The Skulljeep correction Edit for the OP:
TJ SE's and the 2.5 liter engine
There's a lot of discussion around the Jeep 2.5 liter engine and how to increase horse power and torque.
The short answer is, you can! We all like that answer, and it's the truth. There are a lot of reasons to purchase an SE. Lighter weight, lower purchase price, successfully burns anything that looks and smells like regular pump gas, even hydrogen or propane! They can be simpler and slightly less expensive to maintain. Plus they last as long as you maintain them!
The common modifications to gain performance are:
4.0 Throttle body
Throttle body spacer
Cold air induction
High flow air filters
IAC sensor relocation
Ford yellow top, 4 hole injectors
Headers
Cat back exhaust system
Power chips
Electric fans
Some of the less common modifications are:
Roller rockers
High performance cams
Ported and polished heads
Stroker kits
Turbocharging
Wet shot nitrous injection
Also Stand Alone ECU's:
HP Tuners, Megasquirt, Haltech.
Upgraded Ignition Coils, plugs, and wires.
-The common modifications-
Intake modifications:
The TJ SE intake system is designed, and comes from the factory, with a restrictor in the throttle for air velocity to help generate vacuum in the plenum of the intake manifold. Fact. Technically there are restrictions to air flow due to casting imperfections. The engineers did a good job of designing the intake system from the start. However, porting the intake and head to match the gasket opening and porting down the runners to smooth the transition can and will help. Takes time with a Dremel tool, but time well spent.
Putting a 4.0 throttle body on will give better throttle response, and allow the engine to use all the fuel it gets at WOT because these OEM ECU's run rich at WOT. With port matching the TB to the intake manifold, you are gaining potential to flow more air and having air with more density. Any ICE is a glorified air pump, we add fuel and spark to make it run on its own. Air flow going in and getting out boosts the efficiency of that to your benefit. Air density is more important than flow, and that allows a more efficient burn of the fuel in the map for performance gains. Regardless of the RPM range you are operating at. That is where tuning comes in.
Putting a throttle body spacer in has got to improve performance? Yes, except the magic gains are not from the swirling effect (Air Velocity by restrictive material). Increases in torque do come from increased plenum volume, and permit room for additional sensors and spray nozzle modifications. The length of the intake runners and the plenum volume can be adjusted with custom fabrication to improve HP and torque bands. The Jeep engineers did some of that already, hence the intake manifold differences on the 4.0 where in 2002 they made revisions. The 2.5 from the late 80's to early 90's was revised to what we got in 99. Not the best, but not the flagship motor, so there is room for improvement.
Cold air induction has got to give me something, right? After all, cold air is denser than warm air! This is a true statement. The difference in air temperature and air density going into the engine with a cold air induction system compared to the stock system diametrically better than consuming its own engine heat waste and turbulent air from the cooling fan. A stock computer does not always measure air density, but with a tuner, that can be unlocked for better gains.
High flow air filters will get more air into my engine, right? As long as all restrictions are removed from the throttle and intake manifold, it permits your engine to use more air. The stock paper element flows as much air as some aftermarket setups, just remove the restrictor leading into it. It can do a better job of filtering the intake air than some of the aftermarket air filters. It's cheaper for those on a budget, and readily available.
So, how about relocating my IAC sensor to the air box? Heat soaking the sensor is not ideal near the airbox. Once the engine reaches operating temperature and the ECM goes into closed loop mode the following does take place: The ECM receives information on air flow, engine temperature, air temperature, exhaust gas oxygen content and throttle position. This information is used to calculate the proper pulse width for the injectors and fires the injectors for the calculated period.
You do not want the air to be measured from the hot engine bay, and want it closest to the air flow going into engine as possible for accurate readings, hence the plenum location.
High flow four hole fuel injectors are the clear ticket, right? More fuel = more power! Injectors support power, they do not make power. Tuning will allow you to use gains for more fuel, the Stock ECM will adjust fuel moderation naturally based on the map, so larger injectors will just have less duty cycle percentage. However, better atomization of fuel has shown for more efficient use of the fuel, and has contributed to better cylinder cooling because atomized fuel absorbs heat. Which is a good bonus, since fuel is also used to help keep the engine running cool.
