Need advice from someone who really knows fuel injector function

[ratio411]

I have a question that strikes at the very heart of how the fuel injector and ECM work together, and would like to hear from anyone who knows this relationship between parts very well.

I'll give you my theory, and you tell me where the flaws are:

If 19# injectors run 302 cubic inches (everything stock) with an SD EFI system, will simply changing to larger injectors trick that SD system into thinking it is running 302 cubes, when I have actually substituted 351 cubes in it's place?

The only reason the SD won't run more cubes is the preprogrammed fuel curve is optimized for 302 cubes, so a bone stock 351 (or even 408) should require more fuel that the "dumb" (as in blind to change) SD system would not recognize... Correct?

So if I divide 19# by 302 cubes, then multiply that number by the new cubes, which is 23# for 351 cubes, if everything is left stock but the displacement, will the ECM supply the extra fuel for the extra displacement without protesting? The 351 would need a bit more fuel, but the timing and fuel mapping should be perfect so long as the extra fuel delivery is there... ???

I want to keep an SD sytem while upping my cubic inches if you haven't guessed. I just want more cubes, I won't be making other changes such as fancy heads or big cam. So does my theory hold water?

Thanks!
Dave

 

[Chrome_Rust]

Your logic seems sound.
But...
You have to take into consideration pulse width (the time the injector stays open). a 24lb injector is 20% larger then a 19lb, and given the same amount of fuel pressure, will flow 20% more fuel across the board because of pulse width.

At idle you need a very small amount of fuel to run, and then the fuel curve progresses to the max duty cycle at WOT. Would your engine need that 20% across the board? I feel that it would be boggy, and burn to much fuel unless you were at WOT. A 351 is 24% larger then a 302, so you'd think that you would need 24% more fuel. I just feel that it may work, but just not correctly.

Charlie

 

[FastDriver]

Interesting theory. Only problem I can see is that a 302 is not going to have identical volumetric efficiency as a 351. So, there will be some difference in the amount of air-flow. The 351 is not going to flow exactly 16.2% more air across the board. That's as a result of differences in the cam, heads, intake, etc... It still might be close enough to work, though. You'll just have to try it out and see. Hopefully, the EEC will be able to compensate for the difference. It has roughly a 12.5% range that it can richen or lean out fuel. I think there's a good chance that it will run acceptably. Further, I don't think it would be necessary to purchase 24lbs injectors. As long as you have a high pressure pump like the GSS340, you can just up the pressure correspondingly on the 19 lbs injectors. Look for the injector chart that I've posted in the past. It shows the exact pressure required to compensate. I'd say you're relatively safe up to about 55psi, which I an personally attest to running for a couple years without issue on an old car of mine.

 

[ratio411]

Thanks for your input!
I knew the Fox 5.0 guys would be the ones to talk to.

I have a 1990 F150 EFI with SD 5.0... The engine is tired, and it tows like a weakling, so I was trying to figure a way to use a 351w based engine in place of the 302, and not dump more money than the truck is worth into converting it to MAF. By the time I converted to a Stang MAF ECM, wiring harness change, and stand alone E4OD trans controller, it would bankrupt me over an old work truck.

I have a 351w truck lower, and have found that you can reshape the ports to match the 302 upper ports (similar to Holley EFI intake), and I would use stock (or stock-like) heads, cam, exhaust, etc...

Therefore, I was hoping it wasn't much of a stretch to trick the SD into thinking it was still running a 302 in the name of towing reliability.

Thanks again.
Like I said, I knew this was the forum to come to for anything Ford EFI.

Dave

 

[ratio411]

As long as you have a high pressure pump like the GSS340, you can just up the pressure correspondingly on the 19 lbs injectors. Look for the injector chart that I've posted in the past. It shows the exact pressure required to compensate. I'd say you're relatively safe up to about 55psi, which I an personally attest to running for a couple years without issue on an old car of mine.

Curious about this idea...

If I use an adjustable regulator, how much pressure can you pull from a stock pump without jepardizing the pump life? We have 3 5.0 EFIs (Fox, SN95, and F150) and I have done the 'socket mod' to all 3 to increase pressure slightly, and they all ran noticably better, but bumping up pressure another 10 psi sounds iffy... ??? Thoughts?
(remember a truck has 2 fuel pumps to consider, one per tank)

 

[Chrome_Rust]

just get a SD ECU for a truck with a 351.
New one from AZ is 156 with core

 

[ID89GT]

So you want to swap a efi 302 truck to a 351? You don't have to make any changes at all. If I remember correctly the truck uses an entire different system than the mustang on efi. I researched this a couple years ago because I did this exact same swap into a 1991 f 250.

 

[FastDriver]

I wouldn't recommend taking the stock pump up in pressure any more than you already have. The pump is unlikely to fail as a result of the increase in pressure, but as you increase pressure, you reduce the volume that the pump can flow. This is the opposite effect of what will happen with the fuel injectors. The pressure is in front of the pump but behind the injector. So it increases flow at the injector, but decreases flow at the pump. The stock pump in a fox is 88lph (pre'89) or 95lph(89+). That's only good for around 300hp at the stock pressure rating. That's going to decrease as fuel pressure increases.

 

[ratio411]

So you want to swap a efi 302 truck to a 351? You don't have to make any changes at all. If I remember correctly the truck uses an entire different system than the mustang on efi. I researched this a couple years ago because I did this exact same swap into a 1991 f 250.

So you used an EFI 5.8 in place of the stock 5.0?

Did you do any mods, or was it just a stock for stock swap?

Were you happy with the results?