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Turbo'd for Torque

Where you don't try to increase peak power...

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Turbo for torque not power
  • Smaller size turbo with zero boost by redline
  • Immense tractability and bottom-end torque
  • Some major advantages
  • Some disadvantages
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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?

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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 factory reliability...

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.

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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 power figure.)

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?

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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 figure? Irrelevant...

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 all.

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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 quicker!

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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.

Benefits

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.

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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 downsides.

Downsides

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.]

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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.

Conclusion

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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