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Fuel injectors - These are opened by the ECU to add fuel to the cylinders, the longer they are opened for, the more fuel is added. As explained above, the fuel pressure is regulated so if a pulse of say 1.5mS is applied to the injector the same amount of fuel flows from the injector under all load conditions. An injector is rated by its flow in either lbs/hr or cc/min, this is usually measured with a fuel pressure of 43PSI.
1 lb/hr ~ 10.5cc/min
Don't think that you simply select the biggest injectors you can find, as this would be wrong. If an injector is selected that is too big you wont get a good steady idle or a steady cruise, as a small increase in the pulse sent to the injector from the ECU will have a large change in fuel. It then becomes difficult to control the AFR making for a lumpy idle and cruise, and it can be difficult if not impossible to get through emission testing. The injector is basically a solenoid which has 12V ignition feed to one side of it and the ECU switches the other side to ground in order for it to make it open and therefore fuel flow through it. There is no positive or negative to an injector so they can be connected either way round, but it's a good idea to keep all of them wired the same on your setup to aid fault finding, etc.
A rough guide to selecting injectors is:
InjectorSize = (HorsePower * BSFC) / (#Injectors * DutyCycle)
BSFC = brake specific fuel consumption. This is usually between 0.42 and 0.58 at wide open throttle. Normally aspirated engines with efficient combustion processes are at the lower end of the BSFC scale ~0.45, where as a supercharged engine tend to be towards the higher end ~0.55.
Duty Cycle is the amount of time the injector is opened when compared to the amount of time it has available to fire between engine cycles. A max duty cycle of 85% is highly recommended, 100% is the absolute maximum, but is not recommended as you are at the very limit of your fueling, don't feel you can cheat this, you can't!
If your not sure then select a set of injectors from an engine that has the same amount of cylinders you have and that produces a similar amount of BHP.
Selecting the correct size injectors isn't as hard as you may think. If you already have an EFI setup then simply retain those unless you are about to increase the BHP dramatically. Don't think that you can simply increase fuel pressure to increase your injectors flow rate, as there is a limit that your pump and pipes and injectors can take. Also fuel flow doesn't increase at the same rate as pressure, flow increases at the square root of increase of fuel pressure, so you'll need a massive increase in fuel pressure to gain much from an injector.
So using that formula I calculated that for a four cylinder engine at 200hp 278cc/min injectors are required
The standard XE ones are 214cc/mim@2.5bar which indicates they may be too small for my needs.
Could someone check my sums and share their experiences please ... thanks
Fuel injectors - These are opened by the ECU to add fuel to the cylinders, the longer they are opened for, the more fuel is added. As explained above, the fuel pressure is regulated so if a pulse of say 1.5mS is applied to the injector the same amount of fuel flows from the injector under all load conditions. An injector is rated by its flow in either lbs/hr or cc/min, this is usually measured with a fuel pressure of 43PSI.
1 lb/hr ~ 10.5cc/min
Don't think that you simply select the biggest injectors you can find, as this would be wrong. If an injector is selected that is too big you wont get a good steady idle or a steady cruise, as a small increase in the pulse sent to the injector from the ECU will have a large change in fuel. It then becomes difficult to control the AFR making for a lumpy idle and cruise, and it can be difficult if not impossible to get through emission testing. The injector is basically a solenoid which has 12V ignition feed to one side of it and the ECU switches the other side to ground in order for it to make it open and therefore fuel flow through it. There is no positive or negative to an injector so they can be connected either way round, but it's a good idea to keep all of them wired the same on your setup to aid fault finding, etc.
A rough guide to selecting injectors is:
InjectorSize = (HorsePower * BSFC) / (#Injectors * DutyCycle)
BSFC = brake specific fuel consumption. This is usually between 0.42 and 0.58 at wide open throttle. Normally aspirated engines with efficient combustion processes are at the lower end of the BSFC scale ~0.45, where as a supercharged engine tend to be towards the higher end ~0.55.
Duty Cycle is the amount of time the injector is opened when compared to the amount of time it has available to fire between engine cycles. A max duty cycle of 85% is highly recommended, 100% is the absolute maximum, but is not recommended as you are at the very limit of your fueling, don't feel you can cheat this, you can't!
If your not sure then select a set of injectors from an engine that has the same amount of cylinders you have and that produces a similar amount of BHP.
Selecting the correct size injectors isn't as hard as you may think. If you already have an EFI setup then simply retain those unless you are about to increase the BHP dramatically. Don't think that you can simply increase fuel pressure to increase your injectors flow rate, as there is a limit that your pump and pipes and injectors can take. Also fuel flow doesn't increase at the same rate as pressure, flow increases at the square root of increase of fuel pressure, so you'll need a massive increase in fuel pressure to gain much from an injector.
So using that formula I calculated that for a four cylinder engine at 200hp 278cc/min injectors are required
The standard XE ones are 214cc/mim@2.5bar which indicates they may be too small for my needs.
Could someone check my sums and share their experiences please ... thanks
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