Started logging fuel pressure

[furrystump]

So my fuel system has been the same for 15 years and when I installed it a video of the fuel pressure gauge showed 4-5 lbs steady at WOT. Video was SHAKEY to say the least. Thinking about trying Q16 to get my .17 in ET that I need and want to have as much info I can about whether the fuel system can handle 20-25% more raw fuel load. I know someone who went through the change using the same fuel set up and carburetor with about 80 more CI’s. There were struggles along the way for him I’m hoping to avoid. I run a Holley red pump with a big mechanical. 3/8” line from beginning to end. Through an Edelbrock Thunder 800 cfm. I have run this carb with drilled out front jets and no rear jets during a pretty good fall day and it was a bit fat. $20 pressure transducer later and this is my current readings. 6.5 lbs at idle on both a drained gauge and with the sensor. Less than .17 lb of fuel pressure at 5000 rpm. AFR is fine, and it has ALWAYS trended slightly rich with rpm gain. Edelbrock states that it would do that. Next step is going to be to remove the mechanical and install a Holley black pump. Attempt to add some reserve capacity. Black line is engine rpm, green fuel pressure. Was surprised it has run so well with just .17 lbs pressure. Pressure is just restriction but .17! I was not on a track so sample is pretty small and the pump might start to build back some pressure….

 

[furrystump]

Noticed this, spinning the tire….mechanical trying to catch up?

 

[TT5.9mag]

I believe the dip in pressure you’re seeing when it spins the tire and the RPM sky rockets, is the needle coming off the seat. For that short duration you have minimal restriction, so pressure drops, but flow is high, until the bowls are filled again and the needle shuts off flow. Not sure but the little wiggle in the trace might be a pedal. Did you chicken foot it?

 

[furrystump]

Probably or it spun a bit, 3rd gear, street, little tire I’m VERY chicken. It was probably wheel spin though. It does slip the tire if the car unloads a bit on small bumps. I had to dump throttle position to log fuel.

 

[Newbomb Turk]

I would eliminate the mechanical pump. It’s actually a restriction in the line because the duration it’s pumping isn’t as long as the duration it’s not pumping.

Obviously the time it’s not pumping gets smaller with rpm, but it is still a restriction.

I’d get a real bypass regulator and run a return line. IIRC the red pump only has a 10 pound spring in it, so line pressure from the pump to the regulator (or your pump in your case) is only 10 psi. That’s pretty low if you 60 foot much quicker than 1.7x or so.

With a real bypass regulator you can block the pressure bypass in the pump. Line pressure will go up to about 20 psi depending on the pump and how old it is.

That’s huge. Then run your return line. The pump will run far cooler than it does with the bypass in the pump operating.

And the engine will see cooler fuel because it’s not getting hot going through a block mounted pump.

 

[TT5.9mag]

Your engine has to “appear” stock right? Is that why you’re using a mechanical in line with the electric?

 

[Mattax]

I know there's some recommondations in the old MP Tech Tips about running a mechanical pump with an electric pump but I don't have any experience with it.
One rule of thumb is 1 lb per hour of fuel is needed for Hp/2
Even not knowing the Hp your engine is producing, there were several mechanical pumps that should be able to supply the volume needed at 4.5 psi.

Here's Holley's chart for the "Keith Dorton" 110 Mechanical pump.

Free flow is 0 psi in the outlet tube.
4.5 psi is a somewhat restricted outlet and seems to be a common reference pressure.
I've seen cut off pressure listed as low as 6 psi an das high as 8.5 psi. I did not send this pump to Ryan Brown for testing, but my experience with it has been pretty much no issues with it over powering the bowl inlets beyond the usual with Holleys that sit.


Where I'm going with this is that lower pressure on the outlet side of the pump will result in more fuel into the bowl.

The question is whether the lack pressure at 5000 rpm is due to inneffective delivery or some restriction before the gage.

Electric pump delivery is similar.
I think it is worth looking at an experiment @Cudafever did at @PRH suggestion. I contributed pump flow charts and diagrams.
Holley/Demon Carb stumble
His first test is post 488

 

[Mattax]

Here's the tech data on Holley's Red Pump from the 2012 catalog

 

[Newbomb Turk]

Your engine has to “appear” stock right? Is that why you’re using a mechanical in line with the electric?

I forgot about FAST rules.

I might consider taking the guts out of the mechanical pump and the lever off of it and send it.

Unless the rules say the mechanical pump has to function.

