I found this out purely by chance. I inadvertaintly bought a full dash sized fuel presure gauge rather then the small inch sized ones everyone normally used. I said to hell with it and stuck it in rather then return it. Looks stupid having this huge gauge sticking out of the fuel line, but its easy to read.

When I ran the car I found some interesting things out with the large, easy to see gauge. First, the fuel pressure seems to drop when teh throttle is opened. Varies from about .5-1.5 PSI if I rev it to 5K at WOT. I thought it might be the fuel return restricting the fuel flow, but excess pressure in the return shouldnt skew it as the regulator should compensate for this. At most, Id expect to see a downtick during sudden demand, not a prolonged drop in pressure. I ahve a TPI airbox, which is RESTRICTIVE. To me, I think the fuel pressure regulator is acting like a VAFPR when there is static loss through the induction system. After all, it sits inside the air cleaner and is open to external pressure through the vent port and adjustment screw. Thats enough to explain my .5 AFR leanout at high RPM that I had after the airbox swap.

What is really wierd is teh rapid 5 PSI fuel variation that I was seeing. When I revved it WAY up and snapped the throttle shut, the needle would rapidly oscilate between 25 and 30 PSI for about a second or so. I wonder whether is is due to standing waves or some other form of resonance, or some fault within the regulator itself that causes resonance in the fuel pressure?

Definatly have to test some more...

 

It's been over a year ago but Rbob reported some funky fuel pressure issues with the stock regulator. He logged it with a pressure transducer and traced it down to the plunger on the regulator dragging or hanging up on the seat. He swapped to an external reg. and hasn't had problems that I know of. I think this problem shows up more at higher FP since you have more force on the diaphram.

 

I was thinking of finally going to an external. At the very least, Ill probably have to seal the regulator and run a hose to the outside of the throttle body to get the external pressure reference for the regulator. The fuel pressure drop-off at higher RPMs is a kind of annoying issue.

 

I did a little more testing. Its definatly something else. If I block the inlet to the air intake, the fuel pressure will go down as expected, but the pressure needed is alot more then whats in there from static loss. To get a few PSI of drop, I pretty much have to block the whole inlet and it feels like its going to suck me in.

I would assume then that its something odd going on with the regulator. Im using a heavy duty spring in the regulator with high pressure. It could be that the spring coeff. is large enough that itll cause teh pressure to vary with teh ammount of fuel being returned.

 

I was looking at the aeromotive units, and damn theyre expensive... Itll probably cost like 200 dollars to convert it over.

 

It is expensive, but well worth it. When I converted the Impala to TBI I started with an Aeromotive 13301 reg. When you budget for this remember that you'll probably use more AN hose than you think, that always happens to me. $22 for ONE hose end??!? ug

I was running 1 line to the TB, but it was causing issues. RBob suggested I run the supply to the TB, then the "return" outlet to the regulator.

 

I was thinking of placing it on the return side as well so I can get away with using all the factory soft and hard lines with adaptors for the regulator. The only issue is how much pressure loss is there through the dinky return line. If its more then 1 PSI, Ill have the same issue now. The pressure at the TBI will drop under load.

 

Three things occur to me given the observations.

First, pressure oscillations could occur in a closed (fuel) plumbing system especially if it lacks a good damper. The factory FPR in that sense acts both as a pressure regulator and prehaps also as a damper.

Second, a dead-end side branch with a damper in it might be useful. Many people that own homes with forced hot water heating have these installed inline with the hot water pipe; they help resist "water hammer" noises, and those are the result of fluid pressure noise oscillations managing to excite a resonance mode.

Third is that the factory FPR has a finite transient response (associated with the time constant of the diaphragm) so it can't respond immediately to a change in fuel pressure (going to WOT). IOW there may be merit in ramping-up the commanded flow from injectors to be in step with the response of the FPR so that the transient drop in fuel pressure doesn't happen. IOW #2 the injectors can very quickly cause a drop in fuel line pressure, so quick that the FPR/fuelpump system can't keep up... so that would cause a transient lean which could cause some brief stumble. Acceleration enrichment is probably there to offset this, but if the fuel pressure drops too much, the AE isn't going to be able to keep up with the demand.

Since I'm not an tuning guy I don't know enough about what GM put in there to cope with the above, but it seems like it might be important for tuning very sudden changes in throttle angle such as during WOT. FWIW.

