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Engine Performance Advisor

The quest for horsepower is an American tradition and a legacy of the post-World War II hotrod era. It permeates almost all aspects of motor vehicle enthusiasm. It certainly applies to Jeepers, though perhaps this group has more justification than most in seeking out performance improvements.

As a Jeep is modified for trail use it usually gains weight. Performance is also reduced when already atrocious aerodynamics are made worse by lifts and big tires. The mass of those big tires also soak up more power on their own. Even if correctly geared, a built Jeep with a stock engine is almost always slower than the stock Jeep was for these reasons.

Recently another element has come to the forefront… fuel economy. Fuel economy also decreases as weight increases. Lifts and big tires also cause the fuel gauge to drop faster. These effects cannot fully be countered, but the beauty of certain engine mods is that you can increase power and gain fuel economy at the same time. You may not recover all your stock economy, but you can get some of it back.

In this Advisor, we’ll talk you through the choices in the “Big Three” engine performance modification categories, air filtration systems, exhaust systems and chips/programmers. These are three modifications that work hand in hand to offer cost-effective improvements that are easy weekend jobs.

Great Expectations

Evaluating Your Payback

One of the biggest disappointments is not being able to “feel” a difference after installing a performance product. Don’t despair! Even people with a pretty well calibrated “butt-dyno” cannot “feel” a five HP increase on a 4.0L Jeep six. But it’s there and you’ll realize it some day and notice your Jeep just made it over that mountain pass in fifth gear where previously you had to downshift to fourth. Maybe you’ll see it when you get an extra 20 miles out of a tank of fuel.

Another element of disappointment involves advertised dyno numbers versus what you may see in independent tests, or personally at a local dyno shop. Dyno calibration and operator skill aside, you could dyno test a particular product on ten similarly equipped Jeeps and get ten different numbers. Some will be way lower than advertised, other will be equal or “in the ballpark,” some may actually exceed the advertised numbers.

First, realize that any aftermarket manufacturer is going to put its best foot forward by testing the product on a vehicle known to be in tip-top shape. An off-the-street rig is seldom a match. It could have worn spark plugs, a slightly dirty air filter, stretched timing chain that’s retarding cam timing, weak EFI sensors, a partially plugged catalytic converter… the permutations are endless.

The only fair way to evaluate a product is to do before and after tests on the same vehicle. If you add a product then happen to glom a little dyno time, don’t blow a gasket when your 95,000 mile Jeep can’t equal the numbers of a tuned-to-perfection 10,000 mile Jeep.

Also consider this; with any modification, the amount of performance you gain is directly proportional to how much better the new parts are than the old ones. OE isn’t always bad and aftermarket isn’t always better than OE. If a Jeep has a particularly good stock exhaust system, for example, the gains from an aftermarket system, even a very good one, may be modest. Conversely, a poorly designed aftermarket system may not even equal a stock system.

Out With the Old

Exhaust Systems

In most cases, exhaust mods are job one. Not only do they offer power gains in themselves, they prop the door open for other mods in the future. Plus, a performance exhaust systems is a modification with a double payoff; performance and fuel economy.

There are two main elements behind the gains that can be realized from exhaust modifications, improved scavenging and increased exhaust flow. Without good exhaust flow, it actually takes a certain amount of power to push that exhaust out. Flow restriction may not be much of a problem at lower speeds and at stock power outputs. In other words, the power losses from an OE system may be minimal… until they are not. In a good stock system, “not” usually doesn’t come until the upper rpm ranges. In lesser systems, “not” can come pretty early and if you are pushing more exhaust created by other engine mods, then “not” can come even earlier.

The scavenging effect is the even more important part of a well-designed performance exhaust system. The effect is present to some degree in all engines and exhaust systems but maximizing it results in an increased ability to inhale a larger gulp of air and if you can add a little more fuel to that air, you get more power.

What is scavenging? It’s a lot like a siphoning effect. If you place a bucket on a ladder with a long hose in it and start water flowing down the hose, you can watch the “weight” of the water flowing through the hose “suck” the water out of the bucket. Though exhaust gasses are considerably lighter than water, their “weight” of their pressure waves flowing out the pipe creates negative pressure (vacuum) at the exhaust valve. When properly tuned, the exhaust system can create many times more “suction” than the downstroke of the piston on the intake stroke.

Negative pressure at the exhaust valve does several things. First, it clears any remaining exhaust gasses from the cylinder after the pressure from the power stroke has dissipated. Second, during the overlap period when both the intake and exhaust valves are open, it helps draw a larger charge of air into the cylinder. Kinda like “free” forced induction and it can improve volumetric efficiency.

