Timed mechanical fuel injection

[Mark Boxsell]

> Sure, go for it Carter. I've a '70 Porsche 911 with a bosch mechanical
>injection system and dread the day I ever have to replace the thing.
>It has the same rack and variable displacement plunger mechanism.
>Those pumps aren't cheap I hear. So any news on part(just control the rack)
>or full conversion to electronic control would be welcome.
> I'd also like to hear your(or any one else's) experiences on the effect of
>injection timing on the torque curve. Normal setup for the 911 is for the
>timed end of injection to occur at about 40degATDC on the intake stroke.
>The begining of injection is varied by a 3-dimensional cam according to
>the fuel requirements(throttle pos,rpm,air density-barometric cell). However
>I discovered that the previous owner's mechanic had set it up 180deg out.
>ie it was injecting on a closed intake valve during the combustion cycle.
>It ran hard, accelerated well, good low rpm torque, felt noisey and harsh,
>cruised roughly at low throttle settings - lot of transmission noise-like
>the engine was slapping it at every combustion and the motor ran cool.
> When I changed the timing to standard the motor smoothed right out, was
>running way too rich, bottom end torque felt lacking. Even after correcting
>the mixture low rpm torque still didn't feel as strong as before. The motor
>ran slightly hotter, cruised very smoothly and quietly without the
>transmission noise. Fuel consumption was initially higher and only equalled
>the 180deg case after mixture correction.
> My thoughts are that (and I could be totally wrong): 
>	for the 40deg case:
>	- at low rpm the corresponding low intake air
>velocity results in poor vapourization of the tiny injected droplets,
>giving slower combustion- longer flame duration and more cylinder wall
>exposure to the flame-causing the higher operating temp. 
>	for the 180deg case:
>	- the droplets have the whole combustion and exhaust cycles in which
>to vapourize to some extent on the hot closed intake valve before being
>drawn into the engine. A lot may be lost to wetting on the relatively cool
>intake walls, but are compensated for by richening the mixture. Richening will
>also help throttle response to reduce leaning from rapid throttle opening.
>The highly vapourized pocket of fuel burns rapidly-giving a short flame
>duration and less heat loss to the engine.
>	So if anyone has dyno figures on torque vs injection timing I'd love
>to hear from them. Does anybody inject both on the closed valve during
>combustion and during the intake cycle? 
>
>		Russell Hayter
>		Design Engineer, AWAmicroelectronics
>		Email:	russellh at awam.com.au
>
> 
>
 Russell,
          The mechanical stuff works very well. Although I share your
concern about fixing it if something goes wrong. We once converted a BMW
323i to electronic from mechanical (although it was D-Jetronic). Apart from
having complete control (ie. adjustment) of the fuel no real advantage apart
from servicing and if you are going to modify the engine.
Also (for Carter) the early formula 1 systems in the turbo era used
stepper/servo motors to control the fuel cam. Renault and BMW used this type
of arrangement.
              regards,
                       Mark Boxsell.