BMW Top Cars: Reading the VACUUM Gauge

Friday, October 5, 2007

Reading the VACUUM Gauge

Reading the VACUUM Gauge:

Many racers and engine tuners (I get caught myself sometimes) have opted for state-of-the
-art diagnostic equipment but have forgotten one of the simplest, as well as most accurate
tuning tools ... the vacuum gauge.

Of course if your engine is barely able to sustain 2"Hg vacuum at idle, it will be hard to
tune using this method, but it is still useful. In most cases if you use a quality large
faced Vacuum gauge, with some experience you can effectively tune your race car's fuel and
timing systems. This is a starting point, we all know that dozens of passes and laps will
more than likely be required to verify tuning adjustments and compensate for variations.

First, A Few Basics:

Connection of the gauge is to a simple "manifold" vacuum source. "This must NOT be "ported" vacuum that rises as RPM increases". In
most cases this will be a direct manifold source or in many cases the PCV port (larger
port on the carb) on a typical carb.

NOTE: Connection to EFI cars is best done "Tee'd"
into the source line. If you are not careful, disconnecting a particular line can affect
the idle speed as well as information that the ECM needs to see. For this reason it is
best to tee into your vacuum source for the gauge.

You must have the timing "Pre-Set", or at least do all ignition settings "Before"
any carb adjustments.
After each adjustment is made, you MUST RESET your idle speed setting. You do this to have
a standard or baseline to compare against.
Small adjustments are best, and in fact "optimum" carb settings on the vacuum gauge
(highest reading) is usually richer than it needs to be. In other words, after the highest
reading is reached, the best setting (depending on engine) is to set mixture screws back
"lean" approximately 1/16 to 1/4 turn.

Proper Carb Adjusting
Procedure:

With the above items taken into consideration, the first thing you do with the gauge
connected is to lean one of the mixture screws until the gauge as well as the engine
begins to shudder.

Note: With a properly jetted carburetor, turning
either of the mixture screws all the way lean, should kill the engine. If not, you're too
rich! This may require re-jetting, or drilling the primary butterflies to add more idle
air. Many of the newer "race" carbs allow you to change Idle air bleeds to fix this.

You now bring the screw back towards rich, watching the gauge. (I'm not being specific as to "in or out" movement of the mixture
adjustment for there are a few carbs that are actually "In" for rich instead of the
standard "Race style" Holley that is "Out" for richening the mixture). As the gauge climbs, you will stop adjustment when
the gauge reaches it's highest reading.
Now do the same process for the other mixture screw. You may have to repeat this process a
few times to get optimum results, plus it's worth the time an effort.

For carbs with the 4-corner mixture screws, you have to take a bit more time setting
these. You can also run the engine at a "steady-state" RPM of say, 2500 RPM to double-
check your secondary mixture screw settings. Do this with temperament! It takes time to
get used to what you are seeing as well as if it is actually helping. each engine will
behave differently.

Vacuum gauge needle
readings explaination

Normal reading (usually 17-22" Hg. in stock engines) Race engines vary "a lot" and in most
cases will be considerably less.

Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft

Late timing, low compression, sticking throttle valve, carb or manifold vacuum leak
(remember most race engines with a big cam and a tight centerline and high overlap will be
naturally low ... you must decide your baseline vacuum reading)