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Injected Jeep Inline Engine Improvements

Posted in How To: Engine on October 1, 2012
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You’re most likely to find an inline-four- or six-cylinder engine under the hood of many of today’s Jeeps, and the multiport fuel-injected 2.5L and 4.0L are the most common of those still cruising the highways, byways, and trails. So it is no surprise that we get the most questions about these powerplants, namely “How can I get more power out of my existing engine?” Well, here are some modifications we’ve done first-hand that have worked for us.

Synthetic Fluid
The Idea: Just about the easiest potential power-adding modification you can make is to switch over to synthetic fluid. Many tests have shown how synthetic lube can free up more power than conventional lubes because it is slicker. Also, because of the better lubrication you can often use a thinner viscosity in the vehicle, which can result in more power to the wheels.

Real World: In a super-high-powered engine, this one works. Unfortunately, most of our Jeeps have much less available power on tap, and if you’ve got an inline engine in your Jeep you likely won’t see any day-to-day power or mileage increases from swapping to synthetic. The moving parts in our drivetrains create so much more parasitic loss than the coefficient of friction of a given fluid that the percentage gain in power isn’t that noticeable.

Synthetic will better protect your parts from wear and heat than dino oil.

Stays at rated viscosity over a longer temperature range, making your Jeep work like it should at sub-zero or very high temperatures.

Might result in leaks from old dino-encrusted seals


The Idea: A programmer or chip interfaces with the ECU and in effect lies to the computer about either intake air temp or the O2 sensor output. The other way they like to lie is to interrupt the O2 sensor to dump more fuel. Programmers work better on OBD-II Jeeps than OBD-I models, and the later the vehicle, the better the programmer will work.

Some programmers will also advance the timing, which will boost power. The factory Jeep programming already has the timing pretty far advanced, thus there is only so far it can be pushed. The way to tell if the programmer you are looking at is doing that is that it will suggest higher-octane fuel.

Real World: Power out of the gate is much improved, but we notice over time that we lose that initial power increase or we get tired of the loss of fuel economy we sometimes realize for the power we gained.

Easy to do

Easy to reverse

Can get better bang-for-buck with hard parts

Poor gas mileage (sometimes)

Cold Air Intake
The Idea: If we can get more air and/or colder air into the engine, more fuel can be added, and more power is the result. Enter the high-flow intake. The two big problems here are some high-flow intakes feature filters that don’t filter very well and some cold-air intakes make no concession to actually taking a Jeep off-road.

That said, in both cases, not only is this an easy modification, but the factory programming in the ECU will automatically give you more power when in closed-loop mode.

Real World: It works. For four- or six-cylinder engines putting more air into the engine is a good thing. While some kits on the market might look too big for a four-cylinder, we’ve not run into any drivability issues with any of them. Pick your kit based on how much water and mud you see and how good of a filter you want. Be aware that no matter what you decide, you are going to hear more noise coming from under the hood.

More air in = more power out

Easy to install

Can suck in water if left unprotected

Makes an already loud vehicle louder

Lower Temperature Thermostat
The Idea: The idea is that if you can run the engine cooler you can draw more fuel into that nice cold air and thus make more power.

Colder air is denser in oxygen molecules than hotter air. Therefore, colder air has more oxygen and will allow more fuel to be burned, which makes more power. The big difference between multiport-injected and carbureted is the closed-loop, open-loop programming. Most Jeeps will go into closed-loop somewhere between 165 degrees and 180 degrees, and at too low of a temperature it will never start reading the O2 sensor’s input and will run too rich.

If you do get the fuel-injection system into closed-loop, it is optimized for that temperature range. What that means is that more fuel will completely burn at a hotter temperature than it would at a lower temperature, possibly resulting in even less power.

Real World: We’ve run 165-degree thermostats in both OBD-I and OBD-II Jeeps with poor fuel mileage as the result. We have run 180-degree thermostats in both with mixed results in mileage. On the dyno we found that in almost every instance we made more power with the 195-degree (stock-spec) thermostat.

Keep Jeep running cooler overall

Appease the old man carburetor guy

Can lose mileage

Can overheat when working the Jeep hard in hot weather.

Ignition Upgrades
The Idea: A hotter spark will burn more fuel. Sure, the O2 sensor governs how much fuel goes into the engine, so if you start burning more fuel the computer will give you more fuel to burn, until the O2 sensor tells the computer that perfect ratio has been achieved.

