Fuel Pump Differences

[BigBlockMonte]

Hey all, I'm having a issue running out of fuel once I shift into second gear when I'm driving it hard. I just changed the fuel line from the tank to the pump to 3/8. I also just changed the sending unit from the 5/16 to 3/8.

 

I'm about to take it for a ride, now that I changed the sending unit.

 

If this doesn't make a difference, I'm assuming the fuel pump is next.

 

The question I have on fuel pumps is, I see some with a deep bottom to it, and others I see with a short bottom. It seems all the short bottom pumps I see are the higher GPh. How does that work?

 

I would think the deep bottom would have more fuel on standby to keep delivering it, so it wouldn't run out.

 

Someone please help my way of thinking on this. Thanks for your time.

[Sam (Bones)]

I would first check your filters to make sure not restricting fuel

[Mike 57]

I would check the filters as Sam said. The real difference in pumps is the valves. Hi performance pumps have as many as six valves. The fuel pump diaphragm pulls fuel through the one way valves and pushes it to the carb. That is the simple explanation.

[BigBlockMonte]

okay, then what is the difference between the fuel pumps with a deep bottom and a short bottom.

 

I just replaced the sending unit tonight and the fuel filter. which looked clean to me, but I changed it anyway, I took the car out, and it still doing the same thing.

 

I went to my local parts store and picked up a stock fuel pump for a 70 chevelle 454. Tomm I'm goin to give it a try, if it works then my problem was indeed the pump, if it does the same, then I'm really lost. everything is new now.

 

here's the set-up

 

468

10.2 compression

aluminum heads, 320cc intake runners

comp cams XE284h 574/578 lift 240/246 duration @.50

Holley 850 carb

edelbrock air gap intake

hooker full length headers

3 inch exhuast with flowmaster mufflers

3/8 line including sending unit from tank to pump

pump is supposed to be 110gph pump

28" tall tire with a weak 2.73 rear end gear. (this will be a 3.73 before too long)

TH 350 trans

trans specialties 2800-3200 stall

 

 

need anymore info, just ask

[Sam (Bones)]

I'd say you are borderline needing a 1/2" line with that combo, I'm using one of Holley's 170 GPH mech pumps, had no issues with on my 12:1, 468 but I'm using a -10 (5/8") line

[BigBlockMonte]

are you serious??? i just did all that work to put the 3/8 line. I wouldn't even know where to start to find a 1/2 sending unit.

 

this is..... I don't know

 

i have a carter 170gph pump. but it has AN fittings welded on it, one side 1/2 the other is 5/8. I didn't use because I figured it would be more money to get it to work then to just buy another pump.

[Sam (Bones)]

if a bigger pump doesn't solve the starvation issue, bigger line next

[BigBlockMonte]

do they even make a 1/2 sending unit? or could I run a 1/2 line and connect it to the 3/8 sending unit?

 

thanks

[Mike 57]

I ran 11.6s with a eddy 7 psi 6 valve pump and 3/8 line and a 870 vacuum secondary avenger. Over the winter I upgraded to a 1/2 inch pickup and 1/2 line to the pump and carb when I upgraded to the pro systems 1000 cfm double pumper. I don't think the stock style pump is the way to go

 

Here is a link to the 1/2 inch pickup link

[BigBlockMonte]

i know i used a holley 80gph on the monte, it still had the 5/16 line in it, and I never had a problem with that pump. It has to be this no name pump I have. its gotta be.

[BigBlockMonte]

what exactly is the flow rate of the 40727 fuel pump for the ls6 chevelles. I figure it would have to be something okay. Those cars were 450/500 hp cars back in the day.

 

anyone know what the GPH is?

 

and is there a flow difference between the deep & short bottom pumps?

 

Thanks

 

[CrazyDavey]

not sure about that flow rate but the 3/8's line is plenty for your car. A 3/8's took my car into the 10.90's (502 with a Prosystems 1000 carb) so I think your good there. If it's running out of gas when you shift into 2nd that sounds like it's not starting off with enough gas in the first place. Have you checked the float levels on your 850?

