MechRick":38vq9qeh said:
I once removed the turbo to intake tube with the engine running. This was after extensive tuning, so I know it was close. At 1000-1500 RPM the engine was suddenly *very* lean. I worked out that the MAP sensor was reading less positive pressure without the turbo connected, and the engine was using more air.
What this means (I think) is the engine is ingesting less air for a given RPM with the turbo functioning. Less air means it needs less fuel for the same output as N/A. Not very scientific, but I did back it up with hard numbers.
What you saw when you disconnected the turbo outlet tube is correct. Less manifold pressure more throttle for same engine output.
The reason is slightly different
Air and air fuel ratio is measured by weight not CFM.
Example: 400 cfm at standard atmospheric pressure (no boost) is equivalent by weight to 200 cfm at 2 atmospheres or 14.7 lbs of boost providing both air temperatures are the same. (think intercooler)
When you pulled the tube and the manifold pressure dropped it needed more air volume to provide the same amount of air by weight.
A/F ratio should have stayed the same.
Where you gain fuel mileage when turbocharging is by reducing the pumping losses during the intake stroke.
Looking at it from a gauge pressure perspective.
Normally the engine must draw the intake charge into the cylinder by working against intake manifold vacuum. That takes energy.
Under light loads the turbo reduces intake manifold vacuum which reduces pumping losses.
As the engine load increases and the intake manifold sees positive pressure or boost, the piston is driven during the intake stroke which increases engine power.
As you previously pointed out this energy is extracted from the blow down portion of the exhaust cycle which would have been wasted otherwise.
I would like to add that blow down is complete just after BDC and any residual pressure left becomes a pumping loss as the piston moves towards TDC at the end of the exhaust stroke.
Exhaust system design and Turbo turbine housing A/R selection is important to minimize exhaust cycle pumping losses.
Wesman07":38vq9qeh said:
Ok so fuel efficiency can be gained. I would assume the working rpm would stay the same.
The working rpm can be lowered since the turbo can increase the torque in the lower rpm range