GM Truck Engine Bolt Ons - Tow Story

Whoever coined the words, "Getting there is half the fun" obviously wasn't talking about hauling a racecar to the track. Whether the trip is made on back roads or an interstate highway, towing a trailered racecar is never fun, considering the countless brain-dead drivers you encounter along the way. It seems as though most of them think they absolutely have to be talking on their cell phones, drinking a cup of coffee, or both while they're careening down the highway.

The idiots to your right and left, and in front of you are never going to change, but if your tow vehicle has a bit more power, it may be easier to get away from them. As an added benefit, the power that helps you distance yourself from the accidents waiting to happen, also makes it easier for your rig to climb hills and get up to normal cruising speed. And although it may seem to be a contradiction, the same modifications that bring increased power, can also improve fuel economy.

Although the techniques used to improve the performance of different vehicles is similar, there are engine-specific differences. This month's edition of "Tow Story" deals with Chevy's Silverado and GMC's Sierra, virtually identical 1/2-ton pickups that are commonly used for daily transportation during the week and for hauling race cars on weekends. Since the 1999 model year, most of these trucks have been equipped with GM's 5.3-liter LSx "Gen3" small block. (GM's replacement for the old, reliable 350.) Rated at 285-315 hp depending on model year, the 5.3 delivers impressive performance. But when the weight a car, trailer, tools, tires, and spare parts are hung on the back, life in the fast lane gets a lot slower.

As is the case with a race engine, improved air flow and reduction of parasitic power losses are two important keys to improved truck engine performance. The relatively simple modifications we made to our 2002 Silverado test vehicle resulted in horsepower and torque readings of 268 and 308, increases of 24hp and 15 lb-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're also working on a towing-oriented heads/cam package for the 5.3. It will be covered in a future issue.

The horsepower increase we recorded is a bit deceiving and somewhat irrelevant, but it does put some of the advertised claims you may see into perspective. In a tow vehicle, peak horsepower is largely irrelevant because chances are pretty slim that you'll have the gas pedal on the floor, and the engine spinning at 5,500 rpm, when there's a trailer hooked to the rear; mid-range torque is of far more practical importance. As for the test numbers being deceiving, tests on a chassis dyno are typically done with the vehicle's transmission in "high" gear (with a 1:1 ratio) so that driveline losses are minimized. As is the case with many late model vehicles, the speed limiter in our test truck was set to 105 mph, which corresponds to an engine speed of approximately 4,500 rpm, well below the engine's 5,100 rpm power peak. That being the case, raising the speed limiter setting results in a sizable power gain, simply because maximum engine test rpm was increased.

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; 2007 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 the mechanical fan has been removed is further evidence of its negative impact on power-and fuel economy. Our 2002 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.