Turbo 4.0

A Jeep does what it is designed to do very well - climb steep, gnarly slopes at slow speeds. However, if you have to travel some distance to get to your favorite climbing spot, you have probably found that the Jeep isn't designed to maintain freeway speeds in the mountains or when larger tires are added. Many times, with the six-cylinder in our '99 TJ, we've been stuck in the slow lane, especially when heading west on I-70 out of Denver.

The solution seemed obvious - more power - but how best to do it? We tried a number of techniques: an air intake system, a throttle-body spacer, new headers, an after-cat exhaust system. These made a small improvement (or maybe we just imagined they did), but we were still intermingled with the 18-wheelers on I-70.

It was time for more drastic action. We looked into superchargers, but they are expensive and cause a sizable drop in gas mileage. Next we looked into turbochargers, which are commonly found on big diesels and small imports. While a supercharger is belt-driven off the engine, a turbocharger is powered by exhaust gas, an untapped source of power. It takes engine power to spin the supercharger's belt, but there's no parasitic power loss with a turbo. A turbo has no beneficial effect on engine performance below around 1,500 rpm, but at around 1,800 rpm it starts to take off - just what we needed for the mountainous Colorado roads.

We searched the Web for turbos and found a Jeep turbo kit from 505 Performance in Farmington, New Mexico. Designed to push 6 psi of boost through the intake manifold, it is a complete kit that comes with everything you need to install it -- the turbo unit, the exhaust pipes for routing the exhaust to/from the turbo, the fuel-management unit, an oil pan which contains a return oil-line fitting, and various brackets and hoses. The block diagram to the right shows how the turbocharger kit is installed.

To be able to compare the before-and-after performance, we first took the Jeep to Redline Motorsports in Fort Collins, Colorado, to obtain a baseline. Redline strapped it to the chassis dyno and measured 148 rear-wheel horsepower. That was surprisingly low, but up to 30 percent of the horsepower can be lost in the drivetrain, along with another 3 percent loss per thousand feet - Fort Collins is, after all, at 5,000 feet.

How Does It Work?
With the turbo and its components installed, the next step was to adjust the fuel curve, which we did using an Innovate Motorsports computer that came with the kit (it can be returned for a rebate if you don't want to keep it). We plugged the computer into the O2 sensor port just after the turbocharger and adjusted the air/fuel ratio using a screw on the FMU. No change to the engine's computer was necessary .

Initially, our manual-transmissioned 4.0L Jeep started knocking at around 4,200 rpm. Prior to our install, 505 Performance had only put its turbo kit on Jeeps with automatic transmissions. Those Jeeps hadn't experienced engine knocking. To solve the knocking problem, we purchased an MSD Boost Timing Master. It works by delaying the spark a user-set amount for every PSI of boost in the intake manifold. Delaying the spark 1.5 degrees per pound of boost eliminated the knocking.

We took the Jeep back to the dyno for the final precise tuning. Adjusting the wastegate, we decided to run around 6.5 psi of boost, which resulted in 214 hp to the rear 31s.

Now the Jeep flies at 11,000 feet. It's a left-lane, 75-mph vehicle. In theory, the turbo shouldn't affect the Jeep's operation at low rpm on the trail. However, as they say, "One experiment is worth a thousand expert opinions," and we wanted to make sure. During low-rpm off-road climbs, there was no change in the Jeep's factory climbing ability, which was a great relief since that's what Jeeps are for, after all.

Theoretically, the fuel economy should stay about the same under non-mountainous cruising conditions. This is because the fuel management unit only increases the fuel flow if the turbo is generating boost. During freeway cruising, we noticed about a 1-to-2-mpg decrease.

Now that the Jeep is turbocharged, we'll need to observe some extra precautions, such as letting the engine warm up for 30-60 seconds prior to driving to ensure that oil is flowing to the turbo. Before shut down, the engine should be run for 30 seconds to allow fresh (and cooler) oil to displace the heated oil in the turbo. And changing the engine oil every 3,000 miles is now more important than ever.

If you go the turbo route, be prepared for some unusual noises from the turbocharger. As you let off the gas to shift and the rpm drops, the still-spinning turbine will build up excess boost (since the boost is not being consumed by the idling engine at this point) and will trigger the wastegate to open to bypass the exhaust gas around the turbine wheel. This is accompanied by a very loud woooosh. Depending on your frame of mind, this can be either an annoyance or an asset - we personally felt this was an awesome sound, and it certainly grabbed the attention of startled onlookers.

Expect the turbo installation to take about 10-12 hours plus an hour of tuning.

With the installation and testing complete, you might think we're done. However, there are a number of other enhancements we could make, such as adding an intercooler between the turbocharger and the throttle body. Because compressing air causes the turbo to heat up and heated air has less density, an intercooler can increase the efficiency of the turbo by decreasing the air's temperature and thereby increasing its density. A blow-off valve can be attached to vent excess boost pressure when the throttle-body door closes - the blow-off valve would be triggered by vacuum in the intake manifold. Forged low-compression pistons, also carried by 505 Performance, can be installed to allow the engine to operate at higher boost levels. Like all things Jeep, the list is virtually endless, and as usual, it depends on how much money you want to spend.