> 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.
You are of course right Larry, I didn't consider the ramifications of
bureaucracy on engine design of the period. I didn't even think to check my
MGB Haynes, as the TR ones are devoid of C to C lengths for the rods.
> Now as far as your 1.6 rod length/stroke ratio being "ideal".
Actually it was 1.75.
6.5 / 3.5 = 1.85 which is greater than 1.75, but 1.75 seems to have been
proven over time to be the optimum point between engine longevity and
performance, with diminishing returns either side of 1.75. Shorten the rod
or increase the stroke, you'll end up with a lower number.
> 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.
Well that is my point afterall. There are arguments for either short stroke
or long stroke engines. One of the benefits of a long stroke engine is that
its rod angle to the crankshaft angle isn't as high as a short rod / long
stroke motor. This reduces piston sideload on the bores, and you end up
with an engine that lasts longer and is more comfortable at high RPMs
because the connecting rod doesn't have to change its angle as drastically.
So lower the ratio, the worse for the geometry.
I would attribute the rod/stroke ratio as one of the reasons why the Triumph
GT6 motor is so nice at higher RPMs and the TR6 motor sucks above 5500RPM.
GT6 is around 1.92 rod/stroke ratio, while the TR6 is down there at 1.54.
The other reason is probably just the extra rotating mass of the TR6 crank
and flywheel, which are drastically overweight. Surprisingly the TR wet
liner engine had a decent rod/stroke ratio of 1.73.
Lets also not forget that the points of acceleration for a piston in a short
rod and a long rod engine are different. A long rod will be accelerating
towards TDC slower and moving away from it slower, same is also true for
BDC. Which has implications on the breathing characteristics of an engine,
and at least now is proving that a 4 valve per cylinder engine can be made
to work better with shorter rods than 2 valves per cylinder.
There are all sorts of other benefits of longer rods. A longer rod will
also produce a lower piston speed at a given RPM than a short rod. The
reduced crankshaft angle of a long rod will also tend to give off less
vibration than a short rod motor. Ignition timing is also effected by the
choice of either a long or shorter rod.
Anyway, I'm just compiling information. It isn't for any other purpose.
One of the benefits of compiling the information is that I'm seeing trends
among nationalities, which can be explained by things like taxation (thank
you Larry and Michael) that I gave no thought to.
Yes Larry, I have a new VW.... the Jetta has a dramatically better
rod/stroke ratio than my TR6, which was the deciding purchasing factor after
all! ;-) Other cars considered? Well a Ferrari 400 and a Mercedes 280SE,
because they both have 1.75 rod/stroke ratios :P
Haven't gotten access to any Porsche rods yet, but I guess I'll make the
database a public affair once its completeness makes it worthwhile. With
only about 40 different engines right now, it isn't anywhere near complete.
So who owned the 1971-72 BRG BGT that was parked on South Street in
Philadelphia last night and was eating at the Monte Carlo? One of the nicer
BGTs I have seen locally...
Cheers (who now has to go celebrate an anniversary with the SO),
Kai
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