IHRA Chevy Big Block - From Street To Strip

Stock heads must be retained for Top Stock, which is just fine for this engine, as the 502's oval-port 356-T6 aluminum heads boast 2.25-inch intake valves and 1.88-inch exhaust valves. The springs can be changed as long as the replacements are the stock diameter. In this case, a heavy-duty set of dampening springs was added to stand up to the more aggressive camshaft and the engine's high-rpm lifestyle.

No porting is allowed, but a three-angle valve job is okay. In this case, it was performed on McLaren's state-of-the-art Serdi 2000 "valve seat and guide machining center." The heads were also flow-tested to help determine the camshaft specs.

The rules stipulate the heads can't be ported. The 502's oval-port heads (PN 12363399) have good-sized 290cc intake runners and 110cc exhaust ports, along with 2.25/1.88 valves. With the stock-type, small-top pistons, this motor's compression ratio is 9.84:1. That's great for the street, but a race engine could always use more. Unfortunately, the rules forbid reducing the combustion chamber volume, which would increase the ratio.

IHRA says combustion chambers must remain stock, which, in the case of the Chevy 502, is 110cc. So, McLaren cc'd the chambers and discovered some slight inconsistencies between them. The heads were resurfaced slightly to true-up the chambers to ensure they were all at 110cc (most were slightly larger).

Engine builder Curtis Halverson carefully drops the cylinder heads in place. Like the main bearings, the cylinder heads are installed over studs. Again, the idea is to limit the amount of wear and tear on the block if the heads need to be removed.

The lifter bores are drilled out and fitted with stronger brass inserts. The inserts are indexed, too, to ensure alignment with the cam's lobes. Unlike a small-block, the big-block's lifters aren't all on the same plane, so lining them up correctly will reduce friction. Also, the inserts are drilled to match the oil flow holes in the block.

Although the lifters are stock, the stock, stamped steel rocker arms are replaced with GM's (PN 12361323) 1.7:1 roller rockers. The bodies of these rockers are aluminum, but the pushrod seats are 8620 steel.

The 502 engine package comes with a dual-plane intake manifold. For racing it is replaced with a single-plane Edelbrock Victor manifold that matches the heads' oval-port design. Check out the valve covers, too. They're needed to clear the big, 1.7-ratio roller rockers. The stock valve covers are too low.

To limit the cost of competition in Top Stock, IHRA mandates that each carburetor has a choke. This effectively limits carburetor selection to off-the-shelf stuff (with some minor tweaks or blueprinting). For this engine, a Holley 850 Double Pumper was used. On the dyno, it performed well with the factory-fit #68 jets. The floats were simply set up, and the engine starting making full-throttle pulls. Also, the 1-inch phenolic spacer is IHRA-legal.

Though a seemingly simple part, the oil pan of this Top Stock engine represented dozens of hours in design and construction. It was designed specifically for the chassis of Monty Bogan, Jr.'s '69 Camaro. Within those dimensions, however, the pan is shaped to eliminate crankshaft windage, and, knowing how little oil will likely be used, supply every possible drop of oil to the pump pick-up. Although a wet-sump oiling system, the final design was influenced by the shape of NASCAR-style dry-sump pans.

Like the oil pan, this engine's headers involved hours of careful fabrication (by Paul Swindelhurst of McLaren's fabrication shop). The tuned, equal-length design was based both on the headers' fit in the car and the approximate horsepower of the engine. Even the cylinder heads' flow was factored into the design and tubing diameter. Also, they were designed for easy spark plug access. The 23-inch-long headers boast 2-inch primaries and 3.5-inch merge collectors.

Strapped to one of McLaren's Froude water-brake dynos, the Top Stock big-block was tested between 2,500 rpm and 7,250 (ignition system is MSD throughout). The engine was run with AC R42XLS spark plugs, Mobil 1 10W30 oil, and this timing curve: 15 degrees at 1,000 rpm, 20 at 1,500, 24 at 2,000, and 36 degrees from 2,500 to 7,500 rpm. With less than 10:1 compression, this big-block makes more than 430 lb-ft at 2,500 rpm. From there, it's a smooth, gradual progression to its peak at 5,300 rpm, where it's twisting out 597 lb-ft. As for horsepower, it's a sharper rise, from just more than 300 horses at 3,000 rpm to 657 horses at 6,100 rpm. In all it's a relatively simple, but carefully thought-out, combination that should push Monty Bogan, Jr.'s yellow Camaro into the winner's circle.