This article was first published in 2006.
Turbo and non-turbo versions of the same engines...
what’s the power gain of fitting a turbo?
Well, when there are factory naturally aspirated
and turbo versions available to directly compare with one another, the turbo
engines tend to make something like 30-40 per cent more power than the engines
that don’t have puffers. Of course, in the aftermarket, people bolt on turbos
that cause the engine to develop 100 per cent, 200 per cent – sometimes even
more – power than standard.
But they never do so with factory driveability or
So, keeping it within the boundaries of good
on-road performance across most of the rev range, and good reliability without
having to replace all the internals of the engine, you might say that a 50 per
cent power increase over the naturally aspirated figure is an achievable and
realistic turbo goal.
For example, the EF Ford Falcon that I recently
bought develops 157kW in standard form from its 4-litre six cylinder. As with
all engines, quoting just a peak power figure seldom tells much of the story –
the Falc engine is also very torquey at low revs. (So what’s that mean then?
Simply, it develops lots of power without having to be revved hard.) So if I was
going to conventionally turbo the engine, I’d be looking at specifying a turbo
that gives an end-result of 200-250kW of power.
In other words, this turbo would need to have both
compressor and turbine wheels suitable for the airflow that in this engine,
develops that much peak power. (That’s why it makes more sense than it first
seems to talk about a “200kW” turbo. It doesn’t mean it’ll develop 200kW on a
lawnmower engine, but it does mean that with appropriate gasflows going through
it – caused by the engine, remember – there will be enough airflow to make that
So, on the Falcon, I’d be looking at a turbo sized
for peak airflows appropriate for 200 – 250kW.
But hold on! What about looking at the turbo
sizing from a completely different perspective? Instead of aiming to improve
peak power, what about aiming to leave peak power much the same but improve
average power through the rev range?
People tend to lose sight of the fact that when
you’re accelerating through the gears, the engine revs aren’t constantly at peak
power or peak torque. (Maybe an exception is a very high stall torque converter
on an auto trans where the revs stay more constant as speed increases.) But
normally at full throttle, the revs are sweeping through a range of engine rpm.
And even more to the point in a street driven car,
for most of the time, the revs aren’t anywhere near peak power. In fact, if your engine has a redline of 6000 rpm (or 8000 rpm for that
matter), it’s extremely likely that you’ll be at one-quarter (or less) of that
engine speed most of the time. And where does that leave your top-end power
So let’s look at an example of what I’m getting
at. How about on a 157kW naturally aspirated engine like the Falcon,
specifying a turbo that is normally found on a car with a turbo’d 157kW?
Say, on the Falcon fitting an ex-WRX turbo?
Huh? What’s the point? The Rex turbo is from a
2-litre engine and you’re gonna put it on a 4-litre engine? How restrictive will
that be, for Godsakes?
Depending on how you set it up, potentially not at
Let’s say you run the Independent Electronic Boost
Control kit ( The Independent Electronic Boost Control, Part 1 ) - which
allows you to set turbo boost on the basis of injector duty cycle, which is in
fact related very closely to actual engine airflow). You set the IEBC so that
the little turbo boosts to (say) 10 psi at up to 3000 rpm full throttle, and
then tapers back to zero boost at the Falcon’s low redline. That way, the turbo
compressor and turbine never have to flow more than “157kW” of air. (We’ll come
back to wastegate flow in a moment.)
So what would we have? Well, you'd expect the Falcon six to spin up a
small turbo like that extremely quickly - perhaps to the extent of having 7
psi boost by 1200 rpm. So over the first 2000 rpm of working engine revs
(ie from about 1000 - 3000 rpm), roughly speaking, you'd expect 50 per cent
more power. (Or, if you prefer, 50 per cent more torque over this rev range -
it doesn't matter which way you express it.) So, speaking even more
loosely, the Falcon would become something like a 6-litre engine that still
develops only 157kW but with massive bottom-end and midrange power.
It’s already a car you can sloth around in at 1000
rpm in third and fourth and fifth gears: turbo’d in this manner, you could
probably smoke the tyres in second gear at 1500 rpm! Or, if you wish, rev it
right out to get much the same top-end as standard – but you’d get there
You can also look at the same idea with a much
smaller engine. Those current cars with a combination of low power but high
torque developed low in the rev range (the electric-assist Toyota Prius and
Honda hybrids come to mind), seldom feel slow in normal traffic. Against the
stopwatch they are slow, but the instant rush of ‘go power’ whenever you
put your foot down doesn’t make them feel that way.
Taking such a turbo sizing approach has a number
of major benefits. Let’s take a look at them.
First up, the maximum fuel and air flows of the
engine don’t change! That’s right: the standard sized injectors,
standard sized fuel pump, standard airflow meter, standard exhaust – they can
all remain. You’ll need to add a boost-referenced fuel pressure regulator so
that the pressure differential across the injectors remains the same when boost
is present – but that’s it. Even better, in airflow meter’ed cars, the air/fuel
ratio should never run leaner than stock – the airflow meter will pick the extra
airflow at lower revs and provide the fuel to match.
Secondly, fuel economy will benefit. A given
engine develops best fuel economy when it’s rotating slowly. It’s therefore very
likely that by the more frequent use of higher gears in a given driving
situation, the lower engine speeds will result in better consumption than would
be achieved with conventional turbocharging.
Thirdly, driveability should be fantastic. No more
down-changing in the search of power – it will be instantly on tap. And unless
you’ve driven a really torquey and responsive car, don’t dismiss this – in the
real world of urban and country driving, this characteristic is worth a lot.
Finally, if you’re looking at buying secondhand,
there are an awful lot more cheap turbos around that are good for 150 or 180kW
than there are from 250 or 300kW. Throw into the cost equation the ability to
keep the standard injectors, standard fuel pump, standard airflow meter,
standard exhaust – and you can be looking at an overall cost less than a third
of taking the high power route.
Interesting, isn’t it? So let’s take a look at the
A car with a higher average power but unchanged
peak power won’t be able to turn in the drag-strip times of a car where the peak
power has been lifted. That’s because if you redline it each gear, the engine
will drop only a thousand or so rpm each gearchange. In other words, it will be
back near peak power each gearchange and so a substantial lift in peak power
will result in much faster acceleration.
[However, in-gear acceleration, and
where the driver is caught in the gear that’s not absolutely optimal for best
acceleration (like most of the time when you put your foot down in normal
driving!) the turbo’d-for-torque car will be faster.]
Secondly, the much increased cylinder pressures at
the revs where the cylinder pressures are already at their highest (ie peak
torque) will probably result in the need for revised ignition timing and/or
higher octane fuel. Otherwise, detonation may occur. That depends on the
compression ratio of the standard engine and how close it was to detonation in
standard form, but it’s very likely that engine management changes will still be
needed. (They just won’t be the changes that are normally needed when you turbo
a normally aspirated engine!)
The life of the turbo will also probably be
shorter than it would have been in its original application. That’s because it
will be working harder for a much greater proportion of the time. This –
anecdotally – is similar to those factory turbo engines that run quite small
turbos to give plenty of low-rpm boost but not a huge increase in top-end power.
Audis come to mind!
Finally, the above is premised on the notion that
turbo boost can be dropped to zero by the redline. The standard internal
wastegates in turbos may not be big enough to achieve this – although a similar
outcome could be achieved if the standard wastegate can drop boost to a nominal
level (say 2 psi) by the redline. Alternatively, the blow-off valve could be
opened at high revs to also help control boost.
It’s a turbocharging approach that goes against
everything people hold dearest in the aftermarket. But the more that you think
of it, the more it makes sense for a daily driven modified car...
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