Reader Question: Supercharge – or Turbocharge – It?

Here’s the latest reader question, along with my reply! 

Bruce asks: I am interested in making my NB MX5 faster.I am looking at either turbocharging or supercharging (though not sure if I would like the whine as it spools up, but opposed it is basically a bolt on as to opposed fitting a turbo). Or do I go way out and put a rotary in it? I listened to your podcast with George Gammon(awesome) and you mentioned turbos are a lot harder on engines.How hard is supercharging on engine?Many thanks.

My reply: Now this is a healthy way to start my day! Wrenching stuff! Not Diaper stuff!

Turbocharging and supercharging both do the same thing, differently. They pressurize the incoming air – boost – in order to get more into the cylinders, so as to make more power. The turbo is driven by exhaust gas pressure; the supercharger, by a pulley.

The turbo is easier to fit in tight spots but it requires a specialized exhaust system and other peripherals. The supercharger is generally easier to “bolt on” to an engine that didn’t come with one originally.

The chief detraction with superchargers is that they take power to run them; the power gains offset this, of course – but there is an efficiency loss vs. the turbocharger, which makes its boost using the “free” energy of exhaust gas pressure.

The chief detraction with most modern turbo setups is they impart a lot of boost; some OEM systems run 15-plus PSI, which puts a lot of pressure on the engine’s guts, including con rod bearings. They also are prone to “cooking” if not provided with a well-designed cooling system (most factory systems have this).

There is also lag – the slight delay in between when you floor the gas pedal and time it takes for the engine to make enough exhaust gas to spin up the turbo.

Most aftermarket superchargers run less boost – around 6-9 pounds is typical – and that is easier on the engine, especially if it was not originally designed for turbo-supercharging. The supercharger will make the engine run hotter, but the thing itself is less prone to heat-related problems because it is not being literally cooked as part of the exhaust system.

There is no lag with the “blower,” either – it being mechanically driven.

You’ll have to balance these considerations, the pros and the cons. Mazda has, of course, offered turbocharged variants of many of its engines and so there is a lot of factory (and aftermarket) support if you go that route.

As an intermediate course, you might consider a top-end job, with some head work for better breathing and more aggressive cams – plus a more aggressive rear axle ratio and (if you’ve not already done this) a free-flowing exhaust system.

The Miata is so light that a 20-30 hp uptick – easily achieved via the above – plus a bit more leverage from the lower axle ratio – might be all you need to satisfy your performance goals, without the expense/complexity (or pressure) of a boosted setup.

On the rotary: They are very fun, but very peaky, torque-deficient things that generally require lots of revs to make power. This is great on the track and driving fast on the street but it’s not so much in traffic. One of the Miata’s great boons is that it is a track car that can be daily driven as easily, reliably and comfortably as an economy car. A rotary engine would change that.

Of course, it would be a very cool thing, regardless! (Like my buddy’s Miata, which has a Corvette V8 under its hood… somehow).

. . .

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