High Performance Chevy Engines - Dyno Freak

Big boost power is the name of the game, and this month we're bringing the high-performance Chevy crowd a quadruple-digit power play. The test subject is a 496ci Rat witha Comp valvetrain, Dart induction, a Carb Shop carburetor, and an F2 Procharger capable of moving enough air to make a category five hurricane jealous. We wanted to see what a concoction of such magnitude could potentially deliver on an engine dyno, so we commissioned the Westech dyno facility to give our boosted beast a run across its Superflow polygraph.

After many months of preparation, the engine finally made its way onto a dyno cart, only to reveal a number of hurdles yet to overcome. The first was interference between the F2 blower case and the driver-side valve cover. Our immediate reaction was to warm up the TIG welder and clearance the valve covers, but thanks to the quick thinking of one of the guys in the shop, we tried a pair of aluminum Moroso pieces. Sure enough, they fit perfectly and we were ready for the next challenge.

Next we learned that ProCharger's 8mm cog belt wouldn't allow us to fit an electric water pump overthe Comp Cam beltdrive system. So the guys at Westech temporarily rigged up a boat-style setup that carried water directly from the dyno water tank to the engine. If you run into a similar situation for the street, you could use a block-off plate with AN fittings, allowing you to place a remote water pump anywhere you'd like. Matter of fact, most if not all are required to do this, unless you're utilizing a factory timing cover. In our case, we opted for Comp's beltdrive, since it would allow us to advance or retard the timing in a matter of minutes and make for easy cam swaps.

Last but not least, we had to fabricate a 311/42-inch aluminum pipe from the F2 head unit to the 4150-style Holley HP carburetor. We contemplated incorporating an air-to-air intercooler but found it too time-consuming for this month's editorial deadline. Instead, we funneled the air directly from the blower to the carb with a bypass valve (surge valve) in between. After measuring the proper length and cutting it, we bead-rolled the outer lips of the aluminum tubing and covered them with high-pressure hoses. We used a 77-tooth cog pulley at the crank and a 48-tooth cog pulley up top at the blower.

Before attempting our baseline pull on the motor, we added 7 quarts of Lucas 10W30 to our deep-sump Milodon oil pan and pre-oiled the rotating pieces. Up top, an out-of-the-box 750-cfm Holley HP series carb went on top of the Dart single-plane manifold. After a few minutes of warm-up and a quick run of the valves, we made a baseline pull of 569 lb-ft of torque at 5,200 rpm and 631 hp at 6,200 rpm.

Once the naturally aspirated baseline numbers were steady, Bob Vrbancic from The Carb Shop out of Ontario, California, came down with one of the shop's modified 750-cfm boost-calibrated HP-series Holley carburetors. He first connected it to our homemade aluminum tubing and added a few gallons of 114-octane race fuel, giving us a safety margin until we knew exactly the type of boost we were going to generate and to adjust both the fuel and timing curves. For the first pull, we again proceeded with a quick warm-up and dipped into the throttle, only to shut it right back down-it's hard to describe, but the noise we heard didn't sound good.

Entering the dyno cell, we checked the fuel pressure, and it was where we had left it at 7 psi, the timing was locked out at 34 degrees (see MSD sidebar), and plenty of fresh air was entering from the fans above. Everything appeared to be OK, so we headed back into the control room and fired the engine back to life, again. This time around, we decided to stay in the throttle just long enough to record a quick data shot at 3,500 rpm to see what was going on. Suffice to say, the F2-packing big-block wasn't happy, bucking and kicking like an angry horse. What data we did gather revealed roughly 855 lb-ft of torque at 3,500 rpm; however, it was ridiculously lean. The problem: fuel starvation-and we traced the leaks to the boost-referenced fuel regulators.

For the third round, we ran into a different situation; the engine lurched forward, out of control, toward us, making us shut it down again. So the motor was down before we'd recorded any data, but we definitely fixed that fuel problem. Strange as it may be, how do you fix a walking dyno? Simple, with two car-style trailer hold-down straps. In all our years of testing, this was a new one for us. We strapped one end to the engine cart and locked the other into the concrete floor anchors.

