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This article was first published in 2006.
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Single turbos, twin turbos that are simultaneous,
twin turbos that are sequential, turbos on in-line engines and turbos on
V-engines, turbos that are sized small, turbos that are sized big. If you’re
thinking of a custom turbo installation, they’re all ideas that you need to know
at least a little about. So what’s the story on the road? We look at how a whole
bunch of turbo cars really perform.
Turbo Basics
A turbo uses the energy left in the exhaust gases
after they've come out of the engine. In a non-turbo car, the heat and pressure
in the exhaust is usually wasted - sent out of the back of the car with nothing
much done with it. On a turbo car, those exhaust gases are channelled by means
of a turbo-specific exhaust manifold to the turbine side of the turbo. Here
there's a wheel with specially-shaped blades attached to it. The exhaust gases
are nozzled down so that they increase in speed, and are aimed at the wheel.
This causes the turbine to spin very quickly - at up to 100,000+ rpm.
The turbine is mounted on a shaft, and fixed at
the other end of the same shaft is a compressor. This is another wheel with
special blades, but the job of this wheel is to blow air into the engine - to
develop boost.
At idle, there isn't much exhaust gas coming from
the engine. The gases are still all directed at the turbine side of the turbo,
but often they simply flow through the wheel, sometimes not even causing it to
turn. Likewise, the compressor end of the turbo isn't turning yet - if the
turbine is stationery, the compressor must be too, because they're both solidly
attached to the one shaft! Even in cars where the turbo is spinning at idle,
there's no boost yet being developed.
So what happens when you put your boot into it?
The engine revs and load rise, and so the amount of exhaust gases coming out of
the engine goes up quickly. This starts the turbo spinning fast. Once the turbo
is spinning fast enough, it will develop boost - blowing more air into the
engine than the engine can actually breathe. That's why the pressure in the
intake manifold goes up. Then the engine will inhale more than usual, and so the
exhaust flow will also rise. This spins the turbo even faster - so pushing more
and more air into the engine, which in turn produces more exhaust flow....
The biggest problem with matching a turbo to an
engine is in coping with acceleration transients – that’s when the power demand
is rapidly increasing. A turbocharged piston aircraft engine or big stationary
diesel engine changes in power output quite slowly, and so the turbo can be
pretty big to provide maximum efficiency. But a car engine needs to have boost
over as wide a range as possible, and furthermore, the turbo needs to ‘keep up’
with changes in engine load. This is best achieved by using a small turbo, or
when a small turbo can’t provide enough power, two smaller turbos. The lower
rotational inertia of the small turbos allows them to accelerate more quickly,
reducing turbo lag.
So what’s turbo lag, then? That’s when you nail
the throttle, especially at low rpm, and wait-wait-wait for the power rush to
arrive. The longer the wait, the poorer the turbo is at keeping up with the
changing power demands. A turbo’d car will always have softer throttle response
than a naturally aspirated car (well, when both have well-mapped management
systems, anyway) but a soft response shouldn’t translate to a power delay.
High power engines with small capacities have less
exhaust gas available to initially spin the turbo up (or more accurately,
proportionally much less than they have at full power), so these engines are
even harder to provide with a responsive turbo. In order that a turbo can
accelerate rapidly in speed, turbo manufacturers have used ceramic turbines
(less mass so fast acceleration) and ball-bearing turbos (less frictional drag
from the bearing). Electric-assist turbos are currently being developed: except
to see them on new cars pretty soon.
Turbo Approaches
More than any other factor (maybe packaging
constraints aside), OE car manufacturers engineer their turbo systems to reduce
turbo lag and improve bottom-end responsiveness. To achieve this, they place the
turbo(s) very close to the exhaust port, use a turbo that most enthusiasts would
consider undersized, and design the rest of the engine to provide good low-rpm
exhaust gas-flow.
