[67ragtp]

I Just recently did a complete FAST EFI conversion on a 427 sb crate engine from Bill Mitchel. I bought a spare Motown Intake and sent it to Wilson Manifolds for the injector bung conversion. Its a little pricey but man do they do outstanding work. Any way to make a long story short, I just had the engine dialed in by a real good FAST tuner on a chassis dyno and he installed a vacuum line from the port on my return style regulator to a vacuum port on the back of my throttle body. Can someone tell me how and why the vacuum being pulled on the regulator effects the fuel pressure and whats happening at the injector as a result of this?

Thanks Rich

[JimM]

Virtually all EFI systems use a vac (and boost) referenced regulator. At high vac, the reg lowers the fuel pressure, at low vac, raises it (usually 1 psi / per 2" of vac) and under boost on a blown car, raises it more.

It's giving the injector more fuel under higher load at the same pulse width. Choosing the proper sized injector is a juggling act. All injectors have a minimum pulse width (you don't want to get that short at idle) and a saturation point (maxed out, you for sure don't want to be there at WOT, could be leanout city!) Automatically increasing the fuel pressure with load let's you extend the range of conditions that the injectors will work.

[V-12-Camaro]

I've seen problems with non referanced regulators causing a rich idle condition. In some cases when using big injectors, the duty cycle from the computer (controlling the injectors) can't be set low enough and an overly rich idle situation will occur.
So, with a vacuum signal, fuel pressure will be lower at idle/cruising. As the pedal is mashed, and vacuum drops, pressure rises and has the natural ability to run richer w/less computer influence.

[67ragtp]

Thanks Guys, Its all clear now.

[Rodder]

LSX regulators are referenced to atmosphere. The regulator in the fuel tank on LS1 F-bodies. The PCM compensates for difference in relative pressure across the injector.

[sc68z28]

Originally Posted by JimM

Virtually all EFI systems use a vac (and boost) referenced regulator. At high vac, the reg lowers the fuel pressure, at low vac, raises it (usually 1 psi / per 2" of vac) and under boost on a blown car, raises it more.

It's giving the injector more fuel under higher load at the same pulse width. Choosing the proper sized injector is a juggling act. All injectors have a minimum pulse width (you don't want to get that short at idle) and a saturation point (maxed out, you for sure don't want to be there at WOT, could be leanout city!) Automatically increasing the fuel pressure with load let's you extend the range of conditions that the injectors will work.

Damn thats good Noway to say that any better!!!

[JimM]

Thanks Bill!

[Doug F.]

Actually it doesn't "change" the fuel pressure. It changes the "gauge" pressure, but it maintains the same pressure differential between the intake runner and inside the fuel rail. When the intake is under vacuum, say 15 In Hg (7 PSI), there is a vacuum against the bottom of the injector. This changes the pressure differential vs the fuel rail pressure. By running a vacuum line to the top of a 1:1 regulator, you get the same differential all the time. If you set the static/non-running pressure to say 43 PSI, you will have a 43 PSI pressure differential at all times, making the true pressure linear.

Same with boost, if you have a base setting of 43 and have 14 psi of boost, your gauge should show 57, but you still really have a 43 PSI differential.

FMU (fuel management units) that some people use with blowers take this to an advantage by have say a 2:1 fuel pressure rise with boost, truly increasing fuel pressure.

So you don't get more fuel.

Cars like 05+ mustangs that have pulse width modulated returnless systems actually mimic this with their programming.

[Hammered]

Thank you Doug - that's the first time I've heard it explained that way. I had someone telling me I would have idle problems because my engine makes less than 10" of vacuum. Now I know that's not true.

[Doug F.]

Originally Posted by Hammered

Thank you Doug - that's the first time I've heard it explained that way. I had someone telling me I would have idle problems because my engine makes less than 10" of vacuum. Now I know that's not true.

That is usually in reference to tuning a speed density system. The more vacuum you have, typically the easier the tuning is. I like to see about 8" or greater although I've tuned at 6" Hg or so. That is in regards to a tunable aftermarket system. FOr a factory system, you either need similar vacuum to stock or need to do tuning to the factory calibration.

[camcojb]

Originally Posted by Doug F.

Actually it doesn't "change" the fuel pressure. It changes the "gauge" pressure, but it maintains the same pressure differential between the intake runner and inside the fuel rail. When the intake is under vacuum, say 15 In Hg (7 PSI), there is a vacuum against the bottom of the injector. This changes the pressure differential vs the fuel rail pressure. By running a vacuum line to the top of a 1:1 regulator, you get the same differential all the time. If you set the static/non-running pressure to say 43 PSI, you will have a 43 PSI pressure differential at all times, making the true pressure linear.

Same with boost, if you have a base setting of 43 and have 14 psi of boost, your gauge should show 57, but you still really have a 43 PSI differential.

FMU (fuel management units) that some people use with blowers take this to an advantage by have say a 2:1 fuel pressure rise with boost, truly increasing fuel pressure.

So you don't get more fuel.

Cars like 05+ mustangs that have pulse width modulated returnless systems actually mimic this with their programming.

thanks Doug, and welcome! I actually learned something today.

Jody

[Doug F.]

Hi Jody,
What you driving on the PT this year? I'm going on the last 3 legs and if I saw you thought I'd say "Hi".

[camcojb]

Originally Posted by Doug F.

Hi Jody,
What you driving on the PT this year? I'm going on the last 3 legs and if I saw you thought I'd say "Hi".

I'll be driving Mark Stielow's Camaro X with Scott G from Lateral-G. It'd be great to meet you. By the way, you've got mail!

[paulm]

Originally Posted by Doug F.

Actually it doesn't "change" the fuel pressure. It changes the "gauge" pressure, but it maintains the same pressure differential between the intake runner and inside the fuel rail. When the intake is under vacuum, say 15 In Hg (7 PSI), there is a vacuum against the bottom of the injector. This changes the pressure differential vs the fuel rail pressure. By running a vacuum line to the top of a 1:1 regulator, you get the same differential all the time. If you set the static/non-running pressure to say 43 PSI, you will have a 43 PSI pressure differential at all times, making the true pressure linear.

OK, I've read this over and over and I'm just not getting the concept here.

High vacuum, lower PSI because the vacuum pulls the fuel in?

Low vacuum higher PSI and it sprays harder?


P.S. Good to see you're posting here!!!

[camcojb]

Originally Posted by paulm

OK, I've read this over and over and I'm just not getting the concept here.

High vacuum, lower PSI because the vacuum pulls the fuel in?

Low vacuum higher PSI and it sprays harder?


P.S. Good to see you're posting here!!!

I think you've got it basically. He's speaking of the pressure differential between a high vacuum helping to pull the fuel out along with a lower psi of pressure on top of the injector; this is equal to a lower vacuum signal pulling less with a bit more psi of fuel on top. So even though under load and vacuum has dropped the higher psi you see on the gauge is the same as the high vacuum/lower psi as far as pressure differential, and ultimately the amount of fuel that is released at a given pulsewidth.

At least I think that's it!

Jody