View Full Version : The Boss' Brief Explanation of Fueling and Ignition

07-07-2012, 10:26 PM
I thought I would share with my fellow roadster lovers a brief document I wrote on fueling and ignitions with information I learned from school and research I did on the side. Although it is not roadster specific it can be applied to any conventional combustion engine. My hope is that you will enjoy it and maybe even learn a thing or two.

Written by Ben Ellett


If you increase the fuel pressure, how does it increase the flow? If you want to double the flow, you have to quadruple the pressure. Just doubling the fuel pressure will only increase flow by 41.4%. This is the reason why adjustable aftermarket fuel pressure regulators should never be used in lieu of larger fuel injectors unless if they only need to flow a few pounds per hour more. Do not increase fuel pressure over 75psi if you have standard injectors as it could cause the injectors to fail and remain in the full open position. This would cause an extreme rich condition and likely a misfire. The website www.megamanual.com has some great plug-ins for doing fuel calculations.

The two kinds of fuel injection circuits are saturated circuit and peak and hold circuit. Cars are built with either one of them, not both; so when choosing an aftermarket injector not only must physical size and shape be taken into consideration but what design it is must be considered as well. Failure to correctly match injectors to ECU can result in many issues. A saturated circuit requires full battery voltage to be applied to the injector the entire time the injector is open, which is cheaper to manufacture but does not have as quick of a reaction time. A peak and hold circuit has a current limiting driver so that maximum current can be used to jolt the injector open faster while a lower current can be used to keep it open. Typically you can figure out which kind of injector you have by measuring its resistance. A saturated injector will usually have a resistance of ~12-16 ohms while a peak and hold injector usually will have a resistance of ~2-6 ohms.

Brake Specific Fuel Consumption or BSFC is the amount of fuel required to make one horsepower for one hour and is measured in (lbs/h)/(bhp). When the term “brake” is used in reference to engines, it is referring to a measurement that is taken at the flywheel. If you were to get the engine’s horsepower from a chassis dynamometer the resulting value after your calculations would be SFC, where as if you were to get the engine’s horsepower from an engine dynamometer the resulting value would be BSFC. Specific Fuel Consumption can be more useful than Brake Specific Fuel Consumption in some cases because it factors in drivetrain loss so it is more accurate for aiding in choosing fuel injectors. The case where BSFC would be preferable to SFC would be when comparing engines alone where you don’t want the efficiency of the cars drivetrain to be involved. If you cared about an engine’s energy efficiency, this information could be used when deciding what engine to choose when doing an engine swap. Although this may not be practical for street applications, it could be quite useful for racing applications where the further a car can go on the fuel it can carry, the less often you have to pit. In the following paragraphs I will be discussing measurements taken at the flywheel, however, all of the math is the same with measurements taken from the chassis. Just ignore the “B” in “BHP” and “BSFC” if you are taking your measurements from the chassis. BSFC can be determined from engine dynamometer measurements and typically ranges from ~0.45-0.50 on naturally aspirated vehicles and ~0.55-0.60 on vehicles with forced induction. The smaller the number is, the more energy efficient the engine is. It can be calculated using a gauge measuring fuel flow in pounds per hour while the car or engine is making a dyno run. First you need to find the RPM at which your engine makes the most power. Then you measure the rate of fuel consumption at that RPM and use the following formula: BSFC=(pounds per hour reading from flow gauge)/(brake engine horsepower).

Using the numbers you just found you could find injector flow rate in pounds per hour with this formula: Injector flow rate needed in lbs/h=(lb/h of fuel measured earlier)/(# of injectors x 0.8). If you did not measure fuel flow rate earlier, you can make an estimation with this formula: Injector flow rate needed in lbs/h=(BSFC x BHP)/(# of injectors x 0.8). In this case, the 0.8 is representing an 80% duty cycle, which is the maximum duty cycle on most injectors but can be replaced with a different value if you know your injector’s particular duty cycle. However, if the maximum duty cycle is exceeded, the injector may not have time to fully close and it will be very hard to predict how much fuel is being used. Remember, it is always better to buy an injector that has slightly too much flow than one that doesn’t flow enough. The fuel pump should always be the first thing to be upgraded and does not require to be tuned for upon installation, unlike larger fuel injectors.


No matter what, ignition timing will always be before top dead center (BTDC). When I speak of retarding the timing, I simply mean that the timing is being retarded from where it was before; however, it is still firing the spark BTDC. When I speak of advancing the timing I mean that the timing is being advanced from where it was before. On older cars, ignition timing can be controlled using a timing light on the markings on the crankshaft pulley while twisting the distributor or in some cases can be adjusted by changing the position of a sensor. On most modern cars, timing must be read and adjusted through the ECU. On two identical engines, one with higher compression pistons than the other but all else being equal, the engine with the lower compression will need more advanced timing than the one with higher compression. This is due to the fact that the more you raise compression; the faster the air/fuel mixture will burn. The same goes for engines with forced induction. Since forced induction just creates greater pressure in the combustion chamber, timing must be less advanced than a naturally aspirated engine. On cars with distributors as opposed to coils, they typically need to run a richer air/fuel ratio to assist in creating a spark since distributors have a hard time creating a spark at high RPM. An air/fuel ratio of 11:1 is the most conductive to spark ignition. However, on newer cars with coils we can run a leaner air/fuel ratio and still get a spark so we will make more power. Maximum fuel economy and power are always limited by the ignition system. The goal is to decrease misfires because when you have the fewest misfires, you have the best power and the greatest fuel economy. The objective is to have the biggest spark plug gap possible without blowing out spark. This will reduce the ignition lag and allow you to retard timing so that the moment when the air/fuel mixture actually begins to burn is more predictable. Improving the spark profile by increasing the intensity and duration of the spark will also allow you to run a bigger gap, which will also decrease ignition lag.

And remember, lean is mean!

kung fu jesus
07-07-2012, 10:31 PM
Nice work Ben!

07-10-2012, 12:41 PM
Nice work Ben!

Thanks! Now to see if people have the patience or interest to read it haha.