Oh, the agony of the digest! All of these interesting threads and I can't
participate in real time. Well, I can still bore you all a day or two
late.
I haven't done the math lately nor will I since my calculus is a little
rusty by I believe that the bore/stroke ratio cancels itself our of the
calculation for power output. In other words, all thing being equal, the
bore/stroke ratio does not effect engine performance.
>From a practical standpoint, long stroke motors do have lower rpm limits
than short stroke engines because of 3 factors. First is the inertia issue
of the pistons and rods flying around. Second is the limit of maximum
piston speed. I have often mentioned the Acura Integra as an example of a
high rpm long stroke engine. The Type R Integra has the highest maximum
piston speed of any mass produced car ever. I seem to remember back in the
seventies (from books written in the forties) the number of 200 fps as a
good safe limit for piston speed. Above this speed you can't remove the
heat fast enough and you cook your oil. For those of you who are bored,
max piston speed = (rpm / 60) x (stroke (inches) /12) x 2 x pi or rpm x
stroke/115. So an engine with a 3 inch stroke turning 8000 rpm would have
a max piston speed of 209 fps. An MG B series engine has a 3.5 inch stroke
so the max rpm would be 200 x 115/ 3.5 or 6700 rpm. The Integra has about
the same stroke as an MG B series so obviously the piston speed is a lot
higher. I know at least one person who consistently exceeds this rpm in
his MGA so the oils must be a lot better now days.
The third limitation is valve size and, for most cars, valve size is the
limiting factor for rpm range given a reasonable cam grind. In the real
world, all cars idle so you have to be able to pull hard from at or near
idle plus you can't go spitting a lot of unburned fuel into the air so you
can only keep the valves open so long. With present technology, you can
only open the valves so fast or the cam won't last or the noise will be
excessive (see previous posts on performance cams). Two old tricks to
increase workable valve opening speed and thus increase air flow are to use
mushroom tappets or roller tappets. New Corvettes use roller tappets. I
don't know anyone who still use mushroom tappets but they used to be hot in
the 60's.
I wonder if anyone has ever looked into mushroom or roller tappets for
LBC's? Just a thought.
Regards,
Bill Eastman
61 MGA with 0 fps piston speed until April
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