[Shiny]

Humor my rambling...

Seems there are two scenarios being discussed: 1. priming a carb and 2. supplementing a mechanical pump. To me, priming is of interest but needs little flow so is easier. Goal 2., which OP seems to want, sounds difficult.

As to priming through a mechanical pump, I think they just have simple flapper-type valves on inlet and outlet so should be easy to get some flow if electric pump generates the same low pressure. The check valves in the pump operate just fine with the low pressure the mechanical pump diaphragm generates so should also work with an upstream low-pressure electric.

For priming only, my guess is the impulse pumps will be less likely to overpressure the mechanical diaphragm and offer less restriction to the mechanical pump than a rotary-vane pump.

Pushing high volume through a mechanical pump sounds difficult, if not impossible, without overpressuring the diaphragm in the mechanical. Those check valves are probably what limits the flow. Pushing harder on them will certainly flow more but the higher pressure needed would add risk and complexity.

I like the idea of an empty mechanical pump body with an electric pump doing the real work.

[mgarblik]

I used an AC Delco EP12S inline electric pump. Quiet and reliable. Small and easy to mount. They don't lift fuel very well, so mounting it low on the frame is a good idea. Mechanical pump has no problem pulling fuel through it. I like the overall safety and reliability of a mechanical fuel pump. But for priming an engine that sits for long periods or has multiple float bowls, priming with a small electric pump makes starting much easier. Electric pump also helps in hot weather vapor lock situations by keeping the fuel line under a little pressure raising the boiling point.

[61-63]

I've posted this on here in the past because this topic has come up several times over the years, but here is the pusher (priming) pump setup in my '63 Catalina. The pump is grounded on the frame and the wire going up into the floor goes to the toggle switch under the dash.

[78w72]

Sounds like many have done this & the electric pumps do indeed push through the mechanical pump when not cranking or when running... & vice versa that a mechanical will pull through the right type of electric pump.

Ive read about this for years on here and seems to work fine for those that do it, for starting after sitting awhile as well as for added fuel supply when needed.... its not ideal for added fuel supply but it works when set up right.

[Tim Corcoran]

Went with a Carter 4603 electric fuel pump and a By-Pass Circuit some years later.
Very dependable. E-pump was off unless priming carbs or "acceleration runs on the street". Fuel went thru the by-pass to the mechanical fuel pump.

Tom V.

Tom

Would you send a diagram of the by pass circuit? Thanks

[Tom Vaught]

Tim, here you go.

Picture is worth 1000 words

Tom V.

[Shiny]

OK, help me here.

Is the benefit of this bypass config just to avoid a flow restriction when running only the mechanical pump?

If, OTOH, the electric pump is used to add more flow at the carb, it can only do this by adding more pressure, right? So does this mean you MUST also have a regulator to stay carb-friendly?

I measured the inlet pressure that overwhelmed my needle and seat, and it was 12 psi sitting still. I guess if a mechanical pump typically delivers 5-7 psi, there is some margin to allow maybe 4 or 5 psi "boost" from the electrical pump. If flow through the mechanical pump is directly proportional to pressure, then the electric pump could almost double the flow ?? So does that electric pump have a low-pressure output or do you need a regulator somewhere to stay carb-friendly?

Mike

[rolling money pits]

I’m curious about this as well, ever since I heard about fuel evaporating from the bowl.

I’m thinking of the primer need, not a booster when getting on it, and something that works like modern FI cars. To that point, is there a “momentary” relay, or similar animal? A gizmo that is triggered when ignition is in the run position, but will only stay on (closed) for a short period of time?

I searched the web, but I’m not too bright when it comes to detailed electrical jargon. Does this site cover something that fits?

https://www.macromatic.com/support/k...elay-functions

[Jay S]

I don’t think the “momentary” fuel switch will work very well. It takes to long for the pump to build pressure and purge the air out. It will get behind. Even my Walbro in tank carb set up takes a considerable amount of time time when it is first turned on to fill the carb. Longer anyway than a EFI set up, the lower pressure 7 psi doesn’t force they air out nearly as fast as 40 plus psi EFI set up.

Those diaphram pumps like the ACDelco and Mr Gasket are usually rated at 30 gph at operating pressure. I don’t know what the pressure and volume performance curves look like on those. My feeling is the mechanical pumps is doing most all the full throttle work if those pumps are feeding a mechanical pump. But I could be wrong, since I have never seen the performance chart. By itself 30 gph would only be good for 300 HP.

