vacuum Gauge

[sexypete]

I have just completed a manual conversion on the weekend, and my vacuum gauge went from being at 5.0 with the automatic to being at 2.5 in the manual is this correct, have i missed something? Does this mean my engine is drinking more fuel.

Many Thanks
Peter

[vkberlina]

I doubt its using more fuel, your auto used vacume to change gear etc there are vacume driven valves etc in the auto its possible you had a small vacume leak before in the auto.
Vacume is good to a degree it still sounds all right it will change repending on go pedal position, load on engine etc.

[sexypete]

oh ok good but is there a rough number that can be put on it like where it should be a idle or something.

peter

[Demons1964]

If it's any help, my manual VC 202 idles steadily at 450mm Hg on the dash vacuum gauge (however accurate that gauge is!). Here's a link that may help you out with vacuum readings:

http://users.bigpond.net.au/ergoff/vac1.htm

Although the measurement shown on that site are in inches, it's easy enough to do the conversion (1" = 25.4mm).

Mark.

[sexypete]

Very helpful thankyou

[Don Sutton]

I doubt its using more fuel, your auto used vacume to change gear etc there are vacume driven valves etc in the auto its possible you had a small vacume leak before in the auto.
Vacume is good to a degree it still sounds all right it will change repending on go pedal position, load on engine etc.

If you have a vacuum leak the reading will be lower. Is this reading at full throttle? If so then perhaps the engine was reving slightly higher with the auto for any given speed.
High vacuum readings at full throttle mean either:

Not getting full throttle
blocked air cleaner

With a carburettor I would expect the vacuum to be about 0.8" HG at speeds below max torque and would increase at WOT at speeds above max torque.

A low general reading while driving can be a blocked exhaust as well as retarded spark.