Chevrolet Silverado SS Supercharged - Supercharged SS Part IV

From bolt-on modifications and transmission modifications tested in Part III, we were able to push the Silverado SS to a 13.21 e.t. at 101.24 mph. In this phase, we'll test The Other Guys headers, a VaraRam throttle body velocity stack and a new ram airbox, a modified stock airbox, a 2.75-inch supercharger pulley, and custom PCM programs from PCM for Less and Nelson Performance. We also designed a new control circuit for the Radix's intercooler pump and performed some intercooler optimization. Accordingly, the higher power and speed capability of our truck dictated new tires. To qualify the effectiveness of each modification, we tested again at Pittsburgh Raceway Park.

Testing
Our Silverado SS is breathing pretty heavy and we're flowing well over 800 cfm compared to the stock-form 450 cfm. Manufacturers design their products for a stock to slightly modified engine but not for an engine that flows 80 percent more than stock, so our modification test results will not be typical for a relatively stock engine.

We obtained a titanium thermal barrier, ceramic-coated headers from The Other Guys. These headers are considered mid-length, which we felt could best accommodate the characteristics of the supercharger. On a normally aspirated engine, headers increase torque and horsepower by a scavenging method. As the intake valve opens and the exhaust valve closes, the exiting exhaust gas draws more intake air into the cylinder, increasing the volumetric efficiency. Scavenging is based on the length (and diameter) of the header pipe. With a supercharged engine, scavenging isn't as critical; we just want to get the exhaust gas out with minimal back pressure.

To install the headers, we placed the truck on stands and removed the front tires, plastic inner fender wells, and the spark plugs and wires. Extracting the exhaust manifold flange bolts took some extra grunts as well as penetrating fluid. TOG advises to cut the pipes at the weld in front of the catalytic converter, but being wary (measure twice, cut once), we decided to install the headers and re-measure the pipe to the header because new flanges must be welded to the pipe to mate to the headers. Minimal filing and grinding were necessary to mate the flange with the headers. We tacked the flange and re-checked the fit. Then we removed the pipes and completed the welding. Though TOG says it's okay to reuse the exhaust manifold gaskets, we opted for new GM gaskets for about $17. Make sure you use antiseize compound on the original header bolts, tighten them, check for leaks, get the headers hot, and then retighten the bolts. Set aside several hours to do this job on your own. The new pipes enhanced our dragstrip time to 13.15 seconds at 101.6 mph.

The VaraRam throttle body velocity stack simply slides into the throttle body opening. Although the manufacturer claims that it will cut e.t.'s by about 0.1 seconds on a stock truck, this particular stack is geared more for the stock engine and a driver who wants a torque boost for towing and isn't necessarily looking for an increase in horsepower. Though we were told that our flow rate was excessive for this velocity stack, we tested it anyway. Using it along with new Volant airbox from Part III, our e.t. was 13.33 seconds at 100.33 mph. We'd been told that this stack would choke flow and we proved that the manufacturer is correct.

Considering all of the airbox testing we'd done, we wanted to take a crack at a design of our own. Chuck Downs (www.silveradoSS.com) was kind enough to donate his stock airbox in the name of science. First, we smoothed out the ridges in the stock airbox lid as well as the bottom of the box and added a teardrop-shaped opening to draw cooler air from below the airbox. The new opening more than doubled the box's intake area. We inserted a Cool Blue reusable filter, too. Our goal was to provide a low-cost alternative to the aftermarket air boxes, but the airflow gods weren't smiling. Our modified box increased e.t. to 13.23 seconds at 101.23 mph, setback of 0.08 seconds and 0.37 mph compared to the new Volant air box. We think our design may work for relatively stock trucks, but not for our high-flowing engine.

In a quest for more, we installed a 2.75-inch diameter Nelson Performance aluminum pulley on the Radix in place of the 2.9-inch one we used in Part III. A unique feature of this piece is that it has a hex head as part of the pulley, making tightening and loosening the supercharger nut much easier. The smaller pulley (along with the new Volant airbox) increased the boost to well over 8 psi, providing an e.t. of 13.12 seconds and an mph of 101.93. Remember, we were still using the tuning that came with the Radix, which would limit the effectiveness of the smaller pulley and the increase in boost. To take full advantage of the extra boost, you really need to modify the tune-up.

There are mixed opinions about the 2.75-inch pulley for daily use. The concern is basically about forcing the supercharger to spin faster (without producing any boost), thereby inducing excessive bearing wear and heat. Everyone seems to agree that a 2.9-inch one is fine for everyday use, and for those occasional track days, the smaller one should suffice. We chose to run the 2.75-inch pulley all the time. The next question? How can you optimize your engine tuning without having to reprogram your PCM every time you change pulleys? Basically, you optimize tuning for the 2.75-inch pulley. For the most part, the only difference between optimized tuning for the two pulleys is that you're able to increase timing about 1 to 1.5 degrees with the 2.9-inch pulley. Remember that the Radix warranty becomes void when you make any changes to the system.

