Engine Limitations and Redline Determination

The misunderstanding of redline determination, and engine limitations. The answer to that depends upon a whole group of questions – such as, what is the engine's displacement? How many cylinders does it involve? What are the bore and stroke dimensions? How often will the engine reach redline? And for how long? Will the engine power a typical road motorcycle expected to last many tens of thousands of kilometers or a competition-only race bike that has its engine rebuilt every few races?

Those factors and several more weigh heavily in an engine designer's or builder's determination of the redline.

In the end, the objective is to prevent certain engine parts from either tearing themselves apart due to inertial forces resulting from their incredible rates of acceleration and deceleration (usually discussed in terms of “piston speed”), or from being destroyed through unintended contact with other engine parts (“valve float” is the most common instance).

Designers of street bike engines like to keep piston speed below about 1375 meters per minute; if it gets very far above that, piston/connecting-rod life is disproportionately shortened. On the other hand, some racing engines reach piston speeds close to 1850 meters per minute (the equivalent of one and a halve soccer field every three seconds), but they usually do it only in very brief spurts and require frequent rebuilds – if they don't grenade themselves first.

Not only does piston stroke help determine maximum piston speed (and, therefore, maximum rpm), but so does piston weight; obviously, the inertial forces of a 5cm piston moving at 1375 meters per minute are much less than with a 10cm piston at the same speed. But regardless of piston size, the designers try to keep piston weight as low as is reasonable possible.

Then there's valve float to deal with. A valve spring can only keep the valve and its attendant follower in contact with the cam lobe up to a certain engine speed; at some point, the valve's inertia causes it to continue opening on its own, separating from the profile of the cam lobe before spring pressure can force it closed. Valve float can easi8ly lead to the valve getting bent of broken through contact with the piston crown.

These inertial matters are why engineers use ultra-light valve materials when designing high-rpm engines, and why putting four smaller valves in a cylinder head of a given size usually results in a higher redline than using two larger valves. Even with positive-closing valve gear such as the desmodromic system used by Ducati, excessively high rpm can either quickly damage the valve and its seat (best case) due to the pounding caused by too-fast closing rate, or it can just snap the head off the valve altogether (worst case).

There are numerous other factors that govern maximum engine rpm, but these are the two primary ones. Comparing a piston moving at 1375 meters per minute is just above 16,000rpm.