Why do the old saws make more torque?

[chowdozer]

Honda did some testing a few years ago and discovered that a longer rod produced more torque without increasing the length of the stroke.

k

Hot rodders have know for years a longer rod changes the torque curve. I first learned of this in 1984-5. 400 sbc, +327 crank + Ford 300/6 rods = 350 cubes with 440 ft/lbs @ 3800 rpm.

Long rod engines tend to create more torque, move the torque curve up, decrease piston skirt side loading and desensitize the engine to detonation.

 

[Crofter]

I think that the old grunters were mainly reed valve induction instead of piston port. Piston ported intake timing is fixed and is optimised for torque up near max horsepower and at lower rpm than this it is too long and interferes with maximizing base compression and vacumn. Reed valves opens only on demand so this condition does not exist. Shorter exhaust duration of a lower rpm engine also allows a longer power stroke and torque at lower rpms. Transfer duration to allow running at higher rpm also interferes with base compression at low rpms where the ram effect is not appreciable. Generally the stroke to bore ratio was greater. The newer saws could be ported to increase torque at lower rpm but would suffer overall loss of maximum horsepower and then our nineteen pound saw would do the work of a 14 pound one just like in the good old days.

 

[kevin j]

Q Kevin, You seem pretty knowledgeable on engine theory, except on this point
you are off base.
Rod ratios do effect torque curves. Rod ratios do effect the "dwell" time of the TDC phase. It is theoretically documented, and has been test proven by many. Q


agreed. what I was trying to explain with the extremes of 4 inch rod vs. 24 rod. but that changes the engine height and is part of many other changes. Point again, just the single variable of bore and stroke is not the expanation for more perceived torque. Just trying to illustrate that torque and power curves are the sum of many design factors, many of which are invisible.



Q So, the old saws don't actually have more torque, just more available when you start lugging it and dropping the RPM. Q

exactly! torque rise is perceptible in operation, while actual torque and power differences between engines are 'usually' so slight as to need dynamometer measuring. The old 1950-1960's case, minneapolis, john deere farm tractors had huge torque rise as they lugged down, and felt like they could pull through anything and recover. The Allis Chalmers of the same time were much higher rpm, not as much torque rise, and criticised for that, but they actually had more modern designs and more hp for equivalent sizes.


So I think the summary of all these pages is that simple bore and stroke numbers are not the only factor, the entire design variables affect the engines, that seat of the pants power feel is deceptive, that modern saws are way better working tools, and that old saws are fun to run just for the experiences.

and THERE'S NO REPLACEMENT FOR DISPLACEMENT


k

 

[Lakeside53]

It's simple, the old farts that use old saws got to have something to crow about, and if the old saw aint as fast as the new saws, they boy ya better bet they got more torque. Snow was always deeper back in the day too, five miles to school uphill bothways into a head wind...

HP = (torque x RPM) / 5252

So to produce the same HP a saw turning 6,000 RPM is going to need to produce twice the torque of a saw turning 12,000 RPM.

agreed.. but.... the max HP (stock saw!) is not at 12k, more like 7-9K :greenchainsaw: