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Performance with Economy

Cars that go but don't drink

by Julian Edgar

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Over the last 18 months, we at AutoSpeed have changed in editorial direction. Much more on fuel economy and much less on huge engine, heavy cars. But that doesn’t mean that we dislike performance – far from it.

Nailing-it and going hard is fun, fun, fun.

But pulling up at the petrol bowser and handing over a huge wad of cash isn’t the stuff of joy. (In fact, who now pays for petrol in cash? – the amounts are getting higher than most people carry around with them in folding!)

So what can be done in terms of modifications to give good performance – but still with excellent fuel economy?

Let’s take a look.

The Right Car

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First and foremost, pick the right car to start with.

The best compromise of performance and fuel economy is yielded by a small turbocharged engine. So, how small an engine? That depends on how high a boost you want to run and how much off-boost performance you want.

Turbochargers – whether fitted to diesel or petrol engines – effectively increase the capacity of an engine. A 2-litre engine can develop the power and torque of a 4-litre engine, while still in normal driving having the light-load frictional and pumping losses of a 2 litre. A car of around 1300 – 1500kg will have excellent performance with an appropriately modified turbocharged, 2-litre, four cylinder engine.

The greater the requirement for off-boost performance, the larger should be the engine. And, to an extent, the more reliability and less engine stress you want, also the larger should be the engine. But in both cases, invariably the larger the engine, the poorer the fuel economy.

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Amongst others, I’ve owned a 660cc 3-cylinder turbo car, a 2-litre 4-cylinder petrol turbo, a 2 litre 4-cylinder diesel turbo, and a 2 litre factory supercharged 6-cylinder. The smallest engined car was a helluva lot of fun, and in modified form, amongst the quickest of these cars. It was also by far the most economical. But the modified engine was very highly stressed and in fact blew up twice, the only car I have owned in which I have killed engines.

‘Very small with lots of boost’ is fun and can give brilliant economy, but it’s more work to drive and more likely to die. Don’t let that put you off - but be aware of what you’re getting into.

In round figures, in spark ignition engines, a modified power output of 100 kW/litre, with peak torque at around 1/3rd full power revs, is a good goal. Don’t just focus on the power/litre figure – the bit in italics is absolutely critical. It guarantees you a ‘full’ power curve, allowing you to trickle along, changing-up in gear early (or letting the auto trans do that for you) while still getting adequate performance. That early change-up (implying low revs are being used a lot) will help give you good fuel economy - more on this in a moment.

So when picking the right car for fuel economy and performance, remember that:

  • Turbo engines have better thermal efficiency than any other car engines

  • Their ‘variable size’ makes them efficient in both fuel economy and performance

  • Spark ignition engines need sufficient capacity (ie litres of size) to give good performance in the chosen car without exceeding about 100 kW/litre specific power

Finally, the factory standard fuel economy of the car will help give an indication of its rolling resistance and drag coefficient.

Picking the right car to start with makes a bloody enormous difference to how likely you are to end up with a performance car that still gives good fuel economy. It’s the single most important decision.

Easy Modifications

Read that sub-title again: easy modifications. You’d think that there might be no such things, that modifications are sure to guzzle fuel and destroy any fuel economy your newly-chosen car might have. But that’s not the case.

Basically, anything that reduces pumping losses (or in other words, lets gas flow more easily in and out of the engine) has the potential to improve fuel economy as well as performance.

First are the exhaust and intake.

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Air needs to be able to flow as easily as possible to the intake valves or turbo; any restriction harms both power and economy. Suitable modifications include picking up cool intake air from an area of high aerodynamic pressure, free-flowing factory airboxes (eg with a new bell-mouthed exit duct) and reducing pressure drop in under-bonnet tubing.

(We’ve covered each of these techniques in great detail over nearly a decade of AutoSpeed so I am reluctant to try to say it all again in one sentence. in short, do site searchs under cold air intake, Magnehelic and Negative Boost. You will find literally dozens of step-by-step articles.)

The good thing about modifying the intake is that all the techniques are well within the range of a competent DIY enthusiast, and the modifications work very well.

Next is the exhaust.

In turbocharged cars, the less backpressure that can be created after the turbo, the better the car will perform in straight-line acceleration and in fuel economy. Care needs to be taken to maintain cat converter emissions performance (and in diesel cars, particulate filter performance), but the bottom line is just to get the exhaust gas out as fast as possible.

In addition to improving full-load cylinder filling (and so full-throttle performance), a big exhaust on a turbo car will also allow the turbo to boost faster, giving better part-throttle torque and so allowing lower revs to be used – and that’s also good for fuel economy.

