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To Turbo or Supercharge?

Basic choices in forced aspiration

by Julian Edgar

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At a glance...

  • Advantages of forced aspiration
  • Basics of turbos
  • Basics of superchargers
  • Sizing
  • Picking and choosing
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This article was first published in 2005.

Forced aspiration can achieve incredible things. You can think of it as making an engine variably sized, giving the power of (say) a 4-litre engine with the economy potential of a 2-litre. But it’s even better than that, because the power of the 4-litre comes with the weight and size of the smaller engine... so you don’t need a huge car to fit a huge engine. A forced aspirated engine also has an average power across its rev range that is higher than a naturally aspirated engine. The translation is that your 2-litre that became a forced inducted 4-litre goes even harder on the road than a typical 4-litre!

Sound like magic? Well, the outcome can sure be magical.

So how do you achieve these results? Leaving aside chemical supercharging, there are two methods of forcing more air (and so allowing the addition of more fuel) into your engine. The first is a turbo and the second a supercharger.

Turbos

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A turbo uses the waste heat in the exhaust gases to spin a turbine. Mounted on the same shaft as the turbine is a compressor, which forces air into the engine. As the turbine spins faster, so then does the compressor. The result is that once the engine is developing enough exhaust gas flow to spin-up the turbo, the compressor can feed more air into the engine’s intake than it can normally breathe, creating boost.

Both the compressor and turbine are like fans – the exhaust gases drive one fan which in turn spins the other which blows air into the engine.

Turbos make use of the heat and flow of the exhaust gases, energy that is usually wasted in the mufflers and as hot flow out of the tailpipe. This means that theoretically, a turbocharged engine is more efficient than a naturally aspirated engine. The practical outcome of this is improved fuel economy. That might not be the case when compared with the non-turbo’d engine, but it will be when compared with a naturally aspirated engine producing the same power. (In normal road use, anyway. At constant full throttle, the answers change.)

A turbo has no physical driving connection to the engine – there’s no belt drive, for example. Instead, the ‘drive’ is by fluid flow (ie the exhaust gases) down the exhaust manifold.

Superchargers

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Superchargers are also air pumps that push air into the engine. Like turbos, they create boost by attempting to force more air into the engine than it can readily breathe. However, a supercharger is directly driven by the engine by means of a belt-drive from the crankshaft – in this respect it’s like an alternator or a power steering pump.

Superchargers come in three major types.

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Centrifugal superchargers use what looks like the compressor side of a turbo (although it’s bigger!). Like turbos, they cannot develop lots of boost at low engine speeds – instead, their airflow output rises rapidly with engine revs.

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Roots superchargers use intermeshing "paddle-wheels". Usually each "paddle-wheel" has two or three lobes which are curved so the intermeshing is as airtight as it can be without the rotors touching. The lobes can be straight or twisted along their length.

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Screw superchargers compress the air between two intermeshing screws, taking it in at one end and compressing the air as it travels along the screws.

Both the Roots and screw types of superchargers are called ‘positive displacement’ designs, because they have about the same output volume of air for each rotation. On the other hand, being much more like fans than pumps, centrifugal superchargers increase dramatically in output airflow as they rotate faster.

Picking and Choosing

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So which is better? There’s not a lot of point in having a purely theoretical discussion. In the real world, aspects like the availability of second-hand units and the number of cars with factory forced induction have a decisive impact on which way to go. For example, if you can buy a car complete with a factory turbo installation, you’d be mad to instead buy the naturally aspirated version and then fit a turbo yourself. Same with superchargers – never try to do what the factory has done for you.

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However, let’s say that you’re modifying an engine which is not available in factory forced aspirated models. The next step is to look for off-the-shelf kits – either supercharged or turbocharged. Again, if a company has spent hundreds of hours developing a forced aspirated model of your car’s engine, why would you want to re-invent the wheel? (That’s not to say you need to use the whole of that kit. Instead, it’s often good to pick and choose – for example, use their cast turbo exhaust manifold but go your own way with turbo matching and engine management.)

But it gets harder if there’s no factory forced aspirated engine, and no aftermarket turbo or supercharger kit. What then? The next step is to look at what turbos and superchargers are available new and second-hand to suit the size of the engine and the power you’re chasing.

Specifics

In this particular case I was looking at the best approach to forced aspiration on an engine with a naturally aspirated 43kW. (Yep, 43kW!) The aim was to pick a turbo or supercharger that would boost peak power by perhaps only 30 per cent, but would be effective at lifting torque across as much of the engine operating range as possible. In other words, boost had to be available early.

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This is a much trickier matching exercise than increasing peak engine power by (say) 200 per cent. That’s because unless the forced aspirated engine is very carefully modified, it’s likely the boosted engine will drive pretty badly. For example, it’s not at all unusual for a big turbo engine to have decent performance over only half its rev range. It’s primarily for this reason that the selection of a turbo or blower should always err on conservative side. In road cars it is always better to have a lower peak power but a higher average power across the usable rev range – something which people swapping power figures at dyno comps completely forget.

