Some thoughts on planning your bike mods…

I am writing this article in response to several questions about ‘why’ certain parts are ‘required’ when you make internal changes to your driveline components. I have had questions asked about different cams, carbs, pushrods, lifters, etc. and what ‘should’ be done. In response to those questions:

Every cam manufacturer uses a different geometry for their grinds. This causes the base circle of the cam to be different from the stock Harley camshaft in most cases. The first consideration for changing your cams is to make sure you have adjustable pushrods: These compensate for the geometric differences between the stock cam and your new one. Next: beefier cam bearings that can handle the increased loading since the stock bearings will disintegrate and cause major engine damage. Finally, valve springs that will keep the lifters and valves riding on the cam instead of floating into the piston at high rpms. The increased mass and inertia of the adjustable pushrods moving up and down coupled to the valves opening and closing wants to keep the valves open at fast engine speeds. The valve springs are the key components to preventing damage to the valves and pistons. Different cams have differing lift and duration, depending on the targeted performance. Valve springs not properly sized for the cam will cause the valves to remain open, and the pistons will make contact depending on the duration of the valve float.

With clutch assemblies, carefully examine why you want a clutch that is not from Harley. There are numerous aftermarket transmission manufacturers out there. Each one uses different tolerances when having their transmission mainshafts machined. This results in a broad range of dimensional variation for the minor and major shaft diameters affecting the spline fit to the hub. Since Harley has total control of the machining they perform on their hubs and shafts, the tolerances can be held much tighter. This results in an almost nonexistent failure rate for their hub splines or shafts. All of the aftermarket hubs are designed with open tolerances to fit every variation of mainshaft out there, compromising the overall reliability of the clutch. Choose wisely, and plan for additional maintenance as a result of any change. The vast majority of the clutch hubs available are non-hardened steel, or hard-anodized aluminum. The tranny shaft splines can act like a file against these softer materials and will cause hub failure.

When changing your carb, ask yourself why. A properly tuned CV stock carb with a dynojet and thunderslide kit will give you better performance dollar for dollar than what you will spend on ANY of the available aftermarket carbs. Another consideration is that the newer Harleys are using a throttle position sensor on the carb (mid-70’s automotive technology) to aid in ignition timing. This means that you can change the carb and have worse performance unless you change the ignition module at the same time. The cheapest performance increase is to allow the engine to breathe with a free-flowing air cleaner, free-flowing exhaust, and a properly tuned carb.

Fuel petcocks: why change what works? The stock unit delivers a fuel flow that will allow you to run to redline without starving your carb. Consider: an average Harley at 43 mpg = 2.98 ounces per mile traveled. At 60 mph, that is 2.98 oz/minute. At 120 mph, you will need a max flow of 6.5 oz/minute. Is your $90 Pingel really getting you anything other than bragging rights and some chrome?

The various aftermarket exhaust systems will give you a louder bike while not necessarily making any performance gains. Numerous high-dollar exhaust systems sound great but perform poorly. The Thunder Header, V&H 2-into-1 Pro-pipes, and the Hooker 2-into-1 systems are the top 3 performers in that order. Of course, none of them will reach their full capability until the heads are flow-matched to the capabilities of the entire system (air filter, carb, intake, cam, headwork, and exhaust).

Enjoy the ride…

Steve