26 replies to this topic

[Redslur]

Folks

Can anyone please tell me whether there is any real advantage fitting an L34 fuel pump over the standard ones? I know they are slightly better in pressure but do they actually deliver better volume? The reason I ask is that my new 308 drains my 770 Holley dry full noise in third gear. I have since fitted a fuel log and that has improved things quite significantly but want to be sure that fuel volume meets the engine needs. I really don't want to go down the electric pump avenue as I really want to maintain the stock fuel system. I know that I am probably on the border line of exhausting the potential of the standard system but want try and keep the system as stock as possible using the best available stuff. At this stage I haven't been able to check my current fuel pressure but suspect it is ok.

Cheers, Gerry

[MRLXSS]

I would say that there is no harm in upgrading to an L34 Spec pump... I don't know if the flow is actually any different, but they certainly provide much higher pressure.

I am running a L34 spec pump on my 355, I was originally running it with standard fuel lines etc, but I have since upgraded the fuel line as well to 3/8th (-6AN)

With this setup, my fuel pressure sits on about 6psi, which is pretty much what I have been told to set an electric pump to...

Saying all this though, my engine is still fresh and hasn't seen much action... Once I have had it on the dyno, I'll get a better idea if the L34 pump is up to the task... If not, I will go an electric pump.

[GML-31]

have you increased the size of your fuel line yet?

[REDA9X]

have you increased the size of your fuel line yet?

Exactly you need the bigger fuel lines to deliver volume. The L34 pump delivers higher pressure with standard fuel lines, remove the fuel lines replace with bigger lines and increase the flow the pump will deliever around the same pressure as a standard pump then.

[Redslur]

When you mention have I increased the fuel lines yet, do you mean from the pump to the carby? I have been told that the L34 pump has a 3/8 supply to the carby as apposed to the standard 5/16. Am I right?

Thanks, Gerry

[GML-31]

you need to change from tank to pump as well

[ls2lxhatch]

Exactly you need the bigger fuel lines to deliver volume. The L34 pump delivers higher pressure with standard fuel lines, remove the fuel lines replace with bigger lines and increase the flow the pump will deliver around the same pressure as a standard pump then.

I thought the pressure delivered by the pump is dictated by the spring in the pump.

A larger delivery line would have no effect on fuel pressure at idle. The only time the larger delivery line would make a difference is if the carby was consuming fuel at a rate higher than the standard delivery line can deliver at the fuel pump pressure.

[Redslur]

OK so what I will do then is fit an L34 pump, increase the line from the pump to carby from 5/16 to a 3/8 set up (as lonf as I can get a fitting to suit) and see how it goes. As stated earlier I would like to retain as much of the standard fuel system as possible (which means the line from tank to pump). I think that that should suffice as the fuel log appears to be doing its job. I am getting my car re dynoed on Monday after fitting the larger carby so should know a litle more then.

Thanks for all input!

[GML-31]

it would pay to replace the whole line.. it can only deliver what it can suck

[Redslur]

Trying to avoid doing that Kev. I worked on the principal that by fitting a fuel log it would fix my problem because there was a larger reserve of fuel for the carby to draw on. It seems to have fixed the problem but I want to just make sure that there is enough fuel there always. My theory is that with a larger 3/8 fuel line from the pump to the carby it should keep the volume of fuel up to my needs. Worth a try I reckon. I appreciate what you are saying though.

[Redslur]

All stock mate. Just trying to get the most out of all the original components with very little alteration if possible.

[EunUCh]

Does fitting the larger fuel line from pump to carb.tank to pump over come the restriction in the needle/seat area or
just through losses of the 5/16 cross sectional area from tank to pump to carb.
Never understood the bigger line concept hence the quest.

[76lxhatch]

There's nothing wrong with over-engineering your fuel supply system (within reason) if you're changing it anyway, but the standard 5/16" lines are capable of delivering more fuel than most people expect. I had the typical Quadrajet problem of emptying the bowl with the standard pump, and replaced it with one of those Carter electric pumps which is limited to around 5psi - this increased the flow enough to completely solve the problem. The car gets high 13s down the quarter (with a manual and more improvement to be had from my driving), and doesn't exhibit any signs of fuel starvation at all (as measured by the wideband oxygen sensor which reports mid 12s to 13:1 A/F ratio at wide open throttle).

