Originally posted by: fleabag
Originally posted by: Zenmervolt
Originally posted by: fleabag
I was thinking about how one would go about increasing torque in an engine in order to improve fuel economy and I wanted my idea to be confirmed/clarified.
In order to improve HP, you increase the length of the rod, so there is a smaller stroke, allows for higher RPMs, correct?
In order to improve Torque, you'd decrease the length of the rod, and therefore increase the length of the stroke, which would decrease top end RPMs but give more power, correct?
Would doing this yield more torque in the lower end or in the whole range?
If the piston meets at the top of every stroke, how would you increase the length of the rods with out having the piston crash into the valves?
Stroke is determined by the length of the throws of the crankshaft, not by the length of the rod. Changing rod length has no effect on stroke at all.
Longer crank throws allow for more leverage against the crank, which is beneficial to torque but far from a be-all-and-end-all. The main issue is that long stroke limits RPM due to the increase in mean piston velocity that it causes. For a longer stroke, the piston must travel farther for every revolution of the crank, which means that for a given RPM, a piston with a longer stroke will have to move faster than a piston with a shorter stroke.
ZV
I'm hoping I understood you correctly so I'd have to reduce the height of the piston and use longer rods in order to increase the stroke and the reverse to decrease the stroke.
Assuming what I said is correct, is there a particular advantage to using a tall piston over a small piston?
You don't reduce the height of the piston when you increase your stroke. When you put a longer stroke crank in an engine, the pistons you use to replace the old ones are exactly the same....except for one thing: The piston pin HEIGHT is different. In simpler terms, the piston pin hole is drilled higher in the piston, so the longer stroke doesn't shove the piston out of the block.
You CAN increase the stroke a very small amount without changing anything. This is common practice in both NHRA and IHRA Stock and Super Stock Eliminator classes.
Example: Your race engine is a 350 Chevy. Stock stroke is 3.48". Obviously, there are tolerances either way during manufacturing, so no crank has every throw at exactly 3.48". Some are more, some are less....it's a very small variance, but it IS there.
So the rules allow you to be within those tolerances. Well, savvy engine builders build their engines to the HIGH side of the tolerance. So they'll take a standard crank, and blueprint it...which is a fancy way of saying they offset-grind it so the actual stroke is 3.48"+whatever the tolerance is. Just to get that little bit of extra engine size.
But you can't gain any significant size with a longer stroke crank unless you change pistons.
On a side note about the blueprinting of engines such as the one I mentioned above, there are factory tolerances throughout the engine that, if you max them or minimize them, can help get the absolute most out of the combo.
One popular one is with the cylinder heads....combustion chamber, specifically. Let's say you have heads that came as 64cc stock chambers. As always, the tolerance from the factory allows for a couple of CC's either way. So naturally, all the builders machine their heads to the lowest possible side of that tolerance allowable under the rules......to gain that small amount of compression, and therefore, power.
So imagine a Stock Eliminator blueprinted 350, rated at say, 10:1 compression from the factory. After a good engine builder gets through with it, it's probably got a couple extra cubes, and the true compression is closer to 11:1, or at least 10.5:1. All within the guidelines. The head work adds compression. The machining of the crank adds both displacement AND compression. All of it adds power, and if you're only .002 second from the national record in your class, that is important.
