[Kenth]

"WOT full power typically requires 0.5 lb fuel per HP per hour. Thus, a 300 HP engine needs 300 x 0.5 is 150 lb per hour.
Gasoline weighs about 6 lb per gallon.
So, 150 lb per hour divided by 6 lb per gallon equals 25 gallons per hour fuel fuel flow at WOT"

Excerpt from Doug Roe's 2:nd book page 24 of Rochester Carburetors, where you can read the entire equation.

FWIW

[69Customs]

Gm put a single 5/16" dia. Gas line on 350 cu. 2 barrel cars only. If the car had ac they had a 1/4" return line. If they had a 4 barrel or 400 cu. they had a 3/8" fuel line and a 1/4" return line.
If you only have a single 5/16" fuel line and have modified the engine by making more hp you need a 3/8" fuel line with a return or you might be robbing your engine of it's full potential. You can add a return line pump and sending unit and use the 5/16" line as a return line, just add the 3/8" fuel line.
Also if you slowly pump the accelerator pump you will get a more steady full stream than just jabbing it.
Just my experience.

[78w72]

Originally Posted by Shiny

Wouldn't it be great if any of these manufacturers published pump curves! Once you bolt any pump into a system with flow restrictions, they aren't going to deliver their "open" flow.

Robbmc does that, they show their pumps ratings at 0psi or free flow compared to a carter high performance pump, their pumps flow a lot more GPH.... but then they show the GPH at 4.5psi which is closer to a real world operating psi and the carter psi drops a lot, below 60gph where the robbmc 550 is up around 90gph in the normal rpm range... the 1100 pump is rated at 120-150 GPH @ 4.5psi.

Good to see actual GPH numbers under pressure from robbmc vs free flow that most are rated at... another option to consider for guys that want/need a little more flow but not go full electric... and the pumps are much better quality than aftermarket OEM pumps at parts stores, more accurate & stable psi plus might address drain back problems if other cheap pumps are causing that?

https://www.robbmcperformance.com/pr...ontiac550.html

[Shiny]

Originally Posted by 69Customs

Gm put a single 5/16" dia. Gas line on 350 cu. 2 barrel cars only. If the car had ac they had a 1/4" return line. If they had a 4 barrel or 400 cu. they had a 3/8" fuel line and a 1/4" return line.
If you only have a single 5/16" fuel line and have modified the engine by making more hp you need a 3/8" fuel line with a return or you might be robbing your engine of it's full potential. You can add a return line pump and sending unit and use the 5/16" line as a return line, just add the 3/8" fuel line.
Also if you slowly pump the accelerator pump you will get a more steady full stream than just jabbing it.
Just my experience.

Great info, thank you!

Did the fuel pickup in the tank also change?

[grivera]

Agreed - thanks 69Customs- the fuel line info is great to know. Mine is an original 350 2 barrel car, now has 1972 4 barrel intake and a late 70s Buick QJet. 5/16 supply line only - no return.

[Shiny]

Originally Posted by Kenth

"WOT full power typically requires 0.5 lb fuel per HP per hour. Thus, a 300 HP engine needs 300 x 0.5 is 150 lb per hour.
Gasoline weighs about 6 lb per gallon.
So, 150 lb per hour divided by 6 lb per gallon equals 25 gallons per hour fuel fuel flow at WOT"

Excerpt from Doug Roe's 2:nd book page 24 of Rochester Carburetors, where you can read the entire equation.

FWIW

So the key is to have a pump big enough to deliver that gph to the carb bowl in spite of all the pressure losses. Online "guidance" suggests if the pressure into the carb is 4 or 5 psi, it's enough to supply whatever the carb was designed to deliver to the engine. To me, this 4 psi must be what it takes to push the fuel through the inlet filter and more importantly, the needle and seat. The seat orifice has to be the biggest barrier, right?

