[not Steve] Trac

1. Is the turbo working hard, or at all, when you are maintaining steady highway speeds? I.e. if I'm just cruising along at 75mph, which turns about 3400 rpm's, is the turbo working hard?

2. Is the turbo working when you downshift and use the engine to slow the car down?

KrautRice_A4 atl-atl

The turbo works according to rpm's because the exhaust gasses push the turbo. When you are at 3400 rpm your turbo is probably at 25k rpm 5k rpm engine, 40K rpm turbo. When you down shift it is working, though you slow down rpms when you slow down speed, so the turbo may not get a chance to spool up very high(rpm's).Is that good enough?

EricB

There is no boost at highway cruising speeds.

Mach V Dan

Oh me! Pick me! I know this one!

1. The turbo is basically just freewheeling. When the engine is at light load, the exhaust gas flow is light, so it doesn't push the turbine wheel very hard, and the engine ingests air at atmospheric pressure, or maybe a slight amount of boost. (It depends on how fast you're going...)

2. No. When you use engine braking, the manifold pressure is negative, as the engine is being driven by the wheels, not the other way around. When the ECU sees high vacuum levels at the manifold, it shuts off the injectors. And with no fuel in the cylinders, there's no exhaust gas, and so the turbo is not driven at all, except for maybe the compressor wheel being spun by the incoming intake air that is getting sucked by it.

EricB, you may be misunderstanding the purpose of the wastegate. The wastegate is a valve on the exhaust side of the turbo. It stays CLOSED until the car is under boost, then it OPENS to let exhaust gas bypass the turbo so the turbo doesn't overspin and blow up the motor. But the wastegate does not "prevent boost" at light throttle.

--Dan<ul><li><a href="http://www.machvw.com">Mach V Motorsports</a></li></ul>

EricB

Thanks for the explanation. (I deleted that part of my post.)