Turbochargers vs Superchargers

[JoeThaMan]

J.P.
January 1st '05

There's a lot of debate, misunderstanding, misinformation and general ignorance when it comes to making this particular comparison. A lot of people seem to make their decision based solely on the fact that many high powered engines, such as those in drag cars and most notably Top Fuelers use superchargers, so naturally this must be the best form of pressurised induction. Others will use turbocharger 'lag' - defined as the time taken from the opening of the throttle until the turbo provides positive pressure to the manifold, as a reason for supercharger's superiority. Whatever the reason given, it's wrong. Turbochargers have proven to be the superior form of pressurised induction over and over again, both in terms of energy used to drive the compressing device, and in some cases - depending on the type of blower - the actual operating efficiency of the device itself. Bottom line; a turbo will produce more power than a supercharger. Here I'm going to outline the reasons, with help from the experts, why this is the case.

Superchargers are belt driven off the crankshaft of the engine, and therein lies the critical difference between the two systems. The supercharger will turn at a speed with a direct relationship to crank speed, while with a turbo does not - many more factors come into play. There are several types of supercharger - roots, screw and centrifugal to name but a few. The roots-type blower is one of the 'positive displacement' type blowers, which compresses air by a series of moving compartments, created by two interlocking lobed rotors. It differs from the centrifugal blower in that its a 'sealed' type design, which provides boost in a 'linear' fashion and actually presents the compressed air to the manifold in a series of pulses. This type of supercharger has a large amount of internal friction due to the edges of the rotors turning inside the casing and meshing with each other. This friction adds heat to the air, aside from the normal heat associated with compression of a medium, and is one of the reasons why a roots-type supercharger (an early design) is inefficient compared to a turbo. This extra heat is energy taken from the crank and wasted. Not only is it wasted but the heat is detrimental to power production.

A lot of people also seem to think that turbochargers belong on small displacement engines, and that they dont work or are not suited to larger displacements. This could not be further from the truth. A turbo is a method for increasing the amount of air breathed by an engine. They are a fairly efficent device, certainly more so than a rootes blower and with advantages over the latest breed of centrifugal superchargers. Indeed a large turbo is more efficient than a small one for two main reasons; it heats the air less for the same amount of flow, and presents a larger impellor area to the exhaust flow, and hence less restriction than a smaller turbo.

In an effort to improve the poor efficiency of the supercharger, designers have taken the turbo (realising its superior efficiency) and essentially cut it in half, driving it from the crank rather than via an exhaust gas-powered turbine. This is the basis for the centrifugal supercharger, and this design is largely successful in solving the inefficiency problems. However, the centrifugal nature of this blower means it is not a 'sealed' type design, instead it pressurises the air by literally flinging it out of the turbine at very high speed. This generates pressure without the pulses of the roots-type blower. Also in contrast to the 'linear' boost delivery of the roots blower, a centrifugal blower generates boost in an exponential fashion. This is both a curse and a blessing; a curse because it tends to hamper response at low revs, and a blessing because airflow at medium to high revs is greatly enhanced. These are the traditional characteristics of the turbo on which it is based. However, even with the increased efficiency of the centrifugal design over the old roots-style design, a considerable load is still placed on the engine by using a crank driven pulley system to drive the supercharger.

In this form superchargers have become increasingly common in Australia. At least 3 types of kit offering supercharged induction for the Holden V8 have become available in the last 2-3 years. With the efficiency of the centrifugal supercharger (but still limited to engine rpm - more on that later) these kits have become a viable alternative to the standard route to finding more power via a bigger cam, head porting, compression increase, larger intake and extractors, etc. The use of the compact centrifugal design means it can fit under the bonnet with no requirement for holes or bulges in the hood. Both the Australian and American kits available here use centrifugal superchargers to achieve flywheel power of approximately 270-300kW for the otherwise stock Holden 5.0 V8. Indeed Yagoonda Automotive Services in Sydney have made around 400kW (~539bhp) and 700Nm from an HSV 5.7 litre-based engine. These figures would tend to be supported with the performance of 4.7 sec for 0-100kmh and 12.2 over the standing quarter mile. Admittedly this car uses forged pistons in place of the stock components, but I think I am fairly justified in saying this kind of output would be nigh on impossible to extract from a naturally aspirated HSV 5.7ltr engine, particularly the torque figure.

The turbo, in stark contrast to the supercharger, is driven solely by exhaust gas pressure. Where normally the exceedingly hot exhaust gasses would simply be left to exit uninhibited down the exhaust pipe to the atmosphere, a turbo brings a number of finely engineered components into contact with this heat. This creates several headaches for designers, and early turbocharged engines illustrate the problems. Short turbo life, cooked engine oil and the resulting damaged engines - all these were common problems associated with a lack of understanding and preparation for the specific needs of a turbocharged engine. Add to this the problems with turbo lag and response time compared to superchargers and it is easy to see why turbos are misunderstood and commonly disliked. Lag, it is likely, will always be a consideration for a turbocharged engine, but it is possible to minimise this with the manipulation of other factors. Turbocharger sizing is becoming a much more understood art, and together with increases in technology the turbocharger has and is continuing to become more efficient and reliable. Lag is being specifically addressed by reducing the rotating weight, allowing spooling to occur quicker with a given rate and amount of gas flow, as well as new developments such as the VATN turbo. Ceramic bearings allow newer turbos to run cooler and more easily, and impellor and turbine designs are improving.

