plumbing tips for dual fuel pumps

[MST0118]

For a future project I’m planning to use dual MFI fuel pumps in the trunk near smuggler’s box like the 73/74 RSRs and STs had. There isn’t a lot of information on how plumbing works from tank to pumps or pumps to tunnel.

I think there must be 2 Y type hose fittings that go from single hose connected to tank and then to dual pumps and then from dual pumps back to single hose in tunnel.

Does anyone know how factory plumbed these or does anyone have a custom dual set up they can share? I’m looking to run these in parallel (similar to RSR) not in series. Also, interested how to wire them and whether check valves are necessary.

Here’s a pic of 73rsr dual plug from trunk but can’t see how it’s plumbed.

[BoxsterGT]

I made up these hoses for a dual pump setup a while back.....



It requires two supply connections from the Tank and a Tee to the Tunnel Feed Line through the Chassis.

With your MFI pumps, probably would need a check valve for each somewhere in the system as well. These late pumps have their check valves built in.

I can help with Metric Hoses if needed, and have crimped AN fittings to Metric Hose....



Also have AN-8 Female to Metric adapters for your Fuel Cell if needed....



For info, please email me, ONLY. NO pm's, box is FULL.

len.cummings at verizon.net

[Walt Fricke]

I think most fuel pumps act as a check valve when not powered. In any case, on my modified car with fuel cell I use two pumps with two separate inlets (one in each rear corner of the tank - it will drain it dry before the engine stumbles), and T the pumps together. But both run all the time, as the reason for two is to deal with fuel starvation at high cornering Gs, not for reliability or ability to switch out a bad pump on the fly.

I've had one pump go bad, and the other still supplied the fuel, so it didn't just pump right back into the tank.

[MST0118]

Thanks Len for posting setup and I’ll be in touch. I’m using a plastic tank like the RSR/ST plastic reproduction below which apparently has 1 hole for inlet and 1 for outlet. So, confused how 2 supply lines would be plumbed into tank unless there is some special Y or T fitting when exiting tank that goes to the 2 pumps.

Walt, I like your setup best but I would need to replace tank with fuel cell and trying to see if RSR plastic tank can be used for that original look before exploring that option.

Here’s a pic of the inlet and outlet from RSR/ST 110 liter tank that I’m trying to work with.

[BoxsterGT]

Just wondering if you could use both fittings at the bottom for Supply and return the fuel at a breather of some sort at the top of the tank, perhaps at the filler neck if steel near the Cap.

I think those threaded bottom fittings are M18 x 1.5.

Are you using the early MFI style as in your first photo? They are god-awful expensive. The later "Flow-Through" Bosch Pumps are now very reasonable in cost.

Most pumps use a larger hose (10mm or 12mm) for their Supply than they use for the pressure (8mm) side.

I do have 8mm Y fittings and can get 12mm Y's if needed.

Len

[MST0118]

Yes, I’m using the early mfi pumps. One has been restored and I was going to have the other one done. I like your idea of using two supply lines below and return using a breather on top.

If 2 pumps won’t work with tank, I may consider the newer flow through pump. I’ll send email soon. You’ve helped me with oil lines in past to front cooler. Thanks.

[Walt Fricke]

Did Porsche use two pumps in order to get the flow they needed before the MFI added in the injection pressure? Or was it a reliability thing, as the NASCAR guys do (or at one time did)? If reliability, most likely there was a selector switch on the dash?

I have trouble thinking that the "stock" RSR tank outlet isn't wide enough for adequate flow, so the simplest thing would be to use two Y fittings (or two more easily sourced Ts), and keep in and out separate and the same. In stock tanks, the return goes to a swirl pot to de-aerate or something the return flow, and wouldn't work all that well as a fuel pickup - too high up? And the return tube has a smaller ID, too.





But the real RSR tank may be plumbed differently.

[BoxsterGT]

I have a Plastic Gas Tank for a RS out somewhere in the barn. I would like to check both bottom fittings.

I wonder if an endoscope would work to check for the interior baffle.

I thought the baffle came out with the CIS cars, not the MFI models.

Is the FVD Tank still available new?

Len

[Walt Fricke]

Len - you may be right about the baffle - for sure I've never looked inside a real RSR plastic tank (though I saw one in a car once), nor owned a pre-CIS car except one with a fuel cell. Get out that endoscope.

Unanswered (and Mr. Google didn't help me either) is the original question - what was the deal with two fuel pumps on these factory race cars? Doesn't look like, say, the 930 system, with a low pressure pump and a second pump back by the engine.

[BoxsterGT]

The setup we built years ago was for a customer in endurance racing and they had two separate circuits, with switches to control each. I assume used as a backup if one pump were to fail.

Does anyone know if the factory RSR's had switches to run either pump during a race?

Len

[MST0118]

Len, I also researched this issue on the S Registry site and consensus was that both RSR pumps ran all the time but the ST had a switch where one pump ran at a time.

Also, it looks like both the supply and return had T fittings from the tank for both the supply and return openings so it doesn’t seem too complicated. Although the RSR pumps were above the tank, there are other setups where the pumps are lower in smugglers box or below tank for perhaps an easier flow with less suction.

I’ll look for some T or Y fittings and check valves and will have you help me with hoses and fittings. Thanks.