The Carter M6122 mechanical pump is rate for 40 GPH at pressure. Free flow is 120 gph.

The free flow of the Carter vane pumps is not published, but rated at 50 gph at pressure. My guess is it is about 100 gph free flow.

The Holley street pumps have higher ratings, but are rated differently than Carter. Not all that accurate in my opinion. The flow drops a lot with head pressure at the outlet, I think they run closer to 70 gph at a normal carb operating pressure. Just a guess though. Better than the Carter, not as good a 140 Mallory.

The idea of the pumps in series, both pumps do some of the work. The electric pump becomes a charge pump for the mechanical pump. Each pump has less losses. They operate at lower pressure on their performance curves. Not bullet proof for drag racing, but an improvement over a single pump that does not pull from a sump or a in-tank setup.

[pfilean]

Jay,
the "momentary" switch is only a toggle switch that is spring loaded to return to its original position when pressure is removed from the switch. As long as you hold the switch it powers one circuit and when you release it it returns to its original position to power the other circuit. Completely independent of the ignition switch.

[Shiny]

Originally Posted by Jay S

I don’t think the “momentary” fuel switch will work very well. It takes to long for the pump to build pressure and purge the air out. It will get behind. Even my Walbro in tank carb set up takes a considerable amount of time time when it is first turned on to fill the carb. Longer anyway than a EFI set up, the lower pressure 7 psi doesn’t force they air out nearly as fast as 40 plus psi EFI set up.

Those diaphram pumps like the ACDelco and Mr Gasket are usually rated at 30 gph at operating pressure. I don’t know what the pressure and volume performance curves look like on those. My feeling is the mechanical pumps is doing most all the full throttle work if those pumps are feeding a mechanical pump. But I could be wrong, since I have never seen the performance chart. By itself 30 gph would only be good for 300 HP.

The Carter M6122 mechanical pump is rate for 40 GPH at pressure. Free flow is 120 gph.

The free flow of the Carter vane pumps is not published, but rated at 50 gph at pressure. My guess is it is about 100 gph free flow.

The Holley street pumps have higher ratings, but are rated differently than Carter. Not all that accurate in my opinion. The flow drops a lot with head pressure at the outlet, I think they run closer to 70 gph at a normal carb operating pressure. Just a guess though. Better than the Carter, not as good a 140 Mallory.

The idea of the pumps in series, both pumps do some of the work. The electric pump becomes a charge pump for the mechanical pump. Each pump has less losses. They operate at lower pressure on their performance curves. Not bullet proof for drag racing, but an improvement over a single pump that does not pull from a sump or a in-tank setup.

All makes sense but still curious about the pressure at carb inlet if both pumps are operating. I guess I'm asking what the pump curves are by themselves and in series. To me, if an electric pump puts out only 7 psi, it will still add to the pressure built by the mechanical pump and end up maybe too high for a carb needle and seat.

"both pumps doing some of the work" makes sense to me from a flow perspective but without flow, wouldn't the pressure be roughly additive from tank (0 or atmosphere) to carb inlet?

Mike

[Jay S]

Originally Posted by pfilean

Jay,
the "momentary" switch is only a toggle switch that is spring loaded to return to its original position when pressure is removed from the switch. As long as you hold the switch it powers one circuit and when you release it it returns to its original position to power the other circuit. Completely independent of the ignition switch.

So your holding the switch to keep the pump on? I thought the idea was to control the switch by the throttle position.

I will add a couple corrections I found, I was off on what I though the Holley electric pump free flow is. 110 was for the 14 psi blue race pump. 97 gph is for the red street pump, max pressure 7 psi. I found the free flow number for the Carter electric vane pump also, it is rated a 100 gph and max pressure is 8 psi. So they are pretty much the same. The Carter is rated for 50 gph at operating pressure, so I would guess the Holley would run close to that.

The stock replacement mechanical pumps appear to run 30 gph, and 40 gph for a high performance. The free flow on the mechanical high performance pumps goes up from 40 to 120 gph, the mechanical pumps seem to respond more to free flow than the electric pumps. As long as the free flow is higher on the pump that is getting fed by the pusher pump there should be some decent gains running a pumps in series. If the free flow of the mechanical is less than the free flow of the electric I think the mechanical pump becomes a bottle neck.

