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Where's all the power coming from? - Power Tuning

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Historic racing cars are routinely pumping out up to 30% more horsepower than when they were new. How? It's time to get technical.

Engines aren’t just about motive power. They breathe life and soul into a car. The thrill of acceleration and the satisfying response to your command are complemented by glorious sounds, tingling vibrations and – rather like a fine wine – a distinctive smell. But, like fine wine, some engines have improved with age. In historic racing, those engines are producing significantly more power now than when they were brand new. And that’s more than just a little intriguing.

It’s not a small increase either. Alfa Romeo’s twin-cam is undoubtedly a design classic, and not only provides prodigious power for its size but sounds and looks wonderful too. When Alfa campaigned in touring cars back in the 1960s, the 2-litre was tweaked up to about 180bhp, but a similar-spec engine today can squeeze out a positively exhilarating 210bhp on modern fuel with suitable cams and the discreet addition of mapped ignition.Formula Junior cars of 1000cc were producing a respectable 100bhp back in 1968 when the formula ceased, but the same racers battling it out in historic formulae today are producing nearly 30% more, which is quite an achievement without any drastic changes to the engine.
 
It’s not just the small engines: classic Cosworth DFV V8s, Ferraris, Bugattis and Jaguars are all finding new strength in their retirement. The much-loved and iconic Ford Mustang won the Indianapolis 500 in 1964 less than a month after its launch while, the following year, Shelby GT350Rs won no fewer than five of the six Sports Car Club of America divisions, all of which would indicate that it was fairly well tuned at the time. But at a recent auction a near-identical car boasted a dyno sheet showing 380bhp and 335lb ft from its 4.7 litres, a 9% improvement with age over the ’60s race-winning 350bhp.

Now, having spent a couple of decades in the car industry, I am only too aware that there will always be some tuners whose dyno test results are on the optimistic side, and any tuner who is making genuinely fast cars will be a little reticent to tell you their best-kept secrets. Yet the sheer volume of cars posting faster lap times and clutching plausible dyno sheets implies that something is different. You’d be forgiven for wondering what’s going on. And, in short, there’s quite a lot going on.

By far the most influential change since the ’50s is our understanding of the natural world – from chaos theory to quantum physics – so much of our picture of how things work has marched on to previously unimaginable realms. Most relevant to this subject are the revelations about gas flow and how to make an engine breathe effectively. Many of the tuning theories popular back in the good old days, when mechanics wore flat caps and suitable ties, were based on minimal data and spirited conjecture that, to be fair, was the best method available. Fast-forward to the present day and we find that every aspect of engine design is calculated to within an inch of its life: engines are flow-tested and run on a computer well before any metal is cast. It is remarkable to note, however, that even with complex computer-based modelling systems and a massively improved understanding of combustion dynamics, there is still a small percentage of the fierce turmoil in the combustion chamber that cannot be calculated. It’s just a little too chaotic.

Anyway, back to the plot. You will probably have heard of ‘gas flowed’ heads, but do you really know what it actually means? Some cheaper cylinder heads might only have had the ports sanded down and smoothed over, while the more reputable heads may have wider ports with a subtly different shape in order to influence the way the air is presented into the cylinder. Half a decade ago the top teams were using flow benches where, to put it simply, a big fan sucks air through the cylinder head being tested – the higher the flow, the more power could be had from the engine. Or so they thought. But it turns out that life is not quite that simple.

The complication arises because the flow through a port stops and starts very rapidly as the valves open and shut. Half a century ago, engineers had no means of accurately measuring what was going on in detail between each cylinder firing; they could only measure the average flow over several cycles.
 
It’s different now. Picture the scene with an open inlet valve: air is rushing into the cylinder, then the valve shuts but the air column is still moving due to its momentum and slams against the back of the valve. We now know that if you get the timing just right then when the valve opens again there will be a nice head of pressure there to force a little more air in.

This works because air is surprisingly heavy stuff. Each cubic metre weighs about 1.2kg – the air inside an empty shipping container weighs more than an average fully grown man. But, I hear you say, surely there is not that much air in an engine for its momentum to make a difference? Well, for every 100bhp you get through approximately 76 litres of air per second – that’s like drinking 133 pints of beer in one second.

