Upgrading an existing cast-iron block-based engine to an aluminum engine block is a great way to reduce the weight on the front area of a vehicle. It makes it easier for the vehicle to accelerate, brake and corner, adds to the cool factor, and possibly can lead to an increase in power output. The aluminum engine block requires preparation and assembly methods that are a little different from a cast-iron block, but you're about to learn a lot of those details here. So get ready to go light.
An aluminum block "expands"...
An aluminum block "expands" twice as much as an iron block (because aluminum has twice the expansion as cast-iron), so the crank main journal clearances are set at or under 0.002 inch (versus the 0.0025 to 0.0027-inch range for an iron block). This tight clearance when the engine is cold demands quality oil flow at startup, and also that you warm up the engine thoroughly before beating on it. As per the Bow-Tie info card, the main webs were torqued to 65 lb-ft on the vertical bolts, 60 lb-ft on the splayed bolts, and 45 lb-ft on the two little bolts at the front main cap.
Setup: This engine is out of Mark Stielow's Thrasher '69 Camaro Pro Touring ride that he built back in 1999, and will be going into Jim Mulvey's Camaro in place of a supercharged big-block. The improvement in overall weight and front/rear bias is going to be dramatic. The original small-block Chevy engine made 520 hp, 510 lb-ft of torque based on a 406ci cast-iron block with unported 23-degree cylinder heads, open-plane intake, and an ACCEL fuel injection system.
Stielow has decided to upgrade the engine with a GM Performance Parts Bow-Tie aluminum block, Dart CNC-machined 18-degree cylinder heads, and some new valvetrain components to take advantage of the new high-flowing heads. While he drove the decision to go lightweight and improve the flow, the experts at Wheel to Wheel Powertrain (W2W), in Madison Heights, Michigan, weighed in on the details of upgrading the existing engine.
Assembly: As far as foundation choices go, it doesn't get much better than the GM Performance Parts Bow-Tie aluminum block. It is a beautiful piece of equipment with a near-surgical appearance in the cast and machined areas. To add to the mystique, the block is shipped in a special wooden crate with paperwork on the sonic testing so you can "see" the quality of the construction. The aluminum Bow-Tie block has pressed-in cast-iron cylinder liners that can be machined to a maximum diameter of 4.185 inches. But this engine was originally built with 4.125-inch bores and the original Arias pistons look practically brand-new, so the block is being machined to 4.125-inch bores.
An aluminum block has thicker...
An aluminum block has thicker main webs than an iron block, which requires the crank counterweight throws to be machined 0.100 inch at the outer edge, and tapered back to the bearing surface. This requires rebalancing the crank-which you have to do for the piston-and-rod combination anyway. Just make sure to do the backcut before the initial balancing.
Head Games: The aluminum, 18-degree Dart cylinder heads just looked too good to not try. The reference to the "18 degrees" is the amount the valves are "angled" from the centerline of the cylinder bore. While the Gen I small-block Chevy V-8 originally came out with 23-degree valve angles, over the years racers have determined that reducing that angle allowed them to run smaller combustion chambers, improve the angle of the intake port, and generally make more power. The 18-degree angle was pioneered by racers in NASCAR and remains as a common upgrade in performance small-blocks.
Making the change to 18-degree heads requires a new intake, or at least serious machining of an intake built for a 23-degree head (which really isn't recommended, as the intake port shape will not take full advantage of the 18-degree ports). Also, a "spacer" will usually need to be added to the "China Wall" area at the front and rear of the engine. Valvetrain changes will abound to achieve "correct" valve geometry. You'll probably want to replace the camshaft to get the most out of the new heads. Expect to make changes to the fuel and spark application, as these heads usually flow better than any 23-degree head you've run.
The real improvement came once this engine was bolted into Jim Mulvey's Camaro. Stielow reported to us that you could feel the change the lightweight block and high-flowing cylinder heads made to the power and nose weight of the vehicle. The resulting faster acceleration and a more "flickable" performance makes all the work required to make this change worth it.