Fuel Pressure Reg. ?

[We Gone]

I was talking with Gordon K and he mentioned you could remove the Vacuum line from the fuel pressure Regulator, If I was to do this what are the advantages if any and are there any negatives to doing it? Thanks

[rkreigh]

Originally Posted by We Gone

I was talking with Gordon K and he mentioned you could remove the Vacuum line from the fuel pressure Regulator, If I was to do this what are the advantages if any and are there any negatives to doing it? Thanks

car would have to be calibrated in the tune for that and honestly I don't see the purpose

graham gave me a modified FP reg to install on the LSV that tries to lean things out a bit at idle

haven't tried it out yet as the car idles pretty solid with the haibeck tune

[We Gone]

How does the vacuum effect the FP? does it decrease or increase with vacuum?

[XfireZ51]

On TBI cars, a vacuum referenced regulator increases pressure which in turn increases fuel flow. You get smaller injectors acting like bigger ones which helps out in controlling the idle and still being able to supply enough fuel for WOT.
On dry manifolds w MPFI, the VAFPR manages fuel pressure but with the purpose of maintaining the SAME fuel flow throughout the engine operating range. Without the vacuum reference, idle and part throttle would be too rich.

[csavaglio]

Bear with me...I'm working mid shift and this is reaching back about 10+ years to when I used to drag race a lot.

As I recall, this was one of those little (and possibly negligible) tricks guys used to use at the track.

Basically, when you go WOT, the vacuum signal drops way off, increasing the fuel pressure by the regulator. The idea of pulling the line is that you have 0" of vacuum at the regulator and it gives full pressure all the time.

There were some different schools of thought. One was that the higher pressure created a better spray pattern and better atomization of the fuel from the injector. The computer would back off the pulse widths to compensate for the slightly higher fuel output. Because of the better atomization, you create more power.

Another thought was that more fuel equals more power. Higher pressure equals more fuel. So higher pressure equals more power. The reality is that fuel injected cars with a proper tune, injectors, etc are rarely fuel starved (usually the opposite...that's why Mark Haibeck and the others lean out the cars a bit). The computer's trying to keep everything at a pre-tuned equilibrium, so, using the real-time 02 sensor values and the learned block values, it will tune out the extra fuel (as much as its limits allow, of course).

Another thought is that you're adding enough fuel to override what the computer can tune (it can only back off the injector pulses so much) and you end up with a rich idle and part throttle.

Personally and real world, I saw exactly no difference at the track on my time slips with several back to back runs with and without the line hooked up.

A popular bolt on was the adjustable regulator, where you could tune the pressure you wanted but keep the vacuum line so you got the variable pressure. Again, the idea was that higher pressure gave better atomization. Realistically, I don't think I saw any difference here either.

I have no idea what kind of regulator the LT-5 uses, compared to the L98/LT1/LT4. I doubt there's any real gain. I never saw anything. I used to like comparing stuff at the track....paper vs k&n filter, throttle body airfoil/without, street/race tires, and even, when I was running my 383, stock/52mm/58mm throttle bodies.

Even if there's a few hp, it's nothing you'll feel and wouldn't see anything on a timeslip, so why bother.

Chris

[We Gone]

Thanks Chris, Great explanation.

I also ask Marc H about this this was his reply.

Removing the vacuum line from the fuel pressure regulator adds 8 psi
of fuel pressure at low engine loads. This will add about 10% more
fuel at idle. Over about 10 minutes of run time the engine control
system will compensate by removing the excess fuel. This can improve
operation if the fuel injectors have a low flow problem. Either dirty
from cheap fuel or shorted solenoid coils due to ethanol damage.
Beyond about 50% throttle removing the vacuum line has no effect
because the engine vacuum is low enough to increase the fuel pressure
normally with the hose attached. The fuel pressure at idle is 43 psi.
At WOT it's 43 + 8 = 51 psi.

Best regards.

Marc

[XfireZ51]

Here's another explanation of why people may have done this early on. And it's for the same reason the GN guys would disconnect the BATT just before a 1/4 run. If you are running Closed Loop, the ECM uses the last BLM value before the system goes PE (power enrichment) for WOT. All fueling is based on the VE table, whether Open or Closed Loop. If the last BLM THE ECM saw before PE was above 128, the ECM adds fuel to the calibration beyond what would be calculated using the PE table. IOW, it richens it up more. Even tho you may have tuned WOT, THE ECM overrides that because the motor is running leaner than it should. A precaution against running lean at WOT. THE GN guys, by disconnecting the BATT, would reset the BLMs to 128 and so the ECM would not
change the WOT tune.
Pulling the hose on the VAFPR, richens the mixture in C/L bringing the BLMs down to 128 or below. ECM leaves things alone at least for a long enough time to get a run in.

[Harvie]

There was an old TSB on fuel pressure at WOT that dealt with this little problem at the assembly plant.

Seems that the lower clamp was a little difficult to get over the neck in the tight spaces so they just put it on the hose above the barb.

When both pumps were under full load the pump with no clamp would blow fuel right back into the fuel supply causing cavitation and reversion. The engine would stumble than pick right back up.

Guess what the repair fix for the problem was?





Fix:

Drink fewer than 8 beers during 10AM morning break and put the friggin clamp in the right place. Live better drink union....