Bore/Stroke Ratio..

[naderson]

How do engineers choos the Bore/Stroke ratio for a new engine??
some choose a long stroke, and others choose ashorter one...

based on what do they choose these specifications???
what are the advantages of the long stroke???
what are the advantages of the shorter strokes???

what about a square engine (B=S)???
why isnt it used in racing vehicles???

[KiwiBacon]

A lot of it depends on the intended application.

A longer stroke is suited to a lower revving engine which generates more torque. A shorter stroke is suited to a higher revving engine and can be a lot smoother.

[GreyGoose006]

but despite all this, nearly all modern engines are oversquare.
even the stroker versions of some engines are still oversquare.
the question becomes HOW oversquare you want to be.
in addition, engine size is a function of stroke, so it can only get so long.

[curtis73]

Its dependent on several things but these guys have highlighted the main points.

Inertial forces increase at the square of speed. What that means in effect is that increasing the speed of the piston by a small amount can easily double the inertial load seen at the piston pin, crank journals, etc. A longer stroke means that the pistons are moving at a faster speed at any given RPM, so a long stroke engine would not be a wise choice in a high-revving application.

Breathing is a concern. Small bores limit valve size which limits breathing to a lower RPM.

Physical engine size is also part of the choice. Large bores mean wider bore centers which makes a longer engine. Longer strokes mean taller decks which makes a taller/wider engine.

On the street it makes little difference. Even among the American V8s there are wide differences. Most factory engines don't rev more than about 6000 rpms, and you can play around with bore and stroke a lot within that ceiling. The Pontiac 455 can pretty safely rev to 6000 with its 4.21" stroke. Compare that to the relatively short stroke of the Ford 429 at 3.590" and they only rev to about 5000 in stock form.

Within the parameters of mild stock engines, the actual bore/stroke ratio matters very little. One of the highest torque outputs comes from the short 3.90" stroke Buick 455, and some of the best high end HP comes from the BBC with a 4.25" stroke. More importantly is what the heads flow and how the cam provides valve events. If you tune an 455 ci engine for where you want it to make power, the bore/stroke ratio is not as important as you might think from a design standpoint.

[KiwiBacon]

Originally Posted by GreyGoose006

but despite all this, nearly all modern engines are oversquare.
even the stroker versions of some engines are still oversquare.
the question becomes HOW oversquare you want to be.
in addition, engine size is a function of stroke, so it can only get so long.

Honda generally bucks that trend. Many of their engines are undersquare, but I'm having trouble finding any relevant links for you.

[Moppie]

The Honda B and D series engines we're all pretty under square, and generaly very high revving.
8000+ for most of the B series motors, and 7,000 for most of the D series.

Although I believe their current engines have gone the other way, being much close to square, to possible over square.

[GreyGoose006]

Honda generally bucks that trend. Many of their engines are undersquare, but I'm having trouble finding any relevant links for you

most of my knowlege is in domestic or european type cars.

oh, and heres a link to dispute my own statement...
http://www.donsautopages.co.nz/enginespecs.htm

you were right about hondas.


one of the highest revving stock v8 engines i can think of is the chevy 283/302
they are essentially the same engine (all small block chevys are) with the difference being that the 283 has a smaller bore.
i've heard that 283s have been taken up to over 10,000 rpm with modifications.