September 01, 1987

Straight Torque - Running the Numbers

Righto. Heads still reeling from last months little discussion? Good! This month we will take a look at these figures in practise and see how all this waffle actually gets to the wheels.

As promised, CB-2801 revved itself silly on ARB's dynamometer last week and spewed forth a collection of power and torque numbers. Before we look at them, pick up a copy of Suzuki's 1300 Sierra brochure, flip over to the back page and look up the figures for maximum power and maximum torque. You'll find that maximum power is 47 kw (kilowatts, remember them?) at 6000 rpm, and maximum torque is 100 Nm (Newton-metres) developed at 3500 rpm.

Now when I tell you that the Mobile Fueltank developed only 40 kw at 6000 rpm, you might think "What a wreck! It's 7 kw below standard specs!" But it also developed maximum torque of 1500 Nm at 3000 rpm. That's 15 times the standard spec!

Whoa! What's going on here? Down on power, up on torque? Well, the first point to note here is that the output figures quoted in most manufacturer's brochures are measured at the flywheel, not the rear wheels. Remember, that power is "the engine's ability to do work". And turning all those gears, shafts, wheels, tyres, etc, is hard work! So by the time it gets to the back wheels it has lost a bit of its oomph. Around 30% of its oomph, in fact.

And the torque figures? Well, you see, a gear box goes by another name… engineers refer to a gearbox as a "torque multiplier". And simply stated, the torque that goes in one end gets multiplied so there is more of it at the other end. By the time you put in 100 Nm, when it comes out the other end, the gear reduction ratio is responsible for my ensuing 1.5 kNm, or 1500 Nm.

So, you see, Betcher's Beast is not really that breathless after all; in fact it is quite respectable… 47 kw minus 30% drive train loss is about 33 kw. Mine had nearly 40 kw. You will also notice that the Cockroach developed maximum torque at 3000 rpm, 500 revs lower than standard spec. All this can be attributed to, I suppose, the extractors, sports exhaust, Finer Filter, Nulon oil additives, good HT leads and new plugs. All these things add up and they do seem to make a difference.

Now, while we are on the subject of gear ratios, you have all heard the common old sierra whinge "the gearing is too high". What does that mean exactly? We all want to do our hard 4WDing at the optimum speed - very slowly! But when you go down to a nice slow crawl, the motor slows down as well. When the motor is just ticking over, say 800 to 1000 rpm, it is developing bugger all torque, or turning force, and of course it takes very little to make it stall. The motor simply cannot keep turning the load being placed upon it, and so it gives up and stalls.

What you really want is a system whereby the motor is able to get up higher into the torque band, say at least 2000 rpm, preferably higher, but to still allow the car to crawl along at a very slow pace. This is what's meant by having lower gearing. 1.3 Sierras are geared low-low at 33:1. The early 2-stroke Suzukis were geared low-low at around 56:1 … the needed to be geared like that because 2-stroke motors only really develop torque if they are spinning quite fast. But that gearing is about 40% lower than a Sierra. Disgusting isn't it?

You can work out the end ratios by looking at the Suzuki brochure again, finding the gear ratios for Transfer Box, Gearbox and Diff Centre (sometimes called the Final Drive) and multiplying the three together. For the 2-stroke, it's 3.012 x 3.835 x 4.875 = 56.324:1.

Why? Because in the modern day rush to get from place to place quicker, Suzuki (and indeed nearly every other 4WD manufacturer) has made a compromise between a very low first gear and a reasonably tall fifth cruising gear. It is simply easier to build it that way because the transfer case can be made a reasonable size… having a big difference between the high and low range means a big difference in the actual cog sizes… hard to fit in a compact case.

In the 1300s, you can drop a 1 litre transfer box into it, but it will lower your high range a little too. You can pop in some lower ratio diff centres, but again, down come all your ratios in every gear. Maxi Drive in Queensland were talking of designing an interconnecting gearbox to fit between the back of the gearbox and the front of the transfer case, which would be a third box with 1:1 and then something lower again. So, when low-low wasn't enough, engage the interbox and you have low-low-low, if you get what I mean. Sounds like a great idea, but when will it surface? Don't hold your breath waiting - the total percentage of zooks that would even consider the use of such a device is pretty minimal, so it's no surprise it isn't on the 'things to do' list of most manufacturers. Wouldn't it be good if the club could design and manufacture one? Something to think about anyway!

Chris Betcher
September 1987

PS - Since this article was written back in 87, there are a couple of solutions to the gearing problem. The best one seems to the Rockhopper, a set of replacement transfer case gears which reduces the final drive by about 83%!

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