Originally Posted by 78w72

Robbmc does that, they show their pumps ratings at 0psi or free flow compared to a carter high performance pump, their pumps flow a lot more GPH.... but then they show the GPH at 4.5psi which is closer to a real world operating psi and the carter psi drops a lot, below 60gph where the robbmc 550 is up around 90gph in the normal rpm range... the 1100 pump is rated at 120-150 GPH @ 4.5psi.

Good to see actual GPH numbers under pressure from robbmc vs free flow that most are rated at... another option to consider for guys that want/need a little more flow but not go full electric... and the pumps are much better quality than aftermarket OEM pumps at parts stores, more accurate & stable psi plus might address drain back problems if other cheap pumps are causing that?

https://www.robbmcperformance.com/pr...ontiac550.html

Thanks. I'll look at their charts and see if they spell out the losses getting gas to the engine..... basically this "operating point" question is where the supply curve (pump pressure vs flow) crosses the demand curve (flow to deliver HP vs pressure losses inside the carb and through the plumbing).

On the "supply" side, I was thinking about my Delphi mechanical pump and other aftermarket "stock" pumps. I searched for those curves a while yesterday and couldn't find them. I suspect they are out there somewhere...

Which brings this back to how much pressure drop occurs through an inline solenoid pump that is turned off?

Will my Delphi pump rated to deliver 38 psi be able to deliver that 25 gph I hope I need? Will it still be able to deliver it if I stick a little solenoid pump in the feed line?

Mike

[69Customs]

Cars that came without return line you have to change tank sending unit to one with the return built in. Just get a sending unit for gto, 400 cu. They all had 3/8" fuel line and 1/4" return.
I put in a built up 400 cu never thinking about the fuel line with a 4 speed. I just blocked off the return on the fuel pump. Going down the track it would fall flat
on its face in 2nd gear. Run out of gas. So I used the stock 5/16" line as a return. (You can force 1/4 line over 5/16 and visa versa by clamping down hard or use barbed reducer fittings without leaks) then put in a 3/8" fuel line. Boy did that ever wake it up.
Also the line from higher 400 pump to the carb has to be 3/8". I use an inline aftermarket fuel fiter and remove the ceramic carb. Filter. This along with a holley adjustable regulator to keep fuel pressure between 4-6 psi

[Shiny]

With a "known full carb" (shut off immediately after starting) and left in my cold garage, I found:

24 hrs: instant start (enough fuel in the bowl)
48 hrs: cranked a LONG time (fuel gone again, even at 50F)

So maybe my cheap mechanical pump leaks enough to do this? I dunno, but maybe a combination of evaporation and leaking through the pump?

I asked Facet about the pressure drop across their solenoid pumps when they were "off" and my mechanical pump was pulling 25 gph through them. I give them credit for being fast to reply, but the answer did not quantify the loss: "The Facet Cube or POSI-FLO pumps have very little restriction when pulling fuel through them however this all depends on the strength of the mechanical pump on the engine." I interpret this as "good luck to you".

So much for gaining confidence before trying it!

I also figured those 1/8" NPT fittings on the Facet pumps mean any mating fitting will have only a 1/8" inner diameter. This sounds like a big restriction...

I found an online calculator and it suggests about a 0.3 psi drop for a 1/8" sharp inlet at 25 gph. Facet does sell one version with 3/8" NPT fittings, so that sounds better if I go this way.

I also looked at the RobbMc mechanical pump curves. They are in a different universe! No worries about starving them!

The aftermarket replacements for 2bbl pumps (short can like mine) seem to be rated for about 38 gph max and shouldn't push more than 7 psi against a closed needle and seat. I could not find a pump curve that would spell out the flow rate against the 1 to 3 psi loss I expect at between the pump and the carb bowl. RobbMc spells it out, but they have more than a little margin!

The aftermarket 4bbl pumps (tall can) are only claiming about 40 gph. This doesn't make sense to me, so there must be "more to it" if they were feeding 400 HP engines. The Delphi version specs 10 psi max, which makes me uncomfortable about flooding, but again, must have worked out OK for most quadrajets or doesn't replicate the stock pump...