European manufacturers – like Audi, Saab and Opel
– produce four cylinder turbo cars that have peak torque at incredibly low revs,
eg 2000 rpm. In other words, the turbos in these cars are producing plenty of
boost (and maybe have even reached their wastegated limit) by these low engine
speeds. Lots of macho enthusiasts like to pour shit on this idea but in the real
world of driving, this approach is extremely effective.
A torquey, responsive car is easy to drive fast,
often provides very good fuel consumption (because revs are so low much of the
time) and is very user friendly. As an example of the latter, boost in these
cars very seldom arrives with a rush (because the turbo is spinning up as the
engine is), meaning that there’s less chance of wheelspin. Simply, that means
faster times point-to-point. The Astra, Saab 9000 Aero and Audi A3 turbos are
examples of this approach.
In our 2003 test of the Astra Turbo, we said of
its engine:
With a dead-smooth idle and hugely driveable
bottom-end torque, you could be forgiven for thinking that the top-end wouldn't
be that strong. And in a way you're right - with peak power at 5600 rpm, there's
not a lot of point in taking the engine to 6800 rpm... which it will still do
smoothly and happily.
On the open road the torqueiness of the engine
is quite incredible. Helped no doubt by the cold ambient temps experienced
during our trip, there was literally no open-road hill that required a
down-change if the speed was above 80 km/h. But the bare performance times
simply don't tell the story - even though a 0-100 time of 7.3 seconds is quick.
It's in the cut and thrust of urban traffic -
where the instantly available torque is just so unusual for a turbo car and
where it's simply impossible to be caught off-boost - that the performance
starts to assume greater proportions. You can short-change at 3000 rpm and still
be quick; take the engine to six grand and you're seamlessly very quick. The
Astra turbo specs show it to be as fast as a current Impreza WRX, but in the
actual driving, most times it's quicker.
Of the Saab 9-3 Aero, in 2003 we wrote:
"Used to have a bit of lag didn't they, those
turbo Saabs?" queried the bloke having a good perve at the new 9-3 Aero as we
stood filling it with fuel.
Boy, if only he'd been in the car just a few
minutes earlier.
"Nah, mate, not this one," we fire back. "This
one drives like a big six-cylinder; it's got instant get up and go." The fuel
filler clicks. "The only difference," we add, "is it's a lot more economical..."
The new 9-3 Aero has a fabulously flexible and
responsive engine. Throttle sharpness is nothing out of the ordinary, but the
low and mid-range rpm punch is what you'd expect if Saab had stuffed a dirty big
six under the bonnet.
If you don't reckon a 2.0-litre turbo can give
big-cube punch at low revs, you're in for a surprise! From seat of the pants,
full boost arrives at very low revs - and that makes sense when you consider the
engine's torque curve. Torque doubles between 1000 and 2500 rpm, with a strong
300Nm plateau held from 2500 to 4000 rpm. Its no wonder open-road overtaking is
so effortless - just squeeze the throttle and you're steaming ahead.
Of the A3 Audi, we said:
Using a sweet and hi-tech
five-valves-per-cylinder 1.8-litre transverse four, the Audi A3 turbo develops
110kW at a relatively low 5700 rpm and peak torque at 4600 rpm - but the latter
feels much lower. With variable cam timing, electronic throttle control, a
well-matched turbo and 9.5:1 compression, this is one turbo car that never feels
like it has a puffer under the bonnet. Instead, the torque delivery is linear,
with good power available from the tractable and refined mill, even at low revs.
It's quite easy to skip the in-between gears on
the way up through the 'box - the car is happy with a first-third-fifth gear
change sequence. The driveability provided by such a good torque delivery is not
to be dismissed; this is one car that's very easy to hustle quickly because
there're always two gears just right for the occasion!
On the other hand, some manufacturers - like
Subaru with some of their earlier WRX STi models - trade off hugely in
bottom-end turbo performance for higher peak power figures. In other words, they
go for a bigger turbo which – in the case of the Subaru – is mounted a long way
from the exhaust ports. The result is a more exhilarating ‘coming on boost’ but
– and it’s a bloody big ‘but’ - the off-boost behaviour of the car is
terrible. These are the turbo cars that unless launched at high revs, are often
2 or even more seconds slower in their 0-100 km/h times than their published
specs. The R32 Skyline GT-R – yes, despite its twin turbos – is another of this
school, using very low gearing to give response but still being very slow when
caught off-boost, especially getting away from a standstill without a huge
launch.
