Tech Talk #38 – The Truth About Tuning

DavidTechArticlesBy David Reher, Reher-Morrison Racing Engines

I couldn’t tune a piano if my life depended on it, but I have tuned a few engines in my career. The process of tuning is fundamentally the same for racing engines and musical instruments. In both cases, you are making the final adjustments to a complex assembly of precision components. No one tunes a piano with a hammer, but sometimes I see racers take a sledgehammer approach to tuning engines.

How often have you seen about a racer make major changes in carburetor jetting or ignition timing? In fact, big changes in the tune-up can have severe consequences for any racing engine.

A carburetor is really an amazing device. Yes, I know that carburetors have been around forever, and that fuel injection is the wave of the future, but I’m still impressed by how well a carburetor can adapt to changing conditions. A carburetor operates on the principle of pressure differential. When the pressure in the venturi is lower than atmospheric pressure, the gasoline in the float bowl enters the airstream going into the engine. This beautifully simple system is self-compensating: when the barometric pressure or engine airflow changes, so does the fuel delivery into the engine.

You may think that the self-compensating characteristics of a carburetor are adequate only over a small range of barometric pressures. The fact is that we run the same jets in our Pro Stock engines at sea level as we do in Denver. For years I just knew that we needed to change the jetting when we went to the Mountain – until the year we tested there with our sea-level tune-up. To my surprise, the engine ran best in Denver with the same jets that we use in Gainesville and Houston. How could this be possible? Because the carburetor only reacts to pressure differential, regardless of the absolute altitude. We might change air bleeds or squirters to tune the setup to a particular track, but the jets in our Pro Stock Pontiac’s carburetors seldom change.

If you are making big changes in carburetor jetting or ignition timing as part of your tuning procedure, you are almost certainly putting your engine at risk. Virtually every high-horsepower racing engine built by a professional shop is tested and tuned on a dyno before delivery. You may discover that the dyno tune-up is not exactly right when the engine is installed in your car due to differences in the air delivery system on the dyno versus the hood scoop on the car. But after you make the necessary changes, the subsequent tuning adjustments should be very minor.

If you feel that you absolutely must tune the engine, my recommendation is to start by richening the fuel mixture or retarding the timing. For years racers have been ingrained with the idea that you have to lean out the engine and increase the ignition timing to run fast. It’s the macho thing to do: Punish the engine to make it perform. Unfortunately, that approach is dead wrong.

Think about what you are trying to accomplish when you tune an engine. The goal of internal combustion is to burn as much fuel as possible (thereby releasing maximum energy) while maintaining efficient combustion. Therefore your first step should be to richen the fuel mixture, not lean it down, to see whether the engine responds. Similarly, you want to use as little spark advance as possible because it is negative work when the piston has to push against expanding gases in the cylinder. That’s why your first step in ignition tuning should always be to retard the timing, not advance it.

If you lean the fuel mixture or advance the timing far enough to hurt performance, it’s likely that you have also damaged the engine. Detonation has dire consequences in a sophisticated, high-horsepower racing engine: You can pinch the ring grooves or knock the moly coating off the compression rings. When the flakes find their way to the piston skirts, they will eat the cylinder walls. That’s a high price to pay for an overly aggressive tuning change.

I’m often asked what an engine’s exhaust gas temperature (EGT) should be if it’s tuned properly. I don’t have the answer, because EGT is not an absolute number – it’s only a reference point. EGT readings can tell you whether you have a dead cylinder, but they can’t tell you whether the engine is tuned perfectly. EGT sensors are fragile, and their readings vary depending on how far the probe extends into the header pipe, how far the sensor is located from the exhaust port, and whether the probe has been contaminated by lead. Erroneous EGT readings can be caused by calibration errors, probe failures, bad grounds, and other faults. In short, tuning according EGT is inherently risky. Don’t trust EGT until you have established a consistent baseline for your particular engine. And don’t worry if your EGT readings are different from your buddy’s; it’s likely that your installations are different as well.

The same cautions apply to making tuning decisions based on oxygen (02) sensor readings. Oxygen sensors are designed for use with low-lead fuels, so they can be easily fouled by the leaded gas that is still used in racing.

You should be skeptical of any instrumentation when tuning. If you see a reading that doesn’t look right, it probably isn’t. Computers and data acquisition systems aren’t infallible. They only process the data they receive, and can’t discern between good and bad information. Radio frequency interference, malfunctioning sensors, poor electrical connections, voltage spikes and other external factors can produce inaccurate information. Computer programmers use shorthand to describe this phenomenon: GIGO, or “garbage in, garbage out.”

When in doubt, believe the scoreboard, not the computer screen. If you see problems in sensor readings while your car runs the same corrected e.t. and speed that it usually runs, the data is probably bogus. The most important tuning tools are common sense and a timing slip. Everything else is incidental.