Choosing Your Guages

Description

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 "read" your spark plugs so you can find the EGT that works safely for your engine.

Boost Guage

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).

I 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 guage 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 the 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.

Compression Ratios
Engine Type Ratio

2.2L Standard 9.5:1

2.2L Turbocharged 8.1:1

2.5L Standard 9.0:1

2.5L Turbocharged 8.0:1

Return to the Mini-Mopar Turbo Performance page

Compression Ratios
Engine Type	Ratio
2.2L Standard	9.5:1
2.2L Turbocharged	8.1:1
2.5L Standard	9.0:1
2.5L Turbocharged	8.0:1

This page is maintained by Russell W. Knize and was last updated 02/11/99. Comments? Questions? Email minimopar@myrealbox.com.