Stroke, Draw, And General Engine Characteristics
Big cubes make big power, but more to the point, a long stroke makes huge grunt. For an off-road mill, you can mitigate the tradeoff in low-end torque associated with a big cam and a single-plane manifold by using an engine with a long stroke-like the Olds 455. Compared with a 350 Chevy's 3.48-inch stroke, a 454 Chevy's 4.0-inch stroke, a 455 Buick's 3.90-inch stroke, and even a 500 Cadillac's 4.060-inch stroke, the Olds 455 enjoys a monster 4.25-inch stroke. To duplicate that with most other brands means stepping into an aftermarket stroker crank.
What's it translate into? According to Mondello's general manager, Lynn Wellfringer, "You just can't duplicate the torque curve of an Olds big-block with any other brand." Brul indicated that our dyno curve was akin to that of a similarly equipped Chevy 454, only our numbers appeared 1,000 rpm lower. That's outstanding.
The key lies in that monster stroke. As the piston travels down, down, down, it draws huge amounts of air/fuel mixture over a longer period of time. Providing the valve hangs open long enough to accommodate that huge draw, a ton of velocity is created. This tunnel ram effect within the intake ports forcefully fills the cylinder and gives a lot of turbulence within the chamber, which helps to light off the mixture, even at low engine speeds with a big cam. The lazy, slow-speed drawbacks inherent in other engine families don't apply to the same degree as with an Olds 455 built like ours.
Cam And Valvetrain
As we stated, to make the most of our stroke and to complement the modifications to our Edelbrock heads and intake by Mondello (tune in next month), Mondello spec'd a truly ludicrous camshaft for Comp Cams to grind. The solid roller has a whopping 268/276 duration at 0.050 and 0.640/0.649 lift, ground on a 110-degree lobe separation angle. We were scared at first, but the monster cam nicely complements our engine package, proving Mondello really knows its stuff. While there's a chop to the idle, the engine isn't as uncivilized as we had feared. So why does such a monster cam work so well for an engine destined for street and nonrockcrawling off-road duty?
First, the larger the engine, the larger the camshaft can be. A Comp Cams 280 Magnum camshaft may lope like a race car when installed in a Chevy 283, but it will seem like a stock cam in a Chevy 454. Our 468ci engine slightly calms the effect of that much duration and lift.
Second, the lobe separation angle (LSA) of 110 degrees is actually somewhat conservative in a camshaft this large. LSA determines how much overlap, or time the intake and exhaust valve are opened at the same time, there will be. The tighter the LSA, the more overlap and the choppier the idle. The wider the LSA, the smoother the idle. The tradeoff is that an LSA of 106 will make more overall power and higher peak power than a cam with an LSA of 114.
Third, the fact that it's a mechanical (solid) roller allows for the fastest valve event of any camshaft type: hydraulic flat tappet, mechanical flat tappet, hydraulic roller, or mechanical roller. Because the points of contact on a roller cam are round instead of flat, they're able to hold the valve on the seat longer, then snap it open at the last moment more quickly, resulting in more power.
Finally, our camshaft works with, not against, our induction system. The valves open wide and hang a long time to allow the 2.07-inch intake valves to suck in as much air/fuel mixture as possible and to let the 1.685-inch exhaust valves expend the gases.