Con Rod Length Formula

Er.. i forget....

You can find it and lots more interesting but arcane stuff in Des Hammil's book 'tuning Rover V8s' (Veloce publishing - England). The information is extremely extensive and aimed at the practical engineer/mechanic rather than the academic engineer. Out of my collection of over 300 engineering manuals, service books and historical publications dating back to 1860's, I'd put this book plus 'modern engine tuning' and 'Tuning V8 engines' by a chap named A. Grahame Bell at the Top. Their is also an excelent book on Supercharging by an american Corky Bell. I forget the publisher details. You can get them all on Amazon.com.uk for reasonable prices.

Thank you for your help very usefull.

j.o.

Some old-school terminology applied to engine design: Square (bore area is equal to stroke) Undersquare (bore area is smaller than stroke) and Oversquare (bore area is larger than stroke). Not the answer to your question, but some other things to peruse.

From the terminology one would think that the length of the con rod is important as its weight is compared to that of the piston, plus its mass placement during the moments of rotation. It is certainly critical for Harley-Davidson V-twin engines, as the pistons or the rods have to sometimes have a bit of material removed in order to achieve the proper weight differential.

For exact numbers I would have to consult my repair manual, which is not here at the moment. But, I think you see where I am going with this. Surely someone on CR4 is an engine designer who can lend some expertise here. I for one would be interested to know as well.

Ing. Robert Forbus

Robert,there is a optium lenght to achive less force on con rod,piston and rings most race tuners say it shoud be approx 2 to 1 ratio? not excate.there is a formula some where.

j.o.

Not sure what you're asking here. If the stroke is 52.2mm, the con-rod length, measured between centres of the bigend and smallend is 1/2 this = 26.1mm.

If all you need is con-rod length, bore, piston weight, rpm etc don't come into it, or am I missing something?

http://www.simetric.co.uk/si_cc2hp.htm may be of interest.

Ignore most of my earlier posting. 26.1mm is the throw of the crankshaft (c/l main bearing to bigend), not the con-rod length. I suppose minimum con-rod length is crankshaft throw + allowance for bottom of piston skirt being below smallend c/l + 1/2 main bearing outside dia. + any extra needed due to crankshaft details to avoid fouling piston skirt at bottom of stroke.

You're running a quite high RPM figure here, is this for everyday use? I seem to remember that Ferrari use a relatively long con rod on over square, high revving engines. They still make it as light as possible, but a longer rod puts less lateral stress on the piston.