Here's my SWAG at flow losses for these two Delphi aftermarket pumps. The curves are total guesses between the two end points (proportional, based on the RobbMc loss at 4.5 psi), but suggest there isn't a lot of wiggle room if you want that 25 gph for more than a second or two...



So I'm still wrestling with decisions. Time to take a vacation anyway!

Mike

[78w72]

After the evaporation discussions and thinking the fuel doesnt evap that fast (1-2 days in 45-50f temps) I think there is likely some drain back going on also... before buying and plumbing in a primer pump, I would replace the pump since OEM ones are relatively cheap priced, might have a warranty on yours if its newer? Or go robbmc & know its a better quality pump that might fix drain back problems if they are there and definitely will pull enough through a primer pump if you still go that route.

Or consider one of the high flow carter or edlebrock pumps, I had the edelbrock on the 500hp motor at first, it was used for the dyno pulls and first time at the track, worked fine on the street and went 12.2 at the track but showed signs of starvation going any faster. For the prices I would get the robbmc though.

I dont recal what year q-jet you have, but Ive read mid to late 70s can handle 7psi or more and most earlier ones are ok at 5-6, if the pump has too much psi, there are cheap priced regulators to address that.

Hope you get it sorted out.

[Formulas]

You just might need a good ignition tune up, With all thats been stated done to fuel circuit but a starting problem still exist's you may be hunting in the wrong woods

[Shiny]

Originally Posted by Formulajones

......

I'm running the same crap fuel, actually considered worse out west here than many other states, yet it's working fine for me on the Vega, the firebird, the Camaro and chevelle. I don't have extra pusher pumps on any of them. Bowls aren't drying up and even after sitting for 4-5 days and even a couple weeks in some cases I don't have to excessively crank the car to start it. The nomad will sit a month at a time and it still cranks and starts within 5-8 seconds and I don't even use the choke. I find that perfectly acceptable and no different than when these cars were new.

What specific pumps do you use? Do you have a way to measure the drainback through the pump?

If you use aftermarket pumps, what style (2 line, 3 line, short, tall ??) and specific brand?

If you use a tall style pump that deadheads at 9 or 10 psi, do you have issues with flooding at idle?

I like the sound of your 4-5 seconds much better than my 15.

Question remains whether your crank time is short because you fill faster or lose gas slower?

Assuming your Nomad sits long enough to dry out from evaporation, the fact that it fills in 5 seconds tells me your pump is pushing a lot more fuel than mine.. either that or mine is draining all the way back to the tank, which I guess is possible ???

[Shiny]

Originally Posted by Formulas

You just might need a good ignition tune up, With all thats been stated done to fuel circuit but a starting problem still exist's you may be hunting in the wrong woods

Good thought.

Yes, this could be a factor, but I went through it pretty thoroughly before we moved (twice). It starts instantly when there is gas in the carb.

My comparisons hot and cold suggest it's a combination of evaporation and drainback.

The pump is aftermarket as I have shared, so I can believe it doesn't prevent drainback well.

Mike

[Shiny]

Originally Posted by grivera

My 69 Lemans has a factory metal fuel line with no return. New rubber lines, new Spectra pick up in tank, new vented fuel cap, a rebuilt Carter 6907 fuel pump, late 70s Buick QJet built by Cliff that I used on bird years back - marine epoxy used at well plugs. When the car sits even for just an hour or two, I have to prime the carburetor to get it started. I don’t notice much of a pump shot and extended cranking doesn’t help. Once I manually prime it and get it started,! Runs well and will start up as long as it’s not left sitting. Fuel is 87 octane, 10% ethanol (MD). Any ideas on what the issue is?

Did you ever get yours fixed? How did this play out?

