Here’s a counterpoint. Back in March ’06 we
highlighted the advantages of turbocharging to improve torque without gaining
top-end power – see
Turbo'd for Torque.
Well, this time we consider another turbocharging approach that goes against the
grain – fitting a turbo to improve top-end power without gaining low/mid range
Sound nutty? Read on...
Turbo’d for Top-End – Why?
The big question is why on earth anyone would go
through the hassle of turbocharging their car only to achieve extra power at the
top-end of the rev range? Let’s face it, when you’ve spent a wad on mods, you
want to feel a huge surge of grunt the moment you hit the throttle – regardless
where you are in the rev range.
Well, there are some very good reasons to take the
top-end approach – but it depends on the sort of vehicle you’re
In the case of a relatively low revving/high
torque engine (such as a traditional Aussie ‘big six’ or V8) the concept of
turbocharging for top-end has real benefits. Look at this chassis dyno graph
from a 3.5-litre V6 Mitsubishi Magna and you can see how quickly top-end torque
drops away – just imagine how much power there would be if a turbocharger was
used to hold torque through the top-end... In normal driving you won’t feel any
extra urge (and given the already generous low-down grunt, that’s fine) but
you’ll now have the ability to quickly blast past a road train or shut down that
smug WRX driver alongside.
Top-end boosting this type of engine helps widen
its power-band and makes it much more satisfying to drive – there’s no longer
rush of grunt and then, well, nothing. Power is delivered much more linearly
compared to a mid-range boosting turbo install and, as a result, the car is more
controllable and less likely to degenerate into pointless wheel spin. Sure,
there won’t be that immense mid-range urge when you put your foot down when
cruising in top gear but, even in standard form, this type of engine is pretty
strong in this area. In any case, you can fit a shorter diff ratio and make the
most of the newly widened powerband.
And there are plenty of other potential
When turbocharging for top-end, it’s a good idea
to select a turbocharger that would normally be regarded as ‘too big’ –
especially on the turbine side. That’s because unlike most turbo fitments, you
aren’t chasing rapid spool-up or low-rpm pumping efficiency. Ideally, you would
use a turbocharger that achieves your desired top-end power hit without relying
on a wastegate to limit turbine speed and boost. As its name implies, opening
the wastegate squanders overall engine efficiency – there’s more heat being
wasted out of the exhaust.
With a turbo’d for top-end approach engine bay
heat issues are reduced because the big turbocharger will cause very little
backpressure in normal driving. At the same time, it will also be running off
boost most of the time – and this means less thermal load on the intercooler.
The problem of detonation is also reduced because
you’re not trying to force in extra air through mid-range rpm where the engine
is already producing peak combustion pressures. No need to add octane boosters,
colder spark plugs, water injection or any other detonation combatants. It’s
also reasonable to expect long turbocharger life because in all normal driving
conditions the ‘charger is effectively free-wheeling – it’s ‘working’ only
through the top-end.
Perhaps one of the biggest advantages of
turbocharging for top-end is the reduced risk of driveline damage. In normal
driving, the driveline will receive loads well within the manufacturers’ rating
while, depending how much power you’re chasing, top-end torque loads probably
won’t exceed the factory peak. Upshot? You won’t need a new clutch/gearbox/trans
on a regular basis. The relatively modest peak torque output also reduces the
chance of bent rods.
So they’re the benefits of turbocharging for
top-end on an ideally suited car. So what about applying the same philosophy to
a car that isn’t already endowed with strong low/mid range torque – something
like a rev-happy Honda VTEC?
Well, turbocharging for top-end doesn’t make a lot
of sense in this situation. In many instances you’ll have bugger-all performance
through the majority of the rev range and then – whoosh – you’ll have
performance across a narrow band of maybe 2000 rpm. Not what you want (but,
unfortunately, what a lot of people seem to put up with...)
How to Turbocharge for Top-End
So how do you go about turbocharging an
engine for top-end?
Well, when you’re not trying to achieve rapid
spool-up from low rpm you can get away with a lot when it comes to turbo
selection. Just be sure that you pick a ‘charger with a suitably sized
compressor to deliver your required power output together with a turbine that’s
maybe a couple of sizes bigger than would normally be suggested. Remember, the
ideal situation from an efficiency point of view is to avoid using a wastegate.
Intercooling should have a major focus on airflow.
Given we’re only increasing top-end power, it’s important to have a free-flowing
intercooler core and plumbing. Cooling performance is important (as always) but
heat stress on the intercooler is much reduced compared to a conventional turbo
set-up. In a street car it’s likely the intercooler will primarily be working as
a heat-sink, rather than as a heat exchange device.
One of the biggest disadvantages of turbocharging
for top-end is the almost inevitable requirement to upgrade the fuel system. At
minimum you’ll likely need a different fuel pressure regulator and you might
also require an upgraded fuel pump and injectors. And that brings us to the
engine management – this will need consideration on a case-by-case basis. A
simple airflow meter/MAP sensor signal modification might suffice or you might
need to install an interceptor, programmable management or a factory management
reprogram. It all depends how much power you’re chasing and how the standard
management system reacts to the extra airflow.
Finally, you’ll also need to ensure there’s plenty
of flow capacity through the exhaust and air intake systems. You need to ensure
there’s nothing that will restrict the engine’s high rpm breathing.
Also be aware that, in some applications, you
might run into troubles when revving the engine toward the top end of its range.
Inadequate valve spring tension and poor performing lifters can cause valve
float and some peculiar performance characteristics. If you’re regularly
enjoying the extra top-end grunt of your new turbo install, it might be a good
idea to upgrade part of the valvetrain. Speak to your local performance workshop
to find out what parts are available to suit.
It’s not ideally suited to all cars but the
philosophy of turbocharging for top-end has some major advantages – a very
worthy alterative to turbocharging for torque!
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...