Oom Koos Watermeyer, Hennie’s father-in-law, started an argument amongst us the other day. He popped in for a chat and a cup of tea, timing his visit to coincide with our morning tea break under the big tree.
During the conversation he mentioned that he was very pleased with his new 1,8-litre Toyota Corolla because it could crest the long hill outside the town in top gear at a steady 80 km/h, whereas his previous car had needed a change to fourth gear to prevent a serious drop in speed.
I jokingly remarked that it was quite amazing how reluctant some people were to change gear, but Syd put the same thought more forcefully: “Surely it would have been better to select a lower gear rather than using full throttle in top gear.”
Hennie then jumped to the old man’s defence by saying: “Come on, Syd, you know that a lot of revs can often create more stress inside an engine than a large throttle opening. After all, using full throttle at 80 km/h cannot be considered lugging.”
butted in to say that at that speed, and for such a short distance, it didn’t really matter whether you used fourth gear or top gear. Both men agreed, but then Oom Koos said: “This raises an interesting question. In which gear does an engine work the hardest?”
Syd opted for top gear, because if a clutch starts to slip it will initially only slip in top gear, so that you can still drive in the lower gears. Hennie defended his stance by saying that the loads on an engine increase dramatically as the revs go up.
In the end I ‘phoned Jake Venter at CAR to ask for his opinion. He said: “Excessive revs are harmful, but it’s usually beneficial to change down to avoid using a large throttle opening in a high gear, because that puts a very heavy combustion load on the pistons.
A steady speed such as 80 km/h requires a fixed amount of power at the rear wheels from the engine. In a modern 1,8-litre saloon this will be about 12 kW.
When you change gear at that speed, the lower ratios increase the engine speed and reduce the torque required from the engine, but the higher ratios lower the engine speed and increase the torque required from the engine to maintain a delivery of 12 kW. This follows from the fact that the power output is essentially the torque times the revs.”
So it seems Syd is correct. An engine usually works hardest in the higher gears, but under the conditions mentioned above this conclusion is academic. The difference in wear rates would be minimal.
