A modern engine dyno is a data-gathering animal, and it can record or calculate a mind-boggling quantity of data. You'd have to be an expert dyno dude to know it all, but there are some key bits of dyno data it pays to be aware of. Besides the obvious temperatures and pressures, here's a breakdown of the most important stuff on a dyno sheet to know, and what it means.
Airflow SCFM: A measurement of the air volume taken into the engine. Airflow usually means power, so this is a number to pay attention to.
Brake Mean Effective Pressure (BMEP): A calculation that assigns a theoretical average cylinder pressure required to achieve the recorded torque. Cylinder pressure creates torque, so a high BMEP shows the engine is generating torque effectively.
Brake Specific Air Consumption (BSAC): Similar to BSFC, this is an efficiency calculation derived from the measured quantity of air ingested by the engine and the power output, given in lb/hph.
Brake Specific Fuel Consumption (BSFC): A calculation of the engine's efficiency derived from the horsepower and fuel flow rate. Given in pounds of fuel per horsepower per hour (lb/hph) the "brake numbers" give a good indication of how well the engine is working at making power, and in some cases can give the dyno operator insight into whether the air/fuel ratio is in the zone. BSFC will be lowest at near peak torque, with a number in the low 0.4s or high 0.3s there showing excellent efficiency.
Corrected Horsepower: The horsepower is calculated from the recorded torque, using the equation: HP=(TQ x RPM)/5,252. Corrected horsepower simply means that corrected torque was used to calculate it.
Corrected Torque: This is the actual twisting force applied by the engine, given in pound-feet. The corrected number brings the raw torquemeter reading to what the reading would have been if the testing was done under a set standard temperature, atmospheric pressure, and humidity. Correcting the number to a standard eliminates the variables that always exist in atmospheric conditions.
Correction Factor: Shows the correction factor applied to the raw or measured torque data to derive the corrected numbers. The two most common correction factors are STP and SAE. The correction factor will vary with the ambient conditions. On a 110-degree day with steaming humidity and a very low barometer, the correction will be high to correct the power numbers to the standard conditions.
Exhaust Gas Temperature (EGT): Some dyno installations are equipped with thermocouples that will read the exhaust gas temperature, which the dyno can record. The EGT can indicate dangerously high temperatures that can cause engine damage and point to mixture distribution problems. Sometimes it gives some clue to the air/fuel ratio, though the EGT can vary with so many other factors, like valve timing, that it takes uncanny experience to usefully interpret what they are telling you. It is virtually obsolete for tuning the mixture since the advent of advanced Lambda sensor systems.
Fuel Flow (lb/hr): Most dyno installations include a fuel flow meter that measures the amount of fuel entering the engine. Fuel flow is measured in pounds of fuel flow per hour.
Lambda A/F Ratio: The wideband Lambda or O2 sensor is the greatest thing to happen in the dyno world since the invention of the strain gauge. A Lambda accurately provided a very precise reading of the Air/Fuel ratio in real-time. Before the Lambda, the "super-tune" involved lots of guesswork and trial and error, and unbelievable skill to get right. This instrument spells it out.
Observed or Measured Torque: The actual torque reading delivered to the dyno as measured by the strain gauge or load cell. This is the raw, uncorrected number and can vary widely with the same engine if it is tested during widely varying atmospheric pressure, temperature, or humidity.
RPM: This is always the first column on a dyno sheet, and one that most everyone already knows. The same as the reading on the tach in your truck, it tells you engine speed in how many revolutions per minute the crank is turning.
Volumetric Efficiency (VE): Given as a percentage, this is a calculation that determines the percentage of air actually ingested compared to the displacement volume of the engine. A VE of 100 percent means that the engine is pumping 100 percent of its displacement volume at the given rpm point referenced. VE levels of well over 100 percent can be achieved in very trick race engines.