 

I don't remember is you have a fuel pressure gauge but it it holds 5-7 lbs at the carb the pump should be enough...JMO..Dave

[Mike 57]

Just a thought, what type of push rod are you running? I run a hollow push rod, I am wondering how hard you are turning your engine and if the spring in the stock style pump is causing the rod to float.

[wallaby]

The fuel pump rod is supposed to float. First off, the cam lobe that runs the pump is very tame...is doesn't need the rapid opening and closing rates like the lobes for the valves. It looks quite round compared to the pointy lobes that run the valves. The second point with mechanical pumps is the mechanical function: when the cam pushes on the fuel pump rod, the pump is on its intake stroke. The cam pushes the diaphragm against the pumps' return spring and fuel is sucked into the void behind it. After that stroke is done, it's up to the return spring to push the fuel to the carb on the exhaust stroke. The fuel pressure is regulated by the strength of the spring.

Now in the event that the carb is already up to pressure, the spring can't move the diaphragm...so the fuel pump sits nearly motionless and the fuel pump rod is kinda in limbo; it might get tapped by the high-point of the cam lobe as it comes around, but it doesn't run "full stroke" with every rotation of the engine unless the pump is running at zero pressure. The higher your demand for fuel, the longer the stroke becomes. It's a variable displacement pump rated at max flow. When running at less than max flow, the fuel pump rod "floats".

[BigBlockMonte]

I don't know what push rod it is, it's solid. and the fuel bowls I adjusted per the holley DVD that comes with the carb. Have the engine run(warmed up) and open the sight window and verify fuel is at the bottom of the sight glass, right on the threads occasionally trickling out. This is what I did exactly,

 

out of the box when I checked the float level. the fuel was pouring out when the screw was removed. According to holley this was too high and needed to be adjusted. So i did what I thought was the right thing to do.

 

I'm actually goin to summit right now and grabbing up a holley/eddy fuel pump with a ARP push rod. I think its a wasted of time to try the stock ls6 fuel pump.

[680HPStroker]

I'm with same and Mike here. You could probably get away with 1/2" line/fittings all the way from the tank to the carb, but I error on the side of caution and went with a modified 5/8" pickup, 5/8" line from the pickup to the fuel pump(170gph Holley), 1/2" line from the pump to the regulator, and dual 1/2" lines from the regulator to the carb. with a very thirsty 650+HP 489BB to date I've never had any fuel starvation issues. I know this isn't what you want to here, but that is how we all learn. Trial and error.

[kc8oye]

one of the things I ran into was fuel feeding into the carb...

my old holley 600 was a single feed and just wasn't up to the task (I know your 850 is dual) but if you are using one of those one-splits-into-2 lines, that _might_ be a source of trouble.

 

I'm plannning on some major upgrades down the road, so when I put my new dual feed carb on, I used a holley pressure regulator mounted right next to the carb, the regulator has massive ports on it.. I want to say they are 1/2" NPT, but I might be wrong, I know they are HUGE

 

This is the only pic I have of my regulator setup. look closely at the passenger side of the carb.

so right now I just run the 3/8 line from the pump to the bottom fo the regulator, then then each port on the regulator feeds the two float bowls, for me right now, it's over kill, but eventually I will be feeding the reg with an-8 from the pump and then two -6's to the carb :>

 

it's possible that your fuel lines AFTER the pump may not be up to the job.. just a thought.. gotta think about the whole system.

[BigBlockMonte]

your set-up with the regulator feeding both bowls is really cool. why didn't i think of that? anyway I have the problem fixed. went to summit and grabbed up a Holley 13gph pump, holley regulator, & a comp pushrod. got it all plumbed in and bingo. I know waht if feels like goin through 2nd into drive now.

 

I 'm rally thinking about taking your idea on the regulator mounting and feeding each bowl seperately.

 

to mee that the cats meow.

[kc8oye]

oh btw, I got the billet aluminum bracket that's holding the regulator from Summit racing. that's hwy I've got those goofy loops of hose on there.. the straight fittings wouldn't clear the bracket...

[mc71454]

For a drop in 1/2" sending unit, check out RobbMC performance

 

http://www.robbmcperformance.com/products.html

 

Quality stuff