In our 2002 test of the WRX STi we said:
It's pretty sad that the STi gets gobbled up by
ninety percent of traffic in normal day-to-day driving. No joke - caught out at
anything less the mid-range rpm, the Super Rex is an absolute s-l-u-g.
To give you an example, not long after picking
up our test car, we found ourselves stopped at a set of traffic lights with our
lane ending about forty metres ahead. The lights change green, we engaged first
gear and stomped the loud pedal all the way to the floor - just to make sure we
could merge into the next lane with plenty of room to spare. What followed was
tragic. Despite its wide-open throttle, the STi proved unable to muscle its way
past a humble Mitsubishi Colt in the adjoining lane....
We couldn't even pull out a nose length before
we had to abort and hit the brakes...
Embarrassing - not to mention dangerous -
situations like this quickly teach you the MY02 STi is a no-goer at anything
below mid rpm. It's tractable - yes - but don't expect any useable
acceleration
Keep the throttle floored past 4000 rpm,
however, and - bang! - the STi transforms into a head-kicker. It slams your head
back with the subtlety of an uppercut to the chin.
It’s a different story when you’re starting with
an engine twice as large. The Falcon XR6 Turbo, for example, has quite a large
turbo that can keep up with modified power outputs of twice the factory engine.
Which, since the standard 4-litre engine develops 240kW, is really something.
And yet the car in standard form doesn’t drive in a laggy way. Partly helped by
the prodigious bottom-end torque that this engine design develops, the turbo
comes up on boost smoothly and quickly.
On our 2002 test of the XR6 Turbo, we said:
Awesome, simply awesome. Ford Australia has
held nothing back with the release of its new Ford Falcon XR6 Turbo and the
result is good enough for it to become the next cult car. Consider the XR6T 'big
picture' for a moment; here's a freshly styled full size sedan with a modern
tech turbocharged engine capable of 240kW and enough low-to-mid range torque to
make a big 5.7-litre V8 cry.
The new turbo engine is fantastic; it offers
good throttle response (at least as good as any other turbo car on the market)
and it's supremely flexible at all revs. No need to row gears in the XR turbo
because with a 450Nm torque plateau available between 2000 - 4500 rpm she'll
pull away instantly. With the turbo set to deliver just 6 psi, the new engine
can stomp out 240kW at 5250 rpm and a very handy 450Nm from 2000 through to 4500
rpm. Who said all turbo engines are peaky?!
You can't help admire how effortlessly the XR6
Turbo 5-speed can outrun the fabled Impreza WRX and run alongside a 5.7-litre
Commodore. Giving it just a gentle launch with two people onboard we hand-timed
a 0 - 100 km/h sprint in 6.6-seconds. With a bit of practice, though, we reckon
the XR could crack 6-seconds flat - seriously cookin'.
And all these turbo cars that drive so well
achieve this performance without equal-length runner exhaust manifolds that fill
half the engine bay, huge blow-off valves or any of that crap...
When manufacturers do decide to go to a
more sophisticated turbo system, they fit twin turbos. Twin turbos are not
particularly suited to four cylinder cars (although some people have fitted
them), but instead lend themselves ideally to in-line sixes and V-engines.
Because of the way the pulses of exhaust energy can be entrained, the sixes work
very well with two turbos, while the V8s suit twin turbos because of the
proximity to the exhaust ports with which the turbos can be mounted. However, in
the case of a six cylinder engine of say 2.5 or 2.6 litres, you’re really
looking at the exhaust flow of only two 1.25 or 1.3 litre engines and so if you
fit turbos that are too large for these gas-flows, the result will still be
doughy. Especially if the engine is set-up for good top-end power.
