7 Steps To Power & Mileage

Jeep Wrangler 3.8L Upgrades

Verne SimonsPhotographerChristian HazelPhotographer, Writer

If we’ve heard it once, we’ve heard it 1,000 times. The ’07-’11 JK’s 3.8L minivan-sourced engine is a turd. But is it really? Yes and no. Tasked with pushing such a heavy brick, its 202hp does a pretty good job once you get comfy leaning against the rev limiter. And that’s the downside. These little engines are really lacking in low-end grunt. You have to make ’em scream to get anywhere. Most owners tend to lug ’em down low, which makes the JK feel slow and doesn’t do any favors for the engine internals. And the high load placed on the engine every time you encounter a hill spikes exhaust gas temperatures, which heat-cycles the exhaust manifolds. Next thing you know you’ve got carbon buildup, oil consumption issues, and big cracks in your exhaust. So what’s a minivan engine owner to do?

First, drive it like you hate it. Wind that little sumbitch up when you encounter a hill rather than letting it lug. And upping overall engine power doesn’t hurt. Rather than hunt-and-peck on the Internet for parts, we did a quick-draw from the hip and tapped a couple of our old standby companies for some performance parts to (hopefully) turn our ’07 Rubicon Wrangler slug into a…well, slightly faster slug. Any bump in mileage would be a welcome improvement.

For every modification, we tracked our fuel consumption for a couple thousand miles of mixed driving and published the average for each combo. Then, we made a series of 1-mile uphill power runs noting the speed midway through the course and at the finish line. We left the vehicle in Third gear for the entire run, beginning at a slow roll at 1,500 rpm. This tested not only peak horsepower, but overall engine torque through the rpm range. Improvements in low-end torque are more evident from the mid-point speed numbers, with increases in top-end horsepower more telling of the finish line speed.

In the end, we ran into some troubles caused by the header installation that plagued our end-of-test numbers. Between collector bolts that wouldn’t stay tight and periodically burning through ignition wires, our mileage runs were probably slightly compromised, but we ultimately wound up with a 1.7 mpg improvement. Power-wise, we never set the aFe Scorcher to the high-octane tune. Had we done so, our speed runs could have improved a bit, but as is, we were able to pick up 7 mph at the mid-point marker and 9 mph through the traps, which is not too shabby. For us, the ideal combo would be some aftermarket heavy-duty cast iron manifolds that resist cracking better than the factory junk, yet don’t have the fitment issues of tubular shorty headers. We’re leaning on a few places to build ’em, and we’ll be sure to let you know (and take full credit) if such a product ever makes it to market.

Step 1: Baseline
When we took the keys away from our former editor, Jp’s JK had 78,000 neglectful miles under the tires. The engine air filter was clogged, the crankcase was 3 quarts low, and it was due for an oil change. There was also a major exhaust leak from the manifolds and it fell on its face above 4,000 rpm. We suspect the low oil level was due to carbon buildup on the rings and valves. We topped off the oil and resisted the urge to clean the air filter to establish our baseline mileage numbers.

Install notes: N/A

Seat-of-pants: Slight hesitation off-idle and perceived lack of power above 4,000 rpm

Acceleration (midpoint): 54 mph
Acceleration (final): 59 mph
Mileage: 16.1 mpg

Step 2: Oil Change, Fresh Air Filter, Clean Crankcase
With our baseline out of the way, we drained out the dino oil and filled the crankcase with some Kendall 5W-20 synthetic blend and a new premium oil filter. We also installed a new air filter in the factory air box and added a can of Sea Foam Motor Treatment to the fuel tank to help clean the valves. We also did a top-end cleanout using Sea Foam to remove any carbon buildup from the pistons, rings, and valves.

Install notes: For the Sea Foam top-end treatment, we got the engine up to operating temperature and with the engine running at about 2,000 rpm, we pulled the vacuum line from the power brake diaphragm and gently sucked about 1/3-can of Sea Foam into the engine. Then, we shut it down and let the engine sit for about 15 minutes before taking it for a short 2-3-mile drive. The carbon dissolves and/or flakes off, coming out the tailpipe in a plume of white smoke that lasts anywhere from a couple hundred yards to a mile or so.

Seat-of-pants: Off-idle hesitation still noticeable, but power returned above 4,000 rpm.

Acceleration (midpoint): 56 mph
Acceleration (final): 62 mph
Mileage: 17.6 mpg

Step 3: aFe Stage 2 Si Cold Air Intake
We chose aFe’s Stage 2 intake for a number of reasons, primary of which was the excellent fit and the fully sealed design that should keep mud and big chunks of dirt off our filter. When we got our system (PN 54-81252) the company’s Pro 5R oil-impregnated cotton gauze filter was the only option, but aFe has since added two filter options that can be ordered with the system in lieu of the standard Pro 5R filter. First, the Pro-Guard 7 is an oil-impregnated cotton gauze filter with seven layers of increasingly finer media material for higher filtration efficiency. Second, the company’s Pro-Dry 5 is a washable/reusable oil-free synthetic media that never needs oiling.

Install notes: Retains factory filter mounting base and requires no cutting of factory hoses or major changes.

Seat-of-pants: The off-idle hesitation was minimized and the engine pulls more willingly to redline. Mileage probably suffered because engine is more fun to fling to redline under full throttle. Seat-of-pants power seems improved throughout rpm range. During acceleration runs, minor knocking and pinging heard with engine under full load, probably due to slightly leaner air/fuel mixture since engine is getting more air.

