Hypothetica Crankshaft Questionl

Synchros shot? Weird noises while shifting? Not sure what needs to be replaced?
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ClutchFork
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Hypothetica Crankshaft Questionl

Post by ClutchFork »

All else equal, same number cylinders, same displacement etc, lets look at two strokes: 3 inch and 4 inch. So torque at the flywheel is measured at 1 foot from the center of the crankshaft for pound feet. So on a 3-inch crankshaft throw, we are only getting 1/3 the leverage compared to what we are reading. So lets say both engines make peak torque at 2000 rpm. Then lets say the 3-inch stroke engine makes 300 pound feet at 2000 rpm. So to get a reading of 300 pound feet, the exertion on the crankshaft must be 300 pound feet, but since the rods are pushing at 3 inches out, they must be exerting 1200 pounds of force (1200 pound 1/4-feet). So if that is the case, then moving to a 4-inch stroke (the cylinder bore reduces to compensate for the longer stroke to keep displacement the same) and still exerting 1200 pounds (300 pound 1/3-feet) of force through the rods, we should get 400 pound feet at the flywheel.

Now show me that I am full of baloney and tell me how it really works.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...
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Re: Hypothetica Crankshaft Questionl

Post by Rope-Pusher »

Image
Are you going to or coming from the packing plant?

Essentially, yes, if you had the same pressures in the cylinders, and the diameter of the cylinders were the same, just not the stroke, then the engine with the bigger stroke would have a greater moment arm for the piston rod to react against and it would result in more torque at the crankshaft.

There is a lot going on with the geometry of the angle of the connecting rod versus the position of the crankshaft arm, and the pressure in the cylinder going down as the piston moves downward, and the price of tea in China, but in a simple example you are correct.

So, why would one, or two even, want to increase torque when it's still horsepower that quickly accelerates you down the 1/4 Mile or tows your trailer up a mountain, maybe even moreso now that transmissions are tending to have more gears than in the past. With more ratios available, it's easier to keep the engine making more power than when you had only a few-couple gears and the engine dropped down to idle speeds when you upshifted.

There exist a myriad of methods for removing the epidermis from a feline, just as there are a plethora of design possibilities for an engine. There are a lot of ways to arrive at the same displacement. You can change the number of cylinders, you can change the stroke, you can change the bore, you can raise the deck height and change the connecting rod length, you can fire the cylinder every time it comes to Top-Dead-Center or every other time. For a given displacement, you can make changes to increase the strength so the redline can be raised, and improve the breathing to make it worth revving high (or the torque will drop off at a lower RPM and make it not worth revving to 9-Grand.......), you can employ supercharging to cram more fuel-air mixture into the engine, yadda, yadda, Nitrous, yadda, yadda turbocomponding, yadda, yadda, electric motor assist, yadda, yadda, cylinder deactivation, yadda, yadda, VTEC, yadda, yadda, alcohol, yadda, yadda, Nitromethane added to the fuel, etc.
'08 Jeep Liberty 6-Speed MT - "Last of the Mohicans"
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ClutchFork
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Re: Hypothetica Crankshaft Questionl

Post by ClutchFork »

Whelp, to quote the folks at Hot Rod Magazine (Feb 1999):

"...you buy horsepower but drive torque."

"You need rpm for horsepower, but torque is the stuff that shoves you in the driver's seat and blows the tires away."

My 2001 Ranger, wife's 2008 Mazda 5 both have the 2.3L modular DOHC 16 valve four popper. It certainly moves reasonably quickly but with the Ranger, which had a stick, I noticed you needed well over 3000 rpm to get any giddy-up. Whereas my 4.9L torque monster inline six (265 pound feet at 2000) would kick you in the pants at 2 grand. Same horsepower as the 4.9 but way way less torque was the 1992 Aerostar vulcan 3.0 V6 and it was a dog compared to the F150's 4.9L torque engine.

While the S10 2.2L pushrod 4 has 20 less hp and is ultimately slower than the modular Ranger 2.3L, the S10 is more fun to drive because it has more guts at low rpm.

And they can keep the many many gears, my retirement car may well have a big V8 and a 3 speed manual. I got along fine in my youth with a 77 F100 4.9L 3 speed column shift. Any more gears is a luxury.
Stick shiftin since '77
theholycow wrote:Why in the world would you even want to be as smooth as an automatic? Might as well just drive an automatic...
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Re: Hypothetica Crankshaft Questionl

Post by Rope-Pusher »

Give me a big enough lever and I can move the Earth.......but not fast-like.

Torque is a momentary thang. Torque without rpm is me with my lever. Torque with rpm is....Power! Power is the time-rate of doing work.

400 torques at 2000 rpm is the same amount of power as 200 torques at 4000 rpm, or 100 torques at 8000 rpm.

If the folks at Hot Rod magazine don't understand the definition of torque vs the definition of power maybe they are in the wrong business.
'08 Jeep Liberty 6-Speed MT - "Last of the Mohicans"
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