Getting familiar with your racing engine in most motorsports takes effort to learn the ins and outs. Knowing certain basics can take some of the error out of trial-and-error. You may start with a freshly assembled, newly dyno’d, ready-to-roll engine, but how do you translate that to the real-world track setting, especially in changing air density conditions? Early outings consist of test runs and partial runs down the track. You tweak and modify until you have it pretty well dialed in. Many racing tuners adhere to that adage:
“When it runs well, leave it alone.”
What do you do if the vehicle runs poorly at the next outing? Changes in elevation, weather, and time of day can make a good setup bad. These affect air density and the air/fuel ratio. Some setups compensate. Others do not and have to be tuned. After the mechanics of the vehicle are all sorted out, excessive changes in power beyond what is expected can indicate inadequate tuning for the air density change. Trial-and-error is a common method to determine tuning for air density changes from different tuning indicators. Here are a few tuning indicators and what’s going on with each.
Note that this article primarily deals with the racing fuel system. Other aspects of the vehicle are assumed to be in proper working condition or not addressed in this article.
Tuning Indicator: Engine Temperature
If there is a reduction in engine performance over the course of several races, engine temperature may be an indicator of potential problems in the fuel system. If temperatures change from normal for that engine, it can indicate a mixture change to a fuel rich condition.
In a high compression methanol racing engine, the air/fuel ratio should be over 15% rich – at or above 15% more fuel than oxygen to burn it. The extra fuel is for throttle response and cooling. In some sprint cars, however, we’ve seen enrichment as high as 40% rich. Small or poorly maintained cooling systems rely on extra fuel enrichment for extra cooling. Any further increase in the amount of enrichment in long circle races end up with a temperature drop. Power is also usually lower. In drag racing where engines run for only a short time, that level of enrichment dramatically reduces power.
Conversely, cool operating temperatures can also indicate an excessively lean condition. In this case, when there is not enough fuel to burn, the engine can run too cold.
In the other direction, a hot engine temperature can indicate a fuel mixture that is not rich enough. An increase in fuel reduces combustion chamber heat and the engine temperature.
Believe it or not, excessively fuel rich fuel mixtures can cause high engine heat with alcohol fuels. If the engine is run excessively fuel rich, the intake system can be so cold from excess fuel cooling that too much fuel condenses out of the air stream. The small amount left in vapor burns in a lean mixture, and the engine temperature is hot. Even though the fuel system is excessively rich, engine combustion is excessively lean, and it can burn up. Making it richer makes the problem worse.
Engine temperature indicates fuel mixture issues up to a certain point. Beyond that, other information is needed to diagnose the problem of whether the engine is lean or rich. Spark plug readings and other tuning indicators in addition to awareness of current weather conditions help diagnose a fuel rich or lean condition.
Tuning Indicator: Spark Plug Readings
After a race, spark plug inspection is a good way to monitor engine performance. A clean spark plug with a certain number of heat indications is the goal. From that, different visual changes of the spark plug form indications of a tuning window.
Note that if spark plugs were used for several races and come out with troublesome heat indications or damage, they will not be a reliable indicator of what occurred. Since they provide a recording of their entire run history, these spark plug readings may not be a valid indicator of the most recent run if that is what the tuner is looking for.
For example, a fuel mixture that is 15% lean will probably run cool if the fuel filter becomes clogged and there isn’t enough fuel to burn. If new spark plugs were installed and they show no discoloration after a run, that tuning indicator gives a false indication of a fuel rich condition when it is actually way too lean. What else can you look out for? Fuel consumption would be a lot lower in this instance. Throttle response would also be a lot less.
Damaged porcelain reveals very excessive heat indicating there is not enough fuel enrichment. It can also indicate the engine is running out of fuel, causing the engine to lean out under max power. Melted outer electrodes indicate that combustion chamber heat was too great at some point during the run history of that spark plug. A broken spark plug may reveal broken parts. Different discolorations reveal different engine heating phases. There are some potential indicators to look out for. Here are examples:
For gasoline mixtures:
- Excessive enrichment will cause carbon build up on the porcelain and possibly on the spark plug metal face and outer electrode.
