What's the difference between just a regular boost gauge and the boost/vacuum gauge? Like what does the vacuum measure and is it important or is just a gauge for boost good enough?

 

The most practicalday-to-day use for a vacuum gauge was always as a kind of economy monitor. Don't see it really serves much of a purpose nowadays.
That said, the price difference betweena boost gauge and a vacuum/boost gauge must be insignificant.

 

the vacuum/boost type gauge, would
be handy for troubleshooting

i have a vacuum/boost type gauge and
i like having it

 

So.....wat's it really do that sets it apart from a regular boost gauge? lol

 

The vacuum readings are an important part of engine testing and diagnosing. It shows the difference between outside atmospheric pressure and the amount of vacuum present in the intake manifold. At idling speed, an engine at sea level should show a steady vacuum reading between 14" and 22" Hg. Since the pistons in the engine serve as suction pumps and the amount of vacuum they create is affected by the related actions of the piston rings, valves, ignition system, fuel control system and other parts affecting the combustion process, if you understand how to read the actions of your vacuum gauge, you can very effectively diagnose engine issues and tune your engine with just the gauge and track time.

For instance, assuming a normal reading is between 17" and 22" Hg with the needle steady (a race engine will vary "a lot" and in most cases will be considerably less):

1. Intermittent fluctuation at idle could mean an ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft.
2. Low, though steady reading demonstrateslate timing, low compression, sticking throttle valve or a manifold vacuum leak.
3. Drifting needle could mean a minor vacuum leak.
4. Fluctuating needle as RPMs increase could mean an ignition miss, blown head gasket, leaking valve or weak or broken valve spring.
5. Steady, but needle drops regularly might be burnt valves or incorrect valve settings.
6. Gradual drop at idle indicates a clogged exhaust or excessive backpressure.
7. Excessive needle vibration that steadies as RPM increases shows worn valve guides.

And on and on. So the vacuum gauge is a simple and effective way of checking your engines health and do preliminary testing of other components that typically require special tools and equipment. If you're F/I and you're guilty of over boosting, over revving or hitting your rev limiter ceiling too often, then this is a good gauge to see if you've done some damage to the engine or transmission. I use a boost/vacuum gauge and I think the vacuum reading is the more important of the two. JMO.

 

a boost gauge would show the positive
pressure of the intake manifold
(basically 0.1 psi and above)

the vacuum part of the gauge would show
negative pressure as well

 

+1

 

hey very nice explanation redfire, it's
great for troubleshooting and also it just
plain looks cool