Kai, the C to C length of an MGB rod is 6.5", I think the twin cam is
the same (no math I looked it up in Haynes), the engine stroke is 3.5";
I think however, your barking up the wrong tree. The bore and stroke
was determined by taxes not engineering. There was a long and
convoluted tax formula that was applied to British engines. The end
result was the smaller the bore, the less taxes were paid for any given
displacement. That's why the British engines have a long stroke and
small bore.
FYI, this is called the "law of unintended consequences".
Now as far as your 1.6 rod length/stroke ratio being "ideal". I'm not
certain that is the optimal ratio for performance. As the rod length
increases, or the stroke decreases you change the geometric relationship
between the centerline of the crank, the centerline of the big end and
the centerline of the gudgeon pin. Its my understanding that the
shorter the stroke, or the longer the rod, the more efficient the engine
becomes.
As an example.
If you take a long stick (rod) and attempt to push your car (piston)
with it you will naturally put one end of the stick in the center of the
car and you would stand as close to the center as you could when you
push. If you take the same stick and put one end in the center of the
car but you stand off to the side while you push, some of your effort
(energy) will be wasted in creating lateral forces. So if you MUST
stand to the side, then you would choose to use the longest stick you
could find, and/or be as close to the center of the car as you could
(longer rod + shorter stroke = less effort). I don't know if this is a
clear example but it's the best I could think of.
Me thinks you are trying to buy an Alfa or Porsche??? Hmm, as memory
serves me you did just announce you are driving a new VW didn't you? <g>
Happy new year.
Larry Hoy
> -----Original Message-----
> From: owner-mgs@autox.team.net
> [mailto:owner-mgs@autox.team.net] On Behalf Of Kai M. Radicke
> Sent: Tuesday, January 01, 2002 8:02 AM
> To: MG List
> Subject: Connecting Rod Lengths
>
>
> Well I hope everyone had a great New Years Eve!
>
> Anyhow, if someone gets a chance today could they please
> measure the center to center length of an MGB connecting rod?
>
> Center to center would be from the center of the small end to
> the center of the big end. Or, you can take the diameter of
> the small end (without the
> bushing) plus the diameter of the big end (without bearing),
> and then the length of the rod between the to ends. (D1 *
> .5) + (D2 * .5) + L = Center to Center Length.
>
> And if anyone can do the same for a Twin Cam rod, I would be
> quite grateful!
>
> I'm attempting to gain perspective into historic engine
> design principals, and what information I have gathered on
> our British cars so far, shows that they really didn't care
> all that much about things like rod length. Yet the Germans
> and Italians were arriving at "ideal" 1.75 rod/stroke ratios
> in the 1960s (or as far back as my data goes for the moment)
> for their engines.
>
> Happy New Year,
>
> Kai
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