new 911 turbos WTF

[kachok25]

OK does anybody know anything about the new 911s turbos? That crazy thing spools up and hits peak torque (460 ft/lbs!!) at below 2000 RPM, and does not run out of breath at 7000 RPM what do they do to ther turbos to give them such an ungodly power band?

[Dyno247365]

Are you talking about the rear mounted turbos?

[2.2 Straight six]

they're called variable geometry turbos. as the engine's revs climb they blades open out more. so effectively the turbo gets "bigger" as the revs climb.

so at a low rpm they're acting like small turbos that spool up fast, and at high rpm they open up more and act like much larger turbos.

they're nothing new, they're been around for years. but were usually the reserve of diesel engines.

[kachok25]

Cool is the Thermal effeciency as good as regular turbos?? Why don't all turbos spool up at 2000RPM or less?? How much can they change the AR or a turbo?

[KiwiBacon]

Originally Posted by kachok25

Cool is the Thermal effeciency as good as regular turbos?? Why don't all turbos spool up at 2000RPM or less?? How much can they change the AR or a turbo?

The peak efficiency will be a little lower due to the drag of the stator vanes. But the efficiency band is much wider.

So they have no place on an industrial engine (think power generator running one RPM for ever) but on a road vehicle make a lot of sense.

One of the american cars had variable vane T25 turbos very early on. It seems they learnt lessons the hard way that moving parts and high EGT's of a petrol engine aren't friends.
But the Germans could well have found the answer.

As for why most turbos don't spool at sub 2000rpms?
On almost every diesel they do. On some petrol cars they do (twin turbo subarus boost almost that low). But at cruise it's terrible for fuel economy to create boost, then throttle it to a vacuum before the inlet manifold.

Supercars care more about power than MPG.
Here's a cross section showing the variable stator vanes.

[kachok25]

So run a bypass valve while crusing that engages the turbo when you punch it. That does not seem like an engineering problem.

[GreyGoose006]

good thinking... it would have to be computer conrolled and refrence the ECM and throttle position sensors to do that.
the major killer of great ideas like this is R&D funding. sure the technology is out there, but can someone convince their boss to let them spend $12Million on an engine that uses technology that is just now being perfected, and put it in a road car that gets driven anywhere between 3000 and 10,000 miles between oilchanges, and treated like shit on a daily basis, but still has to work long enough that the dealers dont have to repair them al under warranty... No.

you also have the problem of the fuel ratios going way lean, so you would need it to be adjustable automatically based on boost levels, and timing would change, so would the *IDEAL* cam profiles... it'd be a little more complex, but still feasable



How are the vanes actuated... what makes them open and close at RPMs?
is it air velocity, or solenoids or what?

[UncleBob]

a bypass valve wouldn't "solve" the issue. The problem isn't so much with the turbo, its with the engine design. High performance cars have a combination of issues that don't make them very fuel efficient. The biggest one is usually the cam shafts.

Take a VW jetta/passat 1.8L Turbo. They spool very fast, because they have a very small turbo, but they get pretty decent mileage. They don't have the option of increasing the AR at higher RPM's, so the engine is really choked in the higher R's. Much different engine design.

[kachok25]

If a proformance engine is not capable of getting good thermal efficency how do you explain the 1997-2002 Z28 over 3500lbs RWD and 300+hp getting 28mpg (if you don't drive it like it is ment to be driven ) that is the fuel effecency of a mild tune V6 FWD famaly sedan of that time. So I would not say that a proformance engines cannot get good effecency.

[2.2 Straight six]

Originally Posted by kachok25

If a proformance engine is not capable of getting good thermal efficency how do you explain the 1997-2002 Z28 over 3500lbs RWD and 300+hp getting 28mpg

overdrive. insanely high gearing allows the engine to operate at a very low rpm, yet the car's still moving at a good speed. so doing 60 on an interstate in 6th (their second overdrive gear) the engine's somewhere between 1,500-2,000rpm i believe.

[UncleBob]

to keep this in perspective..... "300hp" (and I quote that loosely) out of a 5.7L engine is a tad bit different level of "performance" than the original car mentioned here

[Moppie]

Originally Posted by UncleBob

to keep this in perspective..... "300hp" (and I quote that loosely) out of a 5.7L engine is a tad bit different level of "performance" than the original car mentioned here

Out of a 3.0L V6
But its still completely different to the power level out of the Porsche engine, which is almost twice as much, and makes more torque at 2,000rpm than the Nissan did at peak.

[KiwiBacon]

Originally Posted by kachok25

So run a bypass valve while crusing that engages the turbo when you punch it. That does not seem like an engineering problem.

You've still got a lot of frictional loss there. Still spinning the turbo, still pumping air. Just letting the recirculated air go around in circles getting hotter and hotter.

Seamless transition would also be a major issue.

[kachok25]

Aw forget about seamless transitions, I love a turbo that slams me back in my seat
Oh an considering the proformance on the 97-02 Camaro I would not doubt that 300hp quote at all. And now there is the LS2s with 400hp still getting above avrage MPGs.

[UncleBob]

Originally Posted by kachok25

Aw forget about seamless transitions, I love a turbo that slams me back in my seat
Oh an considering the proformance on the 97-02 Camaro I would not doubt that 300hp quote at all. And now there is the LS2s with 400hp still getting above avrage MPGs.

that one I know about, unlike the camaro reference.

There was a day when the HP ratings were pestimistic due to regulations. If you had a car that was rated for 300HP, often they would have 325HP to the rear wheels.

Those days are over though. The 400hp rating for the LS2 is probably at the flywheel, it definitely isn't at the rear wheels. The stock LS2's I've seen dyno'd were around 350 RWHP.