What is torque?
Probably the best way to start this is a simple explanation of what torque is - I'll start with the most common and useful measurement used, foot-pounds. This simply means what it says, feet times pounds.
If we put a 1 foot long wrench on a bolt and hang a 1 pound weight off the end, we would torque the bolt to 1 foot-pound. If we increased the weight to 2 pounds on the same wrench, we would increase this torque to two foot-pounds, or could achieve the same by leaving the weight at 1 pound and using a two-foot wrench.If we increased both, we would achieve 4 foot-pounds of torque.
It's that simple - torque is just good old fashioned leverage applied in a circle. It means the same thing on a bolt, on a car wheel, a steering wheel or a ratchet.
Why do we "torque" bolts?
Firstly, let me say we don't torque everything - things like tinware screws and small bolts should be simply snugged uo, but more on that later. Generally, we are tightening high-tensile bolts to a particular torque for very good reason.
So, first a little metallurgy. Those who studied mechanics at college may recall placing metal samples in a testing machine and running tests on them.While we will see tensile strength quoted on bolts, there are a number of important characteristics we need to know, the most important being the elastic range, plastic range, breaking strength and resistance to fatigue.
If we place a sample in the machine and start applying incremental amounts of tension (pulling force) we will find initially it will remain at the same length.At some point we will find the sample becomes longer under tension, then returns to it's original length when relaxed - this is the start of the elastic range. As we increase tension we will eventually find a point where the sample stops returning to the same length -it has permanently stretched. This is the start of the plastic range and if we continue increasing tension the piece will break, even if we keep replacing it.
If we repeatedly apply the same tension, then release it, we can test for fatigue - this means that eventually the piece will break, even though the tension never gets to the breaking tension determined above. At the lower end of the elastic range the number of repetitions required may be so high as to be meaningless, but toward the upper end of the range they may be relatively few, or the piece may stretch prematurely.
There are a similar set of characteristics for metal under compression, though of course they do not break and we usually alloys so that we remain within the rigid or minimally elastic range. And again there are characteristics for bending, but these do not concern us here.
When we choose a bolt or stud we aim for it to be operating within the safe part of the elastic range, taking into account any fatigue it might be subjected to. As we screw a bolt in, it will enter the thread of course. When the head contacts the surface and takes up any slack, it begins to place tension on the bolt. Knowing the angle of the threads, we can calculate what torque is required to provide this tension - providing everything was well cleaned and lubricated with oil or thread-locking liquid.
Some people, particularly tyre-fitters, think that putting more torque onto a fastener will make it stronger - in fact the reverse is more likely true! We can actually make up charts of the optimum torque on a particular diameter stud or bolt of a certain composition - though a little lower may be required for, say, a head stud in order to combat fatigue! If we tighten a fastener further into it's plastic range or where it will succumb to fatigue, we actually make the fitting weaker.
If you need to make a fastener stronger, you need to make it bigger, or of a different material! Making a stud longer means it will suffer less tension for the same increase in length, so does make it a little stronger. Making it thicker does of course make it stronger too - the reasons for not using the thicker 10mm studs on VW engines is for clearance around the spigot and pressure on the aluminium head due to the increased strength and less stretch. The head nut washers tend to pound into the head a little, but re-torquing the head a couple of times in it's early life should overcome it and is a good idea anyway. Periodic re-tensioning of VW head nuts is specified in the maintenace schedule, but failure to do this is a primary cause of engine failure -it's a bit of a hassle to remove the tinware and rocker shafts and besides, other engines don't need this, right?
As you probably don't have the ability to easily change the fasteners in your engine, I hope I have convinced you to purchase a quality torque-wrench or two and use it to torque things to manufacturers' recommendations.
- But how does that rate to the torque my Engine produces?
Torque versus Horsepower
While salesmen love to talk about horsepower, you may also have heard torque quoted for engines and maybe even heard that "torque is more important than horsepower" from people such as myself.
This is not quite true.
Peak torque is not much more important than peak horsepower. Torque is important - let me demonstrate with the example of a steam engine:
In a steam engine, we can multiply the steam pressure, in pounds per square inch, by the area of the piston face, to get pounds of pressure on the rod. We multiply this by the distance of the pinion from the centre of the driving wheel to get torque in foot-pounds. Experience gives us a percentage to reduce this by to account for friction and knowing the weight of the train we can easily calculate the maximum gradient it can climb.
The steam train has the advantage that this torque is available from standstill, providing we have enough pistons staggered around the drive axle, or the piston is in the optimal position. I have simplified things, but I hope you get the idea. The torque of the steam engine gradually drops off as speed increases due to restriction of how fast we can get the steam into the cylinder at the same pressure and eventually how fast we can produce the steam.
Our car engine does not reach it's maximum torque until it reaches several thousand rpm usuallythen this declines again. This due to the need to suck the air-fuel in, compress it and exhaust it and is affected by cam timing, intake and exhaust design and head choices. The stock VW engine made peak torque about 2000rpm and heavily modified one might not achieve this until 6000rpm or more.
Power is the rate at which energy is produced. A certain amount of energy is required to achieve a certain speed. A car of mass m will have kinetic (moving) energy of 1/2mv2 Joule at a speed of v , but will also require a certain amount of energy to overcome friction and air resistance. So power affects both how fast a car will accelerate and the maximum speed it can achieve.
The trouble is, the power an engine produces is also dependant on the speed of the engine - the example of the VW is producing 27hp at 2000rpm, rising to 47hp at 4000rpm, at the same time, the torque drops from 72 to 60 ft-lbs. While this amount of torque will still be able to pull up a steep hill at 4000rpm, the engine is capable of only accelerating about half as much at 2000rpm, yet it will spend more of it's time at this end of the range (less power = longer time to accelerate.)
A hopped-up engine tends to have it's peak torque higher in the range so unless a close-ratio gearbox or high-stall torque converter is used to keep the revs high in the range, this problem becomes much worse. An example bored and stroked engine with a fairly mild cam produces peak torque at 4250rpm and peak hp at 5250rpm - if we were trying to use it in the same range we would have 46hp at 2000 rpm and 113hp at 4000 rpm. Of course we can hold it in gear longer to the maximum of 142hp at 5250 but the ratio is now more than 3 to 1. More popular cams used on this engine combination can produce even higher hp of 160-170 at 6-7000rpm, but may end up with less than stock at 2000rpm!
This is why with a stock gearbox and diff ratio it is important to concentrate on having torque and getting it low in the range. Race cars can use low diffs and close ratio gears to keep those revs up and multiply torque -and aren't so worried about fuel consumption, wear or noise.
The "low" diff - it is important to point out that a diff called "low" actually has a high ratio, it is a low speed that is referred to - not only allows the engine revs to be held higher, it multiplies the torque of the engine. The gearing down provides leverage so that, ignoring losses, the torque out is increased by the same ratio that the speed is decreased.
In the same fashion, in order to be "close ratio" the higher gears need to be of higher ratio (lower.) As they are closer in ratio, the engine is changed to the next gear at a lower point and using a smaller range of engine speeds. As the engine is capable of higher revs, the car is not slowed by these higher ratios.
