cold engine driveability, fuel pressure

[Carter Shore]

Franc Buxton wrote:

> .. Injector flow is to a large extent proportional 
> to fuel pressure. I have done this on my Rover 
> Metro (UK), fitted with a 1.8-litre engine in place 
> of the 1.4 and using the 1.4 engine management. 
> ... Fuel pressure increase in this particular case
> was from 3.2 bar to 4.75 bar, surprisingly somewhat 
> more than the proportional difference in capacity, 
> but that's what it needed. ...

I have literature that describes the injector flow
rate increase as proportional to the square root of
pressure increase, ie, to double the flow, use four
times the pressure. In your example this gives:

1.8/1.4 = 1.28 -- displacement increase ratio
(1.28) * (1.28) = 1.65 -- fuel pressure increase ratio
3.2 bar * 1.65 = 5.28 bar -- fuel pressure in bar

Of course, this number does not match what you
obtained for proper operation, so much for theory!

I'm looking at a similar situation, running a 1.5L
motor with EFI calibrated for 1.9L. In my case, I plan
to select smaller flow injectors, then diddle the fuel
pressure if needed to fine tune. Once running, I will
hack the spark and fuel tables.

BTW, what method did you use to adjust the fuel
pressure?

Thanks,

Carter Shore

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