(According to the laws of physics)

Naturally aspirated engines present an interesting challenge for automotive engineers. Unlike their force fed brethren, there are finite limits on how much torque and power a particular engine can make, dictated by the laws of physics, so while forced induction motors are a bit of a free for all, NASP engines are a level playing field for worlds best engineers to showcase what their talents.

There are main limitations at play which we'll have a quick look at before delving into the top 10, as they may be a little different to what you're used to. Rather than use the ubiquitous 'bhp per litre' figure, (which can be easily cheated with high RPM), we're going to go a little more in depth to really even the playing field...

Specific Torque

The first limitation is known as specific torque, the amount of torque produced per litre of capacity. A largely irrelevant figure in forced induction engines (because increasing the boost pressure has the effect of increasing the size of the engine), this figure is of interest in NASP engines because unlike bhp/litre, it cannot be manipulated by high RPM. 

It is a convenient figure which describes the amount of air fuel mixture the engine can get into the cylinders, and how effectively it can turn that mixture into mechanical force. With NASP engines only ever capable of producing a few percent over 100% volumetric efficiency, this has the effect of limiting all NA performance engines to around the 70-90lbft litre region!

Mean Piston Speed

The second limitation is one which affects both turbocharged and naturally aspirated engines alike, and that is the speed at which they can run. Whilst the maximum RPM of engines differs wildly, there is another, lesser known measure of engine speed, which paints a very different picture. It's called Mean Piston Speed.

Mean Piston Speed (MPS for short) describes the average speed at which the pistons are travelling in the bores, and for any given RPM this speed will vary depending on the stroke length. The longer the stroke, the faster the pistons have to travel every RPM, so an engine with an 80mm stroke spinning at 8000rpm would have the same MPS (21.3m/s) as an engine with a 40mm stroke, spinning at 16,000rpm. 

It turns out that the maximum sustainable MPS for a four stroke gasoline engine engine is around 25m/s, the reason for this is that if the piston moves any faster it will actually start to outrun the flame front, resulting in a dramatic drop in torque. With a fixed limit on the piston speed, we can use MPS as a method with which normalise the speed of all engines, with most performance engines running somewhere in the 22-25m/s range. 

Torque and speed.... they make something don't they?


So, we know that the torque ceiling is 90lbft per litre, and that the maximum RPM is dictated by the stroke length and a piston speed of 25m/s. Combining torque and RPM make power, so it stands to reason that we can calculate a maximum power output for pretty much any engine, regardless of it's size or design.

Performance Index

The Performance Index figure we'll be using in this article is a comparison between the theoretical maximum output of any particular engine, divided by it's actual rated output. It's scored out of 1000 and can be used to compare absolutely any naturally aspirated four stroke gasoline engine because unlike HP/litre, it is normalised for engine speed. 

But why does that matter?

Consider an old school NA F1 engine, producing 800bhp from 2.4 litres, giving 333bhp/litre. A truly incredible figure compared to the very best road going cars, which hover around 125bhp/litre.

Now lets imagine we capped the F1 engine at 50% of it's maximum revs, so that it produced only 400bhp. That handicapped engine would still be making 167bhp/litre, which to the vast majority of people would still be considered more highly tuned than say, an S2000, even though in reality it is only running at 50% capacity.

Using the Performance Index figure (in this case it would be 500) would reveal the engine's true character as being heavily restricted, and show the S2000 to be the more highly tuned engine.

Anyway, enough with the science for now, if you want to have a look at the maths and learn how to calculate all this for yourself, there's a link at the end of the article. Those be the rules, let's get down to the top 10 most highly tuned NASP engines!

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