[glhs#116]

Originally Posted by Shiny

OK, help me here.

Is the benefit of this bypass config just to avoid a flow restriction when running only the mechanical pump?

If, OTOH, the electric pump is used to add more flow at the carb, it can only do this by adding more pressure, right? So does this mean you MUST also have a regulator to stay carb-friendly?

I measured the inlet pressure that overwhelmed my needle and seat, and it was 12 psi sitting still. I guess if a mechanical pump typically delivers 5-7 psi, there is some margin to allow maybe 4 or 5 psi "boost" from the electrical pump. If flow through the mechanical pump is directly proportional to pressure, then the electric pump could almost double the flow ?? So does that electric pump have a low-pressure output or do you need a regulator somewhere to stay carb-friendly?

Mike

I think if you had some weird sort of parallel setup between the two pumps you might have the situation you are talking about of the pressures adding. Both would be pushing at once at the carb.

But this isn't that. All of these setups are series. So the mechanical pump has nothing between it and the carb whether or not there is an electric pump behind it. The mechanical pump in normal operation sucks from the tank. In other words it causes a vacuum behind itself (well, a pressure drop, technically) strong enough to pull fuel forward to feed itself. On the other side it produces pressure to the carb that it regulates to whatever pressure. If you have a pusher (electric) pump somewhere between the tank and the mechanical what it hopefully does is take away some of the effort of pulling the fuel from the tank and make it easier for the mechanical to draw. This is why Jay was talking about the "free flow" rates of the pumps. In most cases the mechanical pump could pump more fuel to the carb at the same pressure if it didn't have to work so hard to draw it from the tank. And also, in most cases the electric pump could flow more fuel if instead of pushing against a carb it was having the suction side of the mechanical sucking on its outlet. In other words, in series both pumps "help" each other and both should be able to flow more together than each could alone. But unless the electric really overwhelms the mechanical it shouldn't change the pressure the mechanical puts out on its outlet side against the carb.

I think the way to think about the bypass and check valve is that it does two important things.

First, if the electric isn't running then the fuel has two paths to get to the mechanical from the tank. It can pass through the check valve and it can pass through the pump. Each of these is probably a little more restrictive than plain hose but having both open probably means there's no more restriction than the mechanical had before.

Second, when the electric is turned on and pressurises its outlet side, the check valve prevents that pressure just sending fuel in a loop back to the tank and makes sure it gets upstream where it will help.

Sam

[Jay S]

Originally Posted by Shiny

All makes sense but still curious about the pressure at carb inlet if both pumps are operating. I guess I'm asking what the pump curves are by themselves and in series. To me, if an electric pump puts out only 7 psi, it will still add to the pressure built by the mechanical pump and end up maybe too high for a carb needle and seat.

"both pumps doing some of the work" makes sense to me from a flow perspective but without flow, wouldn't the pressure be roughly additive from tank (0 or atmosphere) to carb inlet?

Mike

If the fuel demand is low, there is a resistant to flow, the system pressure increases, and the bypass is operating on the electric pump, the electric pump is carrying the load. The first bypass from the pump is what sets the system pressure until the second pump says too do otherwise. The mechanical pump can raise the pressure if the bypass on it is set higher than the electric pump, it can lower the pressure also but it really needs a fuel return on the mechanical pump to lower the pressure or it really will struggle to control the pressure. It will do funny stuff with a dead head type pressure bypass.

At idle the electric pump already has the pressure metered for demand at the bypass pressure, the mechanical pump’s bypass really doesn’t need to do much. The mechanical pump can gain a couple lbs of pressure though with the suction pressurized. But usually not much, the relief valves are independent of each other. I think the pressure increase is becuase the mechanical pump is operating a higher efficiency with the suction charged.

Once demand goes up, and the fuel system is operating below the bypass pressure, then the two pumps split the load. Then the pumps are operating at lower pressures than they would be by themselves, and operate at a different places on their performance curves where they are more efficient, enabling them to flow more working together.

Edit. I just read Sam’s comment, and that probably explain it better than what I posted here.

[Cliff R]

Engine power and vehicle performance will determine what you can get away with here.

I had no issues whatsoever with fuel delivery until I started taking steps to make my Ventura faster. Without writing dozens of pages here I'll list below the different fuel systems I had in place in attempts to keep the carb full at higher power levels and faster runs at the track.

