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Supercharge or Turbo me

c1.jpgimg00016.jpgForced induction, while very straight-forward in concept, can become fairly complex in execution.  First off, forced induction is more commonly known as turbocharging or supercharging.  Again, in concept they are very similar, but in practice they are very different.  Forced induction exists to force more air into an engine. More air means more fuel which means bigger explosion in the cylinder, which translates into more power.  And let’s face it, power is good!  Okay, so turbos and superchargers force air into the cylinder.  The next obvious question is: HOW?

Let’s start with turbocharging. It is by far the most prevalent in factory applications and has been on factory road-going Porsches since the mid-70’s.  If you’ve ever put your hand right behind the tailpipe on your car, you’ve notice that exhaust consists of moving air.  If you’ve ever watched Top Fuel drag racing, you’ve noticed that exhaust can consist of a LOT of fast-moving air and a lot of heat too.  What turbocharging does is put that moving air to work for us by sticking a turbine (very sophisticated fan) right in the exhaust flow.  At idle it doesn’t spin very fast, but as engine speed picks up so does the airflow at which point that turbine starts to really spin; over 100,000 rpm in some cases.  The exhaust turbine is attached to a shaft that goes from the exhaust housing to the intake housing.  The other end of the shaft has another turbine on it designed to suck air and force it into the engine.  So turbos use exhaust air to drive an air pump that forces more air into the engine. It’s considered “nearly free” power because it uses what would otherwise be leaving the engine doing nothing, and drives a pump that makes more power-that causes more air to leave the engine, which spins the turbo faster and pumps even more air into the engine which speeds up the turbo more and so on and so forth.  There are a number of limiting factors in the system as well as mechanical and electronic safety measures to keep the system from creating too much pressure and doing damage to the motor

One of the downsides of turbocharging is the fact that you have to wait for the turbo to “spool up” enough to actually start pumping air into the engine.  This turbo lag can be inconvenient and, in some situations, potentially dangerous.  Also, with any forced induction system, there’s going to be heat (air up your car tire with a bicycle pump then grab the bottom of the pump cylinder, be careful, the pump will be very hot).  Turbos add to the heat from compression by having these rapidly-moving parts in very close proximity to the exhaust system.  By the way, hot air is less-dense air, which means less combustible gas in a given space, which means less power.  Intercooling combats that, but that’s for another article.

So, in summary: Turbo Pro:  Nearly-free extra horsepower.  Turbo Cons: Turbo lag and heat saturation.

How about supercharging?  Well, it comes in different flavors: positive displacement and centrifugal, for example, but we’re going to stay away from that discussion for today.  Suffice it to say, AMG, GM, Ford, VW, Chrysler have all used supercharging on production vehicles in the last 15 years or so.  And most of the aforementioned still do on at least one of their cars.  Supercharging works in a very similar fashion, except instead of the exhaust gasses spinning the turbine (or vanes -yet another article), the unit is driven directly off the engine like a water pump or air conditioner.  And just like those devices, it requires additional horsepower to run it.  Some are very efficient at idle and some are not, but they all take power.

Superchargers also cause heat during compression, but the complexity and heating issues of a turbocharger are absent and adding a supercharger after the fact does not require completely redesigning the exhaust system, making them a desirable “bolt-on” addition for more power.

So in summary: Supercharging Pros: Less heat and less complex.  Supercharging Cons: Takes power to make power.

Is one better than the other?  Without a doubt more turbocharged cars roll off factory production lines every year.  So for whatever reason, the major manufacturers strongly prefer turbos.  Superchargers though are much easier to add to a car that is normally-aspirated, and some factory exotics come with these.  Then there’s the new Volkswagen that comes with both-but that’s yet another article.

 

PorschePhD

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