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Performance Tuning with a Vacuum Gauge

Vacuum gauges are a cheap but effective method of ensuring that your classics are well tuned and problem free. The Vacuum gauge is a useful instrument to have on any classic as it can be used to tune for power, diagnose engine faults and for maintaining fuel economy.
This article will cover tuning, with articles below about how to diagnose common engine problems using your vacuum gauge.

Whichever gauge you choose whether its a test gauge or a permanent instrument it will need to be connected to the car's inlet manifold. Often there is a connection or pipe already but if not it may be necessary to drill and tap the manifold.

One your gauge is connected and seen to be registering a vacuum you can begin the tuning process

(normal vacuum at idle should be about 18-22In (4 Cyl), 19-21In (6 Cyl) or 15-18In on low compression engines (In is an abbreviation which stands for inches of mercury))

  • Start by first warming the engine and note the idle vacuum reading
  • Adjust the mixture on your carburettor to obtain the highest steady vacuum reading.
  • Richen the mixture setting until the needle begins to fall. This will give you the correct mixture setting for maximum power.
  • Slacken the distributor clamp bolt, and with the engine still at idle advance or retard the ignition until the highest steady vacuum reading is obtained.
  • Retard the timing until the vacuum gauge reading drops slightly (½In).

Your engine should now be tuned to perfection. Tuning with the engine running compensates for wear in the timing gear and therefore will provide better results than the manufacturers ignition settings.




Fault Diagnostics with a vacuum gauge

Engine in good
condition will
give a reading
of 17 to 21

When the
accelerator is
pressed the reading
will drop to below
5 then go to 25
before returning
to normal

A reading
lower than
normal could
indicate worn
piston rings

If the carbs
are not well
adjusted then
the reading could
drift between 12
and 16
When the reading
drops 3 to 5 then
this could indicate
a sticking valve

When the reading
drops by more than
7 it could indicate
a burnt valve when
that cylinder

When the reading
drops 3 to 4
it could be due to
a leaky valve
whenever that
valve operates
When the needle
vibrates between
14 and 19 it could
indicate loose
valve guides
A low reading of
below 5 could
indicate a leaky
manifold gasket or
carb gasket
When the reading
is between 14 and
8 this could indicate
incorrect valve
A reading of 13 to
16 could indicate
incorrect ignition
When the reading
drifts slowly
between 14 and
16 it could be
the plug gaps are
too close or the
contact breaker
points are not
Wide variations
could indicate that
there are weak or
broken valve
springs, increases
with engine
After a normal
reading which then
drops to 0 could
indicate a blocked

An occasional
drop as the
cylinder fires
could indicate that
this cylinders plug
is not firing or
open valve

If normal is 20
and reading is 14
check timing as
spark could be

Wide variations
could be a
blown head


Installing a vacuum gauge

A Vacuum gauge is a very simple and safe piece of equipement to install in your car and as well as being a great tool for tuning can also be used to help with your driving style to achive a smoother and more economical way of driving.

There are a number of different designs of vacuum gauge available as shown in the pictures below, from a pure readout of vacuum in inches of mercury, to a colourful "performace meter" which gives an indication of the type of driving you are doing. Finaly there is an electricaly opeated gauge but with a readout in Bar. All of these types are availble with prices ranging from a few pounds to about 30 pounds.

  Vacuum Gauge   Vacuum Gauge   Vacuum Gauge  

To install a vacuum gauge a piece of plastic tube has to be attached to the inlet manifold. This can be done by removing the manifold and drilling a hole, then tapping it out and making a fixing so that the tube can be attached. This can be a bit of a chore but the manifold must be removed otherwise debrie from the drilling will find its way into the engine and cause lots of expensive trouble.

  MGB installation   Close up of installation MGB  


The way I did on my MGB GT was to remove one of the tapped bungs that are fitted to the manifold (these tapped bungs fill in holes when things like a brake servo is not fitted). I then drilled a hole through the bung the correct size to be tapped out to take a bleed nipple used for the brakes. Before I fitted the bleed nippled I cafully drilled out the interenals (so that it just hollow), this allowed the pipe to be fitttd easily. It was just a case of fitting the bung back into the manifold and running the plastic tube into the car to the place where the gauge is to be fitted.

Once installed and running it can be used to tune the car, use it to help with economical driving or just to have as an extra gauge in the car (you can never have enough gauges!)