RED HORSE

I posted this in the S8 forum but haven't gotten a response. Maybe someone here will offer some insight for me.

My '15 S8 is my first turbo charged car. My curiosity has been piqued now that I can see the manifold pressure as I drive.

I know the very basic concept of turbocharging, exhaust gas powers a turbine that in-turn powers a turbine on the induction system providing boost. After that I no very little.

My S8 driving down the highway at 70 mph in calm conditions on a level road shows 0" Hg manifold pressure. OK. When I gently accelerate the boos raises to +5-10" Hg and then as I continue accelerating it drops to -10"-15". Why the drip in manifold pressure?

Of course aggressive acceleration yields boost in excess of +15". The thing I don't understand is the drop to negative manifold pressure while still accelerating.

jonas321

Keep in mind that modern cars have layers upon layers of electronic control over all aspects of engine performance.

Short version: there is a dip in the boost pressure because the engine doesn't need the boost pressure for that rate of acceleration.

Long version: You step on the accelerator, this turns a potentiometer which sends a signal to the ECU. The ECU interprets both the signal and how fast you made the change and makes a bunch of changes to the engine settings; throttle position, ignition timing, fuel delivery and timing, waste gate bypass, etc, etc. Even if you keep your foot perfectly steady, the ECU can make changes to the throttle position, and all the other things. In general, the ECU will make adjustments to keep a steady pedal position result is a steady and smooth acceleration. If that means dropping the boost pressure at some points, then so be it. You've just demonstrated that at high RPM/low load/partial throttle that negative manifold pressure still results in acceleration. Something non-turbo folks have known for over a century. :-)

motoguy128

The gauge is most likely measured to atmosphere. Keep in mind that at higher RPM's to intake would normally be under vacuum. So the actually "boost" supplied compared ot manifold pressure without the turbo, may be increasing, but the gauge pressure could still decrease if that makes sense.

In other words, the relationship of manifold vacuum to RPM has one curve, while the output of the turbocharger has a different curve. One is going down, one is going up then leveling off.

This is dependent on tuning, turbo charger size, variable valve timing, engine configuration and other factors. That's with a turbocharged engine often has great bottom end torque. Passenger cars are tuned this was to get the best useable increase in performance AND economy. A race car conversely might use different turbos that work best at higher RPM and sacrifice bottom end power to top end. This is why a 2.0L turbo charge engine with 210 HP can out perform as 3.0L V6 with maybe 240HP, because the 2.0 makes 250 ft-lbs, while the naturally aspirated makes maybe 200 ft lbs and does it at very low RPM's.

Peak ratings rarely tell the whole story.

andrewmr

The bigger question is how do you see the boost numbers? Is that something in the newer cars or does my 13 have it and I just don't know where to look?

RED HORSE

I see the boost numbers using my Garmin Nuvi. I purchased the available Blue Tooth dongle which plugs into the OBDII port. Gives you a selection of displaying throttle position, MAF, rpm, speed, boost, water temp, battery voltage, etc.