I have always wondered what it all means having either a 2L vs 3L or 6cylinders vs 8 cylinder car in terms of performance.. Now when i say performance I strictly mean If for example you had the same car, same weight, same power output but the only one variable was a 2L vs a 3L or 6cylinder vs 8. Which car if any would be quicker in such a scenario and why?

 

Larger displacement normally increases power if all other things are equal. Number of cylinders is not always an example of more power. Now forced induction is also a player, such as turbo's or superchargers.

 

With the v8 s/c guys the difference in the bigger blower and 2 extra cylinders seems to be about 100-150rwhp/tq.

 

More displacement usually means more torque as you are either increasing the bore or stroke. Increasing the bore will make a bigger explosion each time the cylinder fires, and more stroke increases the lever arm. More cylinders usually means more torque and smoother power delivery, as every revolution will have more explosions. Smaller displacement usually allows for a higher redline and more upper RPM range power, as there is less mass in motion with an overall smaller explosion. With a turbo, the number of cylinders will effect how fast the turbo spools, as there will be more exhaust pulses per revolution. Engines with 8-12 and 16 cyl are smoother as the number of cycles per revolution are divisible by 4. There are 360º in 1 revolution, with 4 cycles (intake, compression, ignition and exhaust). In a 4 cyl., there will be 1 cycle per 90º, which is why 4 cyls often feel rough at idle. In an 8 there will be 1 cycle per 45º and so on.

 

Thats a really good write up, i enjoyed, thanks for that!

 

As the other guy said, if everything's identical, including output and weight, then the big difference is smoothness of delivery. Also(this goes against output being the same), a larger engine will make the power at a lower RPM than a smaller one. This increases the amount of area under the hp/tq on a dyno graph. Ideally, it would be a perfectly flat straight line across the dyno sheet. If two cars are identical with the same peak horsepower/torque, the one with more area under the curve will be faster at accelerating to peak and will win. Something else to consider, generally the more cylinders an engine has, the heavier it weighs. Horsepower isn't real, it's a derivative of torque.

 

Regarding # of Cylinders:

If power and torque outputs were identical, and the gearing was appropriately modified to support the rev range of the engine, then the number of cylinders does not matter.

However, where the number of cylinders DOES matter is how long the engine will last at the given power rating, or how beefy the internal components and cylinder wall must be to support the power.

Let's say there are two engines that make 480hp/480 ft-lbs torque. One engine has 2 cylinders and the other engine has 8 cylinders. In the 2 cylinder, the piston, bearings, rod, and crankshaft must support 240 ft-lbs of torque per cylinder @ 4x the cylinder pressure. In the 8 cylinder, the same parts must only bear 60 ft-lbs of torque per cylinder @ 1x the cylinder pressure.

Back to reality:

Now, why not make 2 cylinder engines that make 240 ft-lbs of torque per cylinder? (Theoretically) We could. They're called diesel engines. The problem is that the incredibly huge, fat pistons, cylinder walls,and rods required to resist that much torque makes all the parts very heavy. Because the parts are heavy, they are limited in how fast they can rev before the acceleration forces cause the parts to tear themselves in half and engine oil won't be able to resist the shear force required so the bearings will smear into a blobby mess. That's why you could theoretically make a 2 cylinder, 480 ft-lbs torque diesel engine, but its horsepower rating might only be 40hp.

OK, if that's the case, why not make every engine a 12 cylinder engine so that the components don't have to take much strain? Because the total # of components increases with each cylinder added, the block itself becomes more complicated, and the crankshaft becomes significantly more complicated, as well as balancing to cancel out secondary vibrations.

Regarding Displacement:

If power and torque outputs were identical, then you must compare rpm @ peak power.

If rpm @ peak power are identical, then the larger displacement engine is less volumetrically efficient compared to the smaller engine, meaning that its intake/cam/valve/exhaust can't move as much as much air into the cylinders to burn as the smaller engine.

If the volumetric efficiencies of the two engines are identical, then the smaller displacement engine will have a higher rpm @ peak power compared to the larger displacement engine.

Now, this isn't in your question, but let's say that you had two otherwise identical engines, one with 2L and one with 4L, how would the power outputs differ? The 4L engine would take in twice as much air as the 2L engine, and therefore create (theoretically) twice as much power.

Because there is a (sort of) fundamental limit on the maximum volumetric efficiency of a naturally aspirate engine, NA engines depend on revs and larger displacement to make more power.

However, under forced induction, the maximum volumetric efficiency is limited by the turbo/supercharger, which means that you can use smaller displacements and lower revs (of course, limited by the strength of the components of the engine.

This is sort of an oversimplified explanation, but I figure it's better than nothing.