Better cooling systems mean today's turbocharged engines such as Nissan's 200sx are no longer less reliable than their naturally aspirated cousins. And there are a lot of turbocharged engines sold today. Contrary to popular belief, supercharging is not making a comeback. Turbocharging is bigger than supercharging in every way, worldwide. Audi's A4 and A6, Bentley's 'turbo R', BMW's 745i, Ferrari's F40 and GTO, Ford's Cosworth Sierra and Sapphire, Mazda's RX-7, Mitsubishi's GSR, 3000GT and 300ZX, Nissan's GTR, Porsche's 911, 930, and 944, Saab's 900 and 9000 series, Subaru's Liberty RS and WRX, Toyota's Celica GT4, Supra, Soarer, and MR2, are all turbocharged, and that's not even including the racing cars. At the extreme end of performance, BMW's Brabham F1 cars produced over 1300bhp from 1.5 litres using turbos in 1985. That is approximately 15 bhp per cubic inch. In contrast, Top Fuelers running on a supercharged mix of alcohol and nitro methane produce approximately 10bhp per cubic inch from their engines. I quote from Engineer and proprietor of BEGI, Corky Bell ".. that the champion is crowned is obvious even to casual observers".

The fact that turbochargers are exhaust gas driven brings with it advantages as well as disadvantages. Put simply, a turbo can provide an engine with maximum boost earlier in the rev range than a supercharger can. The only exception to this is if the supercharger is overdriven to yeild greater boost pressure at a given rpm, or in the case of a centrifugal supercharger which may provide less boost at low rpm than a positive displacement blower on which the boost model below is based. It should also be noted that overdriving a supercharger increases the supercharger's already significant drag on the engine.

Because of the direct drive nature of a supercharger, flow and pressure generated by the supercharger increases in direct proportion to crank and hence engine revolutions. If an engine redlines at X rpm, the maximum boost (Z) that the engine will tolerate will also be set to coincide with X rpm, so that you are neither losing performance or damaging the engine. This means that at half X rpm, boost pressure from the supercharger will also be half Z of the total available. This is a disadvantage compared to a turbocharger as a modern turbo correctly sized to an engine can reach Z boost pressure by half X at the very latest. What this means is that if an engine redlines at 6000rpm and is supercharged at 6psi (a common figure), the engine is receiving 6psi boost only at 6000rpm. At 3000rpm the engine will receive induction air at only 3psi, and at 3500rpm would have 3.5psi boost. On an engine with a 6000rpm redline, a modern correctly sized turbo on the other hand will be at full 6psi boost by 3500rpm, which translates to greater torque from the increased boost and hence greater power at 3500rpm than the supercharged engine would have at 3500rpm from only 3.5psi, or just above half boost.

As mentioned before, a supercharger is crank-driven while the turbocharger is exhaust gas-driven. The drive method itself provides a further set of advantages and disadvantages. I've already mentioned the lag phenomenon - how at the application of throttle, the engine responds at first only as a normally aspirated engine before the turbo spools, increasing manifold air pressure and engine torque. The advantage to being exhaust gas-driven however, is that the exhaust itself is largely wasted energy. A lot of energy in the form of heat escapes the engine completely unused, and the turbo goes some way towards fixing this. At the same time however, the turbo presents a restriction to the exhaust flow, which results in backpressure. However the loss in power from this backpressure is minimal, especially compared to the direct and substantial power loss caused by a supercharger's load on the crankshaft. The detrimental effects of the turbo on exhaust gas flow can be reduced further by arranging the turbo to reflect pressure waves and assist exhaust port scavenging in the same way that normal tuned-length extractors function.

Another disadvantage of a turbo is that installation on a previously naturally aspirated motor may be more costly than a supercharger installation, due to a supercharger's relative simplicity. This is for a number of reasons. A supercharger needs only to modify the induction side of the engine, rather than both induction and exhaust manifolds as with a turbo installation. This difficulty is further compounded on a V or boxer configuration engine - to simplifiy plumbing two turbochargers are the norm - one per bank. As space under the bonnet of a modern car is usually at a premium, turbocharging installations with their associated plumbing requirements are not always practical.

I will finish for the moment with a quote from Peter Luxon, creater of the APS centrifugal supercharger kit, one of the first aftermarket supercharger kits for the Holden based V8 to go on sale in Australia. The kit in question was fitted to an HSV GTS sedan with a claimed 215kW and 475Nm stock. With the kit in place, power rises to 307kW at 5000rpm and 636Nm at 4000rpm using .35 bar boost. The quote?

"...that same car running turbos would blow the doors off that thing.."