Started off with a stone stock fuel system using a factory mechanical pump and 3/8" lines, this was good to high 13's around 100-102mph.

I "upgraded" to a Carter 120gph with no other changes when I installed my first 428 engine around 350hp.

Added a Mallory 7psi 110gph electric pump right in front of the tank when I installed DOT's and ran into issues at the track.

Installed a 455 to replace the 428 and ran into issues so bypassed the mechanical pump and ran the Mallory 110 7psi pump straight to the carb.

Next when that didn't cure all the issues I installed a 1/2" fuel line in the tank to feed the Mallory 110gph pump and 1/2" lines all the way to the carb. Very minor improvement but still ran into issues on hard runs in the 12.30-12.60 range.

Upgraded to the Mallory 140gph pump and added a Holley regulator, no improvement. The car was running in the mid-12's with a best ever 12.34 @ 109mph.

Installed a Holley 4781-2 850 DP carb because EVERYONE told me the fuel bowl was "too small" in the Q-jet. No improvement whatsoever.

Ripped out the entire fuel system and tank. Then welded a CE sump on the tank, moved the Mallory 140gph behind the tank, high flow filter just before it, Holley regulator right before the carb, and 8an lines/fittings everyplace. This IMMEDIATELY cured any and all fuel delivery issues and the car picked up to 12.0's at 112mph with either the Q-jet of the Holley with NO OTHER CHANGES.

Even so the pump too a chit after 2 weeks on the street being "dead-head". It also made a LOT of noise even though it was a gear type pump and it got pretty hot as well, so hot you couldn't touch it after 10-15 minutes of street driving.

I added a bypass back to the tank and replaced the burned up pump with another one just like it. This all happened between 1999 and 2003. Went from 2003 till 2021 without any issues anyplace, pump was nice and quiet, and I won dozens of races including 5 wins at Norwalk in August and 2 trips to the King of the Hill race at that event. I'd add here that the pump was dead quiet and didn't heat up at all in use.

In 2021 the pump started making noise so I removed it and installed the first pump that crapped out, it only needed new brushes put in it and cleaning up the motor on the lathe. I rebuild the second burned up pump with new brushes for a spare.

Moral of my story, NOTHING worked until I sumped the tank and installed a rear mounted pick-up and moved the electric pump behind the tank. That set-up was FLAWLESS for nearly 20 years without the first hint of fuel delivery anyplace. I even installed another 455 that took the performance of the car DEEP into the 11's over 120mph and low 7's at 95-97 mph in the 1/8th mile.

Lessons learned here were that no matter how much pump or pumps you put in FRONT of the tank, even sucking thru larger fuel lines stuck into the tank, the front sump location on a stock type tank leaves a lot to be desired on a really fast car. I was fine until the car started running into the 1.6 second range in the 60'. Once I started leaving that hard NOTHING was going to work until I either installed a sump or fuel cell in the truck to feed a rear mounted electric pump and "push" the fuel vs sucking on it from a high mounted pump outside and in front of the tank.

I don't have an opinion here, only the results of DIRECT testing during the evolution of my car from 300hp to over 550hp with changes made to effectively use that kind of power with good traction. I'd also add that the ONLY time I ever had the first hint of a fuel delivery issue was using slicks or DOT's with perfect traction.

Even with all that said above the water quickly gets "muddied" with this topic as some folks "dead head" electric pumps for decades w/o any issues and they claim to run into the 10's with a stock fuel pump and 5/16" fuel lines pulling from a stock pick-up in the bottom of the tank. We also read about folks curing their issues with all sorts of other methods, and if they work I'm happy for you, but NONE of them worked for me while I took my car from barely getting into the 13's to running nearly into the 10's over 120mph at the track.......FWIW.....

[Formulajones]

I've had very similar results over the last 35 years of trying things at the track and on the street with only minor differences.

With the stock large cannister GM mechanical pump and stock fuel lines I've gone as quick as 12.50's and 60's at 108 mph in a 4100 lbs. car. It was at that point I hit a wall with that car, no matter what changed it ran nearly the same.
I'm at that same point with the Firebird currently and based off what I've experienced it won't go any faster without some major fuel system changes.