[Pav8427]

I have been following this from the beginning, and lots of good info here.
I dont have a whole lot of experience, but for a drain back test, could you disconnect the tank line and place a line from the pump into a container and see if and how much fuel drains from the pump/carb.
May not be much or may be.
Might give some insight.

[Shiny]

Originally Posted by Pav8427

I have been following this from the beginning, and lots of good info here.
I dont have a whole lot of experience, but for a drain back test, could you disconnect the tank line and place a line from the pump into a container and see if and how much fuel drains from the pump/carb.
May not be much or may be.
Might give some insight.

Good thought, thanks.

I expect the line to be filled between pump and tank so if you open it, that would all drain out. If it then started dripping, I'm not sure how to know if it came from the pump body or from the carb "through" the pump ???

Jon's suggestion of measuring fuel by pouring into the vent until it overflowed might be the most direct way to monitor the level in the carb. If this works well, it would be relatively easy to compare with and without disconnecting the line at the carb inlet and plugging it... basically isolating the carb from the lines.

Earlier in the thread, I linked to anecdotes where Cliff reported his drying out in 2 days.

As I said, I believe evaporative loss is significant.

Maybe my 15 second cranking time is more about my 2bbl aftermarket pump and not about the loss? I'm starting to lean this way...

But I'm not seeing others measure the crank time after their car sits for weeks, let alone what specific pumps they are using.. your perception may vary!

[Shiny]

Trivia rat-hole for the week...

I hoped it would be easy to estimate the "sensitivity" of my mechanical pump flow to the pressure drop caused by an inline solenoid (or even a rotary vane) pump.

I am not finding any truly insightful nor technical information so I'm not jumping on adding an inline pump to fix my long cranking time. No sense adding a real problem to fix a nuisance.

Kudos to RobMc for publishing real data, though!

If anyone has any similar real data regarding flow vs pressure for the stock or aftermarket 69-70 pumps (short can and tall can) please share. I'm hoping for pump curves and even better, a pump curve with and without an inline pump upstream.

I sent email requests for tech info to Delphi, FirstBrands/Carter, and GMB, who all have active PNs for these applications.

They all responded quickly, but NONE of the support people had access to pump curves...(or didn't volunteer to share if they did).

Carter did share a little more detail on specs:



I find it odd that the max flow is the same for both the 4bbl 2 bbl pumps. The 4bbls pumps from all suppliers are spec'd to deliver higher max pressure.

After all the stories of flooding and high aftermarket pump pressures, I did not want to risk my carb holding against 9 or 10 psi so I stuck with the short can pump.

I guess the difference between short and tall pumps is being able to push more fuel against a partially-closed needle and seat? In other words, the max flow is probably meaningless, given the carbs need to stay full to work well.

Carter was the only one that said ANYTHING about the effectiveness of their anti-drainback/check valves. Unfortunately, their spec says they won't leak with pressure applied... nothing about leak rates with zero pressure in the line back to the carb.

If I ever get my car on the road and find this to be more than a nuisance, I guess I can run a science fair project and measure some stuff... or more likely, just add a pump in a way I can easily reverse and pull it out if it causes trouble. I just hate wasting money on stuff that should be easy to predict...

Dinosaur technology was lost with the dinosaurs I guess... definitely easier to find an electric pump to replace the mechanical pump than to have confidence buying a band-aid!

Mike

[grivera]

Originally Posted by Shiny

Did you ever get yours fixed? How did this play out?

I haven’t yet - been dedicating what little spare time I have to my 66. Will definitely update this thread when I make some progress though.

[78w72]

Shiny- Thats a lot of effort trying to find that kind of data regarding flow/restriction of an inline pump from companies since most dont install their E-pumps on a mech pump system to show those numbers nor do they care. You will just need to try it to find out or go another route.

I think I mentioned earlier in this thread about using a bypass set up around the primer pump as has been posted on here before, think it was tom V? Doing such.. eliminates the question of "will the mech pump pull through a inline primer pump" if the fuel bypasses it when its not on. Do a search or maybe someone else can link to that thread or post a pic of the diagram?