With this particular engine having a starting peak power of 43kW, the maximum power the turbo or blower would need to flow was only about 55-60kW. Furthermore, the engine is a 1.5 litre but (critically!) it revs to only 4000 rpm.

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First up, let’s look second-hand. The Japanese Kei class cars are available in turbocharged and supercharged forms and those turbos and blowers can be obtained from Japanese importing wreckers. The legal maximum power of these cars (in 660cc form – the engine capacity is another legal limit) is 47kW. Typically these engines rev to well over 8000 rpm.

With the 1.5 litre guinea pig engine reving to only 4000 rpm, in rough terms it will have an airflow requirement of about a 750cc engine that revs to 8000 rpm.

So the airflow requirement can be approximated in both power terms (original supercharged/turbo engine power = 47kW, desired engine power = 55-60kW), or in revs/capacity terms (original supercharged/turbo engine = 660cc and 8000 rpm, new engine = 1500cc and 4000 rpm).

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And buying new? The smallest ball-bearing Garrett turbo is the GT12. This turbo has a recommended power application range of 37 to 97kW and is said to suit engines from 0.4 to 1.2 litres. From Eaton is available the MP45 positive displacement Roots-type blower. It’s suitable for engines in the 1-2.4 litre range – so is a bit large for this engine.

So as can be seen, in this particular application there are available both new and second-hand turbos, and also second-hand superchargers. So which way is better – a blower or turbo?

Comparisons

There’s a very wide variety of factors that can be looked at when weighing-up the pros and cons of turbos versus superchargers.

1. Mounting

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A supercharger is mounted on the engine where it can be driven from the crankshaft. In most cases this means the pulley of the blower is in-line with the crankshaft damper and main drive pulley. Heavy brackets hold the supercharger in place so that the drive loads don’t cause flexing, which would throw the belt off. Most superchargers have their own oil supply – no connection to the engine oiling system is needed.

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A turbo is mounted on the exhaust side of the engine, normally on a custom-made exhaust manifold. A high pressure oil supply is derived from the engine (usually via a T-fitting on the oil pressure sender) and a return line is plumbed back to the sump – normally necessitating removal of the sump to do this work. In addition, water cooling lines need to be connected to the engine coolant system.

In most cases mounting a turbo on an engine is more work than mounting a supercharger.

2. Efficiency

As already indicated, a turbo has the potential to boost overall engine efficiency to higher levels than will be achieved with a supercharger.

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However, efficiency also has another meaning – it can be applied to the efficiency of the forced aspiration device itself. In this context, the lower the efficiency, the higher the temperature of the boosted air. Turbos, screw blowers and centrifugal blowers have high efficiencies, while Roots blowers have lower efficiencies. High intake air temps place a greater load on intercoolers, reduce potential power and increase the likelihood of detonation.

Most factory and aftermarket superchargers use Roots designs, so unless you pick the supercharger carefully, a turbo will have a higher efficiency.

3. Boost Characteristics

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There are two aspects to consider: where boost is developed in the engine load and rev range, and how boost can be controlled. Turbochargers and centrifugal superchargers do not generally develop boost at very low engine loads. Instead, full boost is available from about one-third of redline revs (eg from 2000 rpm in a 6000 rpm engine). (Note: this applies only to well-matched turbos and blowers!) On the other hand, positive displacement superchargers (screw and Roots) can develop more or less the same level of boost from idle to redline.

Most turbos have built-in exhaust gas wastegates, allowing the very fine control of boost when it is actually occurring. (At revs and load less than peak boost, the turbo wastegate is normally fully shut, trying to bring boost up as fast as possible.) Superchargers may have a bypass valve that can be used to control boost levels, but in general the peak boost is determined by the gearing relationship between engine and blower speed – something alterable only be changing pulley sizes.

Therefore, turbos have much better versatility in the control of boost levels – but only once the engine is producing enough exhaust gas to spin the turbo fast enough to produce boost. If boost is wanted at low engine loads and revs, a positive displacement supercharger is better than a turbo.

Conclusion

Deciding between a turbo and supercharger can be a very difficult decision to make. In the case of the engine being discussed here, access within the engine bay to the exhaust side of the engine is very tight (making fitting a turbo harder) and boost is wanted at very low revs (requiring a positive displacement supercharger). That seems to spell ‘blower’ as the answer, but it’s also likely that fine control over boost will be needed (turbo), and fuel economy is very important (again turbo).

If you’re looking at forced aspiration and are unsure which way to go, answer these questions.

  • Is there a factory forced aspirated version of the motor already available?
  • Is there a well developed forced aspiration kit for the motor already available?
  • What is the range of turbos and superchargers (both new and second-hand) that will suit the application?
  • Will it be easier to fit a turbo or supercharger in the engine bay?
  • What efficiencies are the turbos and blowers likely to have?
  • What are the required boost development characteristics?

Think through the answers to those questions and you’ll be much better placed to make a decision.

We’ve covered what’s available in second-hand turbos and blowers in specific articles – see

Used Turbo Info - Part One, Used Turbo Info - Part Two, Supercharger Steal - Part One and Supercharger Steal - Part Two

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