Fuel Pressure on roots blown TBI (theory question?)

[5.0Thunder]

I'm not having any issues really, just questioned my initial thinking and wanted some discussion.

The car is an 87 mustang with an aluminum headed 302, Weiand 174 Roots Blower, and a sort of DIY TBI setup using MS2 for control of Four 160lb injectors running e85. Return-style fuel system with regulator AFTER the rails (typical I believe).

As far as the fuel injector is concerned, it will act like it's in an N/A application. The spray tips are right under the throttle valves and above the blower rotors so they're ALWAYS under vacuum, though the engine sees boost.

I thought about this 3 ways and currently have it set up per one of these..

1) *current setup* no vacuum/boost reference on the regulator - This is a 39psi base pressure regulator so the injectors see 39psi from above at all times.

2) manifold reference regulator - This would decrease fuel pressure at idle and increase fuel pressure in boost. Since injector tips never see boost, I didn't feel the need to boost reference the regulator since that method is employed to equalize injector pressure in a Port Injection application. This would give me added fuel pressure though which would make my VE table flatter, which probably makes no real difference.

3) TBI reference regulator - This would allow the regulator to see vacuum between the rotors and throttle blades, which is the same vacuum that the injector tips see. more vacuum at idle, less at WOT when blades are open. This makes me think the injectors would always see less pressure than the regulators base setting (39psi). I feel that this may worsen the injector spray pattern.


What do you all think about this stuff? In port injection, I always reference the manifold to equalize injector tip pressure. in a Blow Through application, it would also make sense to do this. TBI on top of a roots blower isn't done very often so I haven't seen what works best.

[billr]

#3, keep the pressure difference between injector inlet and spray tip as constant as possible.

[5.0Thunder]

#3, keep the pressure difference between injector inlet and spray tip as constant as possible.

That would indeed be the case, though the 10 in-hg vacuum at idle would bring my pressure down from 39psi to 34ish (and lower in over-run). I'm not sure if i'll have spray/atomization issues at lower pressures. WOT would be about the same most likely so no worries there. I may just throw the hose on there and see what happens.

[racingmini_mtl]

#3, keep the pressure difference between injector inlet and spray tip as constant as possible.

That would indeed be the case, though the 10 in-hg vacuum at idle would bring my pressure down from 39psi to 34ish (and lower in over-run). I'm not sure if i'll have spray/atomization issues at lower pressures. WOT would be about the same most likely so no worries there. I may just throw the hose on there and see what happens.

But you don't have lower pressure at the injector tip. You still have the same differential pressure so you will have the same atomization.

Jean

[5.0Thunder]

#3, keep the pressure difference between injector inlet and spray tip as constant as possible.

That would indeed be the case, though the 10 in-hg vacuum at idle would bring my pressure down from 39psi to 34ish (and lower in over-run). I'm not sure if i'll have spray/atomization issues at lower pressures. WOT would be about the same most likely so no worries there. I may just throw the hose on there and see what happens.

But you don't have lower pressure at the injector tip. You still have the same differential pressure so you will have the same atomization.

Jean

Hmm, I understand what you're saying, I wasn't focusing on the differential remaining constant. I'll throw a vac line on it and see what happens. It runs okay now, still in the early stages of tuning, but maybe it will make it act better in some ways.

[billr]

I think the fuel spraying into 10"Hg vacuum would atomize better than when spraying into atmospheric pressure. Something about vapor pressures or partial pressures of the liquid...