- Excessive combustion chamber heat can occur from a fuel mixture that is not rich enough with excessive heat indications on the spark plug.
- A cold looking spark plug without any color can occur from an excessively lean fuel mixture since not enough fuel is burning; or from an excessively rich mixture cooling the combustion chamber.
- The difference between the two can be difficult to sort out without knowledge of where the fuel mixture is and the air density.
- Changes in gasoline blends from different regions affect the weight of the fuel and the mixture ratio that can affect performance and spark plug readings (Richardo & Clyde, The High Speed Internal Combustion Engine)
- We saw some fuel systems that were so messed up with both lean and rich mixture transitions during different loads and RPM ranges. In these cases, spark plugs showed both rich and lean indicators. These are tough to sort out with spark plug readings.
For nitro mixtures below about 87% for typical nostalgia nitro drag racing setups:
- excessive enrichment from a rich air/fuel ratio will reduce power with cold looking spark plugs.
- inadequate enrichment from a lean air/fuel ratio can melt spark plugs, pistons, or backfire.
- an excessively lean air/fuel ratio may not melt pistons, backfire, or even make any spark plug color due to not enough heat if there is not enough fuel such as from a fuel system breakdown.
For nitro mixtures above approximately 87%:
- excessive enrichment from a rich air/fuel ratio will lean the engine from too much oxygen due to high nitro percentage and can cause melted or damaged components – including spark plugs.
- inadequate enrichment from a lean air/fuel ratio will reduce fuel cooling and can damage the components including spark plugs.
- A proper value in the middle is hard to find by trial-and-error and the art-of-the-tuner.
- In drag racing, it is often the combination of the amount of fuel volume, compression ratio, and spark advance that enables a nitro engine to survive a run, often with engine and spark plug damage occurring just at the finish line.
- Backing down on the amount of nitro percentage will reduce engine leanness from excess oxygen, with less propensity to burning up. Lowering compression and/or spark advance will reduce engine temperature with less propensity to burning up Spark plug readings become an art form when the tuner is trial-and-error’ing to find the best combination.
While spark plug readings are all important indicators of a tuning problem, again they only indicate something went wrong. They may not indicate where or when a problem arose in the engine during previous running.
Tuning Indicator: Exhaust Thermocouple Data
Exhaust thermocouples in each cylinder are used with a data recorder to indicate exhaust temperatures. Exhaust temperatures can be recorded for an entire run to get a window into when a problem occurs. However, the indications occur with certain limitations:
- For a well-developed engine combination, thermocouples can indicate exhaust temperature readings for each cylinder that correspond to good performance.
- For multiple cylinders, fuel distribution adjustments can be made to even out the temperature readings.
- A temperature change in one or more cylinders from a proven standard for that specific setup is a good maintenance indicator.
- However, there may be a time delay between when the cause of the temperature change occurs and when the measurement occurs, confusing the analysis.
- Any after-burning in the exhaust from rich fuel mixtures and unburned oxygen from combustion can cause error in those readings. Changing the amount of enrichment can cause opposite effects in the exhaust. Enrichment to cool the engine may cause more after-burning with more exhaust temperature — a confusing outcome.
- Any after-burning in the exhaust from camshaft overlap flow of the air and fuel mixture can cause error in those readings.
Using thermocouples to establish a new setup can be problematic without a prior baseline. Temperature values from 900 deg F to 1300 deg F are routinely reported from different race tuners throughout the pits of a race. One racer tunes for 1000 deg F in all cylinders while another tunes for 1,200 deg F in all cylinders and so-on. With that great range without a starting point for a specific setup, this difference that can occur may be a problematic indicator for a new setup.
