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Injected Jeep Inline Engine Improvements

Jeep On Dyno
Pete Trasborg
| Brand Manager, Jp
Posted October 1, 2012

More power!

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

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