Regulator Position After Carb - Why?

[CV355]

Can someone explain the methodology and logic behind putting a regulator AFTER the carb?  I just got in a Holley Super Sniper 1250 kit, and they are recommending putting a regulator after the carb.  Wouldn't that just allow the electric pump to supply full pressure to the fuel log and then only regulate the return line?  Pneumatics and hydraulics are my livelihood and this is making no sense to me... 

https://documents.holley.com/199r11488r.pdf
Page 12

I have an Aeromotive bypass regulator that I planned on putting in-line BEFORE the carb (or EFI carb in this case, whatever).  Would that arrangement still work?

[allrightmike]

Apparently the regulator can flow a greater volume of fuel than the pump can and therefor is able to maintain the correct pressure in the system.

[CV355]

Apparently the regulator can flow a greater volume of fuel than the pump can and therefor is able to maintain the correct pressure in the system.

But, a differential flow would only normalize at the max flow rate.  If the carb or EFI system isn't flowing enough fuel, wouldn't the pressure spike?

I know I'm looking at this from a perspective from a different industry, but I figure the same principles apply - it's not like physics changes just because it's automotive haha

[427stang]

It is to maintain inlet pressure at all engine loads. Sequential/Bank-Batch fire Injection do the same. The regulator is basically a relief valve to bleed off excess pressure so the injectors see a consistent fuel inlet pressure.

[CV355]

It is to maintain inlet pressure at all engine loads. Sequential/Bank-Batch fire Injection do the same. The regulator is basically a relief valve to bleed off excess pressure so the injectors see a consistent fuel inlet pressure.

That makes more sense- I was thinking of it in the reverse manner, where the post-regulator was trying to maintain or reduce pressure before the carb, which would never work with a traditional regulator. 

[SSdynosaur]

Maybe not completely relevant but I can describe what my fuel pressure data graph indicates on a Super Stock drag car launch where the diaphragm/pintel style regulator is mounted before the carbs (both physically and hydraulically). Sitting on the starting line, engine WOT but rev limited to 6K, the recorded fuel pressure is exactly 5.5psi (reflects static setting without engine running) and the pressure graph is nearly linear. However, the instant I drop the clutch the pressure plot indicates a high frequency oscillation (saw-tooth graph) between 8 and 4psi. As initial forces of gravity diminish from the launch the pressure oscillation-frequency drops continually with the plot indicating a near linear characteristic by around the ⅛ mile (half track) through the end of the run. There are a ton of forces and counter-forces interplaying here but, basically, what the plot indicates is the the limits of the regulator's ability to control the dynamic fuel pressure under this specific set of conditions. There is certainly an optimal regulator discharge orifice size (a compromise) that permits an acceptable pressure control range combined with acceptable volume delivery but mechanically, as it does it's job it introduces these wild pressure oscillations. Apparently, we can make sufficient modifications the the carb delivery system to make it all work. I'm definitely not a FI person but my best guess is that placing the regulator post-throttle body would smooth the observed saw-tooth oscillations effects on the throttle body.

[RJP]

I use a B/G bypass regulator on a twin turbo, intercooled draw thru system using a single 850 Holley C/S so that the volume of fuel is not restricted by the pintle/orface as it would be if the regulator were between the pump and the carb. It is plumbed in reverse from a dead head regulated system. IOW Fuel comes from the carb's fuel log into the "out" port of the regulator and the discharge [fuel return] uses the "in" port and is plumbed back to the tank. Only advise I can offer with this type of setup is make sure the fuel lines are big enough to handle the volume and pose no restrictions at all.

[CV355]

Maybe not completely relevant but I can describe what my fuel pressure data graph indicates on a Super Stock drag car launch where the diaphragm/pintel style regulator is mounted before the carbs (both physically and hydraulically). Sitting on the starting line, engine WOT but rev limited to 6K, the recorded fuel pressure is exactly 5.5psi (reflects static setting without engine running) and the pressure graph is nearly linear. However, the instant I drop the clutch the pressure plot indicates a high frequency oscillation (saw-tooth graph) between 8 and 4psi. As initial forces of gravity diminish from the launch the pressure oscillation-frequency drops continually with the plot indicating a near linear characteristic by around the ⅛ mile (half track) through the end of the run. There are a ton of forces and counter-forces interplaying here but, basically, what the plot indicates is the the limits of the regulator's ability to control the dynamic fuel pressure under this specific set of conditions. There is certainly an optimal regulator discharge orifice size (a compromise) that permits an acceptable pressure control range combined with acceptable volume delivery but mechanically, as it does it's job it introduces these wild pressure oscillations. Apparently, we can make sufficient modifications the the carb delivery system to make it all work. I'm definitely not a FI person but my best guess is that placing the regulator post-throttle body would smooth the observed saw-tooth oscillations effects on the throttle body.

This is excellent info.  Concise, technical, I love it. 

I think I understand it now...  So you are using it as a flow-control instead of a true regulator.  I need to run this by my pneumatics guru- this will blow his mind for sure. 

[BattlestarGalactic]

My .02 from the little bit of FI I have dealt with.  Remember that Sniper only "looks" like a carb, but functions as FI.

FI can NOT have any fluctuation in pressure or the engine will lean/richen as it moves around.  It must have a very large, unrestricted amount of fuel at the injectors, unlike a carb that just draws fuel using vacuum from the fuel bowls.  A carb fuel system can see some pressure swing and not stumble like a FI system(as long as the bowls are full).

The injectors in that "carb" body need to see a very specific pressure ALL THE TIME.  That is why the pump in the tank is a high pressure unit.

[My427stang]

Almost every EFI system I put together regulates after injectors. 

The pressure the injectors see, whether it is rail or TB, will never be higher than the regulator controls

Think of the regulator of a dam with spillway, having the fuel held back for the injectors at a certain pressure, anything above that set pressure goes back to the tank

[cammerfe]

Almost every EFI system I put together regulates after injectors. 

The pressure the injectors see, whether it is rail or TB, will never be higher than the regulator controls

Think of the regulator of a dam with spillway, having the fuel held back for the injectors at a certain pressure, anything above that set pressure goes back to the tank

That's very well said.

KS

[Falcon67]

Putting the reg after the carb allows the pump to use it's full volume capacity to feed the carb(s).  Technically, you could be reading 1 lb of fuel pressure but the pump would be feeding max capacity and the engine consuming nearly all of it.  In practice, sure you don't want to get to that level of draw.  Same as noted with FI - the mechanical systems use a pill to pass unneeded fuel back to the tank, same with a return regulator.  SIde benefit is that the pump runs cooler because it has fuel flowing through it rather than banging against a stop (dead head regulator).  Also reduces surging in the carb feed and foaming in the fuel bowls. 

[CV355]

All good info in this thread.  I appreciate it, guys!

[DubyaTF]

   Diesels are most commonly setup with regulator on the return. This provides the maximum amount of fuel for lubrication, cooling and burn. It should also cut down on the amount of introduced bubbles into the flow for a more steady a/f mixture. Our Army's combat equipment uses a small clacker style pump (sometimes dual pumps) to feed the high pressure pump which feeds the injectors that, in a Cat 3126 or bigger, fires at 3500psi. Where you measure fuel pressure is just as important as where your regulator is I would say. Keeping tabs on inlet AND outlet pressure may be a good idea.

[BigBlueIron]

To add, most older mechanical diesel injection pumps use a fixed orifice on the fuel return to control internal pump pressure. The pump body itself acts as a fuel rail.