OT: Which Engine Configuration has the Most Power?, Curious Minds Want to Know Options

[Britain Smith]

I was discussing the engine layout of a Porsche motor in a horizontially opposed configuration and the question came up on which engine configuration produced the most HP/liter and why? I know that an advantage of a Porsche motor is that the center of gravity is really low, but does its configuration have any power advantages when compared to say a 90 degree V6 or an straight 6? For simplicity, lets keep the discuss focused on normally aspirated motors. What are you thoughts and why?

-Britain

[lapuwali]

Torque is more or less fixed by engine size and breathing ability, and horsepower is torque x rpm, so if engine size is fixed, you get maximum power by spinning it as fast as possible, modifying as you go to get it to breathe best at engine speeds the rest of the parts will allow without breaking apart. Since you lose mass far faster than strength as you make parts smaller, this argues for many small cylinders rather than a few big ones. So, a short-stroke V12 should make a lot more HP at peak than, say, a six of equal capacity, simply before the lighter pistons and rods will allow higher revs because you run up against parts breaking, and higher revs with equal torque means more power.

Eventually, you hit a limit where things like the increased bearing area of a 16-cylinder engine generates so much additional friction that you simply don't see much gain over a 12.

The layout of the parts (inline, vee, boxer, W, H, whatever) are usually dictated by other factors, like packaging. Vee engines tend to be fairly compact overall, so they work well in engines with many cylinders. The VW W engines take the same idea another step farther. As long as the layout doesn't cause harm, esp. for high revs, then it's not really all that important to power production. Renault recently tried to make a wide-angle V10 work, as a flat-10 has aerodynamic negatives (engine and exhaust hang into the underbody airstream), but a 90d V10 has a higher CG. The 115-120 degree engine, however, had serious vibration problems, and problems with block rigidity, so it ultimately didn't work.

The friction/speed balance is going to depend a lot on the strength of available materials. 10 years ago, Ferrari were alone with a V12 in a field of V10s in F1. The V10s could spin fast enough that the additional size, weight, and internal friction of the V12 pretty much cancelled the rpm advantage (at that time, about 16K rpm). Next year, rules dictate V8s, so they're planning on running that at 20-21K rpm using better (more expensive) materials than they used in 1995, in order to reduce the power loss from dropping the size from 3.0 to 2.4 liters. So much for reducing costs (which the rule was intended to do).