Pac It In - Gen III Truck Motor Bolt Ons

Bolt-on Horsepower for Gen III Truck Motors From SLP

John Nelson Sep 21, 2006 0 Comment(s)

We're all in search of the perfect combo, the right mix of parts and tuning to endow our rides with the proper dose of high-performance fun. The process is especially satisfying when the payoff results from our own long hours of research. On the other hand, there's certainly something to be said for letting someone else do the homework. With this idea in mind, we decided to check out one of SLP's PerformancePacs, using an untouched '04 Avalanche as our test subject.

SLP has a PerformancePac for just about anything running a Gen III powerplant: '98-02 Camaros and C5 Corvettes, along with most trucks and SUVs. These packages come in variations for 4.8-, 5.3-, and 6.0L engines. The Avalanche system is a fairly new addition to the lineup and has proved to be a hot seller. "The Avalanche is surprisingly one of our more popular kits," reports Brian Reese, SLP's director of engineering. "We sell three for every truck kit."

One attraction to a package deal like the PerformancePac is ease of installation, and the system we sampled scored well in this department. Superior Automotive in Anaheim, California, accomplished the job--including before and after dyno runs--in a mere half day. Having access to a lift certainly helped with the exhaust system swap, but all other parts of the install can easily be done in the driveway.

The PerformancePac includes a High-Flow Cold-Air Induction System, stainless steel shorty headers, a stainless steel PowerFlo cat-back, 1.85:1 rocker arms, and SLP's dyno-based performance tuning contained in a DiabloSport II programmer. The goal of each kit, according to SLP, is to create a package that significantly improves performance without adversely affecting drivability. Much of the performance increase we saw came from the simple fact that the new setup allows more air in and out of the Avalanche's LM7 powerplant. SLP's Brian Reese agreed, and took it a step further: "With our package," he told us, "the motor is leaned out at wide-open throttle, improving performance over the rich factory settings."

Of course, an engine must be able to take advantage of increased airflow, and that's where the switch to 1.85:1 rocker arms pays off. "The key ingredient in the package is the high-lift rocker arms," asserts SLP's Reese. A stock LM7 cam running 1.7:1 rockers specs out at 0.457/0.466-inch lift and 191/190 duration at 0.050-inch. Bumping up the rocker ratio increases the lift to 0.497/ 0.507-inch, which is certainly a step in the right direction.

The custom programming is the final step, providing the engine with the tuning it needs to take advantage of the other modifications. The fuel and spark advance curves are optimized for performance, and the factory delay in the power-enrichment function that occurs at wide-open throttle is eliminated. "When it's floored," Reese told us, "You get the full thing." One thing the programming doesn't do, before you ask, is remove the factory 100-mph speed limiter, and for good reason. When SLP tried it, our source reports, the factory driveshaft yoke broke--and violently expelled the shaft. You've been warned. So, what did the PerformancePac do for our subject Avalanche? Making our dyno pulls in Second gear to avoid that pesky speed limiter, we picked up 33 hp and 26 lb-ft of torque. Had we been able to make our runs in Third gear, we undoubtedly would have registered greater gains, a supposition backed up by the Avalanche owner's subjective observations. "I didn't think it was underpowered before," he reports. "But I can really feel the extra power."

And from the "have your cake and eat it too" department, our subject's owner reports that the much-improved growl coming from the new exhaust system isn't overly loud. On top of that he's getting better gas mileage--another feature SLP engineered into the equation. Check out how this one-stop shopping trip added to this daily driver's fun factor. CHP

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Ditching the stock 1.7:1 rocker arms in favor of 1.85:1 pieces (right) adds approximately 0.040 inch lift to both the intake and exhaust side of the Avalanche's very mild cam. SLP includes Allen bolts to replace the original 8mm hex bolts.

Removing the stock manifolds on our subject Avalanche wasn't difficult. You may find it easier to remove the driver-side piece from the bottom of the vehicle, but our guys were able to pull both out from the top. All in all, the stock Avalanche pieces aren't bad for stock manifolds, but SLP's coated shorty headers are still an improvement, and they look pretty slick too.

In short order, the factory air-intake rigmarole was removed. The black plate that sits beneath the stock airbox (arrow) must be removed to allow installation of the new airbox.

Compared to the original setup, SLP's Cold-Air Induction System looks pretty simplistic--and it is. Installation is equally easy, and again we'll just touch on a few highlights.

The new airbox bolts in using three of the five bolts that held the plate in place. Installing the new high-flow filter element requires a little finagling to work it into place. With an open side and bottom, the SLP box allows much more outside air to reach the filter.

With the new air tube and the SLP-labeled airbox lid in place, this part of the PerformancePac install was complete.

New header bolts and gaskets are included with the SLP setup. The longer set of bolts is used on the passenger side to accommodate the thickness of the slip-fit flange on this side.

Moving to the cat-back installation, we cut the factory intermediate pipe from the original muffler to facilitate removal. The difference in size between the water heater-like factory piece and the sleek replacement is striking.

Installing SLP's Power-Flo cat-back exhaust requires some changes in the hanger configuration. The first change is made at the muffler. Using the provided hardware, the new hanger is installed into the framerails using existing holes. Some quick work with an air gun allows the bolts to be tightened without spinning the nuts.

At this stage, the new intermediate pipe was bolted to the factory Y-pipe and the new muffler is hanging from its newly installed hanger.

On the driver side, the new tailpipe requires a new hanger. Installation of this piece is similar to that of the other hanger.

One small but important step is to zip-tie the rear brake line and differential vent tube together so that they don't touch the new driver-side tailpipe, which is visible in the background.

While not a true dual-exhaust system, the new SLP setup looks better, flows better, and sounds better, strutting a little visual attitude and emitting a musclecar growl without being unduly noisy.

Last, but certainly not least, the PerformancePac includes a DiabloSport II programmer loaded with SLP's custom tuning. The new programming puts the fuel and spark curves "where they should be," according to SLP's Brian Reese.

Stock
236 hp at 5,200 rpm
262 lb-ft at 4,000 rpm

w/PerformancePac
269 hp at 5,400 rpm
289 lb-ft at 4,400 rpm

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