Boost Referenced fuel pressure

[Taylorman]

How is this done? Just a question out of curiosity? I just bought a fuel pressure regulator that has a port to reference boost. How do you do it and what does it do?
Kevin

[HP350SC]

On a bypass style regulator, you run a line to a manifold port under the blower. The vacuum or pressure in the intake manifold will act on the regulator diaphragm. You set your base bypass pressure, then under boost the pressure will rise 1/1 and keep carb(s) from starving. Vacuum at idle will pull it down some, but it doesn't matter.

[Unchained]

HP350SC,
I dissagree, there would be no reason to reference the boost with a bypass type fuel pressure regulator on a draw through carbureted blower setup. It would likely force the floats open and cause flooding.
The boost reference port would only be for an EFI setup where the injectors see boost pressure or with one of the pressurized carb box setups like a procharger.
It is to compensate for boost pressure that would offset the fuel pressure.

[Beer-30]

HP350SC,
I dissagree, there would be no reason to reference the boost with a bypass type fuel pressure regulator on a draw through carbureted blower setup. It would likely force the floats open and cause flooding.
The boost reference port would only be for an EFI setup where the injectors see boost pressure or with one of the pressurized carb box setups like a procharger.
It is to compensate for boost pressure that would offset the fuel pressure.
I agree with you dissagree-ing (I think I got that right). My Grand National regal had boost-reference for the EFI. A carbed, blown motor would just need CONSTANT pressure and never take a dip. The carb(s) would be boost-referenced to provide smooth, constant fuel flow - per lb. of boost.

[Infomaniac]

I have never found the need to boost ref carb fuel systems. A well engineered system will supply plenty. Pressure does not always equal volume.
Edit. . . . The exception being carb in a box blow thru systems.

[HP350SC]

For discussion purposes, my fuel supply from tanks to carb. is more than adequate. I have posted pics. in gear heads, but it is all -10 lines with 5/8 tank pickups, adequate venting etc. My single 1095 Demon will not keep up on a long WOT pass. This has been corrected by raising fuel pressure at WOT. I DON'T think it would be necessary if there were TWO carbs however. All things being equal, more pressure will mean more flow, up to the point of forcing fuel past needle and seat. This would affect a/f ratio, so not good.
Which brings me to a question. I have heard general consensus is around 11-12 lbs., but has anybody personally tested a carb. to see at what pressure this occurs? I am going to do this, so I know once and for all what my carb does.
Leaving for Shasta tom. morn. but keep up discussion! :D :coffeycup

[hulshot]

Mike I have a very similar bypass regulator and run 2 1150's. at 8-9 lbs fuel pressure i will run them dry and the motor will lay over. I am running 9-10lbs boost and the fuel pressure only raises about 1 lb, not much. I bumped up the presure to about 15-16lbs and it does not blow the seats at idle and seems to do just fine. When I run it wide open for a long run it will fluctuate slightly between 15 and 18lbs, and will run great with no problems and does not run out of fuel. I switched the pullies one time and made more blower pressure but when the boost came up the blower just screamed and went no where. I did this a few times until I realized what wass happening. When the boost hit 13psi the referrence regulator would go 1-1 and the fel pressure would spike and flood the engine. The fix was to simply remove the vaccum hose and run it again. It ran great after the line was removed. With the boost you are running I dont know if the regulator is in fact going 1-1 or if it is slightly raising. Maybe try to raise the pressure and disconnect the reference line.
How do you know you are running the carb dry??? : :confused:

[Infomaniac]

If the carb runs dry then the system is too restrictive somewhere. More pressure after the restriction should not fix the problem. No worries about overpowering the needle and seat during hard running. They are already open and flowing fuel.

[HP350SC]

Mike I have a very similar bypass regulator and run 2 1150's. at 8-9 lbs fuel pressure i will run them dry and the motor will lay over. I am running 9-10lbs boost and the fuel pressure only raises about 1 lb, not much. I bumped up the presure to about 15-16lbs and it does not blow the seats at idle and seems to do just fine. When I run it wide open for a long run it will fluctuate slightly between 15 and 18lbs, and will run great with no problems and does not run out of fuel. I switched the pullies one time and made more blower pressure but when the boost came up the blower just screamed and went no where. I did this a few times until I realized what wass happening. When the boost hit 13psi the referrence regulator would go 1-1 and the fel pressure would spike and flood the engine. The fix was to simply remove the vaccum hose and run it again. It ran great after the line was removed. With the boost you are running I dont know if the regulator is in fact going 1-1 or if it is slightly raising. Maybe try to raise the pressure and disconnect the reference line.
How do you know you are running the carb dry??? : :confused:
Don, I have tried running 11 lbs. with no boost reference. Gauge is rock solid (unless tanks are low)and I know there is adequate volume. What I mean by 1/1 is the fuel pressure raises 1 lb. per 1 lb. boost, that is how the regulators are designed. Since I am running 9 lbs. boost I subtracted where I wanted to be at WOT and set base pressure there(4 lbs.)
There seems to be some confusion from some of the statements unless I am reading it wrong. If you are not overpowering needle and seat, you are not changing a/f ratio through the jets. You are filling bowls faster though. My motor is pretty mild at 750hp but it is tough to feed it with a single carb. Would be similar to 1500hp with only dual carbs.

