how come turbos only boost when in gear?

[pongorulz]

why is that?? i was just sitting here thinginkg about this.

the first time i had my engine in, i hooked the boost gauge to the vac line and didnt get any boost until it was in gear, then it shot up to like 25 lbs. is there a reason for this??

[Jesshwarren]

why is that?? i was just sitting here thinginkg about this.

the first time i had my engine in, i hooked the boost gauge to the vac line and didnt get any boost until it was in gear, then it shot up to like 25 lbs. is there a reason for this??

25lb's make sure you have a good waist gate working properly. A wideband o2 sensor to make sure you have adequate fuel to support 25psi. Hate to see you melt down your motor.

The reason is because it takes resistance to make boost. It don't make boost with no load because the throttle body is almost all the way shut. It takes air and fuel to make boost add them together = HP and HP with no resistance = a free revving motor waiting to blow :=)

[D94R]

Yup, needs a load to "hold back" the engine from free revving. The turbo spins from exhaust energy, but if there is no load (in gear) then the air will be getting out the engine just as fast as it comes in. A pressure difference of 0 means 0 boost. Now, with the load on it, the turbo still spins to the preset wastegate pressure level, but the engine isn't able to spin as fast to catch up. So what you see is an overpressure from the turbo outlet to the combustion chamber. This creates the pressure difference resulting in positive pressure.

That's about the easiest I can put it.

[nagelandy55]

So what does it mean when you can't boost while free revving? Does this mean your exhaust isn't restricting any? You see all the vids of the hondas free revving and blowing off but i can't even get 1psi when i'm free revving.
Thanks.