Reader Question: Supercharge – or Turbocharge – It?

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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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  1. Faster? or Quicker? What sort of hp/tq are you looking for?

    For quicker I would go supercharger but the price is very high.

    Sometimes an entire bigger engine swap can be much cheaper, and simpler, than trying to wring too much power out of a too small engine. High stress usually = shorter lifespan.

    I looked at turbos and superchargers for my Fiero. All way too expensive, not great gains and the 2.8 would just go bang in many cases. My research found 3.8SC or SBC swap were cheaper options with much less engine stress. Picked up an entire(rotted body) Olds with a 3.8SC in good condition for $500.

    Also, if just looking for bursts of quicker, nitrous.

  2. Both turbo and supercharger consume some parasitic power. Traditionally the supercharger shines down low- boost is there right away and tops out due to (most) supercharger physics. The extra pressure of a high boost setup is probably less important than having plenty of fresh cool oil in the system to keep the bottom end happy.

    I suspect that supercharging is harder on bearings since the bottom end boost comes in at lower RPM’s where oil pressure is also lowest.

    Bottom line, run what you want but if you don’t design and build your engine as a system you will find the weakest link.

  3. eric, I’d agree with you 10-20 years go but a lot of cars now have a two-stage turbo that avoids ‘the lag’. Even 20 years ago big rigs had inline turbos. That was sure a “boost”. The first I ever saw was on a huge cat engine on a Peterbilt in Mexico with a slightly longer hood than a US 379. Since Peterbilt and most PACCAR rigs are built in Mexico it wasn’t surprising. They have plenty of tiny two lane roads in the mountains but their common rig is a tri-axle tractor and a quad axle trailer, not to say they don’t run even larger rigs. We were leaving a little mountain town one morning and someone hadn’t been paying attention(probably the truck going downhill). There was this tiny curved that had been widened quite a lot so that those multi-multi axle rigs could pass. The problem being, the rig going downhill had crowded the one going uphill…..very slowly since it was so steep and the curve was nearly 180 degrees. Once that rig going up had to get slow enough to not run into the downhill rig, it simply fell on it’s side the grade across the lanes being so severe. It was my first sight of something like that.


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