Choosing Your Guages
[ A/F Guage | EGT Guage
| Boost Guage | Fuel Pressure |
Oil Pressure |
A/F versus EGT
This page is designed to help you decide which guages you should get for
your car, if any. Much of the information comes from some experiences
racers, especially Gus Mahon (pro-A/F) and Klaus Wilkens (pro-EGT) from
the SDML. Many thanks to the both of them for some interesting debates.
:) Any other comments are welcome.
Guages To Choose From
Air/Fuel (A/F) Guage
Most people install an A/F (air/fuel) guage, which is available from Summit
Racing Equipment as well as many other automotive performance outlets.
The Intellitronix (formerly Cyberdyne) is tested guage (P/N ITC-TH7009
at Summit for about $30) that uses your stock oxygen sensor and displays
it's value on an 10-element, digital LED bar graph. Other manufacturers
include K&N (more expensive). Since the guage's response is instant
(because it is digital) and the oxygen sensor's response is extremely fast,
the overall response of the guage is very fast (immediate, as far as the
human eye is concerned). The more lights that are lit on the graph,
the richer your mixture is. Too rich isn't dangerous, just inefficient
and may eventually fowl your spark plugs. Too lean can quickly cause
detonation at high boost and crack a ringland, or burn a hole right through
your piston from very high EGTs. At WOT 10 lights is safe but a bit
too rich (inefficient), 9 lights is good (safe), 8 lights gives more power
but is close to the margin (no room for error), 7 lights is risky, 6 or
less lights is dangerous. If you are using forged pistons, you can
run leaner, but you may lose power from misfiring and detonation.
If outside temperatures are low, your engine will run leaner than if the
outside temperatures are high. If you want to run leaner than 8 lights,
I HIGHLY recommend that you install an EGT guage. These values are
based on an average temperature, say 75^F.
Exhuast Gas Temperature (EGT) Guage
There is always the EGT (exhaust gas temperature) guage, which will give
you an actual reading of your exhaust temperature. These are available
from many vendors. Tested guages include the Westtach EGT (available
from Aircraft Spruce Co. and Pegasus
Auto Racing Supplies), the Autometer
EGT, and the famous VDO guages.
Most are available with a simple installation kit where you only have to
drill a hole in your exhaust pipe. It is best to install it on the
exhaust manifold by cylinder #3 using the weld-in kit because this cylinder
usually runs the hottest, assuming your injectors flow at even rates.
Don't trust the "matched sets" of injectors because many inconsistancies
have been found. If you are running high EGTs, get your injectors
flow tested. Installing the sensor on the pipe after the turbo drops
the temperature by about 200-300 degrees, but the extra casting that the
exhaust passes through will increase in temperature, so the reading is
not consistant. This is the main problem with the EGT guage.
It must be installed properly, otherwise it is useless. It must be
close to the head and the probe must be in far enough so that it measures
the exhaust gasses, not the temperature of the manifold. The maximum
safe exhaust gas temperature for stock Turbo I pistons is different, depending
who you ask. 1600^F is close to the melting point of aluminum.
I would recommend that you not exceed 1500^F to 1550^F on stock Turbo I
pistons. The maximum EGT for the stock Turbo II Mahle pistons (high
durability cast) is probably closer to 1600^F. If you have forged
pistons, you may be able to go as high as 1650^F or more. Always
your spark plugs so you can find the EGT that works safely for your
Even if your vehicle has the stock boost guage, you should consider getting
a quality guage since these are not very accurate nor responsive.
When you are increasing boost levels, an accurate boost reading is critical.
Autometer and VDO
make very good guages in many styles. Look for something that will
give to up to a 25psi to 30psi reading. Some guages just show boost
while others show vacuum and boost. Either is fine. If you
don't have a boost guage at all, it is a must.
Fuel Pressure Guage (FPG)
This guage is not necessary, but I recommend it anyway. Most major
perfmormance modifications will require additional fuel flow. This
makes having a strong fuel system even more important. By installing
the FPG, you will always know if you fuel system is keeping up with the
demands. If you are using the stock fuel pressure regulator, you
should always see 55psi plus whatever your boost guage is reading.
So, if you are at 15psi of boost, you should see 70psi of fuel pressure.