Great, so how do you get a hotter spark to burn more fuel? Lower resistance is key. A distributor cap with brass terminals and lower-resistance spark plugs will do it. Making a higher-power spark won’t hurt either, and you can achieve that with high-power aftermarket coils or a step-up converter like that available from Davis Unified Ignition (performance

Real World: It works. We’ve seen anywhere from 5-10hp on four-cylinders and 10-20hp on six-cylinders. If your cap, rotor, plugs, and wires are in good shape, spend the money on a coil. If not, get good cap, rotor, plugs, and wires first and a coil or step-up converter as funds allow.

Safe way to bump power

Can also yield more economy if you keep your foot out of it

Ain’t cheap to go full bore

Not as sexy as an intake or exhaust

Increase Injector Size
The Idea: If you increase injector size, more gas will go into the engine, which means more power. This goes back to the carburetor guys again. With a carburetor, if you increase the carb cfm, you can get more fuel in the engine and the engine would usually respond well to it.

Real World: Doesn’t work on stock engines. The computer will open the injector for exactly as long as it needs to, and nothing you do with injectors is going to change it. However, if you are running leaner than acceptable for some reason, this might fix it.

Can fix lean condition

Can cause check engine light

Raise Fuel Pressure
The Idea: More fuel pressure will allow more gas into the engine if the injector is opened for a specific time. More fuel equals more power.

Real World: Doesn’t necessarily work that way. Again, the computer determines how long the injector opens. Unless you have messed with the engine internals, you will never need to do this.

Bragging rights of how much “over stock” your fuel pressure is

Possible early fuel pump failure from working harder than it was designed for

Bigger Throttle Body
The Idea: A bigger throttle body can make more power by letting in more air.

Real World: It works, but don’t go overboard. A stock six-cylinder throttle body measures 60mm. A stock six-cylinder can benefit from up to 62mm before it starts having off-idle hesitation problems. A four-cylinder can benefit from a 60mm stock six-cylinder throttle body, but a bored-out throttle body as shown can have similar off-idle issues.

Makes power

If sized right, no drivability issues

Easy to go too big

Won’t increase fuel economy

High Flow Exhaust
The Idea: The fuel/air mixture needs to get out of the engine and the exhaust is how that happens. The problem is, a lot of Jeep mufflers and tailpipes are too small from the factory without being mauled off-road. A high-flow exhaust can help.

Real World: We often see about 15hp from a 2½-inch-diameter exhaust on six-cylinders. We’ve put a 2½-inch exhaust on four-cylinders and they fall on their faces. They also won’t see a 15hp increase. Four-cylinders work better with 2¼-inch exhaust but the power increase will be more like 5-10hp.

Better power

Better sound

Some can be too loud

Some won’t last as long as stock in rust-prone areas

Adjustable MAP Sensor
The Idea: MAP stands for Manifold Absolute Pressure and is often misunderstood. The computer only “listens” to the MAP sensor before the engine is warm enough for closed loop and between 80-percent throttle and wide-open throttle. The MAP sensor varies a 5-volt source to the computer so that it can access fuel maps and vary the time the injector is open. The Jeep fuel-injection system is programmed to be as lean as possible to help fuel economy and emissions; this doesn’t make the most power possible.

Real World: An adjustable MAP sensor might cure your lack-of-power issue when you are really standing on the gas pedal, but how much time do you spend there? For the rest of the time the MAP sensor either won’t do anything or will dump too much fuel before the Jeep is heated up.

Can cure a lean issue at higher throttle input

Relatively easy to install

Can be difficult to dial in without other gauges and tools

Won’t unlock power where you do most of your driving

Computer Reflash
The Idea: By removing a lot of the over-lean parameters of the stock programming, more power can be had. You can even re-flash a computer for your specific setup, which can bring the ultimate blend of power and economy for your Jeep.

Real World: We haven’t done it on an injected inline Jeep engine, but on Chevy V-8s it works great. Right now, the only company we know of re-flashing the Jeep computer is 505 Performance (, and the company only has the capability to flash OBD-II computers (’96-’06).

Best tune for your Jeep

Easy to do

Core charge

Might not be cost-effective for a stock engine

Water/Methanol Injection
The Idea: You inject a mixture of water and methanol into the intake stream, and it serves to both lower the temperature of the air/fuel mixture as well as provide resistance to detonation.

