Torque Curve Shapes & A Rewarding Engine

Torque Curve Shapes
&
A Rewarding Engine

After driving many kinds of cars and experiencing different engine layouts, displacements and aspiration, a question perplexed me - why do some cars feel alive compared to others?

Within the narrow spectrum of naturally aspirated performance engines that are made today, there remains a gap between those that are simply good and those that are truly visceral. Let's explore this idea using a couple of driver's cars. Pay attention, not to the numbers presented, rather, to the shape of the curves.

Let’s explore this idea with 2 cars of relatively similar displacement, but vastly different in character, the 3.4L 6 cylinder Cayman S engine and the 8 cylinder 3.6L Ferrari 360 engine. Looking at each torque curve it is clear that the 360 revs much higher, owing to the two extra cylinders and the shorter engine stroke that enables the higher piston speed.

But looking just a little deeper, we can see that peak torque on the Ferrari arrives at a higher RPM, and falls off more gradually. What this means is that the natural curve of the engine is pushed towards the right of the curve. This means, as you accelerate through the gears, there is an increasing amount of force that you feel as you chase the tachometer. This combined with sound and vibration creates a sensation of ‘chasing acceleration’.

To further explore this idea, we will now explore this idea from the perspective of a modern turbocharged hot hatch - the Golf GTI. No one doubts that this is a fast car, but how does the engine reward the driver who pushes it further?

As you can see from this graph, peak torque arrives at 3,000rpm way down in the torque curve and starts to fall off dramatically from 4,500rpm. While the low-end kick is appreciated, there is little incentive for the driver to push the engine to its low 6,500rpm redline.

One might argue that the way to combat this is to swap to a larger, more advanced turbo and move the torque curve to the right, however, the added exhaust gasses will further blunt the naturally less responsive turbo motor. So, what if we look at a much more advanced twin turbo motor like the one in the Ferrari 488 GTB?

While the engine generates good torque till 7,000rpm, the fall-off to its lower redline at 8,000rpm is steeper than its predecessor. There is no debate on the quantitative numbers when the Ferrari 488 is compared the 458. However, subjectively, we can see the shape of the torque curve of the 458 to be more rewarding to the driver who will rev the engine out. However, there is a compromise to be aware of.

When the Honda S2000 was launched, it was simultaneously lauded for it's high redline, and criticized for it's lack of low end torque. Power to weight ratios typically compare peak power over weight. This however does not paint the full picture of how the car feels over the entire rev range. Let's explore this through comparison of the 1999 Honda S2000 and the 2015 ND MX-5. The static power to weight ratios are as follows:

Honda S2000: 240hp/1250kg = 0.192 hp/kg

Mazda ND MX-5: 160hp/1050kg = 0.152 hp/kg

The top graph shows the horsepower curve, and the graph below that shows horsepower over weight. Through the data, we can conclude that the Mazda MX-5 has the advantage in power to weight ratio up till 6,800rpm, upon which the S2000 takes over. So we need to balance top end power with lower end torque particularly in cars which have higher weights.

A special note has to be given to the Honda F20C and K20A engines. Francesco Morettini, a Chief Engineer at Ferrari who was responsible for the V12 engine in the 812 Superfast and other engines was asked which engine from other brands he respected the most. No surprises that it was Honda's F20C.

In conclusion, we can boil the specific qualities of a rewarding engine down to some key characteristics:

1. Natural Aspiration

This allows for the maximum engine response and aural experience, particularly on the induction end of the engine.

2. Anticipatory Torque Curve

Camshaft timing and lift control that builds torque as the engine is revved high, with airflow that has a gentle torque falloff closer to redline.

3. Short stroke, large bore.

Reduction in frictional losses, better rod stroke ratio.

4. Lightweight Car Construction

Ensuring that the car is as light as possible, for low end performance.

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References:

https://www.pressreader.com/south-africa/business-day-motor-news/20170817/281582355739962

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