I'm using the Holley 670 TBI w/ AFPR. The most pressure I was able to crank her to w/ the old 305 was 17psi. Now, I have a 383 going in that should be worth 400 ponies, and I'm afraid my 85pph injectors just won't be able to support that power at that pressure. Could I change the spring in the regulator, and in return generate a higher pressure? I seem to remember Jon Prevost doing something like this when he added vacuum to his. I'm looking at running it around 25-30 psi, which should give me enough fuel to support the engine (injectors are rated at 21psi). Fuel pump is stock, but will be a Walbro 255 after I get her running. Thank you, Roy

 

I'd change my pump first then determine if modifications to the regulator are needed.
Also, you might look into a vacuum referenced fuel pressure regulator to add that extra fuel pressure at wot so your injectors arent acting like 100 pounders at idle. This will aid idle tuning where big injectors can cause problems. As an example if you are running a cam that gives you 15" of vacuum, it would allow you to run about 7 psi less pressure at idle then with a standard pressure regulator.... yet still allowing the full pressure at WOT.

 

Very true. I know Jon made his holley regulator vacuum. I will have to do a search and see if I can find it. I do remember him getting some good pressure out of it as well, though I think he had to play around with the springs as well.

 

You may not get much more then 20psi out of the TBI fuel pump. Even if you do the flow rate will be severely less then required.

RBob.

 

I had to play around with the springs because I was using a stock fuel pump.
New fuel pump was able to use the stock spring BUT just the other day I noticed that my car was running lean. Turns out the spring has sagged AND the adjustment screw is backing out . You can play around with different springs and I recommend you do that after you get the walbro. I was just at carparts the other day and was looking at the help section for door hinge bushings when something caught my eye. it was a spring for a Honda exhaust clamp. A lot of transversly mounted motors have springs holding their exhaust slip joints together. Anyways, the spring would obviously need to hold up to heat cycles (hot and spashed with water) so it might be an obvious choice for an alternative. I've got a bunch of springs that I can test if you need one.
To make the Holley AFPR into a VAFPR just put a screw into the bottome hole and use the side hole for a press fit vacuum boss. I tried threading the hole but HD and Lowe's didn't have anything small enough so I ended up just pressing in a fitting.
Only trouble is that the stock spring sagged and the adjusting screw will walk. I'm going to have to figure something out and when I do I'll be sure to post.

 

Just a couple of ideas/thoughts: teflon tape on the adjusting screw, another screw that goes in and locks against adj screw, longer adjust screw w/lock nut, some PST on adj screw threads, an adj screw w/a nylon insert.

RBob.