Article and Photos By: S&S Cycles
Originally Published In The October 2016 Issue Of Cycle Source Magazine
We’ve all heard the saying “There’s no replacement for displacement.” and there is some truth to it. It’s about the most elementary way to make more power. After all, if sucking air and gas into an engine and burning it is how you produce power (think Suck, Squeeze, Bang, Blow), it stands to reason that a bigger engine will suck more and give you a bigger bang. It may seem that this is all pretty low tech and simple, but there’s more to it than that. I talked to Roy Meyer, principal engineer here at S&S Cycle to get the engineering perspective on what we do and why. It turns out that I also found out why it’s not as easy as it looks. There are two major considerations in making more power from an existing engine platform. The first is the physical limitations of the engine, and the second is optimizing performance, or making the most of what you have. This article will focus on the physical issues, and in Part 2 we’ll tackle the optimization. In Part 3 the focus will be more practical issues in the real world. We have two ways available to increase the displacement of an engine. We can increase the cylinder bore size or we can increase the stroke of the crankshaft. OK three ways, you can do both. Adding another cylinder or two would get the job done, but here at S&S we’re working within the structure of a Harley-Davidson® motorcycle with a v-twin engine.
Nothin’s for nothin’. For every W advantageous modification you dream up, there is a corresponding disadvantage. Increasing cylinder bore, for example increases piston weight, resulting in increased stress on the connecting rods. Surprisingly, the greatest stress may be at the top of the stroke where the connecting rod has to stop that massive big bore piston at TDC and make it go the other way. Needless to say, that stress is also transmitted to the flywheel assembly where it puts a greater load on the bearings and tries to shift the flywheels out of true. Increasing the stroke of the engine adds increased piston speed and rod angle, which in turn increase friction and thrust on the piston. Longer strokes also increases the acceleration and deceleration loads on connecting rods and bearings since the piston has to travel farther in the same amount of time. Another factor that limits increases in both bore and stroke is simply physical size. Yes, size matters. Eventually the engine is going to get too large to fit in the motorcycle.
It all sounds awful! All these problems! Why would anyone do these things? The answer is simple. They make more power! Simpler yet, it’s fun! Actually, it’s not as bad as it sounds. There are solutions to the problems of bigger bore sizes, longer strokes, and the resulting power. If there weren’t, S&S wouldn’t still be in business. One of the most fundamental ways of coping with all that power is to build stronger parts, which is exactly what we’ve been doing for the last fifty years or so. If the connecting rods aren’t strong enough we design a beefier set, but didn’t we just say more weight was bad? I little more weight can be justified by a big enough increase in strength, so in addition to making the parts somewhat bigger, we also use stronger materials. The result is a powerful engine with good longevity. Another example. S&S uses engineering grade grey iron, a material renowned for its strength, its heat dissipation, and the fact that it wears like a pig’s nose in our cylinder liners and iron cylinders.
The result is a better cylinder that can take a beating and still deliver great engine life. Let’s consider the valve train. Who builds a big motor and doesn’t install a bigger cam? A high lift cam will put more stress on the entire valve train. If an engine is going to turn high rpm, stronger valve springs are probably needed to keep the valves from floating. Big springs stress the valve train even more because of the increase in spring force. Oversized valves are usually heavier and definitely require stronger springs to maintain control at high rpm. Rocker arms, pushrods, tappets, cams, and cam bearings all take more of a beating as we upgrade the valve train. That’s why S&S valve train components are stronger by design and made out of superior materials. As you can see, there is more to building a large displacement motor than just increasing the bore and stroke. In this first installment we’ve touched on the structural and physical problems of big inch engines and how we approach their solutions. In the next issue we’ll get into the more high tech area of optimizing the performance of a large displacement engine.