Going back to the inlet port, the air’s momentum depends on its speed and, if you open the port out too much, the air speed drops, and with less momentum to give that helping push you can end up with less air going into the cylinder on the real-life running engine, despite having good flow bench results. This, with a little imagination, gives us a clue about how the efficiency of the inlet system, exhaust system, port design and valves is heavily dependent on camshaft design and timing.
 
In a further twist to the old logic, it turns out that a little bit of rough surfacing on the inlet port can be a good thing, particularly on carburettor-fed engines where fuel has a nasty habit of falling out of the air and dribbling along the manifold walls. Then a bit of rough surface encourages the fuel to vacate the metal and rejoin the air flow. So the cheapest of the ‘flowed’ heads would actually make the problem worse.

Applying this knowledge to rebuilding a historic race engine means that, even within tight regulations, valve seat angles, cam profiles and valve spring materials can be optimised to get a bit more wind through the engine. John Crabb from Piper Cams says: ‘The whole intake/valvetrain and exhaust system is considered to be one mechanism, where all the parts are designed to work in harmony. Valve seats, valve sizes, cams and gas-flowing the intake and exhaust are all designed to improve the flow path through the whole engine system.’
 
As well as improving flow, modern valve materials can be lighter, which allows them to move faster. This means that the valve spends less time tied up in the tedious process of opening and closing, usefully spending more time being fully open or closed. But just as importantly it allows the engine to rev higher before the valves start ‘floating’. Valve float happens when the valve is opened very fast and momentum keeps it lifting, losing contact with the cam lobe: in severe cases it can be flung into the piston face, which is potentially catastrophic. Lighter valves stay under control at higher speeds, and higher revs can mean more power, which is generally appreciated.

You might think that changing the valve material to something more modern is cheating, but actually the material can still be steel, just a lot purer, so the thickness of the valve head can be reduced without compromising strength or heat dissipation. Technology’s wonderful.

We all like to admire a nice set of pipes, and similar leaps of understanding have helped to improve intake and exhaust manifolds as well as silencers and air filter housings. A recent historic Porsche 911 race car had its power raised from 204 to 230bhp largely due to improved tubework that better complemented the way the engine worked. Matching exhaust headers and intake manifolds to the port ensures that the momentum is maintained, both for generating a pressure pulse at the inlet and a slight depression at the exhaust valve. Also, where runners join together, the pulses can be combined, either to amplify the effect (high peak power but narrow useable rev band) or to dampen the effect (lower peak power but wider usable rev band). Sometimes a slightly lower peak power figure can make a car faster on the track when the spread of power is more liberal, giving greater average acceleration in each gear and reducing the number of gearchanges needed per lap.

Sometimes a power gain can be achieved by a new tank of petrol. With all the adverts everywhere these days you may be aware that fuel technology has marched ahead quite dramatically. It is remarkable that, even with the same energy content and octane rating as older fuel, some modern ones manage to produce more power. This near-magical feat comes from a combined attack on many fronts: by modifying the way the fuel mixes with air, the accurately engineered blend of around 200 different substances can be tailored to ensure less fuel is wasted as unburnt hydrocarbons, and if more of the fuel burns in the combustion chamber rather than simply falling out of the exhaust port then you get more performance.

Also, if the fuel burns more readily at the start of combustion, there is less burning at the end of the combustion phase and so less heat is transferred into the exhaust valve and its seat. This reduces the burden on the cooling system and, because the chamber metal temperature is a little lower, there is less tendency for combustion knock, allowing more ignition advance which, in turn, makes for more power.

I run the splendid V12 of my long-suffering Jaguar XJ-S race car on BP Ultimate petrol. Initially I was not entirely sure that it was making much difference. Sure, I found I could run a little more ignition advance and the throttle response seemed a little more sprightly, but without a dyno test I couldn’t be certain of a significant improvement. That is until one day I was forced to run on ordinary unleaded, and the performance drop was noticeable. What appeared to have happened is that the Ultimate had made a steady improvement to the engine over a period of time.

