Engine Principles

Because most of us are driving landrovers or road vehicles with regular gearboxes or automatics, I'm not going explain how to build a full race engine here, since the best roadgoing, trials and comp safari engines are not usually in a high state of tune either.  What I will try to do is cover the basics with standard or near standard engines suitable for the above vehicles.  Sorry if some of it seems like teaching some of you to suck eggs, but it's important for what follows if you didn't know it already.  Some of it is fairly technical although I'll try and leave out the tricky terms and maths as far as possible and stick with rules of thumb.  I'll have to assume that you all know roughly how a four-stroke engine works and build on that. You'll need to get through some tricky bits though, Let's start with some terms you already heard.

Torque - measured in pounds per foot (ftlb,lbft) or Newtons per metre (N/m) usually. This is the turning force produced by an engine, measured at the crankshaft, at a particular speed (RPM or revolutions per minute).  Just as when you turn a spanner and apply a torque, so it's the turning force per engine revolution.  Usually quoted as the maximum value measured along with the RPM it occurred at.  It’s more important than BHP when calculating engine performance and gearing.

Power - measured in Brake Horsepower (BHP),Kilowatts (KW) or PS (about 1.1 HP). This is the actual power output of the engine, which can be used to do work pumping water, generating electricity etc.  Also measured at a given RPM and usually quoted as the maximum value measured along with the RPM it occurred at. I'll be using lbft and BHP.


There's a convenient expression to relate these two terms. If you wanted to know the torque at a particular rpm and you know the power, then

Torque = (5250 xBHP) / RPM

Conversely, BHP = (Torque xRPM) / 5250

So if I want to know the torque of a certain 3.5 Litre V8 engine at 5000rpm, knowing the power at that speed is 156BHP, then Torque = 5250 x156 / 5000 = 164 LB/ft And if I wanted to know the power produced by the same engine at 2500 rpm and I know the torque at that speed is 200lb/ft, then Power = 200 x 2500 / 5250 = 95 BHP.

Now the thing is that power is directly related to engine speed if the torque stays constant, but it doesn't.  What you tend to see with all the standard V8's is that the torque climbs fairly quickly to somewhere near it's maximum value and then gradually falls away until it gets up to a certain rpm where it drops like a stone.  The graph of the power curve is going to simply be the torque multiplied by the engine speed.  Consequently, what we are interested in always is what the torque is doing since this is what affects the power at any engine speed, so I won't talk much about BHP and I'll focus on torque instead as we progress.

Compression ratio (CR) - This is a ratio derived by comparing the volume of the combustion chamber against the total cylinder and chamber volume.  So starting with the total engine size divided by the number of cylinders, or 3528cc / 8 = 441cc, if a cylinder + chamber volume of 441cc has a total chamber volume of 47.18 cc then the CR will be 441cc/47.18cc =9.35:1 CR For the purists,I've also included the volume contained in the gasket height, the crevice in the spark plug and the volume in the piston bowl. Torque is heavily influenced by the compression ratio.  Within a certain range, roughly a 10% increase in CR will result in a torque increase of about 5%.  However, for reasons that I'll come to later, modifications to CR are only useful over a small range. V

olumetric Efficiency (VE) This is a measure of how well the cylinder is filled on each induction cycle.  If regarding the engine as an air pump, during the period that the inlet valve is open, the cylinder should ideally be filled completely with fresh air.  In practice this doesn't happen except on engines with forced induction such as turbo and supercharged units.This is because air is forced in at atmospheric pressure against a number of restrictions as follows:

The intake charge will additionally be diluted with fuel. All of these make it impossible to completely fill the cylinder under normal circumstances.  There are methods other than forced induction to overcome this using pulse tuning or ram effects which can overcome this but the usefulness is limited on a road going engine that needs a wide torque spread at relatively low speeds. A typical V8 engine may have a best VE of below 80%.  

Even so it has to shift a staggering amount of air. We can calculate this too: Assuming 4000RPM and VE of 75% (0.75) and remembering that each cylinder is only filled once for every 2 engine revolutions, 3528cc/2 x 4000rpm x 0.75 = 5292000cc of air per minute. That's 5292 litres or over 5 cubic metres per minute.  

Now that really is a lot of air.  Not only has it got to get in easily, it also has to get out.  Worse still, when it comes out it's very hot so the volume is much greater (or the pressure).  So we can see that getting the gases in and out quickly and easily is rather important.  On all Rover V8 engines, this is the one area that needs the most attention and produces the greatest gains, especially the “getting it in” bit. This brings me to Boyles law.  

Essentially it states that if you compress (or expand) a volume of a gas, its temperature and pressure will change.  This has implications everywhere, since pressure and temperature changes occur in the intake, the cylinder and in the exhaust and the pressure in the cylinder is what gives us the torque.  Good control of these effects and attention to the related subjects of gas dynamics and aerodynamics are the keys to high efficiency and economy and consequently power.