If it was me and I couldnt deal with a minor inconvenience of a few short cranks when needed, I would try a new OEM pump of your choice or step up to a carter, edelbrock or robbmc 550. I know from experience the edelbrock & robbmc dont run anywhere 9-10 psi, the edelbrock was 6.5-7 & robbmc is ~7... if your carb cant handle those pressures, a cheap priced small regulator is easy to hook up.

Hope that helps you decide what to do.

[Shiny]

Originally Posted by 78w72

Shiny- Thats a lot of effort trying to find that kind of data regarding flow/restriction of an inline pump from companies since most dont install their E-pumps on a mech pump system to show those numbers nor do they care. You will just need to try it to find out or go another route.

I think I mentioned earlier in this thread about using a bypass set up around the primer pump as has been posted on here before, think it was tom V? Doing such.. eliminates the question of "will the mech pump pull through a inline primer pump" if the fuel bypasses it when its not on. Do a search or maybe someone else can link to that thread or post a pic of the diagram?

If it was me and I couldnt deal with a minor inconvenience of a few short cranks when needed, I would try a new OEM pump of your choice or step up to a carter, edelbrock or robbmc 550. I know from experience the edelbrock & robbmc dont run anywhere 9-10 psi, the edelbrock was 6.5-7 & robbmc is ~7... if your carb cant handle those pressures, a cheap priced small regulator is easy to hook up.

Hope that helps you decide what to do.

Thank you. I appreciate the voice of reason!

I obviously like learning how this stuff works... probably more than the stuff.

14 seconds of cranking is not a good thing (for me), but like you say, may be my new pump leaking. And it may be evaporation. Or both. TBD later to inform my decision when the weather warms up.

Mike

[78w72]

Originally Posted by 78w72

Ok, but this situation is about fuel evaporating, in a car, when hot... totally different environment than a jar of fuel on sitting on a table.

I agree some cranking is a non issue, tried to explain it wont damage a starter doing some 4-5 second cranks.

I also have no intention of adding a pusher pump to any of my cars, but any electric pump will fix or improve the "hard" starting some have whether its from evap or drain back. If someone doesnt like dealing with cranking everytime they start their car after sitting for a few days or a week, then a pusher/elect pump is an option, lots of guys on here & out there with classic cars use them for one reason or another.

Sorry to bump an older thread, took a minute to find this one, theres numerous pages when searching for "fuel starvation" and they all explain the same thing that some cars lose fuel after sitting for X amount of days, usually a week or so, but I just wanted to update this after I did a quick test to see how fuel evaporates when its in a real world situation like in a carb on a warm engine.

Regarding a qjet fuel bowl going dry, the test above of putting a full cup or 2 of fuel in a cool container on the garage or house counter showing it didnt lose any fuel so we all must have something else going on...
Well just last week I remembered this thread and how the test wasnt what fuel does when on a hot engine/carb, so I filled a tablespoon or a little more of fuel in a small metal container, about the size & amount of fuel in a q-jet bowl, I set it right on top the carb after a short drive but fully warmed engine at ~180°. After letting it sit with hood closed for about 1.5 hours I came back to check & sure enough it was completely dry, not even a residue of wet fuel left. Granted inside the carb isnt exposed to open air but the carb is fully vented. This test is more of a real world scenario for fuel in a q-jet than a big cup of fuel at room temp, based on what I found after a short time fuel definitely can & does evaporate at a pretty fast rate when heated like an engine does. This was non ethanol fuel too, 10-15% eth will evap even faster.

I cant say for sure what else would cause the hard starting after sitting for X number of days but its clearly a real issue that many people have on q-jets, not just pontiacs, a quick google search turns up countless examples, maybe its also drain back related to certain fuel pumps, but fuel definitely does evaporate at a faster rate than some may be aware and Im sure it contributes to the problem.