As most of you know we have a video out that explains the proper use of E.G.T. (Exhaust Gas Temperature) information and how to properly use the information this type of sensor supplies. But we still receive calls from misinformed racers about the use or potential misuse of this data.
Since onboard data-logging systems are now becoming pretty mainstream most people are under the impression that they can bolt these systems on and burned pistons, detonation, and incorrect jetting are a thing of the past. Not true.
EGT and Oxygen sensors are wonderful inventions, but they are not an end all be all of tuning. Here’s why.
Every engine has a thermal signature. Meaning it will create “A” amount of combustion heat, it will burn “B” amount of fuel in a given span of time and it will expel “C” amount of unburned or currently burning gases.
“A” is “in the chamber” combustion. Allow me to explain. One of the teams I work with has a system that actually measures the combustion temperature. This is an “in the chamber” reading not and “in the pipe” reading and you can directly correlate chamber efficiency, fuel distillation curve selection and even cylinder head temperature to assist you in burning all the fuel “in the chamber” as opposed to a residual burn. Its really quite an eye opener and it really taught our company a lot about droplet size and fuel preparation.
“B” is the amount of fuel required to match up fuel and air to get proper combustion. Now everyone knows that 14.7:1 is stoic, but this is not a perfect world. Piston tops and intake to exhaust scavenging percentages will alter the required air to fuel ratio for proper operation. Now don’t panic, just because your engine requires .8 more of an air to fuel ratio or EGT’s that are 100 degrees lower than your buddy’s car is not a bad thing. Some engines make excellent power with lots of fuel and although they may seem to be wasting it, you can’t deny proven on track performance. For example: We have a particular carburetor design that makes no more power on the dyno, but for some reason its very quick on the racetrack. When the fuel curve is altered to supply what the engine builder deems is a proper air to fuel ratio, the vehicle slows down. So there are no set rules and EGT and Oxygen sensors have to thought of as base-lining or reference tools. Remember that statement.
“C” is the amount of residual unburned or currently burning gases that are expelled into the exhaust pipe. One of the nitrous teams that tests for our company currently has EGT numbers in the 1760 degree range. You may think that’s high and that sure seems excessive, but it’s a World Record holding tune-up and you don’t deny that. The engine gets torn down regularly and to quote the racer “Its so nice inside this engine, I could even reuse the bearings if I wanted to”. So don’t panic if you’re seeing real high EGT numbers. Oftentimes its residual fuel burning and if you roll some ignition timing into it to get it to burn in the chamber, you could end up hurting parts.
Oftentimes an Oxygen sensor will show you a cylinder that requires more air to fuel ratio than its opposing cylinder or even its direct neighbor. Let the cylinder have the fuel. Don’t get caught in a numbers game. The same thing goes for EGT data. If your engine performs better with higher or lower numbers than someone else’s, look at the A,B,C’s and you can probably pretty accurately determine what’s different from your combination than his/hers.
Also remember, when mounting EGTs or Oxygen sensors symmetry is VERY important. An EGT sensor mounted on the bottom of a header pipe will typically record data 100 degrees lower than the same sensor mounted on the top of the pipe. If you’re using a single oxygen sensor, remember a misfiring cylinder will exhibit an over-rich condition and you may think less fuel is required (or it may even exhibit an exceptionally lean reading as a result of fresh air being pumped into the header from the misfiring cylinder) so don’t forget to look at everything when you’re tuning.
When you open the hood there is a lot of stuff under there and something as simple as a fluctuating fuel pressure regulator can create surging and just plain unusable data. That’s when the real tuner earns his pay. EGTs and Oxygen sensors are tools that can lie to you, but they’re great for educating you on what to look for, so pull that header off or pull out that sparkplug when the car is running good. Get a good look and remember what you see. Sparkplugs and header pipes are harder to read but they rarely tell a lie.
Thanks for reading. Have a great season everyone.