teach me about superchargers

trav_19 said:

so you auctually think the 42's are undersized?!?! wow i thought i read somewhere that people were running 24's with chargers...this info is surprising me lol but im glad you guys are telling me so i dont go out and buy useless parts

Click to expand...

When you run 24's you have to use the fuel pressure/vacuum reference thing - the name of it escapes me at the moment - it sees boost and increases fuel pressure which makes your 24's perform like larger injectors pushing more fuel through.

EDIT: FMU - Fuel Management Unit - that's the name I was looking for... it's been a long day.

Wes

Enough "here's what I think"... here's the help file from EEC Analyzer on calculating injector/fuel pump sizing.

Putting in 350hp (flywheel), 8cyl, 20% safety margin, 24# injectors, 39# fuel pressure, 43.5# rated line pressure says 23.63lb/hr are needed.

You can download the trial of EEC Analyzer and going to the "Fuel Delivery" tab to see this also.

Wes

_____________

Detailed Explanation by AEM
The information below is a direct quote from the AEM EFI basics V1.3 document. I cannot explain it better, so I will give credit to AEM for the contents of this help screen.

Fuel Pump Sizing
To achieve proper fuel delivery, you must select the right fuel pump for your vehicle. In most cases, where the engine has been modified only with “bolt on” performance items, there is rarely a need for a larger fuel pump or larger injectors. Vehicle manufacturers typically design a “safety factor” into the fuel pump to accommodate the deterioration of the fuel system over time. This safety factor is intended to compensate for a fuel filter that is nearing the end its life, or for deposits in the injector orifice. Our research has revealed that generally there is about a 15%-20% oversize in most factory fuel pumps.

If the engine is enhanced via forced induction or nitrous oxide, the stock fuel pump is inadequate. If the engine’s power is increased more than 15-20% fuel delivery must increase as a factor of the power gain.

The way to determine the proper-size fuel pump is based on the desired brake specific fuel consumption (BSFC) of the engine. This term refers to how much fuel in pounds per hour (pph) the engine consumes per horsepower and is a measure of the efficiency of the engine. It is a useful term in determining the total fuel requirement of the engine.
On vehicles equipped with forced induction or nitrous oxide, higher BSFC’s are required as an added measure of safety to prevent detonation or high combustion chamber temperatures. Below is a guide of BSFC’s with standard CR that

AEM uses for various engines that run on gasoline:
• Naturally Aspirated engines have a BSFC of .48 to .50
• Forced Induction engines have a BSFC of .65 to .68

Methanol (alcohol) powered engines require twice the amount of fuel so the BSFC’s are doubled. Calculating the total fuel requirement of an engine requires simple equations that we outline in the following section. You must know how much power the engine is anticipated to make and we recommend that you guess on the high end. The fuel requirement will be determined in pounds per hour of fuel flow. Since most pumps are rated in gallons/hour you must know the weight of your fuel/gallon. (The vast majority of gasoline based fuels run at 7.25 lbs./gallon.)

The equations to determine your fuel requirement is as follows:
• (Power x BSFC) x (1 + Safety Margin) = pounds/hour
• Pounds/hour / 7.25 = gallons/hour.

An example of this equation is:
• 500 hp gasoline engine using moderate boost with a 30% safety margin
• (500 x .625) x 1.30 = 406.25 lbs./hr.
• 406lbs/7.25 = 56 gallons/hour.
• If the pump that is being considered is rated in liters per hour, use the conversion factor of 3.785l/gallon. The pump described above would be rated at 56 gallons x 3.785 liters = 211.96 liters/hour.
In the fuel pump sizing, always use a safety margin greater than 20%.

Fuel Injectors
The AEM PEMS requires the use of “saturated” or high-impedance fuel injectors. If “Peak and Hold” or low impedance injectors are to be used, an injector resistor must be used or you will damage the ECU. Resistors can be purchased from AEM. The PNP version of the AEM PEMS is configured for the stock injectors and no additional parts are required.

To determine the size of the injectors, the total engine power must be estimated or known. The fuel pump calculations and BSFC information mentioned in the previous section provides a good understanding of the fuel requirements for an engine. The following equation will allow you to determine the requirements of your injectors.

Using the same engine as above:
• ((Power x BSFC) x (1 + Safety Margin))/Number of Injectors = pounds/hour

An example of this equation is:
• 6 CYL. engine rated at 500 hp on gasoline using moderate boost with a 15% safety margin on the injector
• 500 x .625 = 313 lbs/6 = 52 lbs/hr/ injector. 52 x 1.15=60lbs/hr/ injector
If we take the flow of the injector (60 lbs/hr) and multiply it by the number of cylinders (6), we arrive at a total of 360 lbs/hr of flow. As you can see, the fuel pump described above has enough capacity to feed the engine with a little room to spare.

It is a good idea to know the maximum operating pressure of the fuel injectors. In some cases the fuel injector will not open if the fuel pressure exceeds the design limit of the injector. Also, at the higher pressures the injector fuel flow may become non-linear and cause inconsistent fuel delivery, usually creating a lean condition. Most injectors can withstand up to 70 psi. Many of the pintle style injectors can withstand higher pressure.

In the fuel injector sizing, always use a safety margin between 15-20%.