[HP350SC]

If the carb runs dry then the system is too restrictive somewhere. More pressure after the restriction should not fix the problem. No worries about overpowering the needle and seat during hard running. They are already open and flowing fuel.
If the needles and seats are the restriction, then they would allow more fuel through by raising the pressure to them right? That is the point I am trying to make. Unless flow was so great at WOT that bowls were filling faster than motor was using AND you had enough pressure to push past needle and seat(not too likely).

[Infomaniac]

Yea if the needles and seats are the restriction then more pressure would flow more. That would be more pressure before the restriction.
But I cannot see that happening with the big assortment of needles and seats available.
And excess pressure not allowing the float to close the needle and seat would raise the fuel level in the bowl and richen up the mixture. At idle is where overpowering the needle and seat causes the greatest problems.

[kojac]

Quote "Mike I have a very similar bypass regulator and run 2 1150's. at 8-9 lbs fuel pressure i will run them dry and the motor will lay over. I am running 9-10lbs boost and the fuel pressure only raises about 1 lb, not much. I bumped up the presure to about 15-16lbs and it does not blow the seats at idle and seems to do just fine. When I run it wide open for a long run it will fluctuate slightly between 15 and 18lbs, and will run great with no problems and does not run out of fuel. I switched the pullies one time and made more blower pressure but when the boost came up the blower just screamed and went no where. I did this a few times until I realized what wass happening. When the boost hit 13psi the referrence regulator would go 1-1 and the fel pressure would spike and flood the engine. The fix was to simply remove the vaccum hose and run it again. It ran great after the line was removed. With the boost you are running I dont know if the regulator is in fact going 1-1 or if it is slightly raising. Maybe try to raise the pressure and disconnect the reference line."
I have heard or read (don't remember where?) that gas starts to foam after about 11 psi? Does anybody know at what pressure it does foam?
It would seem to me that if it is foaming at 11#'s or so that the bowls would not be filling adequately with gas.
I have two 1150 dominators on a 572cu incher with a bds blower and worry that my carbs are running out of gas on a 1/4 mile run. Am considering referenceing my regulator to get more gas pressure (currently 7and 1/2#'s)
Also I am thinking of increasing my boost from 6#'s to 11#'s for when the bad boys come around. Is it necessary to increase the jetting to accommodate more boost?
Kojac

[superdave013]

I don't think that's right. EFI stuff runs in the 35 - 45 psi range. When my hat injector set up was on the flow bench over at Gorr's it was up around 300 psi @ 8,000 rpm

[Maxey]

How is this done? Just a question out of curiosity? I just bought a fuel pressure regulator that has a port to reference boost. How do you do it and what does it do?
Kevin
Kevin, I run a 3300 Whipple on a 502 in my older Sanger Mini Day Cruiser. I use a Malloery 140 GPH pump with the Aeromotive bypass regulator. I set the fuel pressure to ~7-8 psi with the vacuum line on the regulator @ atmospheric. I use -8 lines on everything. If you talk to Aeromotive, they will tell you that you do not use the vacuum sensing line unless you are "blowing through" your carb. I agree with "Unchained". :D
Dave

[steelcomp]

Unchained is right on the money. Carbs before the blower (draw through) don't know there's a blower down there. They simply act on the air passing through them.
Increasing pressure through the needle and seat isn't the way to go about curing a flow (volume) problem. Go to a bigger needle and seat. Holley makes needle/seat assy's with seat dia.'s from .097" all the way up to .150". If that dosen't solve your problem, then I'd say it's not in your carb(s), but in your fuel system. Hose sizes are only part of the fuel system. You need to make sure you've optimized your fittings as well. There's a lot of adapter AN fitings who's thru hole is the small dia rather than the bigger, which means adapting from, for example, a 1/4" pipe to a -8 line when the fitting only has a 3/8 hole (or less) through it, is a waste of time, AFA the -8 line is concerned.
Boost referenced regulators are designed for EFI systems where the injectors are subject to the boosted intake pressure. The purpose is to overcome the increased pressure (under boost conditions) inside the intake manifold trying to push back through the injector. As the injector opens, the fuel is designed to pass through the injector at a given pressure for proper atomization. By increasing the pressure in the intake runner, you're basically decreasing the effective presssure through the injector, reducing the effectiveness of the atomization. To overcome this, fuel pressure is increased behind the injector to balance and offest whatever increase there is in the intake runner, effectively giving the fuel the same positive pressure through the injector. A draw through system is never exposed to any pressure changes, no matter the boost...only volume requirement changes. :coffeycup

[HP350SC]

OK then, my definition is one that applied to what worked for me. How about a generic definition like "fuel pressure that is increased according to manifold pressure".Taylorman has a carbed boat in his signature, so I assumed he was asking about that. I went to .150 alky needle and seats and boost referenced regulator. Zero fuel starvation, but noticed 10.5 lbs. boost :idea: maybe too much fuel. Went back to standard .130 n/seats with the boost ref. and works absolutely perfectly. Fuel supply follows engine demand, and there are no ill effects doing it this way so :p :D