Now about the exhaust system:
Headers will give me more power! Headers provide torque, more torque from a shorty header, and more HP from a long-tube header. A tri-Y header has a happy middle ground. Make sure you use a ceramic coated or use header wrap on your exhaust, factory or not. The cast factory parts are thick, and hold a lot of heat in your engine bay, header wrapping it helps control engine bay temps to expel that heat where it needs to go. Headers and high flowing exhaust tend to do that faster than factory parts when properly installed.
Cat back exhaust systems are the cat's meow for performance gains. Cat back mostly changes the tone and sound, at the 2.5in tubing used it doesn't make the gains, scavenging is mostly done by the header collector, you're just controlling sound after that, its not a V8 or an I6. It is a widely spread myth that some back pressure on the exhaust is good, however this is the wrong conclusion of experimentation. Back pressure on the engines exhaust does not increase performance for naturally aspirated (NA) or turbo applications.
What about an electric fan? Let's get rid of all that parasitic drag! All the new cars have them. There are reasons to have an electric fan, as proven time and time again on Engine Masters, parasitic drag is a thing and can cost you as much as 10-15hp and 5-10lbft of torque. Mechanical fans serve their purpose, but many aftermarket mechanical fans make this even worse or don't flow enough air. Electric is a superior option for reliable cooling, even when you're done wheeling, it will run for a bit after shutting down the engine to assist in cooling things off. You don't get that luxury with mechanical fans.
A power chip will rock my 2.5! Yes and no... If already getting optimal air flow and then floor it to WOT, the fuel mixture will be rich. Any gains from additional spark advance are not terrible, and you will need to run mid grade fuel to 97. It is better to get a professional tune over the chips to fully unlock the potential of a more efficient combustion cycle.
Now we're getting into the more old school hot rodding tricks for generating more power out of the 2.5. Forced air induction, displacement changes, compression ratio increases, that sort of things. These will increase power, and have become super cheap these days!
Let's step back a moment and talk about what generates power. Why we are doing this? I'm not sure, we wouldn't be here if we didn't already understand that.
An internal combustion engine has one or more cylinders in which the process of combustion takes place, converting energy released from the rapid burning of a fuel-air mixture into mechanical energy. Our goal is to do this the most efficient way possible to then make more power by using the principle at its peak performance capability. Or at least that's what Gale Banks says based on all of his research.
It is possible to increase the amount of power generated by increasing the compression ratio and by extension the efficiency of the engine. This can be done by adding boost, or by physically changing stroke of the rotating assembly, or by deck height of the block. Doing so may require a higher octane rating due to a very bad tune since someone was lazy, some folks make it run fat rich to limp it to the tuner. A good engine map will not require dramatic changes at the fuel pump depending on your knock results from the combo you are using for your build. One of the bonuses of any computer controlled engine is that you can tune it to run on just about anything flammable.
Another way to increase efficiency is to eliminate as much parasitic drag in the engine as we can. We can try to make as much power go to the transmission as possible and make it explode like mine did. One of the ways to do that is with hydraulic roller lifter cams and roller rockers. With a four banger, that is a great idea since the revs can be restricted and less responsive due to the drag, freeing up the drag makes it snappy. A full roller valve train is inexpensive and result in an noticeable gain identical to a stock cam but a bit more grunt.
Balancing and blueprinting the engine, so that it operates closer to it's best efficiency is a great idea, and reduces stress on the vibration dampener and engine harmonics. The gain is that you wont be rattling piston skirts, dropping valve spring retainers, or collapsing valvetrain components. You will see an improvement spike throughout the entire RPM range when doing this, and increased reliability due to reduced engine harmonics. It's like an added insurance policy to reduce risk of spontaneous failure.
What about camming the engine? There's the possibility of gaining a lot here! The less duration the camshaft has, the more torque an engine will make; the higher the duration, the engine will make more top-end horsepower, but not as much torque. The action of using the flow of exhaust to initiate better cylinder filling is what overlap is about. without it you would make a lot of low end torque, and that would be it. With overlap you move the torque curve to a higher RPM which makes more Horse Power.
What about higher lift on the cam? Another method of improving cam performance is to increase the amount of lobe lift. Designing a cam profile with more lift results in increased duration in the high-lift regions where cylinder heads flow the most air. Short duration cams with relatively high valve lift can provide excellent responsiveness, great torque, and good power. But high lift cams are less dependable. You need the right valve springs to handle the increased lift, and the heads must be set up to accommodate the extra lift. Rocker arms transfer the cam lobe lift, as well as add a bit of lift themselves. The longest rocker arms that will fit under the valve cover are allegedly 1.6 to 1. You do NOT have to be careful to not drive a valve into a piston doing this because it is not an interference engine. Even if you rev it and float a valve. Valve float only causes bent valves in interference engines, because other wise, the valve float just throws off the effective cam duration/lift and basically wont let the engine rev any higher, because of the loss in efficiency and power when the valves aren't doing what they're supposed to.