The grey import 2.5 litre twin turbo Toyota Soarer
(tested in 2001) is a case in point. We drove it after experiencing the
sequential twin turbo Supra (we’ll cover sequential turbo systems in a moment)
and had expected it to be similarly good:
The gasses exiting its 1JZ-GTE head are
constantly divided into two simultaneously operating turbos; this means boost
response and low-rpm torque are both a fair way behind the sequentially-turbo'd
RZ Supra. You really notice this bottom-end 'hole' when punching the 1600kg
Soarer off the line - it feels quite doughy until just before 3000 rpm.
[Compared to the manual transmission car]
we
found the Soarer more enjoyable to drive when fitted with the 4-speed automatic
transmission. That's because the converter multiplies low-rpm torque going to
the rear wheels as well as allowing engine revs flare (often by up to 800 rpm).
Under increasing engine loads (such as when climbing a gradual incline) the
transmission is also very willing to slide back a gear or two, helping to keep
the engine on song. The engine and trans are obviously a very well calibrated
pair.
The rare 5-speed manual version (which some
have suggested is good for 0-100 in the 5s) is not much quicker than the auto
without a big clutch dump and brutal shifting. Only under these circumstances
will you extract any worthwhile acceleration advantage over the auto. In gears,
we also noted that there's bugger-all separating the performance of the two cars
fitted with the different transmissions.
What we most dislike about the manual Soarer is
that it emphasises the engine's bottom-end torque shortcoming; stomp your foot
on the accelerator at low rpm and you have to wait until road speed equates to
about 3000 rpm before any real urge become apparent.
Running the turbos in sequence, where initially
one turbo gets all the exhaust gases before the turbos are fed in parallel,
potentially provides the best of both worlds. However, engineering the
changeover is very complex: how do you stop the first turbo dying away when the
exhaust valve opens to allow gases to flow to the second turbo? But when it’s
done right, this configuration is simply head and shoulders above any other
turbo philosophy based around a smallish engine.
Our 2002 drive story of the Supra VVT-i RZ said it
all:
The post-1998 VVT-i version of the Supra 2JZ
twin-turbo 3.0-litre six is - without doubt - the best production turbo engine
we've ever driven.
Stroke it along gently and the VVT-i 2JZ-GTE
six behaves as 'proper' as a top-line Mercedes. Despite having 209+ kilowatts on
tap from only 3-litres, there's absolutely no hint of lumpy camshafts or an
all-or-nothing turbo system. The only on-going reminder of the engine's potency
is its throaty exhaust burble from out back. It sounds sensational.
Throttle response (via the ETCS-i electronic
throttle) is immediate and comes backed with a progressive snowball of torque.
And, no matter what revs you've got the thing lumbering at, there's never a time
when you have to row down through the ratios to find accelerative urgency -
certainly a rare enjoyment in a turbo car.
On paper, the sequentially-turbocharged
3.0-litre VVT-i 2JZ-GTE is credited with a long-trunk'd 451Nm of torque at 3600
rpm; to give you a comparo, an R32 GT-R makes 355Nm, an Audi S4 twin-turbo 2.7
boosts its way to 400Nm, a Liberty B4 generates 320Nm and an Evo 6.5 kicks out
373Nm.
N-o-w do you get the picture how much grunt the
mega-Supra has on-call!?
However, it’s easy to get a sequential turbo
engine completely wrong, as our 2001 Subaru Liberty B4 story said:
The newly-released Liberty B4 is powered by an
EJ20B 69H - a turbocharged, intercooled, DOHC, 16-valve, 2-litre boxer four,
which is of the same basic design as the engine found in WRXs and the Liberty RS
of ten years ago. Internal developments have resulted in reduced valvetrain mass
and an increased compression ratio (up from 8.0 to 9.0:1), and Subaru has
endowed the B4 with two sequentially-staged turbos in order to maintain a good
spread of torque.
The B4's focus is on response and flexibility
in everyday conditions.
