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

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.

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.

Thanks Guys, Its all clear now.

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.

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 :hurray: Noway to say that any better!!! :thumbsup:

Thanks Bill! :thumbsup: :beers:

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.

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.

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.

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! :thumbsup: I actually learned something today. :yes:

Jody

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".

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! :D

http://www.lateral-g.net/stielow/X_render409.jpg

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. :confused:

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

Low vacuum higher PSI and it sprays harder? :confused:


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

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

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

Low vacuum higher PSI and it sprays harder? :confused:


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! :clonk:

Jody

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! :clonk:

Jody

OK, OK......I see, I think......

OK if there was no vacuum "pulling", the only factors controlling how much fuel coming out would be PSI and duration of pulse. So when you have high vacuum, the injector opens and fuel comes out because of the fuel pressure AND because of the vacuum pulling it out. Since the vacuum pulls out the fuel you can reduce the PSI and deliver the same amount as if the PSI were higher and no vacuum given the same pulse width.

So I guess to standardize the mathmatical equation that the ECU uses to determine how long to open the injector, it has to know the amount (or at least that it is always the same) of fuel delivered per millisecond. Since this can change based on vacuum, the regulator adjusts the PSI to make up for vacuum "pulling" fuel out.

Vacuum is pressure. Just negative pressure. It all "adds up". 29.92" In Hg of vacuum = 14.7 PSI.

Thanks Jody, I'll get it tomorrow! Sounds like you'll have a fun ride!

one way it was explained to me, I think... Doug, are you from Chevytalk? is that with a low pressure (14 psi & 15" vac) tbi and a blower... without the manifold pressure regulator, you could have a situation where the fuel pressure EQUALS the manifold pressure! lotsa injectors clicking, ZERO fuel moving!

And I did do a beautiful job of explaining it not quite correctly, eh?
Thanks Doug F, good to see you with us, we need all the help we can get.

Hi Jim,
Yea, that's me. I'm not trying to "take over" your site". :) :)

With TBI you don't have to reference as the injectors are in the atmosphere and don't care.

Not referencing a boosted application with port EFI would be a problem. If you had 40 PSI base fuel pressure and had 25 PSI of boost, you only have 15 PSI of fuel pressure. Instant hand grenade.

Vacuum is pressure. Just negative pressure. It all "adds up". 29.92" In Hg of vacuum = 14.7 PSI.

Good, then I do understand.


I'm not trying to "take over" your site". :) :)

Doug, I'm glad that you are posting here. Feel free to "take over" and answer all the EFI questions that you'd like, it's OK by me (not that my opinion matters much though ;) ).

Hi Jim,
Yea, that's me. I'm not trying to "take over" your site". :) :)



no worries here Doug. We're just "meat and potatoes" EFI guys here, guidance from a pro is welcomed. :beers:

Jody

We're just "meat and potatoes" EFI guys here, guidance from a pro is welcomed. :beers:

Jody

Jody, "Meat and Potatoes"? He, He, well there must be varying degrees of dishes because the stuff you build is like....Filet Mignon and some fancy potato dish and the stuff I build is like...um...well more like spam and tator tots, LOL! :D :D :D

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! :clonk:

Jody


This probably sounds dumb, but does this make any sense:confused:

Let's say you needed 1 gal of water (fuel) per minute, from your garden hose nozzel (injector) at 45psi. but you had it screwed into your air compressor tank (Manifold), and the tank had 45psi of pressure in it (boost). You would not get anything water in the tank (lean out). Unless you incress the water pressure.
If the tank had a vacuum in it, the hose would give more than a gal. per min (rich).

The Vac/Boost reff regulator just evens it all out, so we always get our 1 gal per minute flow. RIGHT??

---Bill.

That's how I understand it, Bill.

Doug, you are MOST welcome here. Chevytalk has a great EFI forum, and you are one of the experts. As we get our own efi forum off the ground, we need all the experts we can get!