wowf1 wrote:I guess the main drawback of this technology though is the increased mass of the rotating components in the turbo. The variably vanes must have some form of mechanical actuation, which can only add weight I assume. I must say as well that the unit looks pretty hefty compared to a traditional turbo. I wonder if they had to increase the amount of material on the components to retain structural integrity at these temperatures.
This technology has been used for ages now in diesel engines! Till now these turbos couldn't be used in petrol engines because of its higher EGT's that would damage the blades, locking them in place... Now that at least porsche ones can withstand the petrol engine's EGT's they are obviously the best choice possible!
Notice that the variable vanes do not rotate, they are fixed to the core of the turbo and are actuated by the same type of actuator that moves the wastegate of the common integrated wastegate turbo. The actuator is connected to only one of the vanes, and all the others are connected to that one.
Weight is not an issue as it is pretty much the same. Although it has the vanes it dispenses the wastegate, so it kinda balances out.
More on:
http://www.autozine.org/technical_schoo ... _3.htm#VTG
DaveKillens wrote:Good point wow, I guess that's why they run two turbos, to make each individual one smaller, less mass to overcome.
I think the 911 turbo uses a twin turbo setup like all previous turbo models because it's a boxer engine and it makes sense to have a turbo for each bank of cylinders (just like a V type engine) so that you don't have exhaust pipes going everywhere on the engine bay running under the engine, just like on the scoobie engines! The efficiency of the turbos benefit from being closer to the exhaust valves and being two cuts the sizes down giving a better spool up time due to the lower moving masses.
Currently the BMW 535d (3.0 diesel - 272bhp / 413 lb ft) is using two variable geometry turbos, one smaller one for low down power and a bigger one for higher revs to give that huge horsepower figure that with a single turbo would mean that the engine would be completely dead below ~2500rpm...