Gauges
By Ryan Ballou
All right, how
many times have you found yourself asking someone for advice on what oil
weight to run, or if you need an oil cooler? How about when you hear someone
say, "I think I'm overheating, but I'm not sure." Well there's
a simple way to answer this all by yourself, you just need a way to hear
what your engine is telling you; you need some gauges.
The three gauges I feel to be most important for any car no longer running
a stock engine are oil pressure, oil temperature, and cylinder head temperature
(CHT). These three should be considered standard equipment when you start
modifying your engine from stock form.
Knowing your oil pressure has some very valuable benefits. It gives you
an idea of the condition of the bottom end of your engine provided you've
monitored the pressure over time. It helps you choose what weight of oil
is right for your engine, and along with the oil light on your speedometer,
it tells you when you have no pressure and need to kill the engine. The
most important feature of the pressure gauge is the ability to determine
proper oil weight. The rule of thumb for oil pressure is roughly 10 psi
per 1k rpm. This would yield about 10 psi at idle, and say 60 psi at 6k
rpm. Now this is only a rule of thumb, more pressure is common, but many
consider it wasteful. With less pressure you should start thinking about
servicing the bottom end of your engine, i.e. bearings, lifter bores,
oil pump, etc. Exceeding 10 psi/1k rpm isn't as bad as finding yourself
below it. New engines will often see more pressure while they are still
tight. In my opinion, once you pass about 1.5psi/1k rpm, you should try
running the next lowest weight oil.
When it comes to oil weights, you want to run the lightest oil you possibly
can, but still maintain minimum pressure across your rpm band to prevent
engine damage. Running higher pressures than necessary robs power from
your engine as it makes the oil pump work harder. It makes your oil run
hotter as higher pressures will force the oil to bypass the stock oil
cooler. Many people don't know that excessive pressure can also lead to
premature wear of engine parts. Thick oil can't flow through clearances
as fast as thinner oil. After all, it's actually oil flow that keeps the
bearings and journals alive; oil pressure is just a less expensive (and
less accurate) way to measure the flow.
Oil temperature is arguably an even more important thing to monitor as
far as the safety of your engine is concerned. With oil pressure you either
have it or you don't. With oil temperature, you have information that
lets you know when your engine is warmed up enough to safely 'get on it'.
You also become aware of when your oil starts getting too hot, and generally
will still have time to adjust your driving style to compensate. Oil temperature
is almost solely related to engine rpm. This is because of a few factors.
At higher rpms your oil pressure goes up, as it does more and more oil
will bypass the stock cooler. The idea behind the design is that hotter
oil is thinner, yielding lower pressure. So at lower pressures more oil
is sent to the cooler. At lower temps oil is thicker and thus the higher
pressure will cause it to bypass the cooler and warm up faster. It's a
good design, but extended highway speeds can lead to excessively hot oil
because you're keeping your oil pressure up to a level where it's bypassing
the cooler.
Second reason for the rpm/heat relationship is friction. Friction in the
valve train alone can be responsible for over half your oil temperature.
As rpms go up, heat is generated faster from this friction. Another friction
present is fluidic friction. This is the friction of the oil itself most
notably in the bearing clearances. Again, as rpms increase, so does this
friction.Bottom line, if you notice your oil temp increasing steadily
after you get on the freeway or open road, you can keep it under control
simple by slowing down.
The most important gauge you should have in your arsenal is the cylinder
head temperature gauge, or CHT. Failure in the heads can be one of the
most costly to deal with. The best way to prevent a failure of some sort
is to keep your head temps within a safe limit.Some people will argue
what that limit is exactly, but as far as I'm concerned, once you pass
425F you need to be worried. Right around 425F is when aluminum starts
becoming plastic. What this means is that deformations introduced to a
metal that has become plastic are for all intents permanent. On an ACVW
this is when things like valve seats and guides can become loose in the
head, and I'm sure you know what that can lead to.
Unlike oil temps, head temps fluctuate very quickly. Where it might take
5 minutes for your oil to warm up or cool 15F, your heads can warm up
and then cool off again over 100F span in less than 1 minute. This will
let you know much faster when something just isn't right. Another major
difference in head temps and oil temps is what causes them. Where oil
temps generally go up with rpms, head temps will have a tendency to drop
with increased rpms under a constant load. In a well-tuned engine, head
temps are almost entirely the result of engine load and cooling fan speed.
What this means is that you will tend to see head temps increase as you
climb hills, or lug the engine around in 4th gear at a low rpm. Yet you
could downshift to 3rd gear and pull that same hill with a smaller gain
in head temps, you'll just be revving nearly 1k rpm higher if you maintain
the same speed.
With that understanding of you gauges, all that's left to do is install
them, easy right? Well not quite so. The physical installation of the
gauges is pretty straightforward, find a suitable location for them where
you can easily read their faces, and wire them up. The senders are another
story altogether.
Oil pressure is the easiest; on a stock engine the best, and likely only
place for the sender is at the stock dummy light sender location. You
have the option of either installing a 'T' fitting and retaining the stock
sender, or simply using a dual pole sender. A dual pole sender will have
connections for both the gauge and the dummy light. Whatever fitting you
use, you need to know that the thread size in the case is 10mm x 1.0.