[Redslur]

I tend to agree with you 76LXhatch. I have taken the car for a spin today and so far so good, however I didn't go any harder that 6500 1st and 2nd. In the past the problem was eveident when continuing into 3rd. My car is capable of a very low 13 sec / high 12 second quarter. I have a totally standard fuel system with exception to a fuel log and a 770 Holley. Now all I want to try is a L34 pump for more pressure/delivery using a 3/8 line from pump to carby more as a precaution. It always pays to ask others for their experiences as the info provided is generally priceless. I have had similar combos before, that like you, sucked the quadrajet dry. I used a Holley blue fuel pump to prime the standard mechanical pump and it also fixed the problem. When ever I wanted to play I would flick a switch which would turn the holley blue on and away I go. This time round I want to try and do it without an electric pump. So far so good.

Thanks again for the input.

[Balfizar]

GMH to overcome fuel vapourisation and fuel starvation problems followed this route

for a N/A V8 street application.

fuel vapour separator and return line
increased fuel line size
fuel line heat shielding
increase fuel pump pressure 3 - 7 psi
dual mechanical/electrical fuel delivery system. Electrical intank fuel pump ( EFI style control)and Mechanical pump on the motor.
Big single electrical (at the tank) fuel pump. keeps whole line under pressure!

You don't need fuel pressure:- all you need is volume (3 psi is fine as long as you have volume)
9 psi and higher cause needle and seat premature wear and failure.
High pressure can effect the float level . it can cause a higher float level needed to shut off the needle and seat against a higher input pressure..
(revised fuel pressure means revised float level)
rubber tipped needle and seat is always good @ 7psi or above.

N/A motors are easy. Blown/ injected/ surge tank/ return fuel systems can be a nightmare.

Cheers
Balfizar

Edited by Balfizar, 29 January 2012 - 06:32 PM.

[Orange SS]

when I 1st bought my torana it was fitted with both electric and mechanical fuel pump - and it still starved for fuel in high revs. I removed the mechanical pump and if anything it has become slightly better but still starves for fuel. I am running standard carby, and only a mild motor.

I was told a couple years ago there used to be an external fuel bowl available for the standard carbies to prevent fuel starvation, as the small bowl can heat up and fuel vaporise which leads to the car starving of fuel - does anyone know of this? (sorry if this is slightly off topic)

[Redslur]

What sort of revs are you talking when it was starving, and when you say standard carby do you mean a Rochester?

[Orange SS]

once it got over about 5k, it starts to starv of fuel. and yep a rochester carby.

[rexy]

Nothing wrong with a Quaddy. Ran one on the big block for a few years without incident.

[REDA9X]

The L34 pump will deliver more pressure than a standard pump with the same size fuel lines. If you increase the size of the fuel line from 5/16 to 3/8 what you would find is the pressure drops to around the same as the standard pump delivers with the 5/16 line, but the flow is increased.
To put it in simple terms it's like putting your thumb over the garden hose, pressure increases flow decreases.
L34 had the 3/8 fuel line from the tank to the pump, but it was still 5/16 up to the carby. You could run the L34 pump with 3/8 lines and deliver more fuel to the carby.

[MRLXSS]

The L34 pump will deliver more pressure than a standard pump with the same size fuel lines. If you increase the size of the fuel line from 5/16 to 3/8 what you would find is the pressure drops to around the same as the standard pump delivers with the 5/16 line, but the flow is increased.
To put it in simple terms it's like putting your thumb over the garden hose, pressure increases flow decreases.
L34 had the 3/8 fuel line from the tank to the pump, but it was still 5/16 up to the carby. You could run the L34 pump with 3/8 lines and deliver more fuel to the carby.

Yeah, I have noticed that Since I upgraded my fuel line from standard, to 3/8th the pressure at the gauge has dropped to about 5.5-6psi max now, as opposed to much closer to 7psi when I just had the standard lines...

But As Red says, the flow as increased, which is what the important bit is...

Is there a way that you can measure the GPH of these pumps? Or is that too hard because it fluctuates to much? I have 'heard' that they 'can flow' up to 80GPH, but is that true or just someone making stuff up? haha.

I remember reading in a SM Tech article a while back this little equation.

GPH Required = Desired Horsepower x 0.55 / 6

On a 450HP engine, that is only like 41GPH, which really isnt all that much.