Since I get so many questions about how to upgrade the performance of the 3.7, I thought it would be a good idea to post a list.
1. You need to stop the computer from limiting the power. Stock, it will measure the torque being generated, and plug that into a table that determines if it should retard the timing to reduce the power. You need a B&G PCM flash to correct this. It will also advance your timing, change your fuel curves, raise your rev limiter, and help your shifts. This upgrade can be used on a stock 3.7, or with an intake, exhaust, headers, and a mild cam upgrade. This is really the first upgrade you should do.
2. You need to get more, colder air into it. You need an upgraded throttle body from The Fastman. If you don't off road, or live in a dusty area, you can upgrade to a drop in filter, or a cone intake with a heat shield. You can use an off road bag on the cone in dusty areas or temporary off roading. Just don't go through very deep water. To get optimum performance from the cone filter, you need to at least do the $4 cold air intake mod. That is, use a flexible accordian style gutter drain pipe to bring cold air around the radiator. You can also get a ram air hood from Profinish-Proglass that will dump air right where the opening for the stock intake is if you remove the top curve from the snorkel. It can be enlarged, and will dump air right on top of where the cone filters sit, bringing so much air that it creates a small ram air effect. You can modify the heat shield, and create an increase in the ram air effect.
3. You need to get that additional air, and the increased fuel the computer will compensate with, out of the motor. JBA manufactures headers for the '05 Liberty and up. They are currently working on headers for the '01-'04. These headers will help scavenge some of the exhaust gasses. At least upgrade your muffler, and make sure you get rid of the slight decrease in diameter right by it. A complete cat back system can give you an additional increase.
4. You need to get the power to the ground. The stock 42rle and 45rfe automatic transmissions are set up to shift slow, and comfortable. This is bad for both performance and reliability. A Transgo shift kit will allow the valve body of the transmission to be modified. This will make it shift much quicker and firmer. The quicker shift means less slipping and less time being partially engaged in 2 gears during shifts. It also increases the line pressure and helps it lock up better. An upgraded torque converter from APS will increase the automatic transmission's effeciency to around the same as the manual transmission. Also the torque multiplication can be raised. The stall can be raised if you're interested in being able to launch.
5. You can go to 4.10 ring and pinion gears. This gives you a lower gear ratio and allows the motor to wind up the rpms quicker. The motor will be putting more power to the ground at the same speed, since it's at a higher rpm. This is helpful if you have gone to larger tires, to correct for the difference in diameter causing a power loss. With stock diameter tires, or only slightly larger, it will increase acceleration. You do pay for this though on the high end. There are aftermarket gears for the rear, but you need Mopar for the front, or the front end from a 4 cylinder Liberty.
6. You can upgrade the cams. The Fastman can make bigger cams for the 3.7. To make low rpm power, with a cam similar to the 4.7 HO, you can get some 200's. For mid to high rpm with a higher peak torque you can get some 206's. You could also get the heads ported while they're taken apart.
7. You can upgrade to custom pistons. The bore of the 3.7 pistons is the same as the 4.7 which there are aftermarket pistons for. Would not be difficult to make pistons for the longer stroked 3.7. This is necessary if you are going to run any kind of boost or nitrous. The rings are too high on the pistons and the ring land tends to chunk off.
8. You can add nitrous, a supercharger, or a turbo. This is very risky and should only be done on a built motor with forged pistons. More information on this will have to be in another post.
http://www.bgperformancepcm.com/
http://www.thefastman.com/Throttlebodies37.asp
http://www.apsprecision.com/
http://www.profinish-proglass.com/
http://www.krcperformance.com/krc/
http://www.transgo.com/
http://www.ring-pinion.com/
http://www.knfilters.com/
http://www.aempower.com/