What is volumetric efficiency? It’s the amount of air the engine really takes in versus the total volume of the cylinder. Most engines are 80-85% efficient, meaning that your 242 ci engine at 85% efficiency is delivering the power of a 206ci.

The study of how exhaust flow is tuned to increase scavenging is pretty scientific. It all comes down to choosing the right diameter pipes (both at the header and in the system) that maintains the pressure wave. A poor muffler often is the main culprit in breaking up that wave, so a good muffler is also key. Header primary tube lengths are important as well, though collector length and diameter are more crucial.

Often, even a stock fuel injection system can “sense” more complete scavenging and deliver a little more fuel to go with that extra air and more power is the result. A fuel injection system that is specially tuned to take advantage of the airflow increases will deliver even more power.

Exhaust mods come in two common forms, a cat-back exhaust system (or manifold-back system for Jeeps without cats) and headers. A cat-back system, as the name implies, mounts behind the cat converter. The manifold back is also self-explanatory. There is always a scavenging benefit from increased exhaust flow, but the because the scavenging effect takes place closer to the engine and can be best maximized there, that’s where headers come in.

Headers consist of multiple tubes from the exhaust ports that enter into a collector, or collectors. There are two commonly seen header styles. Some have roughly equal length tubes that run into a single collector. Pipe diameter and length are important. Non-race use small diameter tubes (the exact size chosen according to the flow rate at the exhaust valve) and are best at between 24 and 36 inches of length… according to what will fit. Longer tubes tend to enhance lower rpm performance and vice versa. Equal length tubes are not that important on V8s, V6s and inline sixes, but more important with four cylinders. Fours are also more generally sensitive to the primary header tube length.

Another style of header, often seen on inline sixes, branches and pairs in a couple of places. The tubes are paired to take advantage of the exhaust pulses, according to the firing order, for superior scavenging. This method offers very good low rpm and midrange performance compared to four-into-one or six-into-one headers. At higher rpms, the four or six-into-one do a little better.

If your exhaust purchase is an “either/or” situation, the best choice is to start with the cat-back system. The good effects of a header may be partially negated by a stock exhaust system. In truth, the best results come from a set of headers AND a well-designed exhaust system. Even better is when the two come from the same company because they have likely been tuned to enhance each other.

There are some other key selection elements to keep in mind when shopping for exhaust products. They include the way the pipes are bent. Bends are a necessary evil but how the bends are made can enhance or detract from flow. A conventional pipe bender partially crushes the pipe and restricts flow. Mandrel (or mandrill) benders maintain the same diameter and do not restrict flow much.

Another important element is the steel used. A good grade of stainless steel will last just about forever under your Jeep… excluding rock hits, of course! On the other hand, a good grade of aluminized steep pipe can last a very long while for a lot less money and offers a less expensive alternative in salt-free areas. From the performance standpoint there is zero difference between the two types.

With headers, those that are ceramic coated have been demonstrated to deliver a skosh more power because they hold more heat. Heat loss of the exhaust gasses at the header tends to slow the flow a little. Ceramic coatings also help the header last longer. Ditto for the stainless steel headers on the market.

Noise is an important issue for many… both for the owners and the world in general. Remember this: More noise does not equal more power! It’s very possible to have a whisper quiet system that equals the power output of a open pipe. The key to a quiet system is pipe size and muffler design. A larger diameter system than you need will always be noisier but will not make more power than the right sized smaller diameter system.

As a final element of the exhaust equation, let’s look at catalytic converters. The modern honeycomb designs flow well and there are low restriction performance replacements. On a mildly built engine, a replacement cat isn’t a vital necessity, assuming it’s in good shape. Start considering including a performance cat in the budget if your Jeep has a lot of miles on it because cats tend to plug up over time. Also, if the engine has performance mods that will increase exhaust flow, you could outstrip the OE cat’s flow rate. Most cats are not as disruptive of exhaust flow as a muffler can be.

In With the New

Intake Systems

The air filtration system’s main job is… duh… to filter the air. The OE designs a system to do that job well with minimal restriction for the stock engine. Costs and space are always considered and the end result may be some extra flow capacity built in or barely enough for stock power. The basic credo is that until the engine’s need to breathe exceeds the filter system’s ability to flow, there is little extra power to be gained in replacing it.

It takes a small amount of power for the engine to inhale at any rpm and, just like with the exhaust, it’s called pumping loss. At low speeds, the partially closed throttle butterfly accounts for most of any gas engine’s pumping loss, but at higher speeds, with the throttle open more, the air filtration system begins to account for more. At that point, the engine’s ability to inhale is impaired and volumetric efficiency is reduced.

