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Boost gauge question?
tomgregory2000 - 22/10/09 at 06:17 PM

Why do boost gauges have ranges of say -1bar to +2bar.

Now i understand the + numbers thats why you got the gauge but i dont understand why you have the - numbers.

Can someone explane this to me or am i just being super thick

flak monkey - 22/10/09 at 06:25 PM

Its to show manifold vacuum

So at idle you'll have say -0.7 bar if you have a 30kpa manifold absolute pressure.

0 bar absolute is total vacuum. 1 bar absolute is normal air pressure, which would be 0 bar relative on your gauge.

At WOT on a NA engine you would have 0bar (relative)

Then on a boosted engine at some point you will have your boost come in which will take you above 0.

[Edited on 22/10/09 by flak monkey]

tapkutez - 22/10/09 at 06:27 PM

yes, the - shows vacuum, it means the engine is sucking air

flak monkey - 22/10/09 at 06:36 PM

quote:
Originally posted by tapkutez
yes, the - shows vacuum, it means the engine is sucking air


Not really the engine is always sucking air. It just shows the relative manifold pressure to that of the normal air pressure.

When the throttle plates are virtually closed (idle) or you are on very light throttle the engine will create a realtive vacuum in the inlet manifold. If the throttle is wide open there will be no vacuum and the whole inlet tract will be at normal air pressure 1bar absolute or 0 relative.

When the engine is under boost, which will vary with rpm more than throttle position, then the gauge will show a positive pressure as the manifold pressure will be above the normal air pressure.

tomgregory2000 - 22/10/09 at 06:54 PM

ok thanks guys.

The only reason i asked was that my current gauges are VDO and they have changed there boost gauge to only read 0-3bar.

So the vac part of the gauge is not important then?

flak monkey - 22/10/09 at 06:56 PM

Depends whether the gauge is absolute or relative.

The VDO one may be absolute, in which case WOT on NA engine would be 1 and anything above that is boosted and 0 would be total vac.

tomgregory2000 - 22/10/09 at 07:05 PM

fair enough

I may be asking vdo about that then.

This turbo lark is good fun but steep learning curve