I've been tinkering with my car quite a bit since I converted over to the singleplane and found some interesting results.

I don't really understand the physics involved in my results, so if someone could shed light that would be great.

Lower fuel pressures, 38, 40, 45 seem to run the car a bit rough. Even if BLM is near perfect, it appears this fuel pressure just isn't "right".

Running the motor at 50psi, and tune BLM to be in the same spot and it just runs much more smooth. I've verrified o2 readings are pretty much the same, blm is 128 pretty much everywhere with both tunes and fuel pressures. But with the higher rail pressure the car is just happier!

Also, crank time on cold fireup is shorter with the higher rail pressure. Again, bpc is set to match the fuel pressure, so it shouldn't really be getting any more fuel. Just seems the higher squirt is doing better.

SO.. Why?

I know I've talked to Rich at Cruzin performance, and he's told me lower pressures on higher injectors create HORRIBLE spray pattern on his machine. But I'm running like 10psi over "stock" for a thirdgen.

Btw, my idle pw's are in the 1.40 - 1.60 range.


-- Joe

 

I would think its due to better atomization at higher FP's, particularly when cold. The comment on larger injectors with lower FP's sounds vaild too. I'm no expert, but it just sounds logical.

 

I'm going to say that it has something to do with the way you have the injectors pointing straight into the runner (read in other post). At the lower pressure, maybe some of the fuel wasn't bouncing off the wall, therefore would run down the runner wall. The higher pressure may be letting it bounce back into the incoming air?

Just a theory.....

 

i discovered the 95-96 tbi 7.4 L truck runs at 26-32 lbs FP. smaller injectors. GM must have determined better control of atomization. i think trucks used a dual plane dedicated TBI manifold. not sure it they used a vacuum referenced reg however.

 

two things come to mind for me....

one is the higher pressure makes a finer mist of fuel... and two, if your injector angle makes the spray cone hit anything, it gives the fuel the momentum to "bounce" off the surface instead of running along it.

 

The greater the pressure differential, the greater the degree of vaporization.

Same as running a *heated* manifold, but without the heat.

With the injectors being pointed at the runner floors, I can't see where the pattern is going to make that much of a difference.
Whereas having the fuel better vaporized would slightly lessen the amount of fuel that gets puddled at the runner. Thou maybe having that much more inertia in the fuel would cause it to *splash* more, and that would improve the atomization.

 

Hi Ron,

I wonder if this contributed to why yours ran so poorly in cold weather with the 90's at 9psi?

Do those later model TB's run the same type of regulator, just with a different spring? I have seen the regs blow out at 12psi, if the later ones are the same design, I wonder how reliable they are at 25+ psi??

 

i blew one old diaphragm and another new one when i first added vafpr. . this was on cranking at i assume 9/12 lbs with vafpr plugged in. i never fiqured it out. with 2nd new one never happened again. i eventually replaced the vafrp when large injs went in. i am at 11.5 FP today. when the 7.4L TB goes on i will set the aeromotive at 13 lbs to start and possibly creap up a lb at a time. the ease of adjustment on the aero will be nice. more importantly the wb02 goes on thurs/friday and some honest datalogs will be available to me. i took great interest in your last post of the WB 02. who knows what mine will look like. my stumbles are over so my AE may be in line. now i can look at AE TPS as well as AE MAP by monitering my TPS movement. a whole new tuning world at my r foot.

 

my 95-96 GM 7.4L i recall had the orange spring in it. same diaphragm as GM has used. this confirms the dealer specs at 26-32 lbs. i dont think it was too high FP unless the vafpr contributed to the problem. considering all the TBI's in use i would think it is reliable and safe as far as diaphragm. i will look at my 7.4 L diap again to see if it is of diffferent quality. appeared same exact specs from recollection. when i added large injs i reset FP from 9 to 11.5 and started tuning.

 

plenum design was the issue. it is cold here in hurley wisconsin.