How to make a jeep grand cherokee faster

You’ve heard before that a gas engine is basically a giant air pump. Traditional hot rodding suggests that more air in and more air out equals more power. But would this hold true for a modern engine like the 4.7L High Output V-8 in our WJ Grand Cherokee? The H.O. engine uses a forged crank, increased compression, a different camshaft, larger injectors, and a unique intake manifold to make 30 hp more than the standard 4.7L engine. Did the Mopar engineers leave anything on the table in terms of power or mileage? We wanted to find out.

The full-time NP247 transfer case in our Grand results in a lot of spinning parts and stock mileage around 19 mpg. We decided to toss on an intake, programmer, and exhaust to find out if any gains were to be had. But before we could get started, Editor-in-Chief Péwé stopped us cold. “How will you know which parts made a difference if you add them all at the same time?” he sagely asked. To provide the most value to the readers, each part needed to be tested independently and then the system evaluated as a whole.

“Would the parts play nice with each other, or turn the dash lights into a Christmas tree?”

We rounded up parts from Airaid, Superchips, and DynoMax and installed them one at a time, then documented the mileage on a designated route and documented the power on the AWD Dynojet at Street to Sand Off-Road & Performance. Over a dozen pulls were made to test various parts configurations, and the results surprised and sometimes disappointed us. There were very little gains to be had in terms of mileage regardless of what we did, but every part we tested added horsepower and torque throughout the powerband.

Airaid Intake
Intake bonus: Never have to buy another filter again
The factory intake on our 4.7L engine uses a paper filter that passed air through a corrugated hose into a resonator in front of the throttle body. It does a reasonably good job of keeping the engine quiet without choking the engine, but there was still room for improvement. We added an Airaid intake (PN 311-148) that includes a high-flow air filter with greater surface area than stock, along with a smooth intake tube that runs straight to the throttle body. It sits in the lower half of the factory intake box with an included heat shield to keep hot underhood air from entering the engine. We specified a SynthMax filter that never needs to be oiled and added a prefilter from Airaid as well since our Jeep sees a lot of dusty roads. The Airaid intake made the biggest increase in horsepower of any of the parts we installed.

The 4.7L H.O. V-8 engine in our Grand Cherokee is rated at 265 hp at 5,100 rpm and 325 lb-ft of torque at 3,600 rpm. We set out to see if there was still room on the table for improvement.

Superchips Flashpaq
Flashpaq bonus: Read and clear diagnostic trouble codes
The days of rejetting carbs are gone. With modern onboard diagnostics it is possible to add power without even getting your hands dirty. Superchips offers performance, towing, and economy options in its Flashpaq tuner (PN 3870). These tunes cannot be changed on the fly, so we store the Superchips programmer in the glovebox and plug it into the OBD-II port when we want to use the tow tune. The Flashpaq also provides the ability to read and clear diagnostic trouble codes, raise the rev limiter and speed limiter, and account for the larger tires that we intend to add soon.

The Superchips Flashpaq is available for nearly every Jeep with OBD-II architecture. Some models offer unique features, including a crawl mode for extra torque at idle and the ability to lock or unlock the differentials on Rubicon Wranglers when in high range. The Superchips tunes take only a few minutes to load and add noticeable seat-of-the-pants power. Those gains were born out on the dyno.

Dynomax Exhaust
Exhaust bonus: Fully welded muffler can withstand impacts
The last part we installed was a Dynomax cat-back exhaust (PN 19374). Although the aluminized tubing is 21⁄2 inches in diameter, same as stock, the fully welded Ultra Flo muffler uses a straight-through design for increased efficiency and less backpressure. This is our daily driver, and we were concerned about droning but that wasn’t an issue thanks to Dynomax’s Continuous Roving Fiberglass technology to keep the noise down. The free-flowing exhaust resulted in the biggest jump in torque of all three products tested.

We couldn’t simply cut the stock exhaust off since we swapped back and forth between the stock and DynoMax exhaust several times during our testing. Our More Power Puller came in handy to remove the stubborn exhaust without damaging it.

Every part we tested added horsepower, with the final package resulting in more than 30 extra peak horsepower and 25 lb-ft more torque. Those spikes that you see around 4,500 rpm? They are due to torque convertor lockup.

Scientific Testing
Science requires testing, and engine performance is a science these days. Furthermore, the key to accurate testing is limiting the number of variables to isolate each component you wish to test. Ideally we would test each individual part under exactly the same conditions for hundreds or thousands of miles to minimize the effects of any anomalies or outliers. That means the same fuel, humidity, wind, temperature, speed, and stretch of road for all of the tests.

Since this experiment was performed on the open road and not in a lab, some compromises had to be made. Still, every effort was made to limit the variables that might affect mileage and power. The mileage testing was performed on the same 90-mile loop of rural road with the cruise control set at 65 for as much of the drive as possible, and the same fuel station, and even the same pump, was used to fuel up the Jeep between mileage runs.

All dyno pulls were made with the transmission locked in Second gear, with three fans pushing air through the engine bay to minimize heat soak. Three dyno pulls were made for each configuration of parts and the middle power numbers used for comparison purposes, with the highest and lowest values discarded. While this takes more time, it also makes for more accurate and repeatable results.