RED HORSE

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.

Gauta///M

Red Horse- a good place to start would be to understand what is happening in the intake manifold on a naturally aspirated (NA) engine. In an NA engine, the manifold pressure should always be negative as the cylinders draw in air and maintain a vacuum in the manifold. The amount of vacuum varies depending on throttle opening. Coasting or idling = less negative pressure or vacuum. Accelerating = lots of vacuum.

In a turbo engine or Forced Induction engine, the turbo or supercharger is pushing air into the manifold and cylinders, thus the positive pressure. It may dip into the negatives briefly when you release the throttle after accelerating since the boost isn't needed and the blow-off valve vents the positive pressure to atmosphere. Boost (positive pressure) should come back immediately as you apply throttle. Sometimes, the load on the engine may be so low (cruising on a level highway at 60 mph), that the blow-off valve vents some or all boost.

Hope this helps. Somewhat simplistic explanation of the various complex things happening in these engines.
G.

RED HORSE

Gauta, yep I understand all that. I guess what I'm puzzling to me is the why of the manifold pressure decreasing while more gentle acceleration. I guess I'll have to do some research.

Thanks..

Gauta///M

Manifold pressure and load are correlated. I've seen the same phenomenon on my supercharged GTO. Once you reach steady state and load on engine decreases, boost decreases and manifold pressure decreases.

Ravill

That was a pretty good explanation Gauta.

What's REALLY cool about our cars is that the S8 has what are called "twin scrolled" turbochargers. This splits the cylinders as the exhaust gasses enter the turbochargers and makes sure that the turbo lag is maximally reduced and power delivery is smoother.

And the reason why your manifold pressure can decrease while in "gentle" acceleration, is that you aren't making the turbo's build enough pressure to compensate for the vacuum in the manifold.

RED HORSE

Agree, the turbo is great. I don't have any noticeable turbo lag at all. It is an amazing engine so far. I hoping it will also be dependable.

jro

I'm sure that I could try to understand what all of this means (ok, maybe not, I was a political science major after all), but all I know is that the car can cruise down the road in complete comfort right along side an S Class, a 7 series, or a Lexus, and yet move like a bat out of hell when you step on it and handle like a true sports car!