Once again, we checked the engine's vitals and fired it back to life. Ready as ever, the plan was to have a short pull from 4,000 rpm to 5,500 rpm to see what we had to work with. With our ears plugged and fingers crossed, we recorded an unbelievable 1,100 hp at 5,400 rpm and 1,066 lb-ft of torque at 18 pounds of boost. Even more impressive is that the powerband looked more like a nitrous curve, shooting the power straight up.

The good news about our short dyno pull is that it cut off at 5,500 rpm, just before the power fell dramatically from insufficient fuel pressure. It was also good that we ran 114-octane race fuel or else there was the possibility of causing a bit of damage. As for the fuel pressure, our fuel pump was only capable of 25 pounds, and when 7 psi of initial fuel pressure was added to the 18 pounds of referenced fuel demand, that was all she wrote for the pump. While we were extremely pleased with the dyno results, we also knew our factory two-bolt block's days would be numbered at these power levels, so rather than scrambling for a larger fuel pump and increasing power, we headed in the opposite direction, looking for less boost and to ultimately tune it for the street.

To slow the blower down, we didn't want to stray from the cog setup and added a larger 54-tooth blower pulley and left the high-octane in the jug, since we still weren't sure just how radical the F2 was going to be. With lower boost levels, we knew we could run it up a bit higher on the rpm range, so for the next run we upped the ante to 6,000 rpm and were rewarded with 955 lb-ft of torque and 1,091 hp at a rather high 16 pounds of boost. At 5,400 rpm, we saw slightly less boost, 12 pounds, which made 927 lb-ft and 953 hp.

As much as we wanted to keep flogging the mill, we realized that without being able to slow the blower down, it wouldn't be feasible to put our combination in 91-octane trim. With our two days on the dyno coming to an end, we reviewed the two previous dyno pulls for what could be learned. Foremost, the F2 ProCharger is a serious kick-in-the-pants piece with the potential to make some big numbers, which it's already proven. With a max impeller speed of 65,000, our initial 18 pounds of boost at 5,400 rpm was only spinning the blower at 46,775 rpm, producing just 72 percent of its total power potential. To reach the F2's true maximum power output, we would have to spin our Rat to 7,500 rpm, which is way beyond this engine's capability. Say we had a Dart block with the forged internals; you better believe this baby would put the wow factor well in the neighborhood of 1,500-plus horsepower. How sick is that?

MSD Digital-7 W/Boost Retard Curve
With the support of an MSD Digital-7 ignition control box, we were able to include a special boost timing retard curve that allowed our engine to run the maximum amount of timing from idle to peak rpm. The Digital-7 with boost retard control uses a MAP (manifold absolute pressure) sensor to detect boost in the engine and then references a user-built timing retard program that will decrease the engine's total timing by as much as 1 degree for every 11/44 pound of detected boost.

Fuel, Timing, And Air Temperature
The three key ingredients to making power from any engine are fuel, timing, and inlet air temperature. The least fuel matched with the most timing and the coolest air charge will always net the most power.

Cold, dry air is packed with the most oxygen possible, and oxygen makes combustion. And the fewer fuel molecules present the more room there is for oxygen. Therefore, a powerful engine will compress as many oxygen molecules into a cylinder as possible while mixing with the minimum amount of fuel necessary. When this mix happens, a perfectly timed spark is required to light the compressing mixture as soon as the piston reaches top dead center (TDC). If the spark occurs too late, then maximum compressible power will be lost, and if it occurs too early, the piston will have to fight its way to TDC, placing the engine's internal rotating assembly under extreme stress, also known as detonation.

Detonation will occur if any (or all three) of these key variables is mismatched, and it is the number-one leading cause of death amongst power-adder engines. Supercharged, turbocharged, or nitrous-fed, they will all give up the ghost if the engine's tune-up isn't spot on.