Tow Vehicle Essentials - Drive Or Tow?

Although the engines in most late model 1/2-ton pickups-particularly those wearing Chevrolet and GMC badges-have horsepower ratings that were unheard of a few years ago, they may still struggle a bit when a loaded trailer is hooked to the rear bumper. The techniques used to improve the performance of different engines are similar, so even though our test truck is a Chevrolet Silverado, results are typical of what you can expect from other makes.

As is the case with your favorite Corvette engine, improved air flow and reduction of parasitic power losses are two important keys to improved performance. The relatively simple modifications we made to our '02 Silverado test vehicle resulted in horsepower and torque readings of 268 and 308, (at the wheels) increases of 24 horsepower and 15 lbs./ft of torque respectively. We didn't do individual tests after each modification because we were trying to develop a "package" that would deliver significant improvements. (We've also developed a towing-oriented heads/cam package for the 5.3. Let us know if you'd like more information on it.)

The horsepower increase we recorded is a bit irrelevant, but it does put some of the advertised claims you may see into perspective. In a tow vehicle, peak horsepower isn't of much concern or value because chances are pretty slim that you'll have the gas pedal on the floor, and the engine spinning at 5500 rpm, when there's a trailer hooked to the rear; mid-range torque is of far more practical importance. The same holds true for Corvette engines. Even though you may bounce your engine off the rev limiter occasionally, it spends most of its time at the lower and middle ranges of its rpm band. So although big power numbers are great for bragging rights, trading off some top end power for increased mid-range torque usually makes a car more fun to drive and easier to tune.

Installation of an electric fan is not particularly difficult; removal of the original fan is a different story. The fan/coupling assembly mounts to the water pump pulley by way of a large nut that's virtually impossible to remove without the tool specially designed for the purpose. A 36mm open end (or adjustable) wrench and a big hammer will get the job done, but you'll save yourself a good bit of aggravation, and your knuckles a good bit of skin, if you buy or borrow the proper tool. After the fan is out of the way, the lower fan shroud can be removed and the electric fan assembly set in place. Since we used a "universal" type Spal dual-fan assembly, we had to fabricate the brackets required to mount it to the radiator support. The term "fabrication" is a bit of an over-statement-nothing more exotic than a few pieces of 1-inch-wide aluminum, a vise, and a drill are required to get the job done.

A more direct route to increasing an engine's power output is to improve its breathing capability. That's easily accomplished through exhaust system modification. It's truly ironic that auto manufacturers spend so much time and money developing whisper-quiet exhaust systems, yet installation of aftermarket systems with a significantly more aggressive sound continues to be one of the most commonly made modifications. Although many aftermarket systems are installed in the quest for a "deep, throaty exhaust note," such systems also deliver increased power. That's largely because the same characteristics that restrict sound also restrict flow; within limits, increased flow capacity equates with increased power output.

Original equipment catalytic converters are another known power-reducing restriction. (The conditions under which a catalytic converter can be replaced are defined by both Federal and local laws, so be sure to check regulations before making modifications involving any emissions control device.) If your truck's converters need to be replaced, installing high flow models, such as those offered by Random Technology, will pay off with increases of 8-10 horsepower across the entire rpm range. GM's 5.3-liter engines are equipped with a Y-pipe that incorporates a converter for each cylinder bank. Replacement of the converters requires that the originals be cut from the Y-pipe and the replacements welded in their place.

Another quick and easy method of increasing inlet air flow capacity is to replace the stock throttle body with one that has been CNC-machined for enhanced flow. As is the case when installed on a Corvette engine, a modified throttle body bolted to a truck intake manifold provides improved throttle response and more power across a fairly wide rpm range.

Although the engine control systems used in late model vehicles can adapt to a wide variety of mechanical changes, there are limits. Even within those limits, the systems are governed by relatively conservative factory settings that control air/fuel ratio, ignition timing, and transmission shift points. While typical bolt-on modifications don't require reprogramming of the vehicle ECM or PCM (Engine Control Module or Powertrain Control Module), additional performance benefits can be achieved by doing so.

FlashScan can be used to reprogram multiple ECMs/PCMs, so if you're soinclined, you can use it to improve the performance of your Corvette, as well as your truck (and a variety of other GM vehicles). On the other hand, there's a relatively steep learning curve associated with electronic engine controls, and if you don't want to climb it, you can order a "mail order tune," or have a professional tuner email you a file that you can "flash" yourself, and use a learning tool.

Ideally, reprogramming should be done after all other modifications are made, but in the real world, that's not always possible. One of the benefits of using reflashing software is that you can change settings whenever you like, as often as you like. So if you plan to make additional modifications in the future (such as the ported heads/cam installation we have planned for our Silverado), you can re-optimize the settings as required. Some tuners also offer free or low cost "re-dos," so even if you don't have reprogramming software, you can have your truck's computer updated very economically.

Another benefit of reprogramming your truck's PCM is that you'll also improve its performance when it's not towing a trailer. That won't make your truck as much fun to drive as your Corvette, but it will certainly help.