Impret-Sir

iTrader: (1)

Can someone explain in simple terms what the vacuum section of my boost gauge is telling me, eg at tickover it is showing 20 inches of mercury, but is that good or bad? The pressure rises to a positive level as thr revs increase, so what is the point of the vacuum measurement?

Paul_M

The engine creates vacuum cos it's trying to suck in air but the throttle is creating a restriction which is how it controls the power and revs of the engine. Maximum vacuum is produced when the throttle is completely shut since the only air getting in is that through the idle valve.

A normally aspirated engine will always be either in vacuum or zero relative pressure when the throttle is fully open since the open throttle is allowing the engine to consume all the air it wants.

Adding a turbo gives the issue of positive pressure, as long as there's enough revs and the throttle is open enough the turbo will create pressure in the manifold greater than atmospheric and hence boost. Whenever you're in positive pressure the engine is being fed more air than it would naturally consume so you're getting the benefits of boost. Higher the boost the more air the engine is being fed (well it's a bit more complex, flow and temps to consider but the idea's the same) so you get more power.

Paul_M

BTW the vacuum measurement at idle sounds fine, as long as it remains steady when the revs have settled then it's normal.

Impret-Sir

iTrader: (1)

Thanks a lot Paul, very clearly put!

Claudius

Good summary I dont think Paul answered the question, though!