Real World: We used a Snow Performance ( kit on our Insane Inline 2 and saw about 20hp from it. But, the Insane Inline had a turbocharger on it. We aren’t sure how well it would work on a normally aspirated engine, but the theory behind it should still hold true. It just might be such a low power level that you won’t see any seat of the pants improvement. That said, if you are experiencing pre-ignition knocking and pinging, this might be the ticket.

Relatively easy to install

Available in very compact kits to fit many Jeeps

Might not see much bang for the buck on low-output, normally-aspirated Jeeps

Have to refill the tank with the solution which costs money

Roller Rockers
The Idea: The factory rockers are stamped-steel and have a high coefficient of friction at both their fulcrum and the end of the pushrod. By replacing the friction-causing stamped-steel with a smooth roller bearing, a lot of the friction in the valvetrain goes away, which frees more power. As a bonus, many roller rockers feature higher-than-stock ratios, which can create power due to slightly increasing the effective overall lift of the valve.

Real World: If you don’t want to remove your radiator and grille, pull the cylinder head, and all of the other hassles associated with a cam swap, then this is for you. It might not be as good as a true replacement cam, but it sure is easier to install. If you’ve got a four-cylinder, it might be your only choice because cams can be hard to find.

Less friction

Longer valve opening

Valve cover clearance

Some require feeler gauges to install

Late-Model Intake Manifold
The Idea: Sorry squirrel pilots, you already have the swoopy manifold if you have a factory multiport four-cylinder. For the six-cylinder guys the ’99½ -’06 Jeeps with the six-cylinder came with an intake manifold that featured equal-length runners. Equal-length runners and smoother curves mean that the intake manifold will flow better than the old manifold.

Real World: We’ve heard claims of as much as 25hp for this modification, but never saw it. When we put the late-model intake on our ’98, we actually lost peak power. We put the intake on our ’91 with a stroker engine and saw about 5hp at peak, but the real story was in drivability. We saw a huge increase in power on the dyno from idle through peak, and we felt it in day-to-day drivability.

Big power for highly modified Jeeps

Original Jeep part can be found in junkyards

Have to swap or modify power steering pump

More tired engines might not see an improvement.

The Idea: Many Jeeps came from the factory with cast-iron exhaust manifolds with swizzle-stick-sized primaries. This doesn’t do a good job of letting the spent air/fuel mixture out. Then they went to a tubular header, which cracks and can cause the O2 sensor to read incorrectly and will wreak havoc with the air/fuel mixture.

An aftermarket exhaust manifold will flow better than the factory units will and many are specifically designed to resist the cracking prevalent in factory tubular manifolds.

Real World: In practice you can expect around 10hp for a six-cylinder header and about 5hp for a four-cylinder header. The exact power output will vary depending on what other modifications you’ve made. We’ve managed to crack all kinds of aftermarket headers, so just get the one with the best warranty.

Lets more gasses out of the combustion chamber

Fixes manifold cracks

Requires removal of many components to install

Might crack again

Electric Fan
The Idea: Less driven front-end accessories mean less parasitic power loss. A popular car magazine showed a 10hp increase at the wheels about a decade ago,and people have been talking about it ever since.

Real World: We’ve got heavier moving parts, and our engines produce less power than that car did. The theory holds true, but any potential power to the ground will probably be less than any dyno’s ability to measure. However, the electric fan offers other benefits when wheeling which might make the modification worthwhile on its own.

Potential for more power

Can shut off fan for faster heat up and water crossings

Lot of work to install and wire

Can sometimes have problems cooling at highway speeds

Aluminum Armor and Skidplates
The Idea: Less weight for the engine to haul around means more power gets to the ground.

Real World: The “1 pound equals 1hp” is a popular, but untrue and often misused equation. It refers to the drivetrain, and while there is a correlation between losing weight in the drivetrain (anywhere from the piston to the tire) to putting more power to the ground, most Jeep people use it describe going to lighter hard parts. Let’s use armor for example. The TnT Customs belly skid for a TJ shown is 32.15 pounds. The company’s steel belly skid is 40 pounds heavier. We aren’t saying that you won’t see a drivability difference, but you aren’t making more power. Four-cylinders benefit more from this modification.

