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.
We can boil the specific qualities of a rewarding engine down to 4 key characteristics:
1. Natural Aspiration
This allows for the maximum engine response and minimum restriction on air-flow.
2. Torque arriving fashionably late.
Camshaft timing that builds torque when the engine is revved to redline.
3. Short stroke, gentle torque fall off at high RPM.
Reduction in frictional losses, and good airflow near and at the redline.
4. High cylinder count, over-square engines
Not implying that a square or under-square engine is not competent, it is just not ideal.
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