converting from EFI to carb?

Doug Sainlar · 02-12-07, 02:02 PM

General question here--I'm working on the motor that will eventually find its way into the 40, but for now I want to "store" it and run it in a 1985 T-Bird. The T-Bird is set up from the factory for TBI.

What do I need to do to run a carburetor in this car, which is of course computer controlled? Will an inline fuel pressure regulator be adequate? Wouldn't I need to make arrangements for the return line?

It seems to me I could run the fuel line into a T, with the line from one part of the T going to the return line and the other line from the T going to the fuel pressure regulator, and from that to the carb, but my experience tells me that idea is WAY too simple and logical and so it won't work.

Any advice??

Dazed and confused, I remain....

YerDugliness

Lynn · 02-12-07, 06:05 PM

Doug,

I think you are making it too complicated:

Code:

fuel line -> regulator ─> to carb
                  └─────> return line

The issue is going to be reducing 30+ psi down to ≈8 psi. The question is, will the regulator bypass this much fuel and will the return line be adequate for the amount that will be returned (nominally, of course at WOT this will be less.)

You may want to take out the high pressure, in tank fuel pump and replace with one that pumps at a pressure in accordance with a carb. I don't know if they make such an animal for intank, but certainly there are inline pumps that would be suitable.

Paul Thompson 'Hooligan' · 02-12-07, 06:11 PM

Hi Doug -

Whilst I'm not familiar with the specifics of T-Bird injection - what you propose should be easier than the other way round (Carb to EFi)..

The only real consideration I can see is dropping the fuel rail supply to something suitable for a carb input, e.g. less than 4 to 7 psi max. The Efi rail is likely to be in the region of 3bar (45psi) so the carb would definately not like a direct connection. If the fitted fuel regulator is not adjustable, I would suggest adding one that is and can then be screwed out, so that the rail pressure is as low as possible to a more acceptable level for the carb. (it should be possible to use it on the '40' at some later point too)

You might need to bypass the normal power feed to the high pressure pump as its relay is probably controlled by the motor's ECU, which if not used, would mean no fuel pressure... Of course if you retained the ECU, but dropped the injectors, (might need dummy loads), you could possibly retain a mapped spark, but it would all probably work easier with a Dizzy and MSD in place..

One last thing that comes to mind - if the trans is an auto - It might have some additional input from the ECU too for correct operation - I don't know what system is used so cannot comment for sure.

Hope this helps in some small way - let us know how you get on.

Paul Thompson 'Hooligan' · 02-12-07, 06:22 PM

Hmm - just re-read your posting and Lynns reply and if you are going with carb(s) on your 40, then the suggestion of a low pressure pump to replace the HP unit seems a far more logical solution.

What is it they say - 'cant see the wood for the trees'.....

Lynn · 02-12-07, 06:53 PM

Paul,

Your comments about the all the other issues with EFI -> carb were completely appropriate, IMHO. Not sure there is a fuel rail with TBI though; I am not real up on TBI but I wonder if the regulator isn't built into it?? TBI is probably a speed-density computer, so it should be a little simpler than port EFI.

Lynn

JohnC · 02-12-07, 07:23 PM

Doug, as Lynn pointed out, the existing FPR may not be able to bypass that much fuel, and even if it did, you'd be heating it up too much with all the circulation, and vaporising a bunch of it.

If you wanted to try to retain the existing EFI in tank pump, you could try adding in a little PWM box to give the pump a lower "effective" voltage so it would pump less. These guys make one: Other Control

The PWM method of reducing voltage to the pump is better than using a resistor network, as some pumps (Walbro) won't run at a true reduced DC voltage, but do fine on a PWM "reduced" voltage.

Probably the best way, as already stated, would be to replace the high pressure, EFI type in tank pump, with one that is better matched to carburetor.