[Nevin]

Ok, I read the title of the thread, and I imediately thought of an issue of Sport Compact Car which I still have! In the June 2005 issue, Dave Coleman covers this quite nicely, and I am going to type out a bit of what he said.

To prelude this a bit, people sometimes think that "oh, the turbo is doing 6 psi and so is the supercharger, so wouldn't it come out at 36 psi?"

Taken from page 26...

"That said, two at the same time can work, but not the way that you might think. First, you must think in absolutel pressure, not boost. Six psi of boost is really 6 psi over ambient. Ambient is usually 14.7 psi, but let's call it 15 psi because we're lazy. Second, you have to look at the pressure ratio across each compressor rather than the simple increase in boost.

In the 6 psi example above, the supercharger takes in air at 15 psi and spits it out at 21 psi, (15 psi ambient plus 6 psi boost) so the pressure ratio is 21/15, or 1.4:1. Now, if you feed that 21 psi (absolute) into a turbo running the same 1.4:1 pressure ratio, you'll get 29.4 psi out the other end (21 psi x 1.4). Subtract ambient (15 psi) and you see that you've got 14.4 psi of boost, not the 36 you were hoping for.

Since you could easily make 14.4 psi of boost with either a supercharger or turbocharger alone, there's little point in using both at this level. Run each compressor a little harder, say at a 2:1 pressure ratio (15 psi, or 1 bar of boost in the real world) and things get more serious. Air comes out of the second compressor at 30 psi (absolute) and out of the second at 60 psi. Subtract 15 psi of ambient, and that's 45 psi of boost. Now, any turbo capable of this kind of boost would be very laggy indeed. so using a supercharger and a turbo in series might make sense. Neither compressor would be working abnormally hard at a 2:1 pressure ratio."



Ok, so hopefully that makes a little sense.

Also, he points out that if you're choosing a turbo for an already supercharged engine..."You need to plan for the mass flow rate of the engine with the supercharger installed, not the engine by itself."

And lastly, I apologize if this has already been gone over. I was on the first page and just hit "reply" and then realized there were two additional pages. So, if someone already said this, or you guys already knew since you're super-smart anyway, then my apologies.