VDO offers senders in both this and 1/8" NPT, which is close, but
wrong. The NPT threaded sender is for use in custom setups where you have
a boss tapped for an NPT fitting. If you use another brand of gauge (I
use Autometer), then you will no longer have the option of a dual pole
sender. The Autometer sender is also only available in a 1/8" NPT
meaning a different installation method will be required, generally it
will go somewhere in your full flow setup.
Oil temperature sender location is a topic you will see debated often.
There are so many types of senders and possible locations, but they won't
all yield the same results. Some locations may read up to 20F hotter or
colder than other locations. Ideally you want the hottest location, after
all what good is a reading of 200F if the oil is 220F somewhere else in
the engine.
The coldest locations tend to be the drain plug sender located in the
sump plate, followed by the dummy light location, and finally the pressure
relief valve sender. Air rushing under the car cools the sump plate location
and the sender itself will be under the stock oil screen, away from oil
flow. The dummy light sender location is not in the flow of oil and neither
is the pressure relief valve location. The relief valve is the best of
these three as it is further into the core of the engine and not affected
by outside air, like the other two. Ideally, the place where you will
find the hottest oil is right inside your bearing clearances, yet it's
impossible to measure there. So the next best place is in the sump at
the top of the oil level, right in the return flow of oil. The most straightforward
way to measure the temperature here is with a dipstick sender or a sender
fitted to the type 3 dipstick location. You can drill and tap the type
3 location for an NPT style sender. The next best place would be right
at the oil pump outlet on full flow setups.
Finally, the CHT sender. There is really only one type of sender that
is viable for a type 1 or even type 4 head, the ring sender that replaces
your spark plug washer. Many people will try to mount these sender either
under a head stud or at the stock FI head temp sender location. Both of
these places will read head temps up to 150F cooler than under the spark
plug. Not only that, but they are slow to respond and you will often see
the temperature rise from heat soak after you shut the engine off. This
goes to prove that the location was not the hottest spot on the head,
and as I said above you really want to know the hottest temperature your
head has reached. The best place for this sender is right where it was
intended to go, under the spark plug. Which spark plug matters as much
as sender location. It needs to go under the # 3 plug, as this is the
cylinder that will run the hottest.
For those of you with modified heads or the newer 044 style head that
uses a 12mm spark plug, don't settle for the 14mm sender that comes with
the gauge, and don't listen to the guys who tell you to just run it anyway.
I've tried this and played with it extensively. I always found signs of
leakage, as the ring sender did not seal properly under smaller spark
plug. This will let superheated combustion gasses leak out and give a
false reading up to 75F hotter than you should be seeing. The leaking
gasses can also lead to erosion and damage to the spark plug boss given
enough time. Take the time to make the phone calls and find the 12mm sender,
many shops no longer carry them but they are still out there.
Most importantly with a CHT gauge, don't lengthen or shorten the wire
on the sender. CHT senders are precision instruments known as thermocouples
(TCs). A TC is a bi-metal junction, meaning two dissimilar metals joined
together. As heat is applied to the junction, a current is induced, meaning
you have electricity flowing. The level of this current is directly related
to the types of metal in the junction and the temperature at the junction.
When you try to lengthen the wire and you don't use wire made of the same
material (TC extension wire) you introduce a new bimetal junction into
the circuit. Now it will also create a current and blend with the existing
current throwing off your gauge readout.Shortening the wire is not as
bad as adding wire, but still undesirable. The current flowing through
a TC junction is so small that by cutting just a few feet of wire off
you can change the resistance of the wire enough to affect a small change
at the gauge readout. This change should be quite small, but why risk
it?
Now that you have a grasp on how your engine is performing, and know how
to read your gauges to keep it performing...drive
it like you stole it.
) 1-Here
are your three basic gauges left to right; CHT, Oil Pressure, and
Oil Temperature. It's important that they be fully visible to the
drive with no more than a glance. Location, location, location. |
) 2-This
is how I mounted my oil pressure sender. I chose to mount it after
the oil cooler and filter so that I knew exactly what the pressure
was as it entered the engine case. |
) 3-Here
is my oil temperature sender, right at the full flow outlet. This
and the pressure gauge were installed in a virtually identical manner.
I used a -8 hydraulic 'T' fitting. Drilled and tapped a -8 cap for
the senders, then plumbed the lines through the 'T'. This puts the
sender tip right in the flow of oil for the most accurate response. |
) 4-This
is one of the CHT senders I've tried in the past. Visually, it's almost
identical to the normal VDO sender that I use now (with my Autometer
gauge), but the ring was made of a harder material that experienced
the same leakage problem as the 14mm ring on my 12mm plugs. |
) 5-A
picture of the installed CHT sender. Note that the sender is installed
with the wire to the top of the spark plug; another thing I experimented
with. Placing the wire under the spark plug will give more accurate
results as the spark plug blocks the cooling air from affecting the
sender. |