As soon as you increase an engine’s power output in any way, the intake airflow equation changes. Most stock systems are restrictive in the upper rpm ranges, a little or a lot. Not many of us run Jeeps at high revs, so it isn’t usually a problem. As soon as your engine becomes able to inhale more because of other modifications, such as a performance exhaust and/or an improved EFI program, the point where the restriction becomes a problem moves to a lower rpm and that’s when a performance intake is most necessary.

A performance filtration assembly does several other good things. Most raise the air intake point. In newer Jeeps, the intake brings in cool air from the grille area. That’s a good place… until you have to cross a deep creek. Engines prefer air to water every time. By moving and raising the filter inlet it lessens the chance the engine will be force-fed a liquid diet and rebel by puking up a piston or connecting rod. Another feature is that many systems will still deliver that air in a cool state. Cooler air is more densely packed with oxygen molecules, so when mixed with the right amount of fuel, cooler air will deliver a skosh more power at the same airflow volume than warmer air.

Here’s another side benefit. The trail is a dirty place. As a filter collects dirt, airflow is restricted. That can happen surprisingly fast on a dusty trail. The filter elements used in most aftermarket systems are far superior in dirt carrying capacity than the stock filter and can go lots longer before power is restricted due to clogging. These filters are also cleanable so you can use them for many years. Their ability to filter dirt is usually no worse than the stock filter and often much better.

Finally, in many cases, there is a tuned aspect to the tube between the filter element and the throttle body. Look for a smooth interior surface, wide radius bends for the least amount of airflow disruption.

Hello Mr. Chips

Chips and Programmers

More power can be unleashed via EFI (Electronic Fuel Injection) programming changes than with almost any other single modification you can make to a fuel injected engine. The problem is that extensive changes usually put the engine into a non-compliant emissions state. Even factoring in that limitation, there is a fair amount of power to be had from a smog legal chip or programmer and they are among the easiest mods for a home wrencher.

First lets define the differences between a “chip” and a “programmer.” A chip is a plug-in device that either piggybacks onto the OE EFI ECU (Electronic Control Unit), or sandwiches into the harness between the engine and the ECU. This latter unit is called an “interceptor.” The basic operation of both these items is that they modify the signals to or from the ECU to alter engine performance. A programmer connects to the vehicle diagnostic port, downloads (and preserves) the original program and uploads a modified program.

Most older systems cannot use a programmer but some of the newer system can use either a programmer or a chip. When faced with this choice, which is the best? Chips are generally less expensive than programmers but programmers often have extra features that account for the difference in price. Virtually all programmers contain a code reader, so you can diagnose and erase DTCs (Diagnostic Trouble Codes). Most give you several other options, including the ability to program to compensate for tire diameter increases and gear ratio changes so your speedometer reads correctly. Some will give you an option to tune for regular or premium fuel, with the premium tune offering a bit more performance. Some setups include performance testing devices that calculate power and give you 0-60 or quarter mile times and speeds.

The performance result is about the same from either a mass produced chip or programmer. Any differences between them usually come down to how well the programmer did his job. With either, the big gains come at WOT (Wide Open Throttle). Most often, the new programs will be similar or identical to the OE in the low and middle rpm ranges so that the vehicle passes emissions. In some cases, safe “tweaks” might be added to improve some lapses in the original code. That’s fairly rare in newer rigs because the OE usually does a good job. They have excellent resources for getting it right… generally much better than the aftermarket… but their main goal is meeting emissions standards and then minimizing the performance losses. The aftermarket comes at it from the other direction by tweaking the performance and trying to minimize emissions.


When the benefits from several improvements combined exceed the sum of the benefits from the individual components.

One question that comes up often is whether you can add up the advertised individual gains from several mods to get an approximation of the final value. Most times, yes. Some products may not be well matched and “fight” each other to an extent and decrease the combined result. In the product types we have reviewed here, that’s not likely. An improperly selected camshaft upgrade might do more to upset the synergy apple cart than anything.

In a few rare instances products from different manufacturers have particularly good synergy together. That’s when Jeep life gets sweet but it’s hard to get these products hooked up without testing a bunch of them. You are most likely to find it in packaged products from a single manufacturer that have been designed to work in harmony. Also, one manufacturer might have done some tests that show a particular brand of something else worked best with their product. It’s worth a phone call to find out.

Sometimes, ho-hum results come from NOT adding a certain product. An example of that might be to install headers, a chip and an intake system but keeping the stock exhaust system in place. You can’t get the full effect until you remove that last bottleneck.

Generally, the order in which we presented this info is the best order to “stage” your modifications. Each opens the door for better synergy with the next modification. Above all, don’t be afraid to call the tech lines of the various manufacturers and discuss the particulars of your Jeep. This simple step can save lots of time, money and aggro.

If you are looking for ways to increase your jeep’s engine performance, we suggest the following items: