How many times have you read the ad by the company claiming you'll get 25 extra horsepower by bolting on their widget? You can't always believe the hype. While it's true that most power-adders really do add power, the reality is that the results often fall short of the claims. Here are some of the hottest power mods and what you can expect from them in the real world.
High-Flow Air Intake
It's one of the first modifications many Jeep owners make, whether it's an injected 4.0L or a 327 Vigilante V-8. A good, high-flow filter will really free up some power across the rpm range. Replacing the convoluted factory ducting and keeping the filter away from engine heat usually translates into noticeable power and increased mileage numbers.
Look For: Drop-in replacement filters, whether flat or round, look for a tight fit that doesn't leave any gaps between the sides of the filter and its housing. Also make sure the cotton, gauze, or other filter material is sturdily bonded to the rubber in which it is encased. Cheaper filters can come apart over time and allow dirt and particulates to enter the engine. For entire replacement air intake systems, look for free-flowing bends, good silicone or high-quality rubber couplings, and some form of air dam that isolates the filter element from engine heat.
They Claim: Anywhere from 8-18 hp.We Say: Expect about 5-10 hp at the rear tires with a noticeable improvement in driveability.
Large-Bore Throttle Body
While a larger-bore throttle body allows more air to enter the engine, in reality the factory part offers decent airflow to accommodate most stock engines. Unless you've added a bigger cam, a free-flowing exhaust, or other aftermarket power-adders (like a supercharger or turbo) you're not really going to notice much, if any, difference from the driver's seat. It's a complementary component that works best in conjunction with other parts. In fact, we noticed a substantial decrease in power when we installed a 58mm 4.0L throttle body on our 2.5L Wrangler in place of the stock 52mm unit. This is one item that should really only be added once you've hit a brick wall and nothing else you're doing adds power.
Look For: Aftermarket units machined from billet aluminum are less likely to have throttle shaft leaks than some older, bored stock units. Most 4.0L units are punched out to 62mm and don't feature a tapered bore. This will give you some whistling at certain rpms, so be prepared for it. Quality units will come with new Torx fasteners and an anti-tamper Torx driver to remove the factory sensors.
They Claim: As much as 15 hp.
We Say: On a stock engine, don't expect to notice any difference and only 3-5 hp with air intake and exhaust mods.
Free-Flow Exhaust
Just about any V-8 and most six-cylinder engines will respond favorably to a free-flowing after-cat muffler and mandrel-bent tailpipe assembly. Headers will also go a long way in waking up these engines, letting the engine scavenge spent gasses from the cylinders and rev more quickly.
For the four-cylinder crowd, a free-flowing exhaust is usually going to do more harm than good to everyday driveability. A four-popper will gain a little on the upper end, but the loss in torque below 3,500-4,000 rpm is going to make itself known.
Look For: When selecting headers, Jeep four-cylinder and six-cylinder flange thicknesses are dictated by the intake manifold, so the flanges are going to be thick by design. V-8 applications should look for flanges at least 31/48-inch thick for a good seal. Look for a good, high quality stainless steel construction or a coated 16- or 14-gauge mild steel. Mandrel-bent tubes that are fully welded inside and out, tapped O2 bungs (when applicable), and collector flanges that use a factory-type, ball-style flange gasket rather than a flat paper gasket are preferred. For after-cat systems, look for a fully welded muffler rather than a crimped case design and high-quality stainless steel or aluminized steel tubing with mandrel bends and a diameter of about 2.5 inches. Healthy V-8s can go for a 3-inch or a dual 2.5-inch.
They Claim: 10-15 hp for the header, 10-20 hp for the after-cat system.
We Say: Expect 5-10 hp with the after-cat system and an additional 5-10 hp with the header. However, in some cases we have seen horsepower numbers drop after installation of an after-cat system.
Throttle-Body/Carburetor Spacers
If ever there was a camp vehemently enthusiastic about their modification, it's the throttle-body spacer guys. While many people claim huge and noticeable differences, we've just never seen it on an injected rig. There are several on the market with spiral or helix bores designed to create turbulence in the air charge to better atomize the air/fuel mixture. The theory is sound, but so is the whistle it can create.
On the other hand, adding a 1-inch phelonic or other heat-resistant spacer under your carburetor can deliver a noticeable increase in seat-of-the-pants operation in a vehicle with a free-flowing exhaust and a good ignition system.
Look For: A quality spacer kit is going to include all the gaskets, longer bolts, and any linkage or adapter spacers that may be needed for installation. Some are taller and some are shorter. For a more noticeable gain, go for the taller spacer, although realize that it may cause fitment problems in your aftermarket air intake systems.
They Claim: 5-11 hp.We Say: Probably 1-2 hp for the throttle body, 5-8 hp for the carb spacer.