[MrNiceGuy]

In order for the air to be compressed though , the turbine needs certain rpm to spool up, if you don't push your car to that rpm and beyond all the time you'll get pretty good gas mileage.

 

[MrNiceGuy]

I know hardly anything about boost... but I've heard the term "negative boost". Does that mean that you're not getting any boost, or you could actually be losing compression? Either way, if you're not boosting at all, wouldn't it not hurt gas mileage?

Positive boost : air pressure above atmospheric

Atmospheric pressure : air pressure of 14.7 psi or 105KPa or 1bar

Negative boost : obviously air pressure below atmospheric

Negative boost applies to all internal combustion engines , forced induction or not.

When the intake valves start to open and the pistons descend in the engine cylinders it causes vacuum within the combustion chamber , so air from the atmosphere enters through the intake resonator , passes through the air filter , through the intake pipe and intake manifold , intake runners , past the valves and right into the combustion chamber and cylinder to eliminate the vacuum . At certain rpm the cylinders are filled completely with air (100% V.E.) . At that rpm maximum torque is reached because there no losses in airflow.

Theoritically the vacuum caused in the air intake when the piston descends should climb up to 14.7psi again because of the incoming charge.But does it? No. Why? Answer : restriction.

Any restrictions in the air intake system that cause pressure drop is considered as negative boost.

In turbocharged cars negative boost is exists before the turbocharger.

Hope that makes sense.

 

[MrNiceGuy]

yes they were referring to the boost gauge , but vacuum is a more precise description than "negative boost" .

 

[MrNiceGuy]

Well really the question was, how will FI effect MPG... not should I worry about MPG when I go FI...

I hope you can resist the temptation to mash the throttle