For last year's HOT ROD Power Tour, we set out to get more than 20 mpg from a 425hp 355ci small-block Chevy. Within days of leaving, we stuffed this engine in a '65 Chevelle graced with a Lingenfelter-designed SuperRam EFI intake, which was controlled with an ACCEL/DFI electronic-fuel-injection system. And for the ultimate in highway cruising, we backed it up with a Richmond six-speed overdrive transmission.
The Chevelle successfully averaged more than 21 mpg for the Tour, which was an achievement considering the minimal tuning effort that went into the combination. This challenged us to come up with a small-block Chevy that was capable of both more power and better mileage. Boldly going where few have gone before, our goal became 550 horsepower combined with 25-mpg fuel economy in that same '65 Chevelle for Power Tour '96. Given that a '65 Chevelle offers an aerodynamic envelope considerably worse than an equal-size brick, these are lofty goals. This story will detail the buildup of this hopped-up mini-Mouse. Next month we will dyno test it, drop the motor into the car and see how close we come to our mileage bogie.
PROJECT PLANS
After much pencil-pushing theory, an engine plan began to take shape. Most hot rodders believe that as power increases, part-throttle fuel mileage decreases. But if you design an engine correctly, a high-horsepower, fuel-efficient engine is certainly possible, especially with the help of forced induction and EFI. To enhance fuel economy, a very mild, small-displacement (302 cubic inches) engine would be needed. But since that design would never make the horsepower normally aspirated, we needed a supercharger to push the power up.
After much deliberation, we decided on a centrifugal supercharger, given the centrifugal's superior efficiency over most Roots-type blowers. Of equal importance is the centrifugal's low profile and easier installation, which places it under a stock hood line and greatly increases the subterfuge factor. Best of all, the centrifugal supercharger is a natural when combined with electronic fuel injection.
In an attempt to further improve efficiency, we decided on a short 3-inch stroke for less piston travel and a short-duration mechanical-roller cam. We chose a mechanical roller over a hydraulic roller because the mechanical cam has a shorter ramp design, which means the cam could be smaller than a comparable hydraulic-roller lobe.
POWERPLANT PRANKS
While we could have used a stock production 350 block, we decided on one of GM Performance Parts' new cast-iron Bow-Tie cylinder blocks. This particular block is one of the new CNC-machined blocks used by the Winston Cup teams. The block comes fully machined except for the finish bore size. It also features 8620 steel four-bolt main caps on all five mains and a revised priority-main oiling system. While this block is not cheap, it offers exceptionally thick cylinder walls for good ring seal and considerable machining-cost savings over the standard cast-iron Bow-Tie block that is also still available.
To ensure that our smallish Mouse motor would be capable of handling the power output, we chose a Crower Sportsman 4340 steel 3-inch-stroke crank and a set of Crower Sportsman 4340 steel connecting rods to complement the Bow-Tie block. Since we thought the engine may have to rev as high as 6750 rpm in order to make the power, we felt that these much stronger bottom-end pieces would give the engine the beef to run these rpm levels with ultimate reliability.
For pistons, we chose a set of Childs & Albert (C&A) flat-top forged-aluminum racing pistons. To help the efficiency game even further, we sent them to High Performance Coatings (HPC), where owner Jeff Holm coated the tops of the pistons with a ceramic thermal-barrier coating. He also applied a wettable matrix SDF-1 coating to the piston skirts to lower the friction between the pistons and walls while improving oil retention. The thermal-barrier coating will help retain heat in the chamber while also stabilizing the temperatures across the face of the piston.
In a previous HOT ROD dyno test story ("Closing The Gap," March '94) we found power by using a set of Childs & Albert 1/16-inch Zero Gap Second (ZGS) rings, which dramatically reduce leakage past the second ring. Combined with a moly-faced top ring and a low-tension 3mm oil ring is a set of C&A rod, main and cam bearings. The C&A bearings were also narrowed to clear the larger-radius fillet of the Crower crank.
Cylinder heads play a crucial role in ultimate power, and we've had excellent results with Air Flow Research (AFR) aluminum heads in the past. While it would have been easy to spec a fully ported set of 190 or 195cc heads from AFR, we elected to stick with a set of off-the-shelf 190 heads fitted with AFR's stainless-steel 2.02/1.60-inch intake and exhaust valves. Crane recommended a set of big-block springs to complement the roller cam, and we added a set of Crane titanium retainers to lighten up the valvetrain.
If the cylinder heads are the heart of power for any engine, then the camshaft must be the brain. Proper cam selection was doubly difficult since the cam had to meet two opposing goals. After discussing the problem with John Lingenfelter, he suggested a short-duration timing profile that would also complement the supercharger. We chose a Crane Cams mechanical-roller cam with 212/220 degrees of duration at .050-inch tappet lift, with a lobe separation angle of 115 degrees and .470/.489-inch lift on the intake and exhaust respectively.
