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Turbo'd for Top-End

Turbocharging without increasing low/mid range torque

By Michael Knowling

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At a glance...

  • Turbocharging for top-end power - not torque
  • Eliminate the wastegate for maximum engine efficiency
  • Significant advantages for some vehicles
  • A total waste of time for others...
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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 torque.

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 turbocharging...

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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.

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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 engineering benefits.

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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.

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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?

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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.

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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.

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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.

Conclusion

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!

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