Don’t believe everything you read in car brochures

BACK when electric typewriters and telex machines were cutting edge, I was very fortunate to meet the late Phil Irving MBE, the great Australian automotive engineer and author best known for his design work with the world championship-winning Repco-Brabham Formula One engine and the speed-record breaking Vincent V-twin motorcycle engines.

At the time Phil was consulting on, and I was reporting on, the development of a rotary-valve cylinder head for four-stroke engines by Australian inventors Peter Gabelish and Albany Vial.

On our first meeting Phil asked with hardly an introduction, “Son, what does bhp stand for?” As Phil was near 50 years my senior I wasn’t offended by him calling me “son” but I did think that a silly question given the apparently obvious answer.

EXPLAINED: What is power and torque

“Brake horsepower,” I replied confidently, only to be told, “No son, it’s brochure horsepower.”

Phil didn’t explain what he meant by this at the time but later stressed the point that manufacturers’ quoted power and torque figures are maximums only and, unless you know the shape of the power and torque curves in question, are largely meaningless.

In other words two engines that claim 150kW and 450Nm, for example, aren’t necessarily equal in effective output, and what’s on the sales brochure is just that, a ‘brochure figure’ that doesn’t necessarily tell the whole story.

The problem with sales brochures, be they printed or digital, or manufacturer’s vehicle specifications in general, doesn’t end there however, and many things that may appear to be equal when comparing one set of specifications to another aren’t necessarily so.

Take for example driver-switched rear differential locks, commonly fitted to new 4x4s these days. On paper these would all appear to offer the same benefit, but that’s not the case. In reality they come in what we will call ‘A-Grade’ lockers and ‘B-Grade’ lockers.

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A-Grade rear lockers, when activated, obviously nullify the electronic traction control (ETC) across the rear axle but still keep the ETC on across the rear axle active. B-Grade lockers cancel the ETC front and rear, leaving the front differential effectively open.

The difference is substantial and while A-Grade lockers always provide a traction advantage when switched on, B-Grade lockers don’t, which means 4x4s fitted with B-Grade lockers may perform better off road with the locker switched off, depending on the exact nature of the terrain in question.

Unfortunately, on a manufacturer’s brochure or specification sheet, while it may list ‘Rear Differential Lock’ it will never list ‘B-Grade Rear Differential Lock’.

ETC: The gift that keeps on giving

And, while on the subject of traction, not all ETC systems are created equal and some are far more effective off road than others.

All new 4x4s now come with ETC as ETC is a sub-set technology of Electronic Stability Control (ESC), which is mandatory on all new vehicles, 4x4s now included.

ETC wasn’t initially developed as an off-road traction aid but as a safety feature to limit or prevent wheel spin under acceleration on slippery roads. It was actually developed before ESC and was one of the stepping-stones in ESC development.

ETC designed to prevent on-road wheel spin still helps off-road but nowhere near as much as second and third generation off-road specific ETC pioneered by the likes of Land Rover and Jeep and taken up by others.

Again it’s a case of A-Grade ETC and B-Grade ETC although with the ongoing development here there’s more like A, B and C-Grade ETC now. The difference is in the tuning of the electronic software that controls the strength, timing and duration of the application of individual wheel brakes, the engine power modulation, and the shift protocols used by automatic gearboxes now almost universally fitted to modern 4x4s.

The irony of all this is the most important off-road traction attribute of a 4x4 is wheel travel and that is one specification you very rarely – if ever – see on a sales brochure. If you have long and supple wheel travel then the need for, and importance of, diff locks and ETC is greatly diminished.