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Found an interesting article.
EGT Monitoring

EGT monitoring can be a very effective tool in tuning, but "ideal" EGT can vary from engine to engine dependant upon combination. Usually, 1300 to 1500 degrees F is optimal for normally aspirated engines on gas, while gas turbos will run optimally @ 1500 to 1650. An important note: this is measured before the turbo, not after, as the turbo will reduce EGT by an average 200 degrees F.

Finding the optimal EGT signature for your engine is a trial and error (hopefully more trial than error!) procedure, and other factors such as power output, plug readings and air fuel ratio equipment should be used to corroborate the data. Once you have ascertained this ideal EGT, it should be repeatable regardless of climatic conditions: simply tune for the same previously determined optimal EGT, and your engine should perform at full available output under any ambient conditions.

Easy error for new users of EGT: high temps indicate lean condition. Not always true! Excessively rich conditions will result in "after burn", where the fuel, which was unable to completely combust due to insufficient oxygen in the cylinder, lights off in the exhaust system, causing an unusually high temp reading. If all other indicators still suggest a rich mixture, try leaning in small steps, and you will likely see the EGT go down. Just be sure that the power does not also go down from the changes. If you are on the right track, power should go up noticeably as you lean towards optimal mixture while EGT drops. As you approach and then pass the optimal mixture point, the EGT will begin to climb again. STOP! Richen by one step and you are there! Now, when climatic conditions worsen (i.e.: hotter temperature, more humidity, less air density), lean until you get that optimum EGT again. If conditions improve (colder weather, lower altitude, less humidity), richen for optimal EGT. Bear in mind: if EGT suddenly changes for no apparent reason, you may have an aggravating factor (ignition problem, fuel pressure wrong, clogged air inlet, etc.) which is unrelated to tuning. Be observant, and the indicators should guide you to the right tuning decision.

Another caveat: Air fuel ratios, which are not optimal throughout the entire available RPM and manifold condition range, will mislead you. In other words, an optimal EGT signature at high RPM may not show an incorrect condition at lower RPM or different manifold pressure. Although perplexing, this problem is truly the difference between a happy, powerful and long-lived engine and one that is trying to destroy itself slowly but surely. It is one reason why the precision of fuel injection is usually superior to carburetion in both power production and engine life. Just watch ALL the indicators, and remember: lean is mean, and fuel is power. Instead of continuously trying to lean it for maximum HP, try to find ways to get more air to the engine, and thusly support the combustion of more fuel. There's only so many BTU's in a gallon of fuel, no matter how you burn it. Just try to burn more fuel!

Tuning Via EGT vs, Wide Band/ Narrow Band Meters

There seems to be a lot of mystery and misinformation about using exhaust gas temperatures to tune engines. Claims by many EGT gauge manufacturers about it being the best way to tune an engine must be qualified. The BEST way to tune an engine is on an engine dyno- PERIOD. What EGT is good for is a reference for where the engine made maximum torque at wide open throttle. Once removed from the dyno, a similar air/fuel ratio can be established at a later date by dialing in the mixture to achieve the target EGT. It is really the AFR that is important, not the EGT. Most engines will make maximum power at an AFR of between 12.0 and 13.5 to 1 however, the EGT may vary from 1250F to 1800F and is dependent on many factors.

It should be mentioned that the target EGT is valid only on the same engine configuration as was used on the dyno. If you change the ignition timing, cams, pistons, headers etc., the optimum EGT may also change. Raising the compression ratio with no other changes will drop the EGT at the same AFR. Retarding the ignition timing will generally raise the EGT at the same AFR. One engine might make best power at 1350 degrees while a very similar engine might be happier at 1500. You can't guess at this or you are simply wasting your money on the instrumentatio n. Wankel engines have higher EGTs than comparable piston engines due to their lower thermal efficiencies. 1800F is not uncommon here.

Some gauge manufacturers say you should tune to achieve maximum or peak EGT for maximum performance. This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.

You can sometimes feel a lean of peak condition as the mixture is hard to ignite and power will be down a bit as well. Once the AFR gets close to 17 to 1 at WOT, generally the engine will start to lean misfire. Most tuners always recommend to begin jetting or programming from a known very rich initial setting and carefully leaning until torque falls off slightly, then going back richer to the point of max torque. Note the EGT at this setting. Be aware that altitude, barometric pressure and ambient air temperature may affect this optimal temperature to some degree.

