Competition Fuel Systems


Carburetor Tuning on the Dyno

One of the main objectives in dyno testing is to tune the air/fuel ratio to maximize torque and horsepower. There are some common misconceptions about the interpretation of dyno data with respect to tuning the fuel curve. Reliance on the wrong data, or the lack of the right data, can make fuel curve tuning more difficult.

One of the most common misconceptions is that Brake Specific Fuel Consumption (BSFC) is a direct indicator of air/fuel ratio. BSFC is an overall indicator of engine efficiency. It is affected by many factors, not just air/fuel ratio. These factors include cam timing, ignition timing, fuel/air distribution in the intake, exhaust scavenging and back pressure, flame travel in the combustion chambers, contamination of intake mixture by exhaust gases during the overlap period, just to name a few. To assume that there is some BSFC "magic number" that will give the best power, is a dangerous assumption. I have seen a very wide range of BSFC numbers at peak horsepower, ranging from .40 for an efficient engine to .54 for an inefficient engine, all with an air /fuel ratio of 13:1.

Another common misconception is that exhaust gas temperature is direct indicator of air/fuel ratio. Just like BSFC numbers, exhaust temperatures are affected by many other factors. I use exhaust temps only as an indicator of fuel/air mixture distribution by the intake manifold. Again, assuming that there is some exhaust temp “magic number" is a very dangerous practice. Also, don't assume that exhaust temps in the header pipes are the same as the temps in the combustion chambers. Temps in the pipes can be much higher than combustion chamber temps.

There is no substitute for measuring the air/fuel ratio directly. Whether you do this by actually measuring fuel and air flows, and then calculating the ratios, or by using wide band exhaust O2 sensors with a readout display, your efforts to have air/fuel measuring capabilities will be well worth the time and expense. I have seen dyno operators literally destroy engines because they didn't have air/fuel ratio measuring capabilities, and were relying on BSFCs or exhaust temps to guide their tuning process.

I recently witnessed a situation where an engine builder in another state dyno tested an engine that he had built for a friend of mine. This engine builder uses exhaust O2 sensors to monitor gasoline air/fuel ratios, but did not understand how to use them for the fuel being used by this engine (methanol). The alcohol carburetor for this engine had some serious problems, and because the testing was done without any attempt to measure air/fuel ratios, some serious damage occurred. The carb went back to the modifier who found and corrected the problems, and the engine builder had to repair the engine to correct the damage. The engine went back onto the dyno, and still had problems because the engine builder still did not use his exhaust O2 sensors, and no fuel pressure gauge that was visible from the dyno control panel. This is a lesson in how not to approach dyno testing. By arrogantly assuming that he could dyno tune this alcohol engine without air/fuel ratio and fuel pressure data, this engine builder caused himself lots of trouble and has a pissed off customer.

In addition to preventing engine damage, the benefit of measuring air/fuel ratios is a much better chance at finding the best power curve that the engine is capable of. BSFC numbers and exhaust temps can lead you down the wrong road in your quest to find that power.   © Norm Schenck/CFS


520-241-2787          Competition Fuel Systems, Inc.   Vassar, Michigan