A dirty filter or weakening pump can cause you to lean out. Your
A/F guage will always warn you of this, but having the FPG will let you
diagnose the problem instantly. Your choices are a simple pressure
guage installed under the hood, the guage installed outside the car, or
a guage installed inside the car. Do not run a line from your
fuel line into the passenger compartment if you are going to install a
mechanical guage inside. If you choose to install the simple
guage under the hood, keep in mind that the most demand is placed on the
engine when you are driving it, so you won't be able to see the reading
while you're driving. If you don't mount the guage outside the car,
you should use a liquid-filled guage so it can withstand the vibration.
If you do mount it inside, you can use a mechanical guage with an isolater
(Autometer no longer supplies these for high pressure applications), or
you can install an electronic pressure guage (available from Autometer).
have found a source for pressure guage isolators, which is pictured on
the right. They are made by Bellofram and are available through a
company called Industrial Process Measurement,
Inc. Ask for item number 950001000, which is the Series 8498
Guage Protector with a nitrile diaphram seal and 1/4" NPT connections.
They cost about $31 each and come with installation instructions.
I purchased them using my company name so that it looked like a prototyping
effort. I don't know if they will sell to individuals. The
nitrile diaphram is compatible with gasoline and engine oils, so they can
be used for an oil pressure guage as well. When installing for a
fuel pressure guage, my advise is to run fuel line or steel braided line
to the process side of the isolater. Then run 1/8" copper tubing
from the guage side to the guage. You must fill the guage, tubing,
and guage side of the isolater with fluid. Antifreeze is probably
the best, for most situations. You have to be sure to get all of
the air out of the guage side of the system. The instruction that
come with the isolator will describe this in detail.
Oil Pressure Guage
If your instrument panel does not come with an oil pressure guage (P and
some K bodies), then install one! Even if you are not making
any performance modifications, an oil pressure guage is just as important
to a 300hp Super 60 engine as it is to a 1.6L N/A engine. The guage
will tell you the condition of your oil system. The oil light only
tells you when you have NO oil (below 4 psi of pressure).
Once that light comes on, you must turn your engine off and put the car
in neutral to avoid serious damage to your engine. An oil pressure
guage can warn you of potential problems (weak oil pump, low oil level,
oil viscosity too low) before they can cause serious harm. You have
many choices here. All engines will have a tap on the oil supply
line to the turbo and to the oil light switch. There will be two
other taps available if you do not already have a guage. Electrical
pressure guages simply require that you install a sending unit (usually
supplied with the guage) and a wire to the guage. A mechanical guage
gives you a more accurate and easy to read guage, but requires that you
run a small oil line into the car (usually nylon). As long as you
are very careful about how you route the line (making sure it can't get
kinked or cut), these will usually work. I recommend using some 1/8"
copper line. It is much more durable than nylon. When going
from the chassis to the engine, make a couple of coils in the line so that
it can flex without fatiguing the copper. You can also install steel
braided line but this is much more expensive. My installation uses
steel braided tubing from the engine to the chassis (available from Autometer)
and 1/8" copper tubing from there to the guage. For added safety,
you can put a pressure isolater between the line from the engine and the
line to the guage. See the above section for more
information on isolators.
The Great A/F vs EGT Debate
As was discussed in the previous sections, there has been much debate over
which is better: the A/F guage or the EGT guage. Here, I will describe
the argument from two perspectives and then give my slant on it.
The argument against the A/F guage is that it reads the air/fuel ratio,
and not the actual combustion temperature. The A/F is a good way
to prevent detonation, but it doesn't "say" anything about temperature.
Also, the oxygen sensor upon which the guage relies cannot tolerate leaded
fuel (racing fuel), nor can it read A/F ratios less than about 12:1 or
greater than about 18:1. The problem with that is that most racers
use the high-octane leaded fuel and need to operate at A/F ratios less
(richer) than 12:1. The argument against the EGT guage is that it
reads the exhuast gas temperature, not the actual combustion temperature.
