More on the 2015 Corvette C7 Z06 TVS 1740 supercharger problems

The Roots type supercharger is the most commonly used OEM supercharger system, with the +General Motors flagship C7 Z06 being no exception.

But... the question is always there: why do the auto manufacturers (OEs) nearly always choose a roots type supercharger? The one word answer is price, regarding both OEM cost and supercharger manufacturer profit margin.

Cheap pricing to the OEM’s is compelling enough to make up for all the deficiencies associated with the roots type supercharger.

For the record, the Roots type can be called a supercharger, but IT IS NOT a compressor, because it is not; as a positive displacement supercharger, it is an air mover.

One misconception is about the airflow path for these Roots superchargers: the air does not flow through or between the rotors but rather around, on the housing sides within the voids of the rotors.

The roots supercharger traps air in a chamber between the rotor and the housing, in the void between the rotor’s lobes, and transports this trapped air to the outlet side, at the engine’s intake manifold. By virtue of moving air into the manifold at a rate higher than the engine consumes it, pressure is built.

Every time the chamber of trapped air opens to the manifold, the previously pressurized manifold air reverse flows into the chamber until pressure is normalized, then the chamber is closed and exhausted as the rotors mesh together. This “reversion”, plus the internal leakage between each rotor as well as the housing, combined with the tortuous flow path the air must follow, is why the roots type supercharger is the least efficient of the types available.  As demonstrated by the problems with the new Corvette flagship, attempts to correct this situation through size decrease are bound to cause considerably more problems than they can correct.

 While the aforementioned process can offer reasonable performance at low pressures, about 5 to 6 psi, operation at higher pressures clearly shows the limitations of the device. When operating a Roots supercharger at higher pressures, it is typical that at least half of the input power is consumed to make heat rather than pressure. This would roughly indicate a very low adiabatic efficiency of 50%, or below.

It is possible for other types of superchargers (centrifugal) to operate this inefficiently also, but it would be due to an exceptionally poorly designed and sized installation and certainly not typical.

When defining the term supercharger efficiency, what is meant is the adiabatic efficiency as an expression of how well the supercharging process works. All of this supercharging process is about increasing air density (pressurizing or compressing air) and an inefficient supercharger will put more heat into the air during this process, and thus reduce density, than an efficient one. Further, that heat isn’t free, it was bought by a parasitic power drain on the engine.

Manufacturers like GM attempt to mislead or confuse shoppers by using the term volumetric efficiency instead of adiabatic, so they can display a very favorable number, usually in the ninety percent range. The term volumetric efficiency only refers to how well the chambers are being filled and has little to do with how well the supercharger is working; this term only applies to positive displacement devices and not to centrifugals. Others, even large OEM suppliers, will sometimes offer sales materials or compressor maps showing unlikely performance and/or supercharger efficiencies, (they definitely have a straw in the Kool-Aid).

Of course douchebags like +Tadge Juechter  feel compelled to point out that the roots type supercharger will make boost at very low engine RPM.

This is true; these superchargers make more boost than for example a centrifugal at low speed and less at high speed, but this feature is much less helpful than it would first appear. First, excessive boost cannot be fully utilized at these low speeds. On engines like LT4, the ignition timing must be significantly retarded to avoid destructive detonation that it is very probable that no net increase in power is realized especially considering the parasitic drive losses.

Second, the variable valve timing generates more than enough low speed power to exceed the tire traction available.

Third, low engine speed is not where power should be made.

Fourth, when Z06 supporters and "tuners"  point out the “area under the curve” they usually refer to a huge misrepresentation of what the curve should look like. The higher boost levels that are common today are not shown and no one can explain the mathematical significance of this phantom area!

Further these numbnuts ignore the heat soak and inherent “nosing over” performance of the roots type at higher engine speed, right when a high performance vehicle should pull the hardest.

For similar output, a roots type supercharger will be heavier than a centrifugal by a large margin, and will generally require a larger, heavier charge cooling system. Also it is necessarily placed in the worst possible location: way up high and forward, right on top of all the other weight.

The rotating mass of the LT4 Roots type supercharger has a higher inertia than a comparable centrifugal compressor would have.  The power it takes to accelerate that inertia, to speed it up or slow it down can be huge and this is independent of actually doing any useful work.

So, with the little Roots type turd in the Z06, the buyer gets:

 A noisy, heavy, high inertia, poorly placed, air-moving device that makes too much boost when you can’t use it and not enough when you can, that is as good a heater as it is a supercharger, that shows even lower performance after a full warm up.

And when the tuners flash that big horsepower number on You tube, two things are certain: it was a first “pull” with a cooled engine and they won’t show a second “pull” because it will be much lower, and a third pull, even more so.

What would happened if instead of idiot Juechter there was someone competent in charge and IF NECESSARY to go with forced induction, chose a larger centrifugal compressor?

Chances are very good that the car would be just as quick   There still would be more pressure but the flow would be much more efficient at WOT and high speed situations.  Who knows, maybe fatso Jim Mero would be able to complete that Nurburgring run?