Out first custom tuner was Bryan Herter (PCM for Less). He tuned our truck on the highway using LS1 Edit to optimize the job and to reprogram the PCM. First, he made modifications to adjust the L-trims (long-term fuel correction) and S-trims (short-term fuel correction) to zero, and he was able to practically zero the trims out. This means that the PCM tables were ideally calibrated for our truck and that no PCM fuel correction, i.e., learning, was necessary. He then focused on the timing and air/fuel ratio at WOT (wide-open throttle). He leaned the air/fuel ratio slightly (still conservative) and maximized the timing without any knock retard. Bryan also lassoed the torque management in the PCM. (Torque management limits the stress on the drivetrain during shifts and other abusive maneuvers by pulling timing out.) Our transmission modifications enabled us to drastically undue the PCM's torque management, as noted by the 12.85 e.t. at 102.63 mph.

Our second custom tuner was Allen Nelson (Nelson Performance), who provided a tuned PCM via the mail. Allen's approach was similar to Bryan's and the final tested results showed that they were very similar with their final tune. Since he wasn't able to perform in person, the L-trims and the S-trims weren't exactly zeroed, but he also did well, providing an e.t. of 12.86 at 101.95 mph. The purpose of using two tuners was to show what could be done with programming and to provide you with two custom tuners from across the United States.

VaraRam has designed a new ram air/cold airbox air scoop that takes its cold air from in front of the radiator at high speeds and gets its low-speed air from below, similar to the original cold airbox that we tested in Part III. The installation was easy. First, install the cold airbox as before and rotate the box so the ram air input is facing up. Then remove the pushpins from the facia piece above the radiator and remove the facia piece. Finally, install the ram air scoop assembly over the cold airbox, clamp the ram air scoop to the cold airbox with the supplied hose clamp, and attach the air director to the metal supports. VaraRam's new ram airbox resulted in an e.t. of 12.82 at 102.82 mph with the production air filter, and 12.79 e.t. at 102.89 mph with the race-only air filter.

Heat is a performance-killer, so we decided to do a couple of our own modifications to address it: a circuit that keeps the intercooler pump running for a short while after the key is turned off and a study of intercooler pump speed versus intercooler efficiency.

To meet our first concern, we designed an intercooler pump circuit that keeps the pump running for about 10 minutes after the ignition is shut off. This helps cool the supercharger to reduce the heat soaking. The 1-ohm resistor is rated for 10 watts and should only be inserted into this wire if you want to slow the intercooler pump speed down. We installed the circuit under the accessory fuse box in the engine compartment. In the circuit, the 1,000uF capacitor gets charged when key power is applied. The voltage on the capacitor keeps the IRF510 MOSFET transistor turned on, which powers the relay's coil. When you turn the key off, the capacitor has to discharge (when discharged, the MOSFET and the pump relay turn off) through the 470k-ohm resistor, which takes about 10 minutes. If you don't want the pump on as long, use a smaller-value resistor instead of the 470k ohm. The pump draws about 2.5 amps, which is about equal to that taken by the radio at mid-volume level.

Our second modification dealt with the intercooler pump speed. The supercharger draws in outside air and compresses the air into the intake manifold where the air becomes heated due to the squeeze. The temperature of the high-pressure air is then cooled by the intercooler before it enters the cylinders. A pump circulates water from the intercooler to a heat exchanger (radiator) and as the vehicle moves, the outside air cools the water in the heat exchanger. The tricky part of designing an intercooler system is optimizing the coolant flow rate. The heat exchanger likes a slow flow rate to let the outside air cool the water. However, the intercooler likes a high-flow rate to pull as much heat from the warm air as possible. If the flow rate is too low, the coolant in the heat exchanger gets cool and the coolant in the intercooler stays too hot, which reduces overall efficiency. If the flow rate is too high, the coolant doesn't stay in the heat exchanger long enough to cool down, which also reduces the overall efficiency. We needed to find the happy medium.

For our intercooler pump speed study, we used a separate power supply to adjust the pump voltage while we drove the truck and monitored the inlet air temperature (intercooler air temperature). We adjusted the intercooler pump voltage to control the coolant flow rate. The first test was to vary the intercooler pump voltage while we cruised at a normal highway speed (70 mph), and during this test the supercharger was not producing any boost. We basically saw that the inlet air temperature reached about 30 degrees above the outside air regardless of pump speed. We even turned the pump off and the intercooler air temperature only rose a few degrees, showing that while not producing boost, the intercooler air temperature is cooled mostly by the incoming air and the air outside the supercharger's housing.