Less weight makes more a more fun-to-drive Jeep

Might be able to drag race a ¼-mile faster

Won’t put more power down on the dyno

Cost-to-speed increase ratio pretty high

Camshaft Swap
The Idea: A cam swap will work for either the six-cylinder or four-cylinder. By opening the valve further (lift), at a higher rate of speed (ramp rate), and holding the valve open longer (duration) you let more air and fuel in and let more exhaust gas out. There aren’t many four-cylinder cams around anymore, but Mopar was making them for quite a while and they pop up for sale every so often. If you don’t need the needle-in-the-haystack Mopar part, just call up 505 Performance.

Real World: It works. But, it can be a rough swap, sometimes requiring removal of the whole front of the Jeep. Additionally, it requires the intake manifold, exhaust manifold, and head to come off. Depending on the camshaft and the engine you can see anywhere from 10-30hp.

Makes power

With custom grinds, the power level is up to you

Long project to tackle (have to remove cylinder head to get to lifters)

Can’t tell if anything is wrong until it is all back together

Aluminum Head Swap
The Idea: An aftermarket aluminum head such as that from Alabama Cylinder Heads ( or Hesco ( makes power by out-flowing the factory head. Bigger intake and exhaust passages with smoother curves will result in better airflow. The aluminum head also shaves about 30 pounds off the weight of the Jeep.

Real World: They make more power. Expect to do just about as much work as you might do with a camshaft swap, but you’ll be putting a different head on the engine.

Light weight

Can run more compression

Dissimilar expansion from cast block could cause head gasket leaks

Early engines (’90-and-earlier) need more parts to make them work

Air Adders
The Idea: Nothing is better than putting more air in the engine than a turbocharger or a supercharger. The Jeep inline engines have no problems living with boost. There are numerous bolt-on kits on the market. However, some aren’t that easy to install, there can be teething issues with the factory ECU, and many of the kits aren’t smog-compliant.

Real World: You will always get more power out of the engine, and Jeep engines are great candidates for it because of the stout bottom end.

Lots of power to be had, and 30-130hp is not out of the question

Bolt-on kits make it easier to get power than putting in a V-8

A good kit can cost thousands

Might take it a while to get dialed-in

Increase Compression
The Idea: Compression ratio is expressed as 9:1 where the engine takes 9 units of air volume and compresses it into 1 unit. Increase it and you get more power. A lot of compression makes the air/fuel pretty hot, and too much can actually make it hot enough to ignite on its own. It is called pre-ignition and can result in loss of power and internal engine damage. The right compression ratio for you will vary depending on what altitude you drive and what kind of gas you run. With an aluminum head you can typically run about one more compression point than with an iron head.

You can offset the danger of pre-ignition a little bit by adding higher-octane fuel, but by and large it is good to plan an engine that can go anywhere you might go and run regular fuel. The easiest way to change the compression ratio is to either shave the block or the head, but you need either part out of the Jeep to do so. You can also change pistons, the stroke of the crank, length of the rods, and (to a much lesser extent) the bore diameter.

Real World: It makes more power, but it is a lot of work to get done. And, we wouldn’t do it on an engine that didn’t already need to be rebuilt.

If you are handy and can disassemble and reassemble your engine yourself it can be a cheap way to get power

There are many ways to do it successfully

Lots of work and Jeep downtime

Easy to go too far and end up needing high octane or race gas for the Jeep to run well

Stroker Crank
The Idea: Taking a crank from a 4.2L inline-six and putting in a 4.0L results in a longer stroke and from 4.6L to 5.0L of displacement. There is no replacement for displacement. There are plenty aftermarket companies making bolt-in long-block strokers, or you can take the long way around and build your own. Got a four-popper and absolutely love being the underdog? Call up 505 Performance for a stroker kit.

Real World: In all but the most extreme cases, your factory computer will work with a stroker. The low-end grunt is awesome, and the overall driving improvement is great too.

Makes more power than your current engine does

Fits in your Jeep and works with all existing accessories

Can cost almost as much as a V-8 to install

Doesn’t quite deliver the V-8 driving experience

Port/Polish Head
The Idea: This is another tried-and-true power trick that still works well with Jeep engines. Anyone can attack a head with a die grinder, but it takes a guy who knows what he is doing, and where he can remove material from to not screw it up. Sure, just about any polishing of the intake/exhaust passages will result in more power, but you need to make sure every intake and exhaust flow the same.

Real World: It is a lot of work, and for best results someone with experience should be involved. It takes a long time but it is virtually free if you do it yourself. It will work for both four- and six-cylinder Jeeps.

There is always power to be had by porting


Easy to mess up if you don’t know what you are doing

Lots of work for something that you might not know if you will get right or not.

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