Because the engine parts (head, block, manifolds, etc.) serve to cool down
the exhaust gasses, the exhaust gas temperature is not representative to
the combustion temperature. As the engine parts heat up (during long
WOT accelerations), the EGT rises because it is getting closer to the actual
combustion temperature. So if your combustion temperature is too
high, by the time you get a meaningful reading on the EGT guage, it is
too late. Mounting the EGT probe on the downpipe renders this guage
useless because now the gasses have to heat up the entire manifold, the
turbo, and the pipe. Since the turbo is cooled, you never see a real
combustion temperature there. If the probe is install properly and
is close to the head, the EGT guage become muchmore accurate, but unfortunately
it doesn't respond as fast as the A/F guage.
So what good is the A/F guage, then? Well in reality, the A/F
is "telling" you what your combustion temperatures are, just
not directly. The temperature of the combusting air and fuel mainly
depends on four things: initial temperature of the air and fuel as it enters
the engine (air charge temperature), the overall amount of air and fuel
entering the engine (manifold pressure and air charge temperature = air
density), the compression ratio of the engine (see chart below), and finally
ratio of the air to fuel entering the engine (the A/F guage).
The two unknowns here are the inital air temperature and the air density,
which depend on atmospheric conditions (air temperature, humidity)
If you think about it though, these variables sort of cancel each other
out. If the intial temperature is high (raising the combustion temperature),
then the air density entering the engine is low (hotter air is less dense,
which lowers the combustion temperature). A lower air temperature
has a lower inital temperature, but a higher air density, so the engine
will run leaner, but initially cooler. These variables definately
don't cancel each other out completely, but if you also consider the fact
that the ECU compensates the mixture using the charge temperature sensor,
you can see how this effect is largely cancelled out. With all of
this said, the A/F guage is giving you a fairly consistant, relative indication
of the combustion temperature because the A/F reading does not change as
the exhaust gasses pass through the manifolds, etc. An immediate
change in A/F ratio has an immediate response by the A/F guage, therefore
you are getting a real-time reading of your relative combustion temperature.
As discussed in the previous section, 8 or 9 lights on the Cyberdyne is
a good A/F for a good combustion temperature, based on the experiences
of hundreds or racing hours. Keep in mind that a cold day will still
lean out your engine somewhat, and that can lead to detonation.
In contrast to that argument, the A/F guage, though more responsive
and accurate, is useless to most racers because they need to see a wider
range of ratios and also need to use leaded race gas. Wider-range
sensors are available, but are extremely expensive. The leaded gas
contaminates the oxygen sensor anyway and causes its output to quickly
decrease (indicating a lean condition whenone does not exist). This
shows why the EGT guage is important and why the A/F guage is not practicle
for some applications. Again, if the probe is installed properly
and in a location where it can measure the EGT without having to account
for too much loss through the manifolds, the EGT guage works very well
and is generally accepted by most racers. Neither of these guages
say much about detonation. Typical ways of reducing detonation include
reducing compressions ratio (different head, head shim, etc), running higher
octane fuel, running richer mixtures, reducing boost pressure, using the
EGR to reduce EGTs and detonation (by recirculating exhaust into the intake),
etc. There are a couple of detonation guages available, but it is
difficult to measure detonation at high RPM because of all the noise caused
by moving engine parts. Even at safe EGTs and A/F ratios, detonation
can still be a problem.
Much of this argument is based on discussions I had with Gus and Klaus,
plus my own understanding of engines. Klaus has been racing FWD Dodges
since the mid 80s and has been using the EGT guage with much success.
He runs EGTs of 1600^F to 1650^F at high boost using a reducd compression
ratio and high octane race gas. Gus uses only a small part of high
octane race gas mixed premium unleaded gas to save his oxygen sensor.
He runs at high boost by reducing the compression ratio and running at
a richer mixer to keep EGTs and detonation down: "9 lights for 9 years
with no problems"
So with all that said, here is the conclusion I came to that will hopefully
help you decide: It all depends on the application. If you
are racing on a track and need to use race fuel, then you need an EGT guage.
If you are not using race fuel or only a small portion mixed with unleaded
fuel, then the A/F guage option opens up to you. Even so, I recommend
that racers get an EGT guage an addition to the EGT guage to help you know
what is going on in there. Those who are mostly interested in street
performance will do just fine with the A/F guage alone. Maintaining
a rich enough mixture should be sufficient for most street applications.
This page is maintained by Russell W. Knize and was last updated 02/11/99.
Comments? Questions? Email
Copyright © 1996-2003 Russ W. Knize