Practical Engine Airflow. John Baechtel

be properly sized for optimal performance, and the carburetors are often improperly sized. So these engines, with higher performance than most, may still only achieve a VE somewhere in the 90- to 96-percent range.

      Your efforts to improve engine airflow and thus VE center on removing those compromises and substituting optimal flow efficiency across the engine; not just in the intake path, but also in the exhaust path, and most important, past the valves. Unless restricted by rules, you are free to employ any means possible, including some pretty formidable technologies that make racing engines breathe better than ever.

Understanding BSFC

      Combustion efficiency is typically indicated by BSFC. This expresses fuel usage in pounds per horsepower per hour. BSFC is frequently misunderstood. Many people mistakenly believe that it is an indicator of rich or lean fuel mixtures, but it actually is a measure of efficiency that indicates how well the engine uses the fuel it burns. More specifically, it is the rate in pounds of fuel per horsepower per hour that a given engine consumes to make power. Most engines have a range of optimal efficiency, and BSFC defines that range.

      As you may have already surmised, the term “brake” is the first word because BSFC is usually measured with the engine running on a dyno. BSFC figures are typically quoted for WOT conditions, but it is also a measurable quantity that relates to fuel economy at part-throttle operation. In the performance world, it is used to judge the efficiency contribution of various engine combinations and to predict certain requirements such as fuel-injector flow rate.

      A particular cylinder head may make more power with less fuel, and that’s an indicator of higher efficiency, most likely because of improved cylinder filling and a more efficient combustion chamber that extracts more energy from a given fuel mass. Guidelines for evaluating BSFC are well established and are frequently used to predict engine performance. One-half pound of fuel per horsepower per hour (0.50 BSFC) is the default norm for most calculations, but you can adjust this for competition engines.

      Herein lies part of the problem if you think of BSFC numbers as indicators of mixture ratios. At 0.37 BSFC, a Pro Stock engine may be thought to be running too lean when, in fact, it is operating at the highest level of efficiency. In contrast, supercharged engines run richer mixtures to complement boost pressure and discourage detonation. They run richer; not because they are inefficient, but to complement specific combustion characteristics inherent to boosted applications, not the least of which is charge cooling and the need for more fuel to augment the greater volume of air being supplied by the supercharging device.

      When evaluating BSFC numbers, lower is almost always better (even when supercharged); 0.60 is still more efficient than 0.65, as long as the combination supports safe combustion without detonation or overheating. Any engine still needs to run at the air/fuel ratio that produces the best power. That’s usually about 13:1 in naturally aspirated engines and 11.6 to 12:1 in supercharged applications. One engine with poor efficiency may generate a BSFC of 0.55 while another may run at 0.40, and yet both may have the same air/fuel ratio. You can’t just run the engine lean and expect to get a low BSFC number.

      Remember that there is no magic BSFC number that guarantees max horsepower. However, there is a BSFC number that your particular engine generates when performing at its best. The lower the number (within reason), the more efficient your engine is at converting fuel into power. Tune for maximum torque and let the BSFC indicate how efficiently you generate that torque. For example, at an indicated BSFC of 0.50, the engine burns 0.5 pound of fuel per horsepower per hour (lb/hr). If the engine makes 500 hp, that’s 250 lbs/hr. If you’re building a racing engine, your BSFC should be way better; something on the order of 0.38 to 0.42.

      These figures are for gasoline only; they differ considerably for methanol applications. With methanol the engine uses much more fuel, something on the order of 1.0 to 1.3 BSFC at the minimum and in some cases as high as 1.7 or more depending on the application. It’s very much dependent on the application and the efficiency of the particular combustion chamber. In drag racing, most applications are supercharged so the BSFC trends higher in, say, an injected car or an injected circle-track car. Short-track cars typically run around 1.0; sometimes even less.

      Read your plugs and tune accordingly. Whatever BSFC your engine generates is what you get when you have your best tune onboard. Don’t worry about it for track purposes. And if you don’t think it’s good enough go back to the drawing board and figure out what part of your combination is causing the inefficiency. Determine where that falls in the horsepower chain and take appropriate steps to remedy it.

      Remember that BSFC is affected tremendously by the burn characteristics of a particular combustion chamber and the filling and emptying abilities of the flow components. You can chase that free lunch all over the engine, but the horsepower gods have dictated that you have to pay for it somewhere. That’s where optimizing each of the processes in Hale’s horsepower chain becomes so important. Optimization and efficiency go hand in hand.

Contributing Engine Systems

      The induction and exhaust systems are the primary contributors to the total engine airflow equation. The camshaft plays the role of traffic cop, determining when and where air flows and the specific timing of air movement during the power production process. The induction system is the most important, but exhaust system efficiency plays a critical role in the efficiency of overall air movement, and the camshaft still pretty much runs the show.

      BSFC Comparisons

       Here are a couple of handy equations you can use to calculate BSFC for comparison purposes:

      BSFC = lbs/hour of fuel consumed ÷ uncorrected HP

      BSFC = (BSAC ÷ A/F ratio)

       Where:

       BSAC = brake specific air consumption

       A/F = air/fuel ratio (typically at peak torque)

       When checking dyno results, BSFC is always calculated from the uncorrected, or raw, power figures. If you divide the fuel usage by corrected power numbers, the calculation will be incorrect, and it will throw you off.

       Larry Meaux at Meaux Racing Heads provided the following handy equation for making ballpark estimates of fuel consumption when planning a fuel system:

      Fuel Consumed = ci × rpm × 0.0001

       Where:

       0.0001 = mathematical constant

       For example, a 565-ci engine running at 7,500 rpm would consume 423.75 lbs/hr (565 × 7,500 × 0.0001).

      The induction system comprises everything upstream of the intake valve including the valve, intake port, intake manifold including plenum and distribution runners, and any associated spacers that may be employed. It also incorporates the air and fuel metering device (carburetor or throttle body), air filter, and air scoop or in some cases various types of cold-air packages that redirect cooler air to the induction system via underhood passages and strategically located air inlet openings. In the absence of a supercharging device air moves through the engine based on the creation of pressure differentials or voids (empty cylinders) into which it naturally flows (naturally aspirated) because of atmospheric pressure.

      When contemplating engine systems that contribute to maximum VE we typically think of only the induction system. But as Hale’s horsepower chain of processes demonstrates, every link in the chain contributes to or influences the overall VE equation. The induction system is the most familiar player. But it goes nowhere without the exhaust system and all the subtle and less recognized contributions made by the correct combination of short-block components to gain the optimal rod-stroke ratio, effective ring seal, and proper valve and ignition timing. To be sure, most of the important VE actions occur in the cylinder heads where the air/fuel charge is introduced, processed, and discharged with the short-block providing convenient means of transferring the power generated into a rotational force at the flywheel.

      In this book I spend a great deal of time on cylinder