Here is a post by somebody I have spoken to on many occasions about engine dynamics. This was posted on www.eng-tips.com, and basically echos what I had said. It might, however, do a better job of explaining then did my crack-infested drawings. At 1-20 degrees past TDC, the short rod engine will generate force in a direction more tangental to the rotation of the rod journal on the crank. This is a more direct way of exerting force on the crank. Well, go ahead and read the post. Hope it explains better then I do.
-=B-=



Most of my opinion is based upon looking at what happens when you change R/S ratio. For my examples, I assume a stock 350 Chevy, 4.000 bore and 3.480 stroke with zero wrist pin offset. Assume stock rod of 5.700 inches (1.638 R/S) and a 8.700 inch rod (2.5). The first thing Smokey said is that a piston dwells longer at TDC with a large R/S ratio. Let’s look at the numbers. I assume dwell at TDC occurs while the piston is within 0.200” of TDC displacement.

Down R/S Ratio
in 1.638 2.5
0.200 335 334
0.150 339 339
0.100 342 342
0.050 347 347
0.050 372 372
0.100 377 377
0.150 381 381
0.200 384 385

The super long R/S ratio only gets a 2 crank angle degree (CAD) advantage over a stock R/S ratio. In time, that’s a only a 0.045 msec advantage at 8000 rpm. Not much. When it comes to allowing more time for combustion to occur, the difference between these two drastically different R/S ratios is not much, hundredths of a millisecond.

Now, let’s look at what I think are the most important parameters R/S ratios change, combination unit force and “how hard the piston sucks on the intake port.” Combination unit force is the percentage of combustion pressure force that causes a torque on the crankshaft. More torque on the crankshaft is the goal of anything to increase power and torque of an engine.



Combination unit force is determined by the geometry of the slider/crank assembly. If you keep stroke constant, combination unit force is driven by R/S ratio. So, if our goal is to increase power and torque, the higher the combination unit force, the more power and torque we’ll make. However, you also have to remember most torque transferred to the crank occurs during the first 20 to 25* after TDC. I think it’s more important to have a good combination unit force during the 20 to 25* after TDC than to have a good number later in the power stroke, say 75 to 100* after TDC. Looking at the example below, a shorter R/S ratio is much better at extracting power from the combustion chamber force than a longer ratio WHEN IT COUNTS. The peak values increase with increasing R/S ratios, but the peak occurs when virtually zero cylinder pressure is available to push on the piston.

“How hard the piston sucks on the intake port” is also important, and I have to get a better way to describe it. The plot below shows the change in cylinder volume with respect to crank angle. This helps to determine the volumetric efficiency of the engine. But, you can’t just look at how hard it sucks, but when does it suck in relationship to valve lift. A shorter R/S sucks harder, but it’s peak is roughly 70 degrees after TDC, the longer R/S doesn’t suck as hard, but they suck when the intake valve is open more. Translated, it’s easier to get more mass flow and higher VE when the R/S ratio is higher.

So, we have two parameters that want two different R/S ratios. To take advantage of the cylinder pressure you have, you want a short R/S ratio. To get more mass flow and higher VE, you want larger R/S. It’s all a trade off.

Does this analysis solve much? No. I don’t think an ideal R/S ratio exists for all engines. It’s all dependent on the rest of the engine. If an engine is under valved for its displacement, it may benefit with a longer R/S to enable more air and fuel at higher rpm. If airflow is sufficient for the desired speed, a shorter R/S may give more power by utilizing the cylinder pressure you already have. It’s all a trade off and compromise.

On the short deck pro stock engines, I agree with your conclusions. Plus, I’ve spoken to some Comp guys who run old pro stock engines, and he concurs. The reason for short decks is to get better airflow mainly with a side benefit of stiffer pushrod.

On F1 R/S ratios, I looked at paper I have. On the 1989 Honda F1 engines running 2.5 bar of boost. It’s R/S ratio is roughly 2.28. But, I would imagine the engines of today are more concerned about a physically smaller engine to get better aerodynamics rather than a longer R/S ratio.

Chris

Dude, Boeing called. They want their wing back.

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