Respectfully, I disagree. You can't spin a turbine for free. Period. If the following logic does not convince you, please just remember the most basic laws of physics.
Just like the pully being spun by the crank takes energy away from the engine, so TOO does the turbine take energy away from the engine. I'll grant you this...it does not do it all by itself. A turbo is being spun by gas that is being forced (read using energy) out of the engine. It adds to the omnipresent exhaust losses.
Of course it takes something to turn it and it will add 'resistance' to the exhaust, however, this isn't dragging off the motor. It's not robbing power that is being made like a supercharge will. That turbo is also what is giving the exhaust the extra flow as it revs up and give it more power.
Pretend there was no turbo. Just a regular, quiet, low performance exhaust system. Does that exhaust "rob power" from the engine? Yes. We all know it does. If you open up the exhaust and make it free flowing, you can get some of it back...but not all of it.
Actually, you can use an exhaust to gain and make power more usable... Two-strokes operate on this theroy. A proper exhaust will help the exhaust evacuate without turbulance from the head port. If that were the case, we'd all run 4" exhausts.
To get all of it back, you would need to make the heads open up straight to the atmosphere...even then, you won't get all of it back. That's why naturally aspirated or supercharged RACE CARS use open headers....or even ZOOMIES. To minimize generic exhaust losses.
Headers are an exhaust. It HELPS in getting those exhaust gases out.
So the exhaust gas needs to spin that turbine to what, in the neighborhood of 50,000 RPM? It cannot do that without causing the gas to compress, relative to the other side of the turbine blade. I'm not in to quoting the laws of physics, but I'm pretty sure one of em covers that. That air speed "slow down" perpetuates the high pressure zone like a traffic jam perpetuates bumper to bumper traffic...it goes right into the cylinder and plants it's happy ass on the top of the piston...making that piston work harder to expel it. I'm not saying that the exhaust goes back into the cylinder, just that it has to wait on the onramp longer for traffic to clear.
With a properly sized (mapped) turbo, that turbo isn't going to be causing much of a back-up in it's efficiency range. The turbine is already howling along, it doesn't take a ton of to pick up the pace.
Before I hit "post", lemme ask you a question.
If you could make an electric motor spin a turbo (in the open atmosphere..no pressure against the outlet) to 50,000rpm, how much power do you think it will take?
Remember...you are spinning TWO BLADES...no cheating and removing the exhaust side of the turbo...you have to spin that, too.
In a sence, that turbo is spinning itself. It's being turned to force more air down the motor and not taking (like underdrive pulleys) the power off what the motor is making. I've personally found that you can get away with a smaller injector with a turbo with just working with SHO's than you can with the trusty Vortech. I'm maxing out 48# injectors (95%+ DC) on my Vortech and scott is in the 70% range DC on his turbo running 44# injectors (MY old 44's that I was maxing out at less power than he's got). Our motors are pretty closely built.
Power is not being lost by a belt... Yes, BUT power is being lost from backpressure. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. Which is a loss in power. The loss is not as great as a s/c but none the less a loss.
However, it's not making the extra power to turn it. IE: 400hp turbo SHO is consuming about 400hp of fuel. A 400hp SC SHO is probably actuall consuming 450hp of fuel.
Sizing a turbo (turbine) only increases or decreases the amount of backpressure. For example on t/c'ed road-track car the turbine is alot bigger to allow exhaust gases to go around the turbine, which means less back pressure at low rpms. As rpms increase back pressure increases as more gases are pushed to the turbine. As backpressure increases u are lossing power, which some of it is made up by the psi being produced, but not all of it. Ie at 10 psi it has to fight the exaust pressure(which increased) which produced the 10psi to begin with. ( ok I made that last sentance up but it makes sense) anyone verify this.
As that backpressure increases (not as much as you think since the turbine picks up speed), the compressor side is also increasing and forcing that air into the motor.
No. It means at low rpms it will do nothing. It will spin(duh) but will not produce any pressure. Also making the backpressure alot less until high revs are seen. IOW lots of lag...... Thats all. Some race turbos actually have much larger turbo housings than the turbine itself. Better flow charactersitics at high rpm. hence a race turbo. suffer down low but its a race car WOT all day.
They use larger turbos because they are road-track cars. They're never under 5k RPM... No need for the low end power. If they're sizing the housing larger than normally needed, it probably because of expansion properties of 2,000+ degrees of constant heat.
And let me make this clear. I am not aruging that a s/c does not steal its power. It does. I know that. but A LOT of people think a turbocharger is FREE energy. That is 100% false. IT IS more efficent than a s/c. I said that on my 1st post. But both forms FI are not free. Plan and simple!
And I know the pro/cons of a s/c'ers and t/c'ers.
Considering that it's essentially not burning any extra fuel for something you'd never see, I'd call it 'free'.
When I boost my next toy... whenever that is, I'll go the turbo route this time. With the SC, I do like that I don't melt anything around it, but I don't like the extra bearing wear from a tight belt and the extra power it takes to turn it to reach the same 'seeable' power as respective turbo.
