Two-bolt vs. Four-bolt Mains If your engine is not run at sustained speeds greater than 7,500 rpm and horsepower output is less than 500, a properly set-up, two-bolt main block may be sufficient. Always use studs to fasten the mains. As maximum engine speed increases, you may wish to consider replacing the cast main caps with billet steel parts. In fact, it's good insurance.

Big Bore vs. Long Stroke An over-simplification of bigger bore vs. longer stroke comparison is that a big bore engine will run better as engine speeds increase, and a longer stroke engine will run better at lower rpm. Graph B shows the piston velocity of a 4-inch stroke vs. a 3.25-inch stroke.

Mean (average) piston speed is traditionally calculated with the formula: 2 x stroke x rpm. Graph C shows how stroke affects mean piston speed as engine speed increases: As mean piston speed increases, so does friction (and power loss). Ducting losses (due to more rapid descent) also increase as mean piston speed increases, which negatively impacts cylinder filling (volumetric efficiency or torque output). Overall, a long stroke engine is a cost-effective way to make power at lower engine speeds but is one not likely to be efficient or reliable at higher engine speeds.

Connecting Rod Length Much has been written about the effects of rod length on engine performance. The following data in Graph D shows a rod length comparison on a NNC-type engine. In this case, the shorter rod made better low speed power and the longer rod made more power at higher engine speeds. Graph D shows the results of the test. For this test, pistons and rods were changed as a unit, so the CD (compression height) of the piston did change with rod length. The results are presented as the change in power from the 6.200-inch rod to the 6.080-inch rod (bhp of 6.200 to bhp of 6.080) plotted against engine speed in rpm.

Graph E shows how rod length affects piston position. Only to make the difference appear obvious, the plot compares a 4-inch rod to an 8-inch rod. While this is an absurd difference, it illustrates how the shorter rod moves the piston away from TDC more quickly. Long rod lengths reduce piston side-loading and, therefore, friction. Valve timing (camshaft design) should be considered when contemplating connecting rod length. Primarily, variations in piston motion away from TDC on the intake stroke (as rod length changes) should be linked to intake valve events. If your camshaft provider of choice is unable to respond to this concern, we suggest you look for another supplier.

Main and Rod Bearing Sizes and Clearances Oil film thickness increases as bearing clearances are reduced, provided everything is perfectly round and straight. This trend continues until the clearance becomes too tight and the oil film goes away. As the oil film thickness increases, friction is reduced.

Smaller bearing diameters reduce the surface speed of the bearing. A reduction of bearing speed will reduce friction. This effect will be most noticeable at high engine speeds.

Small crank and rod journal sizes with relatively large strokes will reduce the stiffness of the crankshaft and cause bending loads to increase. This will also increase loads on the bearings. As a result, the best time to try smaller journal sizes is when using shorter strokes and big bores.

The trend in Cup today is to have bearing sizes at the minimum with very tight bearing clearances. This is very difficult and expensive to achieve. The crankshafts, connecting rods, and bearing shells must be ultra-precision components which turn out to be extraordinarily expensive. Plus, this combination leaves no room for error. Small errors in a component or during assembly will most often result in a spun bearing.