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7 Steps To Power & Mileage

Jeep Wrangler 3 8l Engine
Christian Hazel
| Brand Manager, Four Wheeler
Posted October 1, 2012
Photographers: Verne Simons

Jeep Wrangler 3.8L Upgrades

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


Performance Distributors
Memphis, TN 38132
Corona, CA 92879
Rusty's Off Road
Kendall Motor Oil
Sea Foam Sales Company
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