In the last installment of Mileage Master, our '98 XJ, we added armor, skidplates, and recovery points front and rear so we could take it wheeling without killing vital components ("Project Mileage Master", Part II, October '08). Unfortunately, with the added weight we paid the piper in average mileage over a 5,000-mile test period.
So rather than remove the armor (some of which we'd welded to the body), we decided to try cranking up the power on our 4.0L engine by using some simple bolt-on power-adders. The theory is by making more power out of the same engine, it's more efficient. With everything else in the Jeep unchanged, that efficiency should translate directly to mpg.
But wait, there's more! We couldn't let the addition of a bunch of power adders go by without running them on the dyno to see what actual power numbers we made, as well as testing mileage gains.
While we could see instantly that we were making power thanks to the dyno, we didn't really see huge mileage gains. However, this is only the first part of the power adding tests and while we added a freer flowing muffler, the stock down pipe is still crushed in half (from the factory) and the stock catalytic converter has seen better days. Once we open it up more, we should see even bigger numbers in both mileage and power.
The first thing we tried was...
The first thing we tried was a low-buck redneck-style homemade air intake. Our seat of the pants dyno said we'd made power, but the real dyno disproved it. We also didn't see any change in mileage out on the highway; in fact we started seeing the mileage drop off.
The next step was to put an...
The next step was to put an AEM Brute Force dry element intake kit on the Jeep. We did see more power numbers than the redneck intake on the dyno, but over the next 1,000 miles we noticed our mileage starting to tank.
At about 2,000 miles into...
At about 2,000 miles into the AEM intake test, we discovered why we were losing mileage. We were down to almost 13mpg cruising on the highway. One day on the way to the office, a horrible squealing started under the hood. The A/C compressor pulley/clutch assembly had died. We swapped in a pulley from a junkyard donor and were back in the mileage ballpark again. We were told repeatedly to swap the entire unit, that the fix wouldn't last, but over 5,000 miles later it is still going strong.
Crank It Up
We didn't get back to stock mileage numbers with all this stuff, but we did make more power and find more fuel efficiency. Face it, if you are buying power adders and expecting them to pay for themselves, it isn't going to happen. But, we should be able to get back to stock mileage numbers and maybe even get better than stock mileage while having a Jeep that is much more capable off-road.
We went from a flat and level highway average mileage of 16 mpg back up to the 17-18 mpg we had been seeing when the Jeep was bone stock. One to two miles per gallon doesn't really seem like a good return on the parts invested, but we knew going into this we'd not see the parts pay for themselves. However, the Jeep is more enjoyable to drive and we are hoping that with the next round of power adders we'll see even better than stock mileage.
Since we were going with a...
Since we were going with a low-buck theme for this installment, and our factory muffler was leaking, we took a Gibson muffler we had laying around for a TJ down to the local muffler shop and had them bend up a 21/4-inch tailpipe to match the Gibson muffler. It's not mandrel bent, and not pretty, but again we saw power numbers and we almost always see 18mpg on the highway now at 72mph.
We tapped Performance Distributors...
We tapped Performance Distributors (DUI) for a cap, coil, and wires. The company included its new mini VIP, which steps the voltage up to the coil to about 18 Volts. We opened our plug gap up to the suggested 0.075 and saw definite power gains. We didn't see much on the mileage side, and we suspect it is because the OBD2 Jeep programming already runs so lean.
Next an Edge module with throttle...
Next an Edge module with throttle body kit found its way onto the intake of our Cherokee. By replacing the stock throttle body with a 62mm unit and adding a tuning box that basically lies to the computer, we picked up more power. We dyno'd the Jeep with the throttle body and the box separate just for curiosity's sake and found that both made power.
Test 1: Catback
Test 2: Catback and AEM Brute Force intake kit
Test 3: Catback, AEM Brute Force intake kit, and DUI ignition
Test 4: Catback, AEM Brute Force intake kit, DUI ignition, and Edge throttle body
Test 5: Catback, AEM Brute Force intake kit, DUI ignition, Edge throttle body, and Edge Trail Jammer
| Engine Speed (RPM) | Test 1 | Test 2 | Test 3 | Test 4 | Test 5 |
| | HP | Torque | HP | Torque | HP | Torque | HP | Torque | HP | Torque |
| 3,000 | 99.4 | 174 | 105.1 | 184 | 99.4 | 174 | 108 | 189 | 112 | 196 |
| 3,100 | 100.3 | 170 | 106.2 | 180 | 111 | 188 | 110.4 | 187 | 113.9 | 193 |
| 3,200 | 104.8 | 172 | 107.8 | 177 | 112.1 | 184 | 116.4 | 191 | 117.6 | 193 |
| 3,300 | 106.2 | 169 | 113.7 | 181 | 115 | 183 | 120 | 191 | 120 | 191 |
| 3,400 | 108.1 | 167 | 115.9 | 179 | 117.8 | 182 | 121.7 | 188 | 123.6 | 191 |
| 3,500 | 112 | 168 | 117.3 | 176 | 125.3 | 188 | 123.3 | 185 | 128.6 | 193 |
| 3,600 | 114.5 | 167 | 120.6 | 176 | 127.5 | 186 | 127.5 | 186 | 130.9 | 191 |
| 3,700 | 116.9 | 166 | 124 | 176 | 128.9 | 183 | 129.6 | 184 | 134.6 | 191 |
| 3,800 | 120.8 | 167 | 126.6 | 175 | 131.7 | 182 | 136 | 188 | 136.7 | 189 |
| 3,900 | 122.5 | 165 | 130.7 | 176 | 133.7 | 180 | 138.1 | 186 | 141.8 | 191 |
| 4,000 | 126.4 | 166 | 132.5 | 174 | 139.4 | 183 | 138.6 | 182 | 143.2 | 188 |
| 4,100 | 127.2 | 163 | 134.3 | 172 | 140.5 | 180 | 140.5 | 180 | 144.4 | 185 |
| 4,200 | 131.2 | 164 | 136.7 | 171 | 140.7 | 176 | 142.3 | 178 | 145.5 | 182 |
| 4,300 | 130.2 | 159 | 138.4 | 169 | 141.6 | 173 | 144.1 | 176 | 147.4 | 180 |
| 4,400 | 130.7 | 156 | 139.9 | 167 | 142.4 | 170 | 144.1 | 172 | 146.6 | 175 |
| 4,500 | 131.9 | 154 | 139.7 | 163 | 142.2 | 166 | 143.9 | 168 | 146.5 | 171 |
| 4,600 | 131.4 | 150 | 139.3 | 159 | 141 | 161 | 142.8 | 163 | 145.4 | 166 |
| 4,700 | 131.5 | 147 | 139.6 | 156 | 139.6 | 156 | 141.4 | 158 | 144.1 | 161 |
| 4,800 | 131.6 | 144 | 139.8 | 153 | 138.9 | 152 | 138.9 | 152 | 143.5 | 157 |
| 4,900 | 129.7 | 139 | 137.1 | 147 | 137.1 | 147 | 137.1 | 147 | 141.8 | 152 |