| Watts Shop Performance |
| Offset Grinding continued |
| For example, offset grinding a 3.48 stroke 350 Chevy crank and you can destroke it to as little as 3.400 or stroke it as much as 3.560 or pick any number in between. You can make your 0.060 overbore 350 into a 369 mill for dirt cheap or cut the engine size down to meet class rules. We know of 52 combos you can create with off the shelf rods and pistons (many of the combos based on a 305 block will need custom pistons). You can't use a stock 283 or early 327 crank because they're small journal cranks to begin with. We can do tons more with just a little math. In fact, make sure to do your own calculations and verify that pistons are available,and/or call us before attempting any of these combos. |
| Destroking |
| Keep in mind that the stroking basics can also be used to decrease the crank stroke. Why would you want to? Perhaps you run a weight per cubic inch class or a displacement limited class. Let's say the limit is 305 ci. You sure don't want to run a pathetic 305 Chevy because the cylinder bore is too tiny for decent size valves and the is bore is to close to the valves that airflow is compromised. Instead, use your newfound knowledge to build a small block with a 4.010 bore and 3.000 inch stroke for 303 ci. Be extra tricky and do it in a GM low desk block with an 8.700 deck height to save weight, have a compact size and keep the pistons short. You could still have 6 inch rods with custom pistons with a 1.200 compression height. With a 2:1 rod ratio, it'd be a 10,000 rpm screamer! Why not? |
| Cons of Stroking |
| Many believe that cubes equal horsepower and that's true, though long strokes are not as effective at pure horsepower production as large bores are. Why? Horsepower is a function of torque [horsepower = (ft-lb x rpm)/5,252] and stroke alone is rpm- limiting. Stroking increases piston speed (a 3.48 stroke engine at 6,500 rpm moves the piston at 3,770 feet per second; a 3.75 stroke does it at 4,063 fps), and the fast piston speed reduced the time available for cylinder filling at high rpm. Therefore, the torque production drops off at a greater rate than rpm increases and horsepower is cut short. Strokers can be made into high rpm race engines, but there's a greater cost in heads (to flow with less pressure drop at high rpm), valvetrain (to survive the more aggressive profiles required) and lightweight internal components. On a street engine, lengthening the stroke (beyond a certain point) and changing nothing else tends to favor low end torque over high end horsepower. That's less true in the case of very small engine such as Ford 289's, Mopar 273's and Chevy 283's, which case the sheer increase in displacement is worth power everywhere. |