By Archie Bosman:
Over the years most of us have toyed with the notion of acquiring a desirable engine or two and sometimes, foolishly, we’ve asked the question, “Which one would you recommend?” Of course the inevitable answer comes: “It depends on what you’re trying achieve.”
Often that answer leads us to even more uncertainty than when the conversation began. And worse, insiders would often start blustering on at great length about displacements, bore and stroke specs, connecting rod lengths, intake valve sizes and so on, leaving the average enthusiast looking on in bewilderment.
To bring some simple logic to the complexities of this topic, examining deck heights is probably the best starting point, for everything else seems to be determined by it. And since World Products is currently engaged in the introduction of a range of new engine blocks it seemed an excellent time to find out…why so many?
Though their latest replacement range of four engine blocks is designated for Fords, most of the fundamentals that follow apply to any range of engine blocks regardless of their origins.
To begin with, deck heights are measured from the crankshaft centerline to the block deck. Usually this measurement is captured by some form of dial gauge caliper. Here the engine builder uses a steel ball as an aid to obtain an accurate reading.
Then he deducts the diameter of the ball and adds half the crank mains journal diameter. On a Kaase Boss Nine engine, for example, the diameter of the mains journal is 3.193in. Therefore, to establish the block deck height he takes his dial gauge measurement, subtracts the steel ball dimension, and adds 1.5965in.
With help from World’s engine block designer, Dick Boyer, here in descending order of deck heights are the main statistics to be considered in order to satisfy the demands of today’s race engine builder.
Why a 9.5in deck?
In the small-block Ford arena, creating a 9.5in deck height addresses the desires of those who favor small blocks with large cubic inch displacements (cid). Street performance enthusiasts and drag racers cherish big small-blocks, particularly those of 400cid, 427cid and 454cid. In fact, drag racers favor a slightly shorter connecting rod (6.250in or 6.200in) and a little more compression height on the piston. Compression height is the dimension from the pin center to the deck of the piston and additional height in this area allows them to locate the piston rings slightly lower on the piston body, thus increasing the robustness of the ring lands to combat power-adder forces.
But short track oval racers have use for neither power-adders nor large displacement because they cannot use them—the futility of having more power than traction—as they say: “we cannot hook it up”. Plus a 9.5in deck block is probably too heavy to be competitive on the short track ovals.
“However, if you have access to a 9.5in deck and wanted to run it in 400-plus cu in form,” says Boyer, “you’ve got to use a 6.300in connecting rod or a long piston that likely weighs 500grams”. Although efficient intake manifolds are in abundance, neither option is ideal for the circle track racer. In fact this was Boyer’s chief motivation for creating the new 9.2in deck block.
The 9.2in deck satisfies circle track racers who run 415cid and 420cid engines, because the shorter deck eliminates the handicap of running longer heavy pistons. Encouragingly, this block uses the 302 mains journal, the diameter of which measures 2.248in compared to the heftier small-block Chevrolet’s 2.450in. This introduces a lighter crankshaft and with it comes the benefits of a more desirable rotating mass.
Displacing 415cid to 420cid, this block is ideally suited to a 4in stroke with a 6.2in connecting rod and therefore a lighter piston. CP-Carrillo’s Bryce Mulvey says that because the pin is located higher on the piston there is less skirt thus less weight. “Pistons weighing 350g are available,” he says, “but for durability, most engine builders aim for pistons that weigh in the low- to mid-400g.” Indeed, some circle track sanctioning bodies mandate 460-gram minimum piston weight.
Building a popular 360cid engine using a 9.2in deck with a 4in bore requires a 3.500in stroke with a 6.125in connecting rod. Longer strokes develop more torque and place less piston pressure on the cylinder walls; shorter strokes promote faster acceleration—and mindful of this, World is currently in the final stages of development of an 8.7in deck that accommodates an even shorter stroke.
This new 8.7in block will appeal to the 360cid racers. Scheduled for December 2015, its unveiling is planned to coincide with the PRI show. This unit will accommodate a 4.125in bore with 3.335in stroke (356cu in). “Engine builders like this combination,” according to Boyer, “they prefer the 5.850in connecting rod, which allows the engine to accelerate faster. Additionally, the bore size allows them to introduce a bigger intake valve, making more efficient power.”
Naturally, the shorter deck not only lightens the engine but also the weight is positioned lower in the car. Because the deck height is shorter, so, too, is the intake manifold. In common with the 9.2in, the 8.7in deck block, uses 302 main bearings (2.248in diameter).
Alternatively, this engine also lends itself to a longer 3.625in stroke, thereby taking full advantage of the taller deck. Combining a 4.125in bore with a 3.625 stroke delivers 387cubic inches.
Finally there’s the delightful 8.2in deck block. Delightful for circle track racers because they can reach 368cid by using a 4.155in bore x 3.4in stroke with a 5.4in connecting rod. The piston speed is fast, the block is light, the rod-to-stroke ratio is desirable and intake manifolds are readily available.
The formula to calculate displacement is as follows: bore x bore x stroke x .7854 x no of cylinders = displacement.