On the other car I did go through all the changes Cliff mentioned with some variances here and there. For years I settled on ditching the mechanical pump completely and running a Mallory 140 back at the tank, still using the stock lines and stock pickup in the tank. I pushed the car to 11.40's at 118 mph with that deal. My problem with it was the need to rebuild that pump with new brushes about every 3-4 years. Quite frankly I got sick and tired of it and the fact that I didn't feel I could trust it for any real distance. Inline pumps have always been that way from my experiences.

That's when I broke down and did the Tanks Inc setup, a 450 LPH pump in the tank, and ran 1/2" lines to and from with a full length return. Been that way for a bunch of years now with no issues to date, and will support all the HP I'll ever throw at it. Ever since every car I setup now, whether a stout street car or strip only, it just gets a Tank Inc setup right from the start so I don't have to go back and monkey with that deal anymore from that point forward no matter what's done to the engine.

I never really got into the whole "add a pusher pump" deal while running a mechanical. I just never found a need to. I don't understand how sitting for periods of time causes such an issue that the car won't start without a pusher pump. I still run several cars here with stock mechanical pumps. Some do sit for periods of time and I've never found them difficult to start. They don't crank for any length of time that I'd consider abnormal, maybe 4-5 seconds. Just not a big deal.
Both of our dailys still run stock mechanical pumps as well and it just seems to work fine. None of these stock mechanical pump cars make more than about 400hp either so the stock pumps supply fuel to these cars just fine at that power level. Like I said I've gone 12's with stock fuel systems with no need for an added pusher pump, so unless I start adding more HP, those particular cars will stay with the stock pump setups.

[Tom Vaught]

Originally Posted by Cliff R

Engine power and vehicle performance will determine what you can get away with here.

I had no issues whatsoever with fuel delivery until I started taking steps to make my Ventura faster. Without writing dozens of pages here I'll list below the different fuel systems I had in place in attempts to keep the carb full at higher power levels and faster runs at the track.

Started off with a stone stock fuel system using a factory mechanical pump and 3/8" lines, this was good to high 13's around 100-102mph.

I "upgraded" to a Carter 120gph with no other changes when I installed my first 428 engine around 350hp.

Added a Mallory 7psi 110gph electric pump right in front of the tank when I installed DOT's and ran into issues at the track.

Installed a 455 to replace the 428 and ran into issues so bypassed the mechanical pump and ran the Mallory 110 7psi pump straight to the carb.

Next when that didn't cure all the issues I installed a 1/2" fuel line in the tank to feed the Mallory 110gph pump and 1/2" lines all the way to the carb. Very minor improvement but still ran into issues on hard runs in the 12.30-12.60 range.

Upgraded to the Mallory 140gph pump and added a Holley regulator, no improvement. The car was running in the mid-12's with a best ever 12.34 @ 109mph.

Installed a Holley 4781-2 850 DP carb because EVERYONE told me the fuel bowl was "too small" in the Q-jet. No improvement whatsoever.

Ripped out the entire fuel system and tank. Then welded a CE sump on the tank, moved the Mallory 140gph behind the tank, high flow filter just before it, Holley regulator right before the carb, and 8an lines/fittings everyplace. This IMMEDIATELY cured any and all fuel delivery issues and the car picked up to 12.0's at 112mph with either the Q-jet of the Holley with NO OTHER CHANGES.

Even so the pump too a chit after 2 weeks on the street being "dead-head". It also made a LOT of noise even though it was a gear type pump and it got pretty hot as well, so hot you couldn't touch it after 10-15 minutes of street driving.

I added a bypass back to the tank and replaced the burned up pump with another one just like it. This all happened between 1999 and 2003. Went from 2003 till 2021 without any issues anyplace, pump was nice and quiet, and I won dozens of races including 5 wins at Norwalk in August and 2 trips to the King of the Hill race at that event. I'd add here that the pump was dead quiet and didn't heat up at all in use.

In 2021 the pump started making noise so I removed it and installed the first pump that crapped out, it only needed new brushes put in it and cleaning up the motor on the lathe. I rebuild the second burned up pump with new brushes for a spare.

Moral of my story, NOTHING worked until I sumped the tank and installed a rear mounted pick-up and moved the electric pump behind the tank. That set-up was FLAWLESS for nearly 20 years without the first hint of fuel delivery anyplace. I even installed another 455 that took the performance of the car DEEP into the 11's over 120mph and low 7's at 95-97 mph in the 1/8th mile.

