Fuel Pressure regulator location and direction of flow through the rails

[CDMBill]

As part of my ongoing campaign to prove I know nothing, two questions have occurred to me recently following my blown head gasket and lean cylinders issue (different sides of the engine BTW)

1) Does it matter whether the fuel enters the fuel rails from the front of the engine or the back in a dynamic situation, think g-forces at launch or under heavy braking? At launch seems more important as you aren't at WOT under braking, but you may be about to be in transition to full throttle from full braking, and.

2) I've always mounted fuel pressure regulators on the fender well, originally near the carb inlets and now where the output from the fuel rails meets the return line about centered in the engine front to back. I've seen very clean setups with the regulator mounted between the fuel rails on the engine and I wonder if vibration or heat could affect the regulator's function or efficiency.

I run a #10 supply line from an Aeromotive A1000M to a y-block with #8 to the fuel rails at the back of the engine and then on to an Aeromtive four port EFI regulator to the #10 return line.

Given this all operates at 43 PSI nominal it shouldn't matter about any of these questions but since I'm rebuilding at the moment it seemed a good time to ask.

[TheSilverBuick]

Excellent questions CDMBill, I look forward to the answers.

[min301]

Fuel could enter from any angle or direction in the engine bay,
it's under pretty good pressure.

With a carb you have float bowls that slosh, EFI doesn't work like that.
Just keep it away from heat.

[dieselgeek]

Good questions! I wish I had better experience to give an answer... but I don't...

Curious what a guy like 427LS or dynoroom would have to offer here.

[Matt Cramer]

I haven't tested the effects of vibration on fuel pressure regulators, but I can answer on the G loads affecting fuel pressure. The pressure changes under acceleration would be the same regardless of inlet location. Luckily it's not very much. Gasoline weighs 0.026 pounds per cubic inch, so under 1 g acceleration, you'd see a change of 0.312 psi per foot along the car's length. A fairly small change, although on a returnless system that puts the regulator in the tank it could add up to something significant.

[horsewidower]

Assuming your .312 per foot, with my almost 24 (?) feet of supply and return line I would see a pressure reduction of 7.488 psi. I'd call that substantial.

Am I using your number correctly?? Would I just use the 12 foot supply line, because the return is irrelevant and essentially free flowing past the FPR? It'd still be 3.744 psi. Wouldn't the FPR compensate?

Its an interesting topic.

Bob

[min301]

I would think under a load, it should up the pressure.

[dieselgeek]

I would think under a load, it should up the pressure.



that's what vacuum referenced regulators do.

[CDMBill]

I'm familiar with the idea of boost referenced FPR's, my Aeromtive unit is set up for that, but I haven't connected it to manifold vacuum or any such thing. Is this something I need to be thinking about?

What I really need is the ability to datalog fuel pressure (hint, hint Gee). I have figured that if the AFR log is stable then the fuel pressure issue must be moot. Again my recent experience with a failed channel in the AFR meter left me exposed to what later actually happened without the ability to track it down effectively. If the change in the mass of the fuel column moves by 3-4 PSI coupled with the change in the internal dynamic of the fuel rail itself makes me wonder if I could have transient lean cylinders. That is why I was thinking swapping my setup such that the fuel enters the rails at the front and would 'stack-up' against the rear mounted FPR.

It may not matter, but I'll feel better.

[horsewidower]

My AFPR is post fuel rail. You can't have it pre fuel rail or it won't keep pressure in the rail. Whether it is physically in front of the rail (ie at the bumper side) or behind the fuel rail (ie on the firewall side) should be irrelevant.

Or is it, if we assume that Matt's number is correct, what we really are talking about is the relative difference in pressure from the front of the rail to the back of the rail. 3 feet at most. So using his number, you'd see about a 1psi difference. Right??

I still can't help believing that a properly designed FPR will compensate.

Bob

[Matt Cramer]

Assuming your .312 per foot, with my almost 24 (?) feet of supply and return line I would see a pressure reduction of 7.488 psi. I'd call that substantial.

Am I using your number correctly?? Would I just use the 12 foot supply line, because the return is irrelevant and essentially free flowing past the FPR? It'd still be 3.744 psi. Wouldn't the FPR compensate?

Its an interesting topic.

Bob


That isn't the case all through the lines, though - it is just in one direction, front to back, and is sort of like height. So if you have a regulator mounted on the rear of the rail and have 12" between your forwardmost fuel injector and your rearwardmost, you'll only have a 0.312 psi pressure difference no matter how far back the tank was. It only comes into play if there's a large, forward distance between the injectors and the regulator.

Leaving the vacuum line off the FPR isn't terribly bad on a naturally aspirated engine; it will affect how your fuel table looks like and will cause you to need less pulse width at idle (which could potentially make for a rougher idle). But if you have a steady AFR it's not much to worry about.

[horsewidower]

Thanks Matt.

I think I was slowly coming to that conclusion. Operative word being "slowly." ;)

[Matt Cramer]

Fluid dynamics ain't easy, but someone's got to do it.

[JeffMcKC]

Bill, I made some huge rails for what I needed, thinking as fuel passed by the injector hole it might pull fuel out like a pan vac, or solvent sprayer, if it was to small.

I think it does not matter what direction the fuel comes from or angle into the rail.

I would have my injectors flowed, or I have some low use 93's ;) from PGD's

[CDMBill]

The generic Aeromotive fuel rails I started with are supposed to support up to 2000 HP with their .5" ID. The injectors are a set of RCI matched flow 96's which were sized to allow me to run dry nitrous up to 300 on top of the 800-900 N/A window, which is why I don't quite get to 60% duty cycle N/A. I had the same concerns you did on the fuel rails so I went big.

I'm getting the injectors cleaned checked while the short block rebuild is finished up just to be sure, but I think my minor cam change required more fuel than I expected and combined with high alcohol content Winter pump gas and good air it was too lean.

That said I'm changing my fuel routing anyway and going to a crank trigger with the Pantera EFI coils. I also pumped the compression to 11:1 from 10:5 so I'll be trying to get everything I can from it N/A. I'm going to baseline it with the existing cam and then change to a Mike Jones grind so we'll see if I can get to 10 flat on motor in the barge.

If that works then its time for the Fox body something that weighs 1000 lbs. less., maybe then I'll be as fast as your SBC Malibu.

[horsewidower]

Adding lightness is always good.

I did my last buildup without thinking about using E85. I'm currently at 9:1 and I wish I was up at the 11:1 CR. I like your strategy.

Bob

[CDMBill]

My build is still a pump gas deal. No E85 in this part of the world. I was able to drive around on 87 octane in a pinch before with 10.5:1 compression. The combustion chamber is pretty efficient and with aluminum heads and with 268/276 @ .050" and 112 Lobe sep I wasn't really squeezing it all that hard. It would hit 175-178 cranking compression.

Now I won't run less than 91 pump premium and I have a new cam that is 269/277 with a 109 lobe sep that should help out a little bit.

Brian Rock's 428 Pontiac had 11.6:1 comnpression and had no trouble on Drag Week with pump 93 all over the Midwest and even in Cali with our 91 "premium".