Tow Vehicle Essentials - Drive Or Tow?

Driving Your Corvette Is Definitely More Fun Than Towing It, But Sometimes You Don't Have A Choice

Dave Emanuel Jan 11, 2010 0 Comment(s)
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If you're like most Corvette owners, you enjoy the driving experience that's only available when you're behind the wheel of America's sports car. But if your Corvette is a race car, show car, project in process, or simply in need of repairs, the occasion may arise when towing it makes a lot more sense than driving it. If that's the case, having a tow vehicle with ample power is essential to safer and easier towing.

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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.

Chevrolet's Silverado and GMC's Sierra, which share the same powertrain, have been equipped with 4.8-, 5.3-, and 6.0-liter LS-series engines since the 1999 model year. Rated at 285-315 horsepower depending on model year, the 5.3 is the engine most commonly found in pickups that are used for light towing. Even if you don't have a truck, or have no interest in modifying one, you might find the following of interest because the 5.3 truck engine is essentially a cast iron, small bore version of the 5.7-liter LS1 Corvette engine.

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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.

One of the easiest methods of increasing both available horsepower and overall fuel economy is to replace the mechanical fan with an electric model. (This obviously applies only to engines equipped with mechanical fans; '07 and later model GM trucks are factory equipped with electric fans.) Even though the stock fan is mounted to a viscous coupling (that's supposed to allow the fan to free-wheel until engine temperature reaches a predetermined level), it still spins fast enough to consume a significant amount of power. The dramatic improvement in throttle response after we removed the mechanical fan is further evidence of its negative impact on power-and fuel economy. Our '02 Silverado test vehicle picked up a solid mile per gallon after its mechanical fan was replaced with a Spal dual electric fan assembly. In addition to a wide selection of fans, Spal also offers a unique pulse width modulated controller that allows easy setting of "fan-on" and "fan-off" temperatures.

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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.

Setting up the fan's switch-on and switch-off temperatures using Spal's fan controller could be called programming, but that would be another over-statement. All you have to do is press the High and Low buttons on the controller when coolant temperature reaches the desired level and "programming" is completed. Unfortunately, the temperature sensor included with our controller was not of the proper metric persuasion required for installation in an LSx cylinder head, so we had to find one with the proper thread dimensions (available through most GM dealers and many parts stores). If you can't find nor particularly care about using a "genuine GM" sensor, BWD Automotive part number WT5132 is a suitable replacement.)

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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.

Personal preferences vary greatly with respect to exhaust system configuration, so there are many "right" ways to modify a stock system. (Don't consider that statement as a license to do whatever is quick and easy-some modifications are just wrong.) The advantage of installing a complete exhaust system is that cutting and welding are typically not required. The disadvantage is expense. The original equipment systems used on 5.3-liter GM trucks incorporate mandrel bent 75mm tubing, so there's little to be gained by replacing pipes-it's the muffler that constitutes the major flow restriction. Aside from being large enough to house a small family, the stock muffler has enough internal twists and turns to confuse a seasoned navigator. That obviously creates a good deal of exhaust restriction-which can be eliminated by replacing the stock muffler with a low restriction model.

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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.

One of the best ways to take full advantage of flow improvements made to an exhaust system is to make similar improvements to the intake system. Replacement of an original equipment air filter element with a low-restriction type is a no-brainer. However, a number of studies have shown that some brands of high performance air filters don't do a particularly good job of filtering. Issues seem to arise only under extremely dusty conditions, but it's a good idea to periodically check the intake ducting between the filter and the engine regardless of the air quality that normally surrounds your truck. Not only will you be able to determine whether the filter is performing satisfactorily, you'll also uncover any damage that is allowing unfiltered air to reach the engine.

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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.

The best way to optimize ECM/PCM settings for your particular driving requirements is with a programming/scanning system such as EFILive's FlashScan. This system includes software, which will run on virtually any Windows-based computer, and the cabling necessary to connect a vehicle to a PC. The cable includes a "black box" which not only handles PC-to-vehicle communications, it can also serve as a stand-alone data logging device.

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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.

Regardless of the method you select, your end goal is to optimize operating efficiency at the rpm and engine load levels that your truck encounters when it's hauling your Corvette. Keep in mind that most control settings are contained in "maps"- two- and three-dimensional arrays with individual values associated with specific rpm levels and amount of engine load. Consequently, you don't have to be concerned that changes made to improve towing performance will have a negative impact when you're not pulling a trailer-or vice versa.

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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.

Sources

Random Technology
Loganville, GA 30052
770-554-4244
http://www.randomtechnology.com
SPAL USA
Ankeyny, IA
800-345-0327
http://www.spalusa.com
EFI Live
Auckland, NZ, AK
http://www.efilive.com
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