How about a stroker engine? That increases the amount of fuel / air mixture being burned and generates more power! A stroker permits you to use more fuel and air, its doesn't automatically increase it. ha-ha You can order your own stroker kit rotating assembly parts for around $1,300.00. That puts you in the same HP and torque range as a stock 4.0.! That's NOT a lot of money to turn an SE into a stock Sport or X. But you still have the AX5 to deal with in the SE, a good used ax-15 for another 1,500.00 and a 99-01 dodge dakota bell housing solves that however.
There's always head modifications as well. These work standalone or with a cam setup to your application that you are building. It optimize the use of the fuel / air mixture in the combustion chamber. You can do things like bigger valves, CC'ing the heads so that all four combustion chambers are as close to the same size as possible, polishing the combustion chambers, raising the intake ports and port matching, contouring the intake valve bowl, all of that works for gains.
Forced air induction. More fuel into the engine to accommodate the use of significantly more air. Some people's preference would be something like a roots type positive displacement blower. It would generate power from 1500 RPM once you overcame the parasitic loss of the mechanical system in torque. Not to mention heat soak the air intake charge. Turbo charging with the correctly size turbo, you can achieve better low end torque than a supercharger, and with an intercooler you can have air that is 30 degrees cooler than ambient temps. Any real Tuner will tell you about "Torque Shove" and how it can break parts before you even get up into the RPMS.
With supercharging and turbocharging the 2.5, you are faced with tuning the ECM with something that would control spark and fuel management. It's out there and available, it is dirt cheap and easy to find. Megasquirt you may need to program some parameters from scratch to match your engine. It's a good way to learn a new skill around tuning!
Wet shot nitrous injection. This technology has been around since WWII, and you can totally do this with the right tuner! I personally prefer a Water Methanol Injection system, it operates as a great boost cooler and as an added bonus, it covers your added fuel demand for the more dense air! You can also do a hydrogen system, which I am still tinkering with, and once I get that up and running correctly, I can do a write up on it.
If you have the time, patience, cash, or skill, you can totally do any of this yourself, you can do it when it works for you. No rush.
I have never seen an automotive enthusiast upset about what they have done with their vehicle and what they haven't, don't judge others if it takes them longer to get to where they want to be with their respective builds.
These are Jeeps, have fun with them the way you want to, it's your property, don't let someone else tell you different on how to run your life.
You can swap a V8 if you want to or a 4.0L but you're going to spend more on the swap than you would building what you have in today's money.
When someone asks "How do I get more power out of my 4 cylinder?", the way to answer that is with this simple question: "What is the goal you have or what application are you intending to use it for?"
Nobody knows the answer to that question, so it's neat to hear perspectives from other people.
What else can you do to increase the performance of your 2.5 powered TJ SE?
Keep the weight down. The 2.5 is able to push the 4,100 pound TJ around, especially on 31” tires. Around town, off road, on the Highway (no faster than 4th gear will let you). Keep it Light for best fuel efficiency.
Tires, Tires, Tires. Keep your tires to 31”, 32” or 33” tires, unless you built it up to the extreme. This effects the lift you require, the weight of the rotating mass involved and keeps you in the range of available gearing for the Dana 35 / 30 that comes with your 2.5.
Gears. I never ended up changing gears out for one of two ratios. 4.88 or 5.13. I'm running the factory 4.11/4.10's The 4.88/5.13 may sound pretty extreme, but those are the gear ratios that normally work the best with the stock 2.5 SE but will run you what stroker kit will cost to swap, and at that point, why change gears? Build up! Remember that the 2.5 is happy at whatever RPM you choose to build it for if you get into tuning and engine building.
The 2.5 SE is a good little Jeep that can be purchased economically in good condition. It's the ultimate lego set, so grab your wrenches and get to it!
As a professional engine builder in my past line of work, I believed this post deserved a bit of more accurate information. Building 3000 HP Alcohol Dragster Engines was my day job for some time.
