EFI newbiee ideas, few questions.

[Mary or Stephen Burgess]

At 06:51 PM 2/23/00 -0800, you wrote:
>I just discovered this list a few days ago and I am reading everything I
>can. Fascinating stuff. Just got a few questions and "what ifs"
>
>First off my project is to "convert" a 1969 Austin Healey Sprite (1275 cc
>inline 4) to Throttle Port Injection. I would hopefully keep my carbs and
>just use them as throttle bodies and fabricate a piece that would fit
>between the carb body and the manifold with the injector. X2 for both carbs.
>I'm still in the research stage BTW.
>
>As I understand it, fuel injection lets you keep the fuel ratio to a perfect
>14.7 mixture throughout the RPM band. The injectors know how much they are
>injecting therefore the only other variable is knowing how much air is going
>into the engine. As I understand it, there are 3 ways to do this.
>
>1) A "mass air flow" sensor that somehow measures the amount (and pressure)
>of air coming into the engine and then in turn lets the ECU know and the ECU
>calculates how much fuel the injectors need to inject.
>
>2) An o2 sensor down the exhaust pipe inorder to tell the ECU how much more,
>or less fuel in needs too inject on the next stroke to maintain the exhaust
>o2 levels low therefore indirectly keeping the ratio correct to a 14.7
>
>3) A throttle position sensor that just tells the ECU to inject more (pedal
>to the metal) or less (idle) fuel into the system. This last choice seems to
>be the "dumbest" method of doing it. It also depends on some kind of
>variable in the ECU to know how much air is going into the engine for a
>certain throttle position.
>
>So my question is, as I understand it, modern cars have all 3? is this just
>to be redundant and to MAKE SURE the mix is correct? Couldn't I just get by
>with some sort of mass air flow sensor (#1). If I would ever supercharge in
>the future, or I took the car too a higher altitude, I wouldn't have to
>touch anything (just make sure the injectors are big enough to be able to
>inject that much more fuel for SuperCharging.)
>
>Thanks for your help. Let me know if my logic is no good either.

Just knowing how much air is moving into the engine, does'nt really give you
much more control over the fueling than carbs. Air has temp and density
differences that would throw out your perfect 14.7

Modern cars are using a multiple of sensors that may seem redundant, but
serve the purposes of providing performance, and economy - for example the
"stupid" throttle position sensor is used as part of engine loading
calculations in a throttle position vs. rpm comparison - a 14.7 mixture may
not be best torque for your engine, and at 1100 rpm with WOT you would want
best torque for performance, while partial throttle at 1100 you might want
some economy and the ECU should select a leaner than 14.7 mixture for best
lean cruise (that's a naughty thing now, but I don't know what else to call it)

O2 sensors provide closed loop feedback of what really happened in the
combustion chamber, and adjustments if necessary are made to the ignition
timing or mixture, depending on the condition of the other sensors too.

You may want to check out Kinsler Fuel injection's web site www.kinsler.com,
they sell mechanical fuel injection systems that would serve your purpose as
easily as a air flow only fuel injection system - basically mechanical
injection has the engine drive a mechanical fuel pump, that as the engine
speed increases so does the pump speed. The output of fuel to the engine is
increased as the pump speed increases - kinda like a MAF only system would
see the engine breathing more air as the rpm's increased and deliver more
fuel regardless of any other conditions. 

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