Are EGT gauges better than AFR meters? Conventional narrow band oxygen sensors and digital LED meters are not the best devices to measure AFR in the richer ranges but they certainly warn of a too lean condition immediately and obviously, without translation by the driver and they are affordable. Meters combined with wide band sensors are supposed to be highly accurate and everyone has jumped on the bandwagon with these lately. Unfortunately the naive and impressionable often don't question the accuracy of these devices. We have seen some dyno plots indicating best power was achieved at AFRs of 9.7 to 1 on gasoline. This is PHYSICALLY AND CHEMICALLY IMPOSSIBLE and shows that either the sensor was bad (leaded fuel used possibly) or the meter was not calibrated properly. Again, the wide band sensors have the same limitations as the narrow band- leaded race gas quickly fouls them. We have heard and read many stories now indicating that certain brands of wideband meters differ as much as 2 points AFR in readings between each other. In other words, the accuracy of some of these devices is highly questionable. Extensive testing with laboratory quality instrumentatio n on aircraft engines universally indicates that best power is NEVER made at AFRs richer than 12 to 1. Airflow and fuel flow rates are independently measured and each cylinder is instrumented with EGT probes.

We recently dynoed a shop road racing Celica on a DynoJet equipped with a wide band meter. The meter was saying that the engine was going super lean (17 to 1) at high rpm so we kept upping the fuel there. The engine lost more and more power as we added fuel. The dyno operator was convinced that the meter was right but logic told us with no serious dip in power on the curve and the fact that the engine was still alive that the meter was not correct. We started leaning the engine down more and the engine started gaining power. Finally, when confronted with this information, the operator checked the water trap for the wide band sensor. Once this was emptied, the AFRs looked reasonable again. We didn't need the wide band to tell us this, only the torque curve from the dyno.

We have heard of several other instances with people using wide bands getting erroneous readings and tuning their SDS based on these readings. Then they phone us saying that the system is crap. Look at the dyno curve, when the engine makes its best power at a given rpm, that's where it likes the AFR irregardless of what other instrumentatio n is telling you. Remember, a bad sensor whether O2 or EGT equals bad information. When the engine sounds crisp and makes great power, you're there.

I would suggest that mixture meters and EGT gauges are complimentary. EGT gauges have the advantage of working long term with leaded fuel which will clog oxygen sensors. EGT gauges are widely used to set mixture on engines used for steady state high power applications where operation has been carefully documented such as in aircraft. The choice would depend on the application. Both are better if you can afford them.

Which is the best exhaust temperature and how can we use this temperature to tune up a car? Also can we install an air/fuel gauge on a non ECM controlled car?

An air fuel gauge displays the air fuel ratio your O2 sensor is registering. The O2 sensor is monitored by computer controlled cars. Tuning to a particular EGT is probably one of the least consistent ways of tuning your car. An EGT gauge will somewhat help tune a car for full throttle mixture, but isn't as good for idle or cruising mixture. Also, mods like timing, cams, headers, etc can change the optimal EGT. You're best off tuning with a dyno. However, engine temps do give a good idea of how a car is running. Low temps suggest the engine is running rich while high temps indicate a lean mixture although maximum power (and peak EGT) typically occurs at an air fuel ratio of around 12.5:1 to 13.5:1 on a naturally aspirated car. Running leaner than 13:1 is risking engine damage. Turbo motors need to run richer, running leaner than 12:1 on pump fuel is risking damage on a forced induction engine. As a very general rule of thumb, you don't want to exceed an EGT of 1350 degrees F for very long on a naturally aspirated car. With the advent of cheap consumer available wideband A/F meters on the market, tuning by EGT is foolish. A much better choice is to use a wideband air fuel ratio meter like the ones from AEM or Innovative Systems. These are true wideband O2 sensors that are accurate enough for tuning. Do not use A/F meters like Autometer, Halmeter, MSD that use the stock O2 sensor, they are very inaccurate and you risk engine damage if you use them as tuning devices.

Hope it is of some use to y' all. ;)
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