We performed the second test on the highway but it was more demanding. When the intercooler air and the engine coolant temperature had reached a steady-state value, we stopped and did a 0-70 mph WOT blast. After we reached 70, we engaged the cruise control to maintain that speed. During this time and for several minutes following, we logged the intercooler temperature using AutoTap. We repeated this testing for various intercooler pump voltages in 0.5-volt steps from 7 volts to 14.05 volts (stock). The graph shows the inlet air temperature versus time for the normal pump voltage of 14.05 volts for our system's optimized pump measurement of 12 volts, and for a lower-than-optimal pump voltage of 8 volts. The data in this graph begins directly after our WOT blast and shows the next 30 seconds of data. You can see that running the pump at 12 volts (providing about a 30 percent lower flow rate than with 14.05 volts) results in faster cooling than at the normal 14.05 volts.

Running the pump at 14.05 volts (fast coolant flow rate) and 8 volts (slow coolant flow rate) took about 30 seconds to drop the air temperature 24 degrees. However, running the pump at 12 volts only required about 18 seconds to drop the same 24 degrees. To reduce the pump voltage to about 12 volts, we inserted a 1-ohm resistor in series with the one of the intercooler pump leads as shown in pump circuit schematic. When you decide on a mounting location, remember that this resistor will get warm. Our intercooler system uses two heat exchangers, and the hot coolant was first routed to the lower heat exchanger then to the upper heat exchanger. We used a coolant mixture of 25 percent antifreeze, 75 percent distilled water, and some Water Wetter. Your optimum pump voltage will depend on your particular setup.

For safety reasons, we opted for tires with a higher speed rating and the same load rating. Goodyear's Bob Toth recommended the 285/50HR20 Eagle GT II to replace the stock 275/55SR20 Eagle LS, which are good for up to 112 mph. The new meats are HR-rated and specified for 130. Although they are marginally smaller in diameter, they shouldn't create a speedometer/odometer problem.

Dyno Testing
We performed dyno testing after all of the Part IV modifications. Previously we dyno'd in Second gear because in stock form the speed limiter wouldn't let us wind out Third gear. The corrected horsepower and torque curves as previously published and after Part IV with the VaraRam airbox (with the race-only filter) are shown in the graph. As before, the more aggressive transmission shift kit we'd installed made it difficult to catch Second gear at low rpms without downshifting, and this is why the horsepower and torque curves look odd below 4,000 rpm for parts III and IV. We picked up more than 20 hp and about 32 lb-ft of torque at the wheels. Third-gear dyno results would be about 5 percent higher than the Second gear results. If you have an OBDII scanner and a flat-level road, you can measure your own truck's performance. Log the mass airflow sensor rate (in g/sec) and engine rpm. For these trucks it works out that the g/sec value is very close to the wheel horsepower.

Conclusions
How does the Silverado SS's 12.79 e.t. compare? It's faster than an Audi $85,000 RS 6, a $177,000 Ferrari 360 Spider, and is less than 0.1 seconds slower than the $57,000 Corvette Z06. The killer 4.3 second, 0-60 mph blast (partly due to the 1.78-second, 60-foot time) is 0.1 seconds faster than the Corvette Z06, 0.1 seconds slower than Porsche's $118,000 911 GT3 and 0.2 seconds slower than Dodge's $84,000 Viper SRT-10!

Another good measuring stick is the Lingenfelter 427 twin turbo Escalade. The engine upgrade costs $55,959 (bringing the total cost to about $125,000!) and is able to propel the Escalade to a 12.7 e.t. You can repeat our modifications for about five times less and obtain nearly the same performance for a total cost (truck and modifications) that is less than the base price for a stock Escalade. Our gas mileage around town actually remained at 13.4 mpg compared to 11 mpg for the Lingenfelter hellion.

We began with a 5,500lb stock truck and a 15.63 e.t. at 86.37 mph and provided a roadmap for a 12.79 e.t. at 102.89 mph. During our journey we showed you what worked and what didn't. As a rough guess at our base horsepower we can ratio our final measured horsepower and our measured stock horsepower and multiply by the stock base horsepower of 345 (i.e., 441 hp/250 hp x 345 bhp = 608 bhp) while still maintaining respectable gas mileage. We've made more than 200 quarter-mile runs and have had countless undefeated street adventures with no drivetrain problems. So what happened when the Silverado SS lined up against the Ford Lightning (with intake and exhaust modifications)? Well, the Lightning crossed the finish line about a second after we did.