Last month we assembled the Mighty Mouse 302 small-block Chevy and discussed all the pieces that would help us make our goal. The key was the combination of a Vortech supercharger and ACCEL's EFI. Then came the critical moment: firing up the engine and listening to the beautiful noise. But would it make the power?
Right now you're probably thinking about fast-forwarding to the dyno charts to see how we did. We'll save you the trouble. The bottom line is that our little killer pumped out 549.2 horsepower at 5400 rpm on pump gas. In our book, that's 550 horsepower. Then we stuffed the engine in our Power Tour '65 Chevelle, tuned the part-throttle and managed to pull off 23.3 mpg on the freeway at 65 mph.
Our aim was 550 horsepower and 25 mpg, so we fell a little short of our mileage goal. But by the time Power Tour kicks off (at this point, roughly 40 days away), we might be able to squeeze that last smile per gallon out of the car. Now that you've had dessert, let's get into the main course.
POWER PROGRAMMING
If last month's engine buildup passed you by, here are the Cliff's Notes on our Mighty Mouse 302. We started with a CNC-machined GM Performance Parts Bow-Tie 4-inch-bore block, adding a Crower 4340 steel crank and Sportsman 6-inch rods to a set of Childs & Albert 9:1 forged pistons, Zero Gap Second rings and bearings. A Crane mechanical-roller cam and valvetrain move the valves in the Air Flow Research aluminum heads. For induction, we used an ACCEL SuperRam EFI manifold fitted with 36-lb/hr injectors sealed up with Fel-Pro gaskets. We designed the engine mild for mileage and relied on a Vortech V-1 centrifugal supercharger with an S-Trim impeller to supply the boost to make the horsepower.
With the 302 bolted on Ken Duttweiler's Digalog-controlled dyno, our first test after a 30-minute break-in period was to see how much power the 302 could crank out without the supercharger. This would give us a solid baseline to evaluate how much power the blower was actually contributing to the engine.
After tuning with the ACCEL/DFI electronics package to create the proper air/fuel ratio and spark curve, the little 302 cranked out an excellent 331 horsepower at 5000 rpm. The low peak-rpm point is due to the small 212/220 degree duration at .050-inch tappet lift camshaft. Still, this is 1.1 horsepower per cubic inch, which is an acceptable power level for this mild combination.
The critical part of creating a fuel and spark curve when testing a supercharged engine is preventing detonation, because increasing boost and air temperature out of the supercharger increases the potential for the dreaded death rattle. After discussing this with Duttweiler, we decided to do the initial spark and fuel development with 100-octane VP unleaded race gas. This would give us a more detonation-resistant fuel to allow room for mistakes and prevent a broken engine. In addition, John and Shirley Pizzuto of J&S Electronics contributed a slick knock-control system that we employed to keep the engine from doing the death rattle.
The J&S unit is called the SafeGuard Individual Cylinder Knock Control Ignition. It uses a GM detonation sensor to signal when the engine is experiencing detonation. When the ignition timing is close to ideal, perhaps only one or two cylinders will detonate rather than all eight. The J&S SafeGuard unit senses which cylinder is detonating and pulls a small amount of timing out of that individual cylinder, leaving the original timing curve intact for the remaining cylinders. We relied on this system to keep the engine out of detonation during our test sessions, and it did a superb job. Duttweiler said he probably would not have been as aggressive with the ignition timing without the use of the SafeGuard unit.
After a few hours and a bunch of dyno pulls, Duttweiler had successfully dialed in the spark and fuel curve. We had to be very conservative with the boost and spark timing, even with the race gas. After a few more dyno pulls, Ken brought out the best in our little blown motor. By experimenting with a smaller blower pulley, we could spin the blower a little faster and make up to 13 psi of boost. Carefully adding timing and leaning out the mixture using DFI's timing and fuel maps, Duttweiler squeezed a stout 566 horsepower at 5400 rpm with a peak torque of 561 lbs-ft at 5000 rpm.
Rather than continue to tune the combination on race gas, we decided to switch to 92-octane pump gas to see how much power we could make. Still running with 13.5 psi of boost and limited to 30 degrees of total ignition timing, the 302 cranked out 533 horsepower at 5500 rpm. Next, we bolted in a set of 1.6:1 Crane rocker arms and reset the lash at Crane's recommended .020 inch. That change alone was worth 16 horsepower at the peak and even more in the midrange. This pushed the power level to 549 horsepower at 5500 rpm, with boost still at 13 psi and still on 92-octane pump gas. That, folks, is 1.8 horsepower per cubic inch!
At this point, we had wailed on the engine long enough. Overall, the tests were a complete success. We made the power and didn't break a single part. Everything worked the way it should have, including the Vortech supercharger and the ACCEL/DFI electronic-fuel-injection system. In fact, it was the ability to easily tune the engine with the ACCEL/DFI EFI that helped make the dyno tests such a success. Now it was time to see if this mini-Mouse could squeak out miserly mileage as well.
MILEAGE MASTER
The second half of our small-block challenge was to see if this 550 horsepower engine could attain 25 mpg at 65 mph on the open road. We had previously worked with Vortech to ensure the blower would bolt into the car without modification.
The dyno testing dealt with only the full-throttle side of the ACCEL/DFI EFI spark and fuel maps. In order to squeeze that last ounce of fuel mileage out of the 302, we had to tune the part-throttle side of the electronic package as well. This involved connecting our laptop computer to the ACCEL/DFI computer and then fine-tuning the fuel and spark maps while driving the car. We shanghaied HOT ROD staffer Rob Kinnan to drive the Chevelle while I sat in the right seat manipulating the fuel and spark curves. This took a few hours to complete, but it was well worth the effort.
After the tuning session and calibrating the odometer to ensure our mileage figures were correct, we filled the fuel tank and hit the road. Our goal of 25-mpg highway mileage is based on pure highway mileage, based on an average speed of 65 mph.
What makes this mileage attempt even more of a challenge is the downright hideous aerodynamics of a '65 Chevelle. While there are cars with worse aero, the Chevelle is certainly among the bottom feeders in this category. While this may not seem significant, a cleaner car like a new Camaro or Corvette could easily be worth 1 to 2 mpg or more.
Just like optimizing dragstrip times, we also worked to help the mileage as best we could. For example, we pumped the BFGoodrich Comp T/A tires up to their highest recommended air pressure to reduce rolling resistance. We didn't have time to try all our tricks, but we do intend to add synthetic gear lube to both the differential and the Richmond six-speed box along with Mobil 1 5W30 engine oil. If you catch up with us on Power Tour, we'll be able to fill you in on all the details as well as our current mileage achievements.
As this story is written, the Chevelle achieved a best of 23.3 mpg based on one 300-mile trip. While we didn't quite meet the mileage goal, we've come very close. Best of all, none of our efficiency changes will affect our 550 horsepower potential. So we can cruise along at 23 mpg, instantaneously hit the throttle and put down 550 horsepower in the blink of an eye. This comes pretty close to our ideal definition of a hot rod—but only if it had just a little more horsepower!
