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What decreases BSFC and how

EFI Tuning Fundamentals

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What physical characteristics of an engine increase BSFC and how do they do so? Andre lists the cooling abilities of an engine as an example of something that can improve the force output of combustion over sound and heat. How does this work? How does head design contribute to getting more force out of combustion? Thank yall so much for the help I’m sure you’ll give me. I’m so so thankful to have this awesome resource 🙏

Compression Ratio is probably the single largest contributor to BSFC. Next is likely efficiency of the fuel mixing and/or burning characteristics. So head design can affect all those, combustion chamber volume can control Compression Ratio; Fuel Mixing is determined by valve sizes, angles, port shapes; and spark plug location has a big influence on the burning characteristics.

Of course it's not just that simple, and that's why folks spend their whole lives / careers chasing this stuff.

Minor confusion - typo'? - you want to minimise BSFC* while maximising torque.

As David said, it's just not that simple. In short, though, it's using different engine design details to maximise the useful work that can be produced from the fuel used. A rough guide is, at maximum power/torque, 1/3 is useful, 1/3 is lost to the cooling system, and 1/3 is expelled out the exhaust as heat and noise.

I would assume, not being familiar with the discussion, that Andre is talking about -

a/ heat rejection from the hot combustion gas, during the combustion process, into the coolant (and to, usually, a lesser extent the oil) - the higher the coolant temperature the lower temperature different (delta) and so less energy is lost meaning more pressure (hotter gases) and useful work.

b/ effective charge cooling which reduces the pressure head the turbo' is working against, increasing efficiency by reducing pumping losses, so more useful power is gained.

... or both.

As David said, efficient mixing of the fuel can allow less timing to be used and faster fuel burn - so allowing less timing which means less "negative torque" from the pressure build up before TDC and greater pressure shortly after TDC which should increase torque. This is the primary reason high swirl, "tumble" port, and "wedge" head designs are used.

Smaller combustion chambers mean less heat lost through the chamber surface as well as the improved compression which aids gaining useful work, etc.

A VERY common mistake is people running (often much!) richer than actually required for peak torque/power while maintaining 'safe' combustion temperatures - this has only negative affects of which high BSFC is just one.

Compounding this is the very common practice of running over-rich at cruising and light throttle when a little leaner than stoich' will give the best fuel economy - this is sometimes referred to as "lean best power", or "lean best torque" - depending on the engine's characteristics, this can be from ~15:1 all the way up to 17:1, or higher (petrol/gasoline fuel). Some dyno's have the ability to monitor fuel flow and give live BSFC values for when tuning the part throttle parts of the maps.

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