BP has the following to say: ‘BP Ultimate Unleaded can help reduce friction in two different ways. First, it has an immediate action through the fuel delivery system: it coats the upper cylinder walls with friction-reducing components every time the fuel is introduced into the cylinder, meaning that less energy is lost between the piston rings and cylinder walls. This reduction in friction is more than can be achieved by the lubricant alone. But beyond this, the friction-reducing components accumulate in the lubricant over time, so BP Ultimate Unleaded can help improve the properties of your lubricant as well.’

Other racers swear by Shell Optimax or some other modern fuel. As well as peak power being raised by a few percent, the throttle response and performance over a wide temperature range are significantly improved. Some historic engines can be prone to a hefty cough when you floor the loud pedal due to petrol’s tendency to condense and simply fall out of the air when the pressure rises, but this seems to be almost eliminated, showing the new fuels’ better ability to cope with difficult circumstances.

But that is not the end of the contribution the oil companies have made. Engine oil has a very hard job, so perhaps it’s worth explaining a few aspects. Most bearings in the engine are the ‘plain’ type, where the oil is used to form a layer, just a fraction of a millimeter thick, separating the two parts such as the connecting rod big end bearing and the crankshaft journal. It has to resist phenomenal pressures when the piston is experiencing accelerations of over 1000g, which can result in a force equivalent to a couple of Range Rovers hanging off the bearing. But this force reverses every stroke, battering the bearing up and down more than 100 times per second, and the oil has to stay in the thin gap doing its job without failing. It also has to cope with rotation (which in effect rips the oil apart) and, as crank speeds can exceed 6000rpm, the oil is being sheared between the two surfaces at a speed of more than 15 metres per second. Oil does a fantastic job but some of the engine’s power is wasted in shearing the oil apart.

In the current range of purpose-designed oils there is some very clever engineering of its viscosity, ensuring superior lubrication while reducing both drag and the loading on the oil pump. Older engines use the principle of low pressure at a high flow rate to compensate for wide gaps in bearings. In order for the oil pump to supply the engine reliably at idle it is usually quite large, so at full racing speed it will be pumping too hard and a relief valve usually has to be incorporated. Even so, it can account for a good few horsepower and reducing the loading thus releases a little more.

But there is more. Oil has to cope with ferocious temperatures just under the piston rings, where it is licked by flames blasting past. Modern oils for classic engines are engineered to lubricate the cylinder walls without leaving much in the combustion chamber area, thus reducing carbon deposits, one of the main causes of pre-ignition. It also enables the use of better cylinder honing, which reduces friction, releasing a little extra power.

Oil also makes a major contribution to cooling, particularly in the piston area, which has a big effect on knock tolerance and how much ignition advance can be tolerated before meltdown. Air-cooled engines rely heavily on this effect, and its heat transfer ability has been improved significantly over the years. Put all these little improvements together and new oil can make  a significant difference to power, depending on the original design.

While on the subject of cooling, that coloured fluid mixed with water in your radiator has also improved, mostly in the way the coolant molecules contact the metal of the engine. That boundary layer between the surface of the metal and the flowing coolant can impede efficient transfer of heat, but modern coolants break this down and allow the fluid to do its job. As well as taking more heat out of the engine, it also transfers heat more readily to the metalwork of the radiator, further improving efficiency.

Spark plugs have improved too, running at up to 40,000 volts and having to cope with cyclic pressures of up to 100 Bar while resisting temperatures of over 1000ºC. They now have better tolerance to the age-old problem of carbon build-up and also manage better heat control, meaning they can run reliably without the risk of hot spots, allowing a tad more advance. In fact most of the ignition system can be improved with modern components. The coil benefits from quicker energy build-up to give a more reliable spark at high revs: the Jaguar V12 in the XJ-S was originally produced with two coils to feed its single distributor but now a single unit does an even better job.
 
That’s a lot of technological advances to take into consideration, but in combination they mean that an engine that looks fundamentally the same as it did 50 years ago can breathe more easily, form a better fuel/air mixture and burn it more efficiently. They said, back in those days, that the future would be an amazing place. Looks like they were right.

The late Bob Freeman’s wonderful artworks are still available as A2 prints at £58 each – at that price, Octane says every enthusiast should own one. Order online at www.bobfreeman.co.uk.

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