Fuel return line routing question

[mattpatt]

I'm installing an MSD Atomic EFI on SPF3033. Engine is a 554 hp SB so I went with the hi horsepower optional pump (good up to 620). The pump installation recommends a 1/2" feed line from the tank to prevent cavitation from too much pressure drop to the pre-filter - but the car has only 3/8" hose. There is also the concern that cavitation could occur at the pump inlet if the pre-filter is too fine a mesh (I picked a 40 micron - may be too fine). Seems that running the return line to a TEE between the pre-filter and pump (instead of to the tank) solves all of this and greatly reduces the flow thru the pre-filter (only the engine demand) so it might last longer. It would also reduce flow into the tank pickup which might help during acceleration (sloshing starving the pickup). Also, the plumbing would be easier. This would, however, double the range of pump inlet pressure from ~.3 psi to ~.6 psi. I don't have a vapor lock problem so not an issue but this routing would at least partially help.

Bad idea??

Thx

Matt

[Gaz64]

I'd run a surge tank with your main pump drawing from it.

Feed the surge tank with a smaller pump as below:

http://i135.photobucket.com/albums/q...fries/fuel.png

[scottj]

Not a good idea to return fuel to the pump instead of the tank. The pump is cooled by fuel flow. Returning fuel to the tank will provide the coolest fuel possible. It's also not a good idea to run low fuel level in the tank for the same reason.

[PLDRIVE]

i agree with ScottJ "bad Idea". U need cool fuel. Also do not return the fuel near the pick up.

[lippy]

If I were you I would run a new 1/2" fuel line and avoid the extra complication. You can use a stainless-braided PTFE hose. I prefer Aeroquip because of the way the fittings attach to the hose.

[WardL]

Not sure if this adds anything to the conversation, but all gasoline dispenser pumps in reputable retail stations have 10 micron filters. Your 40 micron filter is not too fine. Add to the picture, you can't see particles less than 50 micron.

[mattpatt]

Thx, guys.

All good info.

[CHANMADD]

Mercedes use a similar system to what you describe.......early .C class had two pumps and qfilter and I think they called it an accumulator............essentially allowing the fuel to recirculate around the pump...........worked for them ........... The main line is about 1/2 and the little recirc line about 1/4.......

[Cheesehead]

I am using the same EFI. Works great. I had to install a surge tank. Under hard acceleration (not too difficult with a Cobra) all the fuel in de tank moves to the back (the fuel pickup is in the front) and your pump (and engine) will run out of fuel. It happenend to me every time I had half a tank of fuel or less.

I am using a return to the surge tank. I have a pump (standard low pressure pump)to feed the surge tank. This surge tank has an overflow back to the main tank. From the surge tank the fuel goes to the EFI-pump and then to the EFI.
So even under hard acceleration or hard cornering there is always enough fuel.

[mattpatt]

Where did you locate the pre-pump and the surge tank? Also, what pump and tank did you use?

Thx

[Cheesehead]

In the trunk/boot. The surge tank is small, it contains not more than 1 liter. Any surge tank will do, as long it has enough connections for all your fuel lines and fit the connectors you want to use. I bought one locally at a racing shop. I have also seen this in the wheel well (this is the english word, there is a more correct word for it. Can't remember right now). There is enough room, but it might be a problem if something happens to your rear tire.

The pre-pump is a Carter. I had an old, but still very good one laying around and used that one. I think any pump for a carburetor will do (maybe an intank-pump is the best). You have a return line from the surge tank back to the main fuel tank, so don't worry to much about its capacity.
It is wired parallel to the EFI pump. So switching on the EFIpump also switches on the pre-pump. Works for me. All problems with acceleration and hard cornering were gone.

Chris

[mattpatt]

Thanks, Chris.

I will probably follow your recipe.

Matt

[PANAVIA]

What about the Aeromotive stealth pump arrangement ->?

- with a AN8 feed - and AN6 return.

-- if you had an in-tank pump with a shell / baffle around it , you could save valuable undercar space, which is very scarce in a SPF.