Please tell me if these readings are normal. This is on a stock b18b, not boosting yet. At idle the gauge reads vac. 20. When I give it a quick rev to 4000 rpm and let off quickly the needle hits 0 and then dips down to vac.26-28 and then back to 20. Is this normal??? I thought that it should only read between vac.20 (idle) and 0 (full throttle)?

Also, when I rev the engine to about 5000 rpm and leave it there, the needle goes from vac. 20 to 0 and then quickly falls back down to vac. 22-24.

Ive already tried this on two gauges cuz, I thought that the gauge was broken, but maybe it has something to do with my car?

 

bump

 

yes its normal

 

Weird, mines goes from Solid 18 to 0 to 18....

 

This is totally normal.

 

Are you guys sure??? Why would there be more vacuum than at idle? With the throttle closed it would seem that this is the greatest point of vacuum (vac. 20). What is the reason for the vac. drop as the revs go down? I dont get it, but Im glad that you guys have had this same experience. I was getting worried.

 

my vac at idel is like 18-19 but then on the highway and i let off it gos to like 23-24, iguess it has somthing to do with your compression ratio also.

 

When the throttle plate is closed, there's very little atmospheric air (and pressure) to fill the manifold, so the vacuum reading is high.

The amount of vacuum the engine can pull is dictated by its valve-piston timing events. The amount of vacuum your gauge and MAP sensor reads depends on that above and the amount of atmospheric air coming into the manifold (determined by throttle position).

The higher the rpm, the more air the engine can pump out of the manifold. This is still assuming that the throttle position doesn't change. With no extra air to take its place, the vacuum reading goes up.

Comparison:
When you're cruising and take your foot off the throttle (throttle plate closed completely), the vacuum will hit its highest point. Compare that reading to when you're at idle (throttle also closed) and you'll see that at higher rpm's, the engine creates a greater amount of vacuum in the manifold.

When you're blipping the throttle, you open the throttle to atmospheric air (gauge reads roughly 0) and then it immediately closes, but the engine is still revving down from whatever rpm it was at (gauge reads high vacuum) until it hits idle, where the vacuum will eventually settle to its regular idle vacuum.

I'm really bad at putting together coherent thoughts into words when I'm hungry, so I hope you can comprehend it easily.

 

my needle jumps to 0.1BAR when I rev

but yeah...sounds pretty normal. Gauge should be reading vacuum in neutral, and jump to 0 (which is atmospheric) if you turbo doesn't spool in neutral.