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In naturally aspirated cars, the key is to modify the exhaust to provide best torque – or in other words, best low-rpm power. Extractors designed for this invariably run very long primary and secondary pipes of relatively small diameter; if you see an aftermarket exhaust with big diameter short stumps, you can be certain that the system has been tuned for top-end performance – not what is wanted if you desire performance with fuel economy.

Many people suggest that the exhaust diameter on a naturally aspirated car needs to be kept small for torque and fuel economy; based on our experience, we’d suggest otherwise. After the tuned section of exhaust is finished (ie at the first resonator or muffler – they act as pulse reflectors and homogenisers), the larger the exhaust, the better. For cars that have been properly tuned, we’ve never seen a case that contradicts this perspective.

Camshaft?

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Older car enthusiasts, especially, love changing cams.

“Whack in an economy cam,” they say. Trouble is, in cars of the last 15 years or so, it will be a very special cam indeed that gives better fuel economy without deleteriously affecting performance or emissions. In fact, so special that it probably doesn’t exist outside of the car manufacturer’s factory...

We’re not saying that cam change will never give a performance/economy benefit, but it’s very unlikely that perceptible improvements will be able to be easily made with a cam change.

Electronic Modifications

Electronic modifications involve tweaking the engine management system. Three aspects are most popularly changed - leaning-out full load mixtures, leaning-out cruise mixtures, and advancing part-load ignition timing.

Let’s take them one by one.

In many cars, full load mixtures are extremely rich. The manufacturer has set them up like this to cool combustion and the cat converter – rich mixtures don’t burn as hot. However, if you’re not going to hold full throttle for minutes at a time or race your car on a track, it’s certain that you can get away with leaner full-throttle mixtures. In addition to improving fuel economy, this modification will also often improve power.

Cruise mixtures in some cars go leaner than the norm of stoichiometric (or about 14.7:1). Engine management mods can do two primary things to lean cruise – (1) put the car into lean cruise mode earlier and/or more frequently, and (b) make the lean cruise mixtures leaner than they normally are. Both approaches have emissions implications, especially for raising drive cycle NOx, but these approaches can yield major improvements in cruising fuel economy.

Advancing the part-load ignition timing can considerably improve part-throttle torque, especially if you also run higher octane fuel than the car was originally designed for. The outcome is that the driver can change-up earlier (or the auto trans will do that for you).

Low revs have already been mentioned. By keeping engine rpm as low as possible, the engine works more efficiently because the frictional loses (that increase with rpm) are lower than they’d otherwise be. However, the low revs still need to provide the required performance and the engine must not be allowed to detonate. Furthermore, the engine cannot be loaded to the extent that the ECU automatically enriches mixtures – which undoes all the good work of low revs!

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Another, quite different, engine management modification can be used to reduce pumping losses in throttled (ie most spark ignition) engines. If the exhaust gas recirculation (EGR) valve is under ECU control, part-throttle EGR can be increased substantially – especially if advanced ignition timing is also being used. This reduces pumping losses because the engine is able to ‘breathe’ gas that has not had to squeeze past the almost-closed throttle blade.

A Schizophrenic Car

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Most stories on fuel economy mention driving style. Invariably, they say the same things - drive smoothly, read the traffic, don’t use full throttle, and so on. However, here I want to say something quite different – and it’s about how you set the car up.

If you want performance and economy, you want a car that can deliver, as requested, one or the other – and not necessarily both at the same time. That is, when you’re driving for economy (say, on a freeway commute) you want very low fuel consumption. Then, when you’re driving hard, you want very good performance.

To use the medical term incorrectly (but in its most widely understood meaning), you want a schizophrenic car.

The best way of achieving this is to deliberately configure the car to achieve these aims. For example, in ‘economy mode’, at small loads run lots of EGR, lean mixtures and advanced ignition timing. This will give very good fuel economy but driving aspects like throttle response will be dulled. And then for ‘performance mode’, lots of boost, very good intercooling and relatively rich mixtures. Plenty of power will be developed, but at the expense of fuel economy.

If the car modifications are built on a good base and done with care, the ‘mode selection’ will be as easy as altering your driving style.

Conclusion

By picking the right car to start with, and making careful mechanical and electronic modifications, a car that can deliver – as requested – either good fuel economy or good performance is quite possible.

It can also be a lot of fun because it can achieve an outcome on the road twice as good as a traditional car modified for just performance...

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