Does Stroke Matter?

[olddog]

I considered calling this mental masturbation for engine geeks.

I read a book a while back. I believe it was a college text book for a course on internal combustion engines. It was written in the late 1950’s. A very difficult to read book, it was. It mainly looked at commercially successful engines, and pointed out research done at several universities.

The professor pointed out that the overwhelming factor for making power is the bore diameter – not the displacement. To illustrate his point he compared the Hp of a model air plane engine with a ¼” bore to marine diesel with a 3 foot bore and 9 foot stroke. The Hp per piston area was very close on both engines; however the Hp per displacement were not even in the same ball park. He did take into account the BTU/lb of the fuels being used.

His explanation for why this is the case, boils down to a few simple principles. The bore limits the size of the valves. The valves limit the air flow into the cylinder. The amount of air dictates the amount of fuel. The amount of fuel per unit of time is the amount of power per unit of time available. The efficiency of converting the available power to Hp changes very little, in piston engines.

He went on to explain that on a given engine, the amount of air being pulled into the cylinder increases with piston speed until the flow through the port and valves become the bottle neck. As piston speed increases beyond the maximum flow point less air is pulled into the cylinder. Since air flow is proportional to power, this makes perfect since. We all know there is a peak in Hp and spinning the engine faster results in less Hp.

Now here is where it gets a bit interesting. He always talks in Mean Piston Speed. He said in controlled laboratory experiments, changing the stroke does not significantly change the Hp. In fact the peak Hp will be reached at the same piston speed regardless of the stroke, when all other factors are kept the same. A long stroke reaches a given piston speed at a lower rpm than a shorter stroke. So the long stroke will have more torque at a lower rpm than the short stroke, but both will make the same Hp.

So if we had a 302 cid (4x3 bore/stroke) and its peak was 300 Hp & 197 lb-ft at 8000 rpm and we change only the stroke to 400 cid (4x4 bore/stroke), we could expect 300 Hp & 263 lb-ft at 6000 rpm..

Today we have canted valve heads that allows a bigger valve for a given bore. We also have another 50 years of advancement. However I have to wonder if the old boys understood some things pretty well. His work did not include radical cam shafts or engines designed for racing. His work suggests that if I stick a set of heads that would work best on a 400 cid stroker on a 302, I could make the same power at a higher rpm with less torque. I wonder if this is true today.