 

One thing Ive noticed is that its a sustained pressure drop under load. It lasts for at least a few seconds, or however long the engine took to rev to 5500 RPM at WOT. The oscillations are also weird. As soon as the throttle slammed shut, the needle on the gauge went crazy for a second, and then leveled out. Maybe its driven oscillations in the regulator during a step response in fueling like you said. The last odd thing is all the noise in the regulator/return. Its sort of like teh noise you get with an old water faucet when you have the hot water on. Sounds like vortex shedding or something similar. Not continuous, either. Comes and goes...

 

Regulator on the return side wont work... The pressure loss through the system with a completly open return port is 12 PSI!!!

I guess Ill have to figure out how to make the stock regulator work, or do a regulator with a returnless run to the TBI.

 

i converted to carb and used the universal bypass style aeromotive regulator $120 thru jegs... i just used the stock TBI pump and most of the stock lines... worked out great.

I dont see any pressure creep but i keep finding a fine layer of fuel on the reg.. sigh i keep trying to seal it too.

 

I have the walbo 255, which is part of the problem. I guess it takes alot of pressure to ram 70 GPH through all those tiny little ports in the TBI unit. The caveat is that anything less then the 255 will be outstripped by my injectors. During high duty cycle, only around 20-30 percent of the fuel is being returned, which seems to be part of the issue.

 

I tried teh spring along with the ported holley pod. Still having the same issues. Its definatly not from a lack of flow. At 12V, the pump managed to move 71 GPH of fuel at 28 PSI. Guess thats a lesson learned. The holley TBI and high flow pumps dont mix. The passages in the holley unit are too small. In the GM unit, the return is a large manifold rather then two little passages, so with that pod it probably wouldnt be an issue.

 

Just to be sure that we're on the same page, your using the Holley Injector Pod? I have had nothing but nightmares tuning the holley pods no matter new or old style injectors. I also have the 255lph walbro, and changing over to the 454 pod has helped me tremendously.

I am very interested in your ideas for a returnless system. I wouldn't be as concerned about the Holley Throttle Body, as the actual injector pods on the holleys. I love my Holley TB Base. :-)

Take Care,

- B

 

Well, I went crazy with the grinder in an attemp to fix this. I ruined my spare holley pod when I broke through the casting, but on the second attempt at totally hogging everything out, I think I got it. With the engine off and fuel pump on, I put a set of hose pliers and slowly cut the supply line off untill the pump dead headed and the regulator maintained 30 PSI throughout, so the injectors are now seeing the same pressure as the regulator. The pressure no longer drops when I rev the motor, but its still screwy when I shut the throttle after revving. Maybe it now needs a pulseation damper. I think some of the later TBI setups came with these.

 

I messed with this some more. I put a dense neoprine foam vibration damper inside the regulator within the spring to dampen alot of the crap out. It does work, but it also causes the pressure to be 1-2 PSI higher at idle because its not linear like a spring when its compressed. Pressures much more steady, but still fluctuates a little.

I also ordered a pulse damper for a late model BBC TBI truck. Anyone know anything about these? From the picture, it just looks like an old school metal master cylinder reservoir. Where where these located in the fuel system?

 

if its like all the other GM pulse dampers i have seen, it goes in the tank & replaces the hose.

 

What Denn said. It may be difficult to use that style damper outside of the tank. The fittings are a slip fit from the pump to the damper, and then from the damper to the exit pipe.

There is one that GM put on the engines, it was used on the '3.1L V6 Firebird & Camaro's. Has metal lines with a small metal can that is the damper. Not sure what the actual form factor is regarding pipe length, diameter, and fittings.

Some interesting trivia regarding the use of dampers. GM put them on vehicles that came with the high pressure fuel pumps. These pumps are a positive displacement roller vane style. And deliver fuel in pulses as the vanes pass over the outlet port.

The lower fuel pressure TBI units came with a turbine pump. Which because of the smooth operation did not require a damper.

RBob.

 

I sort of figured that it probably mounted at teh pump and Id have to get creative...

I do remember seeing that the stock pump was a twin turbine type when I took it apart. The walbro unit is a gerotor, so its probably somewhere in between noise wise.

Damping the actual diaphram motion does help alot. The needle is now confined to a small area rather then all over the gauge. It may have been banging the seat during sudden changes in fuel pressure, causing spikes. My last fuel pressure gauge actually broke from all the fluctuations. Tore the tube clear off the drive mechanism. Now at WOT, the fueling seems to be much more consistant. I dont know how long the rubber foam will last when continuously exposed to fuel vapors, though. I was thinking of maybe rigging something up to use silicone oil inside the regulator housing along with an orafice to dampen the motion of the regulator, but the math involved to design it would be damn complicated.

Ahhh... the joys of running high flow injectors at high fuel pressure.