Everyone once in awhile we get asked about how much power the supercharger takes to operate vs. the "free energy" that a turbo uses. So, we came across this article that described the engine losses because of the turbo and how to estimate them. This is not "supercharger propoganda" and I am not knocking turbos here, as we like them also
, just wanted to throw out some information for everyone.
"Myth 1: The energy it takes to spin a turbocharger is free.
I love turbochargers as much as anybody, but if I had a nickel for every person that incorrectly claims the energy to drive a turbocharger is "free" I would be a rich man. The turbocharger's drive system is more mechanically efficient than the belt on a supercharger but it's not free. The most basic laws of physics say that nothing is free. Some people have changed the terminology to say that the turbo is driven by "wasted energy". That's more nonsense. The energy needed to expel the exhaust from the combustion chamber is not wasted, in fact it's rather essential. Many makers of Turbo kits phrase it another clever way. They say that the energy needed to drive the turbo is "not taken directly from the crankshaft like it is with a belt driven supercharger". Ok, well at least that's basically true although somewhat misleading. It may not be taken "directly from the crankshaft" but it is taken from the engine. So how is the energy taken, and how much is used?
Here are the facts as reported in many N.A.C.A. reports. Exhaust drives the turbocharger and that causes exhaust backpressure robbing power as surely as if the compressor section was connected to a belt. It may not rob as much (it's possible under some conditions it will rob more) but it does rob power. Here is a quote from N.A.C.A. " When an exhaust turbosupercharger is used, the net engine power is the total engine power supercharged less the reduction in power due to increased exhaust back pressure." So there you have it, turbos cause exhaust back pressure taking power away from the motor. The report goes on to say just how much the loss from exhaust back pressure is going to cost and I assure you it's not free. If it was, then all the efforts made in the performance industry to reduce backpressure via special mufflers, headers etc. would be wasted. So just how much power will the turbo consume? They compiled a ton of data on that. It will vary somewhat from motor to motor, even between two theoretically identical motors. The number they came up with is 6.2% of total engine horsepower when using 10 pounds of boost. That's assuming exhaust pressure in the manifold between the combustion chamber and the turbine is equal to intake manifold pressure. It's imporant to understand, that's a best case scenario. A turbo car will need very good unrestrictive equal length exhaust manifolds and a perfectly sized turbocharger to achieve this. I suspect the best manifolds made for race cars by masters like Jim Steck or the mad South Africans are good enough to get these results. In practice when limited by engine compartment space, strength requirements and other factors it's not likely you will find a manifold this good on a street Alfa. Many turbo cars have double the intake pressure in the exhaust manifold prior to the turbine. At 10 pounds of boost that would cost about 15.3% of the engine's horsepower using N.A.C.A.'s formula.
Time to plug in some real world numbers. A MP62 supercharger providing 10 pounds of boost on an Alfa 2.5 will use about 27 horsepower. It could potentially use less with optimal piping, but on my car it uses 27 horsepower so that's real world data. My car has about 250 flywheel horsepower so using N.A.C.A.'s formula a very efficient turbo set up would need 15.5 horsepower to drive the turbo. While there is no doubt that 15.5 is less than 27, it's a factor that's small enough to be offset in other areas. Now using the same formula a turbo with a restrictive type of exhaust manifold will use 38.25 horsepower.
My point here is not to say that belt driven superchargers are better or worse. The point is that according to the best engineers in the U.S. at the time working with a nearly unlimited wartime budget determined that the energy to spin a turbocharger is not free. I am sure the brainwashed free energy crowd will state that these reports are old and turbos have come a long way. Of course if free energy was used to spin a turbo then it wouldn't matter how much they improved in this regard. An improvement of any percentage times zero is still zero. In other words if they are free now because they are twice as efficient as the older units the math says the older units must have been free to. "
This is quoted from :
Supercharger Kit
Thanks!
Dave
