An engine’s bore/stroke ratio can tell us a lot about its operating characteristics and intended application. Here’s Jason Fenske, the Engineering Explained guy, to break it down.
Why are some engines designed with a large bore and a small stroke? Conversely, why do some engines employ a relatively small bore and long stroke? And why are so-called “square” engines, using similar bore and stroke dimensions, so popular for automotive use today? These are complicated and fascinating questions, and they have inspired countless technical projects and papers. It’s a meaty subject indeed.
Fortunately, we have Jason Fenske, the wizard of Engineering Explained, to break it all down for us, explaining the critical factors including piston speed, valve area, surface-to-volume ratio, and so on. He covers a lot of ground in this video in a highly efficient manner—most impressive. Armed with this information, we can examine an engine’s bore/stroke proportions and gain a greater understanding of its design strategy and intended application.
One factor not covered in this video that probably isn’t of much use to modern enthusiasts, but of interest to vintage car people like us: historical. Decades ago, some nations, notably Great Britain, classified and taxed vehicles by horsepower, and cylinder bore was a key element in calculating taxable horsepower, as it was known. These ratings had a definite impact on vehicle development, encouraging long-stroke, small-bore engines, but that’s a story for another day. Video below.
I understand how by increasing the bore you increase the displacement using the same engine components except pistons, however how do they increase the stroke without the piston hitting the cylinder head example increasing connecting rod length or increasing the stroke with a longer crankshaft throw. thanks. What parts are in a stroker kit?
You are correct in your thinking. Good question. The engine will need different lengh connecting rods or different pistons with the piston pin’s location either higher or lower. Or both piston and rod to get the combination you want. Jim W.
The traditional hot rod industry stroker kit will usually include pistons with the piston pin bore located closer to the crown to compensate for the longer stroke of the crankshaft. This is usually a better solution than shorter connecting rods, which increase wear and stress.
Sort of explains it right. Though not completely. Rod ratio moves torque and power around, though sometimes at the sacrafice of more engine wear.
Hot Rod engines,, most V8s are around the 4″ bore. A 3″ stroke is around 5 litres and fairly torquless low end, longer rods help torque. Though often at decreased engine life.
3.5″ stroke produces a ‘nicer’ engine often from the same size of package. And many may be even better with a little more stroke.eg 3 9/16. After that really the engine is too crowded or becomes bigger, wider and taller. Not ideal
A few 4″ stroke engines, big lazy engines that move big cars around with far less effort.
The true big blocks which are larger engines, meaning heavier for the most part. Same as the others, shorter stroke revs harder with less low end torque. Though a 4 1/4″ bore engine generally has plenty to start with.
But in reality the gains are not the same from 400-500 ci as from 300-370ci and the engines are too big and heavy for a smaller size of car. The car IF the engine will fit becomes and front heavy pig. Not so much in bigger cars and todays med size cars are near as big as my 71 Galaxie!! Camaros, Mustangs and Challengers are huge!
And the Ford has that huge OHC monster that is so top heavy.