The hottie Liberty is set up to offer good
throttle response and backs it up with a strong surge of torque anywhere in the
rev range. The primary turbocharger is arranged to quickly deliver boost up to
4000-4500 rpm, with the secondary turbocharger then kicking in to add flow in
the higher ranges. It's a cunning system - but is it perfect?
No.
During the transitional phase - where the
secondary turbocharger is starting to pump in addition to the primary unit -
there's an ugly 'hole' in the torque delivery. Under full throttle, a
significant dip in manifold pressure identifies the 4000-4500 rpm transition; we
measured a full 0.25-0.3 Bar boost pressure dip.
It's enough for first-time passengers to ask if
there's an engine problem....
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Turbo
and Supercharger
If
you want to fit a large turbo, also fit a supercharger to fill the bottom-end
hole. It’s not as hard as it sounds, and the changeover valving is a lot easier
than running sequential turbos.
The
Nissan March Super Turbo does it as standard. We drove it in 2004:
Under
the bonnet lives Nissan’s fabulous MA09ERT 930cc in-line four. It’s not a
particularly sophisticated engine with just a 7.7:1 static compression ratio,
SOHC and 8 valves, but its bolt-on forced induction hardware is nothing short of
amazing. The March Super Turbo employs one of the few OE
supercharger/turbocharger set-ups in the world.
And
it works a treat.
At
low-to-mid revs, the 930cc four is boosted by a positive displacement blower. At
higher revs, however, a relatively large turbo kicks in to give great top-end
performance. Note that the transitional stage is very smooth thanks to a
relatively simple supercharger/turbo control system. At low rpm the turbo blows
through the supercharger which is effectively ‘free-wheeling’. Then, at high
rpm, the supercharger is disengaged by an electro-magnetic clutch and the turbo
feeds the engine via a supercharger bypass passage. Airflow through this passage
is controlled by a differential pressure valve which begins to open as the
turbocharger nears operating speed.
Give
it just a whiff of throttle at low rpm and you’ll see the supercharger deliver
up to 7 psi of boost. This means wonderful part-throttle performance but,
curiously, flooring the throttle at this stage doesn’t necessarily equate to
greater acceleration. At more than 4500 rpm, however, the turbocharger steps in
and boost pressure swells to 13 psi. That’s when it’s go-go-go!
With
81kW at 6200 rpm and 130Nm at 4800 rpm the March Super Turbo – which weighs just
770kg - is an absolute rocket. Performance varies depending on the temperature
of the top-mount air-to-air intercooler, but high 7 second 0 – 100s are
easy.
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Thoughts
So where does all that leave us? For
driving – as opposed to dyno comps for peak power figures – a
turbo that leans towards a smaller size will give far better on-road
performance. That’s pretty well the opposite of what you’ll read anywhere else.
So if you’re running a car that is already factory
turbo’d, leave a turbo upsize to the last step in the modification process...
don’t make it the first. If you’re starting off with a naturally aspirated
engine that you’re turbo’ing, seek good advice as to the turbos that would suit
the engine - if you want to make peak torque at (say) one-third of the
redline revs. Usually, that will mean reaching peak boost by that rpm – and
then of course holding it at that value. This approach will limit the top-end
power, but the higher average torque through the rev range will make a powerful
(sorry!) difference to the real world performance of the car. (Or, if you want
to trade-off some more bottom-end response, move the peak torque request up to
half of the redline rpm.)
If you need more power than can be provided by a
single, reasonably sized turbo, and you have a V or six cylinder engine, use
twin parallel turbos. Of course, sequential would be better but we’ve never seen
a twin sequential turbo install done in the aftermarket (most people are too
busy talking off twins to fit a single huge, laggy turbo!).
And finally, be a little wary of dyno curves that
show a fantastic boost curve from a big turbo on a little engine. On the dyno
it’s often easier to load-up the engine by the use of a high gear or slow ramp
speed in a way that results in a completely different boost curve than you’ll
find on the road in first and second gears.
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