A calibration check of thermocouples is a good practice. A check is to swap thermocouples occasionally from different cylinders to see if measurement differences go with the thermocouple or stay with the cylinder number. The former indicates a possible thermocouple calibration need.
Double-checking thermocouple differences can be done with an infrared heat gun. An exhaust pipe with a cool thermocouple reading should be in a cool surrounding mounting area indicated by the heat gun. That is compared to other exhaust pipes with hotter thermocouple readings, indicated by the heat gun.
Another is to put thermocouples into an oven and turn up the temperature. Check that the same readings occur between the thermocouple sensors / data recorder and the oven control or thermometer. Another method is to return them to a calibration service or the original manufacture for calibration check. These are routinely done by several high end racing teams.
Tuning Indicator: Oxygen sensors
Oxygen sensors are a device in the exhaust for tuning, and in some cases automatic tuning. They are common in EFI for highway vehicles since they operate around the stoichiometric ratio of the fuel. Highway engines do not run very rich mixtures common in motorsports.
Oxygen sensors provide a measurement from the exhaust that results from fuel mixture control. If the engine is too rich, that provides a unique exhaust species that is measured by the oxygen sensor. The engine management system responds by leaning the fuel mixture. If the exhaust species is too lean, the engine management system richens the mixture. This universal basis of modern EFI takes the error out of trial-and-error tuning so that the engine management system can be automatic.
If a malfunction in the fuel system occurs causing even a momentary excessive enrichment in gasoline fuels, the oxygen sensor may become contaminated. It may not clean up during subsequent lean corrections, and error in operation results.
Oxygen sensors are also used for alcohol fuels used on the highway such as E85 that operate around the stoichiometric mixture ratio as well. They are reported to be not very reliable for rich air/fuel ratios with racing alcohol or nitro. Most racing combinations run a fuel volume that is extra rich for cooling. These mixtures are rich in oxygen:
- that contaminates the exhaust with excess oxygen and after burning that can make oxygen sensors not accurate.
- oxygen sensors would also be susceptible to premature failure due to excessive heat from after-burning.
Air/fuel Ratio as an additional tuning indicator
The fuel mixture can be numerically controlled with an air/fuel ratio value. Air/fuel ratio is a combustion engineering standard measured as the weight of air to the weight of fuel. For racing, the air/fuel ratio would be based on the stoichiometric ratio plus an amount of richness for the fuel that is used.
A lean mixture setup from the stoichiometric ratio would likely never apply to racing unless the motorsports competitive goal was fuel mileage where horsepower was secondary. An example is the historic Mobile Gas Economy Run from 60 years ago. Here engines were leaned out, running at low RPMs and low HP levels for maximum range.
That would not be the case for racing today, even where fuel limits are imposed such as F1, Indy Car, NASCAR, and other similar events with fuel tank size limits. Power loss is too great to operate on the lean side of stoichiometric. Fuel conservation is done at throttle lifts and under part throttle.
Air/fuel ratio guidelines exist for standard engine types. For fine tuning AFR numbers for a specific engine, added factors are considered.
- for a normally aspirated engine, AFR is determined from the engine size and the volumetric efficiency.
- for a blown engine, AFR is determined from the blower size, overdrive, and the efficiency of the blower.
- for a centrifugal blower, AFR is determined from the intake temperature, humidity, and boost after the compressor or after an intercooler if so equipped.
- for a turbocharger, AFR is determined from the intake temperature, humidity, and boost after the turbocharger compressor or after an intercooler if so equipped.
In engines with forced induction, there should be consideration for the exhaust back-pressure. A high back-pressure will reduce mass flow for the boost that is being measured or targeted. However, an engine with high back-pressure may make less power than one with lower back-pressure at the same boost level (ref. Gale Banks Engineering).
Knowing an optimum air/fuel ratio and utilizing tuning indicators mentioned above will help maintain engine performance while minimizing engine failures. Contacting the racing fuel system supplier to get specific information and understanding some tuning basics can go very far.