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
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
First and foremost, pick the right car to start
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
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
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
‘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.
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.
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
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
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.
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
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
car enthusiasts, especially, love changing cams.
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...
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 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
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!
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
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
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