Fitech Dual Quad fuel plumbing with return line

[JjyKs]

Got sick of my carbs being out of tune and not finding anybody with expertise to tune them without half a year of waiting here in Finland and finally bought the dual quad set. I have experience with EFI systems (megasquirt) from my earlier Porsche 924 Turbo, but this kind of dual throttlebody injection is new to me.

I've been trying to find how these are supposed to be plumbed, but no luck so far. I'm using external fuel pump and return line. Most of the installations use a sump tank and no return line from throttle bodies.

So here are the options I've figured out: Note there are 3 possible inlets, but only one outlet that's

In parallel

Possible cons: In theory all fuel might go through one of them, causing the other to vapor lock? Also a tiny difference between pressure regulators might cause another one to starve.


Inputs in parallel, second return blocked

This one was actually something I saw in a youtube video of a running a car so it should work. However somehow this seems like a bad idea to me. Might have problems filling the 2nd throttle body with fuel after sitting in a hot environment?

In series

Cons: No idea how 2 fuel pressure regulators work when connected like this. Does the 2nd TB get enough pressure?


In series, using input port of 1st TB as an unregulated output for 2nd one

Possible cons: Is the TB designed to flow like this between 2 inputs, or would it be possible to starve the 1st TB of fuel?

 

[Syleng1]

There are a few ways:
Option A: two feed and two separate returns- both back to the tank. A bit extreme but it works.
Option B: call fI tech and ask them how they recommend since they built them.
Option C: “y” both lines just make sure they are over sized to handle the flow. Your idea of one unregulated flowing into the second is great but is a single 3/8” line enough flow for both at WOT?

I removed my single fI tech due to its inability to hold a tune, having to replace the regulator twice. And the O2 sensor once all within 300 miles. Tech service is not super but I’ve delt with worse. There is an actual website that a guy has on the fI tech sets ups where he covers all of the issues and has down loads and replacement part numbers so you can buy parts locally. Like the regulator was a VW Bosch part number and was better than the factory. Also verify the temp sender is good. It’s a Chevrolet temp sender but again in that website they give you a bunch of part numbers that are better.

 

[92b]

There are a few ways:
Option A: two feed and two separate returns- both back to the tank. A bit extreme but it works.
Option B: call fI tech and ask them how they recommend since they built them.
Option C: “y” both lines just make sure they are over sized to handle the flow. Your idea of one unregulated flowing into the second is great but is a single 3/8” line enough flow for both at WOT?

I removed my single fI tech due to its inability to hold a tune, having to replace the regulator twice. And the O2 sensor once all within 300 miles. Tech service is not super but I’ve delt with worse. There is an actual website that a guy has on the fI tech sets ups where he covers all of the issues and has down loads and replacement part numbers so you can buy parts locally. Like the regulator was a VW Bosch part number and was better than the factory. Also verify the temp sender is good. It’s a Chevrolet temp sender but again in that website they give you a bunch of part numbers that are better.

We had one on the dyno with a temp sensor that was way off. It migt be a "thing" with fi tech.

 

[G-Fish]

Here's how I did it.
No issues after several hundred miles.

 

[Syleng1]

We had one on the dyno with a temp sensor that was way off. It migt be a "thing" with fi tech.

Yes. They give you a very cheap sensor with the kits. If I am to try it again I will go Holley but they have their issues as well.

 

[67Dart273]

View attachment 1715921673 Here's how I did it.
No issues after several hundred miles.

Do you mind telling us what you did? I cannot tell from the photo. You seem to have fed both inlets from one pump, right? What did you do with the returns? Do they go into one line? What size? And, did you ever measure the back pressure on the return line?

 

[Brooks James]

Enough bench racing, go for it.

 

[G-Fish]

Do you mind telling us what you did? I cannot tell from the photo. You seem to have fed both inlets from one pump, right? What did you do with the returns? Do they go into one line? What size? And, did you ever measure the back pressure on the return line?

Sorry.
Yes, one in-tank pump, 340lph.
For the supply side, I used the same distribution block I used with the carbs I was previously running (one inlet, two outlets).
For the return side I used a second, identical distribution block (two inlets, one outlet).
All lines are 3/8".
I did not measure back pressure.

 

[64mopar]

I set mine up the same as your first picture.

 

[JjyKs]

Fitech answered very quickly to email I sent them. Their official stance is the 2nd picture:
"On the duel quad system for plumbing the fuel lines. You can bring the feed line in to the rear throttle body and tee off to the front throttle to supply it. Then for the return line you can return off just the rear throttle body as it will set the fuel pressure. Then you can block off the return on the front throttle body."

However I might still do the 4th, passing the unregulated fuel pressure through the first one and using the 2nd one as the regulator. That way there won't be any dead ends where fuel might vaporize after shutting the car down when hot. Should be fine if it's also ok to plug the 2nd in without any return line.

 

[G-Fish]

Fitech answered very quickly to email I sent them. Their official stance is the 2nd picture:
"On the duel quad system for plumbing the fuel lines. You can bring the feed line in to the rear throttle body and tee off to the front throttle to supply it. Then for the return line you can return off just the rear throttle body as it will set the fuel pressure. Then you can block off the return on the front throttle body."

However I might still do the 4th, passing the unregulated fuel pressure through the first one and using the 2nd one as the regulator. That way there won't be any dead ends where fuel might vaporize after shutting the car down when hot. Should be fine if it's also ok to plug the 2nd in without any return line.

I like their solution. It sounds cleaner than mine.
I might update my installation.

Thanks for posting it.

 

[Coyote Jack]

Here's how mine is plumbed. Seems to work very well. It's hard to see but the inlets both go to the drivers side of the throttle bodies. The return lines come off the back of the passenger side of the throttle bodies.

Jack