[Diy_efi] efi conversion

[Tlsalt at aol.com]

Gene wrote,

 My question is in injector sizing because it seems that fuel pressure and 
timing would be more a factor in conversion than size of injector. Or am I 
over simpifing it? Anyway, as far as controllers are concerned, it looks like 
a GM of a mid 90's v-6 will work and put the leads on the injectors as needed 
to correspond to firing order. 
comments?

Hello Gene,

Either you are over simplifying it or over estimating the intelligence of 
ECU's. To further over simplify, EFI injectors are very different animals 
from your mechanical injection. The amount of fuel they deliver is controlled 
by the pulse width, not the timing or the fuel pressure, which is held at a 
relative constant in most systems.  You need to control the pulse width to 
build the proper fuel curve for your specific engine. The injectors are sized 
so that their duty cycle (on-time in relation to engine cycle) does not 
exceed 85-90% and so that the pulse width at idle is not so short to be 
unstable. Most of the injectors you will find on domestic cars will have the 
same nozzle dimension, physical size is a minor issue. A quick and dirty 
estimate of HP potential is (lbs/hr x #inj x 2 x 0.85). The Ford 5.0 
injectors are usually 19lbs/hr, so 6 of them would be(19x6x2x0.85)=194HP. 
This is probably at the high end of what you could expect from this engine 
NA, but probably insufficient for a turbo.  The same size Ford injectors are 
also available in 24,30 and 36lbs/hr.

I would say it is pretty wishful thinking to expect a EFI system from a 
3-4litre GM engine to have anywhere near the proper fuel curve for a 2.5 
Triumph motor with a very different VE curve. Add a turbo on top of this and 
the probability drops further. You need to be able to hack into the ECU and 
control the injector pulse width.  The best option is to use a programmable 
MAP based ECU, but even then you are probably looking at an injector size 
change and MAP sensor change when you go with a turbo. The nice thing about 
MAP is that you don't need anything (MAF or barn door) in front of your six 
throttles. Even if you could find a system from a same displacement engine, 
with the same # cylinders (2.5 BMW ?),you may be in the ball park, but far 
from ideal.

Your fuel rail idea sounds good cosmetically, but not so good from an EFI 
stand point. The injectors need constant pressure and the rail should be 
designed to minimize fluctuation. That said, there are hard adapters for the 
injector top Orings to connect to your metering unit, the GM blower guys use 
them.  Fuel needs to constantly flow through the rail, exiting through the 
regulator and back to the tank.  There is someone in the UK doing EFI 
conversion on this motor using this manifold, have you seen it?

Paul Saltwick



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