Acceleration (midpoint): 59 mph
Acceleration (final): 65 mph
Mileage: 17.5 mpg

Step 4: aFe 2.5-inch Hi-Tuck Exhaust
You’ve got to be careful when selecting an exhaust system for your four- or six-cylinder engine. First, you don’t want your Jeep to sound like a Honda with a fart-pipe. The aFe after-cat system doesn’t. Also, you don’t want to kill the exhaust gas velocity by going too large with the tubing diameter. The mandrel bends and 2.5-inch diameter of the Hi-Tuck exhaust we selected for our two-door JK (PN 49-46208) improve flow without killing velocity like a larger 3-inch system could. If you’re running additional displacement (like with a V-8 conversion) or have added a supercharger or turbo, then aFe’s 3-inch exhaust will be right for you.

Install notes: The factory exhaust is an absolute bitch to get off, but with a lot of pulling, twisting, shouting, and swearing it comes out. Use plenty of lube. The new aFe exhaust has nice band clamps and excellent fit and finish. Don’t tighten band clamps ‘till the system is installed and aligned. The tailpipe on the Hi-Tuck version is up high and out of the way and the aFe muffler is a much smaller target for trail obstacles, but the system didn’t fit with our 1-inch rear swaybar drop brackets. We had to swap to extended rear swaybar end links instead.

Seat-of-pants: There’s a noticeable growl under acceleration but no increase in cruising noise and only a slight burble noticeable at idle. No real difference noticed in power, though.

Acceleration (midpoint): 60 mph
Acceleration (final): 66 mph
Mileage: 17.6 mpg

Step 5: aFe Silver Bullet Throttle Body Spacer
In theory, throttle body spacers increase the plenum volume as well as maximize intake air velocity. In practice, they usually make a lot of noise and rarely if ever any power. We decided to give aFe’s Silver Bullet spacer (PN 46-35002) a shot since we had to pull off our throttle body to clean the carbon crud off the rear of the throttle blade anyway.

Install notes: Insert the spacer and make sure the throttle body is seated squarely before tightening the longer aFe-supplied bolts in an X-shaped pattern. The factory throttle body is plastic and easy to crack if you gun the bolts down too tight.

Seat-of-pants: We got a very annoying off-idle whistle and other intake noise throughout the rpm range, but no perceivable improvement in power and we saw a decrease in mileage. We removed the spacer and our numbers returned to previous levels.

Acceleration (midpoint): 57 mph
Acceleration (final): 64 mph
Mileage: 17.2 mpg

Step 6: Rusty’s Headers & Performance Distributors 3.8 Firepower Ignition
When we say our stock exhaust manifolds were cracked, we mean both were nearly severed the entire circumference of the Number 1 and Number 2 runners. The best price on stainless steel headers we could find was Rusty’s Off Road Flow Daddy units (PN FD370) for about $250/pair. The welds are nice, the port and downpipe fitment were good, and the passenger-side header plunked right in, but the driver-side is a nightmare to install. This is true for just about any short-tube JK header on the market. We had to do some modifications and fought loosening collector bolts for a while. While more durable than factory manifolds, the headers come with their own range of issues, so it’s sort of a trade-off in one hassle for another.

Since the header installation required the removal of the ignition for access, we installed our Performance Distributors 3.8 Firepower ignition kit, including the Screamin’ Demon coil and low-resistance Livewires plug wires. Fitment was perfect and the high-energy ignition allows the plug gap to be increased to 0.065-inch for a huge blast of spark energy.

Install notes: Remove the inner fender liners for installation and drop the steering shaft for more room. The bolts for the driver side are almost impossible to get started thanks to the heat shield. We wound up cutting off the heat shield and still barely managed to get the header installed. Without the shield, we burned through a couple plug wires before using an extra-long wire with a super high-temp sleeve and some creative wire routing. We’ll probably have to drop the header if we ever need to change the spark plugs on the driver side.

Seat-of-pants: The crazy-annoying exhaust leak was gone, but we noted no differences in drivability even though our speed runs increased. We had to tighten the collector bolts every 150 miles or so for the first 3,000 miles.

Acceleration (midpoint): 61 mph
Acceleration (final): 68 mph
Mileage: 17.2 mpg

Step 7: aFe Scorcher Tuner
Many electronic tuners are programmed to work with factory components, but aFe’s Scorcher employs programming by Superchips tailored to aFe’s performance products. That means things like increased fuel delivery and injection timing, more aggressive shift timing (for autos), a bump in ignition timing, and of course, all the other things like speedometer correction, and reading and clearing check engine lights that we’ve come to expect from electronic tuners. There are many different tunes available, but after correcting our speedo for our 305/70R17 tires and factory 4.10 gears, we loaded the 87-octane performance tune and then the Economy tune.

Install notes: Plug it in and follow the prompts. It takes about 5-10 minutes to load a tune.

Seat-of-pants: With the 87-octane performance tune you could really notice an increase in power in the lower rpm range, especially off-idle. The old dead spot as you started from a stop was finally gone and throttle response was sharpened. However, with the Economy tune, the dead spot came back along with an overall lackluster feeling in engine performance. We really could tell a difference between the two tunes from the driver’s seat.

Acceleration (midpoint): 60 (Economy tune); 61 (87-octane tune)
Acceleration (final): 65 (Economy tune); 68 (87-octane tune)
Mileage: 17.8 mpg (Economy tune); 17.6 mpg (87-octane tune)