the pivot points, but don’t go overboard with the grease and risk contaminating the clutch. We re-used the original clutch fork because it was in good shape.
Sometimes it goes easy, and other times it doesn’t. Take your time and don’t force anything into place. Keep the crank trigger well out of the way to avoid damaging it (it’s expensive) and never use the transmission bolts to suck the transmission to the back of the engine. It’s important to note that the mounting bolts should always be tightened to the proper torque specifications.
We haven’t put a whole lot of miles on the Jeep since the clutch upgrade, but so far we’ve been very impressed. Pedal pressure is much lighter than with the old clutch, and engagement is much more smooth and controlled. The extra mass of the flywheel was immediately noticeable. We can now just let out the clutch at a stoplight rather than having to feather the gas. There’s a noticeable improvement off-road as well, with less stalling and struggling to get going on a hill. Overall, it’s was a big upgrade over stock replacement parts.
3.6L Pentastar Impressions
In addition to the 4.0L inertia flywheel detailed in this story, Centerforce has also recently released flywheels for both the 3.8L and 3.6L V-6 engines in the JK. Where the 4.0L has always been known for torque, the higher-revving V-6s in the newer Jeeps are sorely lacking in power down low. The benefits of an inertia flywheel are even more noticeable with these newer engines, as we found out.
We love to drive stick, and the revvy nature of the 3.6L can make the Wrangler JK a playful implement in the right hands. However, the added power and efficiency of the DOHC 3.6L does come with a price when compared with the venerable OHV 3.8L—a lack of low-end torque.
While we love the way our Jeep works, we always felt that it was held back by the stock clutch, which we’ve found to be substandard in many situations. We’ve easily smoked one in a stock Unlimited while climbing obstacles in four-wheel drive, requiring us to shift into 4-Lo in order to get up and over the obstruction, and this is before adding a lift, 37-inch tires, upgraded axles, heavier bumpers, and 4.88 gears. With a Jeep in modified form, the limits of the stock clutch are even more apparent.
Fortunately for those who love to row gears behind a 3.6L, Centerforce has come to the rescue with the company’s new Duel Friction clutch option. When compared to the 53.95-pound factory clutch assembly, the Centerforce parts weigh in at 67.95 pounds. Part of that weight is in the more liberal use of friction material in 10.95-inch Dual Friction assembly, but most of it is in the 13.5-pound-heavier flywheel. Increased friction surface area should give the clutch increased longevity, while the clutch itself is rated to hold a staggering 477 lb-ft of torque at the flywheel, a figure we aren’t anywhere close to topping with our DOHC V-6, giving us plenty of reserve capacity. The heavier flywheel not only increases inertia, but also acts as a larger heat sink, necessary for our 1,000-pound-heavier-than-stock Wrangler Unlimited.
Since the install, we’ve rolled about 2,500 miles of street, highway, and trail on our JK and have been very happy with the upgrade. Thanks to the new parts, the Wrangler is more forgiving when pulling away from a stop, despite a friction zone that feels like stock. The clutch is quiet, easy to modulate, and has no more noticeable pedal effort than the clutch assembly it replaces, and we’ve yet to find ourselves a situation where it slips. Even with the heavier flywheel, any loss of acceleration is negligible through the seat of the pants. On the trail, the engine can lug a couple hundred rpms lower than usual before stalling, and the Jeep has a better ability to maintain momentum while crawling.
Overall, the Centerforce upgrade to our ’12 Jeep Wrangler is one of the most seamless and has really improved the overall drivability of our Jeep with no additional effort or noise. In fact, we haven’t found a drawback yet and highly recommend this setup to anyone with a modified manual Wrangler. – Sean Holman