AutoSpeed - Constant Torque Performance

When it comes to modifying a car there’s invariably little thought given to ‘the big picture’. Most people are content to fit a high-flow exhaust and intake, bump up the boost and deal with whatever clutch and gearbox consequences that result. But there is a much more driveline friendly approach – the quest for a constant torque output.

Too Much Torque?

Talk to the owner of a modified Subaru WRX and they’ll tell you a sad tale of gearbox destruction. Some owners will explain it’s because the car is now making “so much power” - but they’re very much mistaken. It’s peak torque - not power - that’s breaking their gearboxes. That and the high level of traction.

Here’s a typical gearbox breaking scenario... You nail the throttle in second gear to exit a tight roundabout; the engine rapidly builds revs, boost pressure overshoots and greater than usual torque is channelled through to the gearbox. B-A-N-G! This failure typically occurs at about 3500 to 5000 rpm – not coincidentally, exactly where an engine produces maximum torque.

From this scenario we can see that its peak torque - not top-end power - that, together with enough traction, destroys drivelines. If you’re a little unsure on the difference between power and torque, we suggest you first read Power versus Torque - Part 1

Building a Faster Car - Without Increasing Peak Torque

There is one way to make your car go faster while avoiding driveline failure. The aim is to maintain the factory peak torque output all the way through the high revs – but you don’t want to exceed the factory torque peak at any point. Put simply, all we want to do is eliminate the engine’s torque drop off at high revs.

High rpm torque can be maintained by improving the breathing efficiency of an engine. A high-flow exhaust and air intake will certainly help this cause. A custom tailored boost curve is also a very effective way of achieving a constant torque output in a turbocharged vehicle.

So how do you determine the top-end power that’s achievable without exceeding the factory torque output? This is the all-important formula...

Power (kW) = Torque (Nm) x RPM

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Here are some real-world examples of a power increase without a peak torque increase...

Mazda BFMR 323/Ford TX3 Turbo

In standard form, the B6 1.6-litre turbo 323/TX3 produces 103kW at 6000 rpm and 186Nm at 5000 rpm. Using the above equation, we can back-calculate that torque has dropped from a maximum of 186Nm to just 164Nm at peak power rpm. If we can maintain all of those 186 Newton Metres to 6000 rpm you’re looking at a healthy 117kW of top-end power – a tidy gain of 14 percent without pushing the fragile gearbox past its limits.

Mazda 626/MX6 2.2 Turbo

Rated with 108kW at just 4300 rpm and 258Nm at 3500 rpm, the Mazda developed F2 turbo engine is very much a low-rev slogger. Its power output is abysmal when you look at its peak torque output. However, by holding peak torque further around the tacho – to the 4300 rpm max power rpm - it is possible to achieve 116kW. This represents a gain of only 7 percent – but things needn’t stop there in the case of the short-revving F2 engine... If you can hold peak torque all the way to 5000 rpm (1000 rpm short of F2’s redline) you’re looking at a much more creditable 135kW. The breathing characteristics of the engine and turbocharger may make this a challenge, however.

Mitsubishi Ralliart Magna 3.5

In standard form, the factory-tuned Ralliart Magna 3.5-litre V6 cranks out an impressive 180kW at 5500 rpm and 333Nm at 4000 rpm. In this case, the above equation reveals there’s very little scope to increase top-end power without adding to the peak torque load on the driveline – Mitsubishi has already done a great job of tuning. If we extend the 333Nm torque maximum to 5500 rpm the Ralliart Magna can muster only another 12kW - a gain of barely 7 percent over standard.

Subaru MY94 – 96 WRX

A car notorious car for blowing gearboxes is the early series MY94 - 96 Subaru WRX. With 270Nm of torque at 4800 rpm, the standard 5-speed gearbox can be destroyed even before fitting go-fast mods. If we stick to the factory 270Nm torque output, however, we can elevate power from 155kW to 170kW at 6000 rpm. This represents a gearbox friendly power increase of 10 percent.

As you can see, it is possible to achieve handy top-end power gains without exceeding the maximum torque that a specific driveline was designed to handle.

Honda’s VTEC Approach to Power

Honda’s VTEC - along with Toyota’s VVT-iL and Mitsubishi’s MIVEC equivalents - are the ultimate examples of high-power, constant torque engines.

When Honda decided to make a go-fast version of its Civic, Integra and Prelude they steered away from forced induction and the mountainous torque it delivers. By simply extending the rev range beyond 7000 without dropping off torque – as achieved with their innovative variable valve timing and lift system – they achieved extremely impressive power figures without needing to drastically upgrade or reengineer any of the driveline.

Take a moment to consider that the Honda Civic VTi-R makes 118kW at 7600 rpm and 148Nm at 7000 rpm; its peak torque output is only 4Nm greater than the everyday 88kW Civic GLi...

Building a Faster Car – With Only a Slight Peak Torque Increase

So far we’ve examined the power gains that are possible without venturing beyond the factory maximum torque output. Now let’s look at what’s possible with a very controlled increase in torque...

The all-important question that you’ll keep coming back to is, “how much torque can my car’s driveline tolerate?” Unfortunately – unless you have access to the gearbox engineering papers - we can only take a guess. Generally speaking, the gearbox in most front-wheel-drive and constant AWD vehicles are not as robust as rear-wheel-drive ‘boxes. Space limitations invariably require a compact gear cluster with relatively narrow gears.

Based on what we’ve seen in the automotive industry, it’s fairly safe to assume that you can increase peak torque by around 20 percent in most FWD and AWD cars without sacrificing day-to-day reliability. You might need a gearbox rebuilt sooner than usual but you are unlikely to strip any gears. For a peak torque increase more than about 20 percent we’d be looking very closely at the factory gearset and adopting a gentle driving style.

In the case of rear-wheel-drive cars we’d be generally confident increasing peak torque by 30 to 40 percent. Many Nissan gearboxes, Supra ‘boxes and Getrag units are noted for their ability to withstand significantly more torque than the factory-spec engine can muster. On the other hand, there are a couple of RWD gearboxes that are pretty marginal even in the standard application – the BA Falcon XR6 Turbo 5-speed, for example.

Okay, so we’ve established that the vast majority of vehicles – regardless of driveline configuration – should be able to tolerate a 20 percent increase in peak torque. Now let’s return to our real-world examples where a 20 percent peak torque increase has been held to max power revs...