Lessons learned here were that no matter how much pump or pumps you put in FRONT of the tank, even sucking thru larger fuel lines stuck into the tank, the front sump location on a stock type tank leaves a lot to be desired on a really fast car. I was fine until the car started running into the 1.6 second range in the 60'. Once I started leaving that hard NOTHING was going to work until I either installed a sump or fuel cell in the truck to feed a rear mounted electric pump and "push" the fuel vs sucking on it from a high mounted pump outside and in front of the tank.

I don't have an opinion here, only the results of DIRECT testing during the evolution of my car from 300hp to over 550hp with changes made to effectively use that kind of power with good traction. I'd also add that the ONLY time I ever had the first hint of a fuel delivery issue was using slicks or DOT's with perfect traction.

Even with all that said above the water quickly gets "muddied" with this topic as some folks "dead head" electric pumps for decades w/o any issues and they claim to run into the 10's with a stock fuel pump and 5/16" fuel lines pulling from a stock pick-up in the bottom of the tank. We also read about folks curing their issues with all sorts of other methods, and if they work I'm happy for you, but NONE of them worked for me while I took my car from barely getting into the 13's to running nearly into the 10's over 120mph at the track.......FWIW.....

Great info.

That being said, the reason why I had the electric pump and the by-pass circuit in the first place was because if the car sat a few days the carbs needed fuel as the fuel bowl fuel evaporated over time. PIA to add fuel to the center carb and outboard carbs as longer crank times before engine start.

Got really old to have to remove the chain from the hood to the sway bar to keep scum-bags from stealing my tri-power set-up in the parking lot in college. If you can't get the hood open to steal the battery/carbs then the thieves will go elsewhere.

So for drag racing, Cliff's summer race car set-up would work fine.

I also had a battery kill switch under the hood if they tried to hot-wire the
engine under the dash.

Lots of scum bags in college in the 60s/70s looking for a few bucks to get
another Tim Allen "convicted dope pusher" drug buzz.

Tom V.

My Dual 450 l/hr Tanks inc set-up works just fine and no sump in the back. It has proper baffling in the aftermarket fuel tank.

[Keith Seymore]

Put me down for mechanical pump on the block with an Holley red pump at the tank.

GTO has been set up that way for about 55 years. Car goes 12.20's at 118mph.

Chevelle has been that way for about 40 years. Car goes 9.80's at 136 mph.

I drive around on the mechanical pump and then flip the electric pump on via a toggle switch for periods of high demand.

K

[78w72]

Originally Posted by Keith Seymore

Put me down for mechanical pump on the block with an Holley red pump at the tank.

GTO has been set up that way for about 55 years. Car goes 12.20's at 118mph.

Chevelle has been that way for about 40 years. Car goes 9.80's at 136 mph.

I drive around on the mechanical pump and then flip the electric pump on via a toggle switch for periods of high demand.

K

What brand/model mechanical pump? Any bypass system like shown above or just pulling through the electric?

[Shiny]

Thanks for the explanations. I apologize, but I'm still struggling to understand the pressure at the carb inlet when an electric pump is in series with a stock mechanical pump and there is low fuel demand.. (at idle).

Originally Posted by glhs#116

...On the other side it produces pressure to the carb that it regulates to whatever pressure....

This is the part I'm trying to understand. The term "regulate" is more like "self-limiting" I think. Because the pump is all diaphragms and springs, is the max pressure it can put out imited by the diaphram spring in this image:



But isn't the max (idle) output pressure of the mechanical pump always relative to the inlet pressure?

Similarly, doesn't an electric pump build pressure at its outlet when there is low flow at idle (or dead-headed)? It may be internally regulated or limited but I have to believe it is going to be 5-7 psi, correct?

So if the pressure above atmosphere (which is what a carb cares about) is 5 psi at both the outlet of the electric pump and the inlet of the mechanical pump, how can the pressure at the carb not be the sum of both "dead-head" or low-flow pumps?

I think I am missing something here about how the mechanical pump works because no-one on this thread has mentioned flooding at idle because the pressure got too high.

I think if someone measured idle inlet pressure at the carb with and without a series pump powered, I'd at least see whether the two pressures added or if the mechanical pump somehow delivers the same output pressure regardless of inlet pressure.

Mike