The lobe separation angle is especially critical in this engine. Lobe separation angle is the number of degrees of separation between the intake and exhaust lobe centerlines. A "tight" lobe separation angle of 110 degrees or less creates more valve overlap, which helps create that lumpy idle characteristic of big camshafts. Since the overdrive transmission would dictate an 1800-1900-rpm engine-cruise speed, excessive overlap would only hurt mileage. So the lobe separation angle, even on this short-duration cam, was widened to 115 degrees to minimize valve overlap.
Previous experiments with supercharged engines seem to favor camshafts with less overlap. Apparently this minimal overlap prevents the pressurized incoming charge from blowing right out the exhaust valve rather than filling the cylinder. So limiting the valve overlap was one condition that could benefit both causes. Unfortunately, the short cam timing can only hurt the high-rpm horsepower potential, which means we might have to spin the supercharger faster to create more boost in order to compensate. This is where the advantage of the excellent AFR heads should help by improving airflow into the engine even with a short-duration camshaft.
Last year's Power Tour engine sported an ACCEL SuperRam intake, and we decided that this little-block could use all the torque-enhancing tricks it could get, so we added the SuperRam to this blown package as well. The combination of boost and the long-runner-length SuperRam should be worth some torque in the midrange.
PIECES IN PLACE
Assembling this small-block turned out to be a challenge. While we could have farmed this job to a professional engine builder, the author wanted to take on the greater challenge of assembling the engine in his crowded two-car garage. All the Bow-Tie block needed was to be honed, since it was within a few thousandths of the standard 4.000-inch bore size. Precision machine work was ably accomplished by our friends at Jim Grubbs Motorsports in Valencia, California, and included torque plate honing, balancing the crank assembly and milling the tops of the lifter bores to create sufficient clearance for the Crane roller tappets. In addition, Grubbs also suggested that we have the crank snout and the damper broached for a second keyway in anticipation of the loads presented by the crank-driven supercharger.
The Bow-Tie block comes partially drilled for a right-hand dipstick, which required the use of a matching Chevrolet right-hand-dipstick oil pan along with a modified '69 Z/28 windage tray. Four of the original ARP main cap studs in the block had to be substituted with longer ARP windage-tray studs. These longer studs complicated the torque sequence required by Chevrolet on the four-bolt main caps, since the longer studs prevented using a socket to torque the outer splayed main-cap bolts. A complete set of ARP fasteners was used to assemble the engine, including rod bolts, main studs, head bolts, oil pan studs and a complete upper engine stainless bolt kit.
We also customized the Crane roller-cam endplay by using a Moroso fiber-tipped cam button. We needed this type of button to prevent damage to the Edelbrock aluminum two-piece timing-chain cover. This handy two-piece cover allows quick access to the timing chain area for cam timing changes without having to drop the oil pan. This was completed with the addition of a Pro/Race SFI-approved damper.
Once the short-block was assembled, the AFR heads were bolted on with stainless-steel ARP head bolts and sealed with a pair of Fel-Pro composition head gaskets. With a tight .005-inch deck height (the distance from the top of the piston to the block deck surface), a 64cc chamber and the .041-inch compressed thick Fel-Pro head gasket, the compression ended up at 8.9:1, which we felt was also a compromise between good part-throttle efficiency and cylinder pressure under boost with the blower.
With the SuperRam intake in place, it was time to install the Vortech supercharger. Previously, we had sent a small-block Chevy with a SuperRam intake installed to Vortech to mock up a supercharger mount and drive system to ensure that the blower package fit properly on the engine. Initially using an '88-'92 F-Car TPI mounting bracket, Vortech fabricator Jay Chance modified the mount and the drive system to align with the SuperRam throttle body. Vortech also spec'd an eight-rib beltdrive to reduce belt slippage and combined that drive with a set of V-belt pulleys from March Performance. ARP also supplied a high-strength stud to snug the pulleys up to the damper.
The supercharger Vortech recommended was a V-1 model, an internally gear-driven centrifugal supercharger using Vortech's latest S-trim impeller design. Originally, all Vortech blowers were fitted with A-, B- or later, R-trim impeller designs. The S-trim is Vortech's newest impeller design that is designed for street use yet offers many of the higher-airflow advantages of the race-only R-trim. The idea is to generate higher boost through more efficient and greater airflow without dramatically increasing the blower's output air temperature. Higher outlet air temperatures reduce air density while also increasing the engine's detonation sensitivity.
CONCLUSION
We really wanted to show you the test results of our littlest small-block to complete the story, but instead we're going to leave you hanging. You'll just have to wait until next month, when we put Mighty Mouse on Ken Duttweiler's dyno and then stuff it in the car and test it for mileage.
