Thursday, July 24, 2008

68 Inch Racer, part 2

If you read my post entitled Paper Engines, you will recall that I solved the problem (at least on paper) of building a high compression 61" Knuckle lower end with a 74" top end.

There still are a few things to be considered in the bottom end. For one thing, there is the subject of flywheel weight. That will be partly determined by the end use of this motor. Whether it is for drag racing, "street" racing, Bonneville, flat track, or some sort of road racing. For most of these scenarios, I would lighten the flywheels (the one exception being drag racing with a slick). How much to lighten them is the question. The minimum would be to cut down the heavy left side flywheel to the thickness of the right side wheel.

stock 61" flywheels on left , lightened S&S on right

Bike weight is also a factor here. Flywheels "store" energy, so the heavier the bike, the harder it will be to get under way from a dead stop with extremely light wheels. Still, the gain in the rate of acceleration is too good to pass up in most applications.

Since the UL rods are an aftermarket imported item of somewhat unknown quality, I would at the very least have them shot peened. Possibly polished and shot peened. The purpose of polishing is to remove any possible surface imperfections that could lead to a crack. However, polishing also removes the surface left from the forging process which increases the strength of the rod. Shot peening puts that harder surface back on (so to speak). So, just shot peen, or polish and shot peen, but do not polish only!

Of coarse the rod change alone would dictate that the flywheels be balanced, but it becomes even more critical when lightened.

Since the point of this engine is to turn some RPMs, the crankpin and mainshafts are probably not a place to skimp. S&S makes all of the early shafts, including the stepped crankpin, and are probably the highest quality available. Yes the rod set will come with a new crankpin of unknown origin, but that would be better set aside to go in a restoration that will not see such severe service.

This is to be a "Racer" and not a restoration, so why not take advantage of the built in extra strength of Panhead cylinders. Even bored to +.060 (which makes it a true square engine 3.5" bore x 3.5" stroke) the Pan cylinders are undoubtedly stronger than a Knuck cylinder. Sure, there are reproduction Knuck cylinders that are stronger than stock, but they aren't cheap. I would spend the money elsewhere. Besides, there are plenty of Knuck heads out there that have been converted to the Pan headbolt pattern already, so why not use them.

For pistons, I would start with a cheap Pan/Shovel cast piston which is called a 10:1. The dome volume on them is about 52cc which will get some compression ratio (I'll discuss that more when I get to the heads). Once the domes are ceramic coated and the skirts Teflon coated they should work very well.

More to come.

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