http://www.stockcarracing.com/techar...ue_production/

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9. Mechanical Losses

There are plenty of places within an engine where friction can seriously reduce torque, and consequently, power. The piston/ring pack is the number-one culprit here. If you are building an engine for a class that legislates stock-type pistons, be they cast or forged, you need to be aware that the clearance such pistons are designed to use is close enough to limit piston noise to a minimum. When running, many street-type pistons expand to such an extent that they take up almost all the clearance, and their frictional losses are high. Losses from piston friction can be cut by increasing the clearance. Optimal clearance will vary from piston to piston because of differences in material and design. However, you can be sure that the stock clearances used, which typically range from 0.001 to 0.0027 inch, are way too tight. A good starting point with a 4-inch bore engine using a cast piston is 0.004, and 0.005 for an engine with a forged piston. Don't assume that because a minor increase in clearance is good, more must be better. The more the clearance is increased, the more difficult it is to get typical stock wide rings to seal.

If you want to reduce piston and ring friction and the time it takes to break-in the engine, try this for a torque-enhancing move. Use a Scotchbrite pan-scouring pad to polish the rings until they feel smooth and slippery to the touch. Next, use a pad on the bores in a vertical motion, and use Gunk engine cleaner as a lube. Continue the vertical polishing until the bores feel really slippery. This action takes out a minimal amount of metal and removes or smooths the microscopic tears left behind by the honing process. In effect, this process is similar to a fine plateau honing job.
The next culprit for big frictional losses is a slightly bent or twisted rod. You can usually detect such a rod at teardown, as the wear pattern on the piston skirt will have a slightly diagonal direction to it. Any such rod should be discarded.
As for bearing clearances in a semi-stock wet-sump motor, using top-limit factory clearances seems to be the best thing to do. Although this makes for a little more oil flailing around in the crankcase, it seems to allow production machining errors and stock component distortions.