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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.  

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