Originally posted by: Mwilding
Originally posted by: notfred
Originally posted by: Mwilding
Originally posted by: notfred
You're only running in boost at WOT anyway. If you want to get good mileage, just be a bit lighter on the gas pedal.
No boost at all until full throttle?
😕
Thje function of the throttle plate is to limit the amount of air coming into the motor. When it's closed, it doesn't matter how much boost you make, it's not going to reach the engine. Even as it opens up, you don't see any boost until it's pretty much completely open. A forced induction car still drives around drawing a vacuum 95% of the time.
I am not knowledgeable in this area so forgive my obvious questions...
I have seen boost guages and they always seem to be in "VAC" - matching what you said. What does this mean exactly and what effect does it have on performance? If you add a turbo to a NA car you up the HP significantly, but is that solely at WOT?
A normally aspirated car drives around drawing a vacuum most of the time. That is to say that as the intake valve opens and the cylinder moves down, the motor attempts to suck in enough aior to fill the cylinder. However, the throttle plate is closed (at least partially), and the motor can't really get that much air. So, in the intake between the throttle plate and the intake valves, a vacuum develops where there is very low pressure air. This shows up on a boost guage as "vacuum".
Now, the further you open the throttle plate, the more air you let in. This means more air in the cylinders, so the motor makes more power, and accelerates, requiring even more air to fill the faster moving cylinders. However, the further open the throttle plate is, the closer the pressure inside the intake tract gets to the ambient pressure. What the turbocharger does is increase this ambient pressure from 14psi 1 atmosphere of pressure) up to 17psi at 4psi of boost, or 28psi at 14psi of boost. So, when you open the throttle all the way on a N/A car, you'd see a boost guage go to '0'. On a turbocharged car, you'd see the guage go to '4' or '14' or whatever the boost is set to.
But most of the time the throttle isn't all the way open. When you're cruising on the freeway at 2500rpm and 70mph, you need a certain amount of air going through the motor to maintain that speed. That amount of air is far less than the amount of air you see at even 1psi of boost. So, the throttle plate at that constant sped on the freeway is mostly closed, and the car is still running in vacuum. It's not until the throttle plate is nearly entirely open that you'll start to approach that unrestricted state where you'll start registering boost in the intake tract and therefore on the boost guage.
I hope that makes sense.
