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Old 04-21-2002, 12:02 AM   #1
Scooby Newbie
Member#: 2564
Join Date: Oct 2000
Location: utah
Question injector questions

what is the difference between peak and hold and saturated injectors?

also, i've noticed that there are varying opinions on proper BFSC values for turbo charged cars. Bell in Maximum Boost said that a good BFSC value was .55 while RC Engineering says .60-.65 is correct? what are most people using to calculate their injector size?

ok, one last question...this one also relates to the injector size question. RC Engineering says that peak injector duty must be included in the equation. that makes sense, but what if the injector is already bigger than what the formula calls for? example: for HP levels of x, using the formula without the peak duty cycle calls for 127cc/hr injectors. i would then round up to, lets say 150cc/hr, to create a margin of safety. that way, the 150cc/hr injectors would never be at their maximum duty cycle because even at the HP peak of my theoretical engine, the fuel demands would be less than what the injectors are capable of. it's my impression that RC includes the duty cycle variable to make sure that people aren't buying injectors that are at their ragged edge of performance. does this sound right to anyone else?

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Old 04-21-2002, 06:35 PM   #2
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1999 Impreza 2.5RS
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I’m no tuner – so I don’t fully understand how BSFC numbers are best conjured up or how to best size an injector.

But here is how I understand about the two types of injectors:

Peak/hold injectors, and saturated injectors differ in the way they are driven, and in turn, the injectors themselves are optimized or matched to their respective driver.

In order to achieve very fast turn-on times, peak/hold injectors employ drivers that supply a high-powered initial turn-on pulse for the period an injector makes a transition from fully off to fully on. Once the injector is deemed on, it can be held there by a much lower current; the driver lowers the power for the balance of the required injector-on pulse width.
To take advantage of this technology, peak/hold injectors are designed with lower impedance (resistance) in their winding. The lower impedance allows the peak pulse to do its job more effectively; the injectors turn on even more rapidly on high pulse currents while keeping the heat dissipation down for both the driver and the injector since the hold current is comparatively low.

Saturated injectors employ simple V-I drivers. A pulse of fixed amplitude (power) is applied to the injector for the start and the duration of the entire injector on-time.
For this type of driver, the injectors are designed with higher impedance windings. This keeps heat dissipation for both the driver and the injector at a fixed, but manageable amount for the entire duration of the on-time cycle. Since the driver supplies the pulse at fixed amplitude, it can do this in a ‘saturated’ mode, i.e. the driver turns on fully (keeping V-I drop low across it) – dissipating very little heat of its own. This is a slower but more efficient, cost-effective way of driving injectors.


Last edited by edekker; 04-21-2002 at 07:00 PM.
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Old 04-21-2002, 09:18 PM   #3
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Lightbulb thanks!


thanks for the reply. i appreciate your help.

does anyone think my theory regarding RC's injector formula is sound?

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Old 04-21-2002, 09:20 PM   #4
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Question one more thing...

does anyone know what type of injectors the MY93-01 impreza's have? thanks.

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Old 04-22-2002, 08:12 AM   #5
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They aren't all using the same injectors. For instance, the MY98 RS with the DOHC EJ25 uses side feed (also called galley feed) injectors, while the MY99+ RS with the SOHC EJ25 uses top feed injectors. Both are using high impedance injectors, however, as almost all production cars of late have.

The biggest real world difference between the 2 injectors is the minimum injector on-time. If the injector receives a signal that is too short to actually get the injector to open, then the injector actually does nothing (called a stall). The low impedance injectors can have minimum on-times around 1.0ms while the high impedance injectors will be around 1.5ms. (FWIW, I am using high impedance injectors with a TEC2 and have found the minimum on-time to be around 1.8ms!)

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