 

[TT5.9mag]

I forgot about FAST rules.

I might consider taking the guts out of the mechanical pump and the lever off of it and send it.

Unless the rules say the mechanical pump has to function.

Thats exactly where I was going with my question.

 

[furrystump]

Your engine has to “appear” stock right? Is that why you’re using a mechanical in line with the electric?

Yes, in the past I ran a gutted pump. Then thought all I was doing was having a small aluminum pot to heat the fuel. So I drill a hole straight through a pump and ran a line straight through it. Then went with the current setup. I have printed this to hide the regulator.

 

[TT5.9mag]

Yes, in the past I ran a gutted pump. Then thought all I was doing was having a small aluminum pot to heat the fuel. So I drill a hole straight through a pump and ran a line straight through it. Then went with the current setup. I have printed this to hide the regulator.

View attachment 1716459320

It’s incredible the amount of effort you guys go through. I applaud it. I absolutely love hiding the regulator inside the body of a printed stock appearing mechanical pump. Back on topic, so your data above shows an electric pushing through a working mechanical pump? Or a gutted one?

 

[PRH]

Rhett, have you ever tried flow testing your complete system, up to the carb fitting, at what you’d consider the minimum pressure you’d be happy with?

 

[furrystump]

For over the last 10 years it has been electric pushing through a working mechanical. I went this way because I was chasing a stumble in the upper rpm’s and knew someone running the electric/mechanical setup successfully. Turn out to NOT be my issue, was collapsing a hyd lifter, but the system worked so it stayed and was forgotten. The sensor is in the line 5” from the carb inlet without anything else in between. The “stock” filter on the engine isn’t a filter. (There’s a big Canton filter back at the tank) The hard fuel line just goes straight through it. I want to be able to make sure I can put enough fuel in the bowl to handle Q16, just want to be able to bump the pressure/velocity if needed. I did do the free flow fuel into bucket test back when I was just running a blue pump. Checked out okay. Still nosed over above 4500-5000. Thought the cheap regulator might be the issue. Went solid lifter and the red with mechanical at the same time. Problem solved. This weekend should be able to swap to the black pump with an aeromotive deadhead SS regulator. Return style would be better, but will probably go to an in-tank deal at some point.

 

[furrystump]

This filter is fake, hard line goes through it.

 

[PRH]

The way I like to do the flow test is………
(Easiest with a helper)
Disconnect line from carb, and attach a hose that can reach over the fender or grill to a can.
Attach a ball valve at the can end, with a gauge just before the valve.
Prior do doing the actual timed test, with the pump running and the hose off the valve going into a container, adjust the valve to set the pressure at the valve to your preferred minimum(4-5psi).
Shut the pump off, don’t touch the valve.

(I like to use a metal one gallon can for the timed test.)

Time how long it takes to fill the one gallon can while maintaining the desired minimum pressure.

Then calculate how much hp that fuel flow will support at a bsfc of .5
Example: For a 475hp engine the system needs to pump .66 gallons in a minute…….. which if I’ve done the math right is a gallon in 90 seconds.

I have seen several cars fail that test miserably.

 

[Bewy]

With A/F correct, a 300 hp engine will consume 25 gph. A safety margin is needed. This is carefully explained in the original Holley book by Mike Urich.

 

[furrystump]

So I did the flow test with the Black pump through the regulator. It will deliver 1.5 gallons/min to the carb inlet. Also made a run around town, pressure drops 1/4 lb from idle to 6000 rpm run through 3rd gear. It’s the most boring graph line on the log! Idle pressure set at 5.5 lbs.

 

[PRH]

It will deliver 1.5 gallons/min to the carb inlet.

Is that free flow……or while maintaining pressure?

Just disconnecting the line from the carb and discharging it into a container is free flow, which is essentially the flow at zero psi.

 

[fishmens67]

Is that free flow……or while maintaining pressure?

So I did the flow test with the Black pump through the regulator.

 

[PRH]

If there is no restriction after the regulator(as in the hose disconnected from the carb), it’s free flow.
The gauge after/on the regulator will read “0”.

The number that’s important is how much flow there is……..while maintaining the desired system pressure.

 

[furrystump]

That is a free flow number.

 

[Mattax]

Look again at the posts where Cudafever was testing. The last valve before the collection can is the one which simulates the bowl inlet. My little line drawings there may help. If thats the same as you did then we're on the same page.

 

[furrystump]

I’m going to get a valve and retest.