Fuel Return With DCOE Carbs

[MREDDLE]

I am putting 40mm DCOEs on my 280z. My question is should I have a fuel return line or not. This has been brought up before. I couldn't fine were these were or what the result was. Also, I think I remimber 'TonyD' talking about the return fuel should be use to cool the carbs to keep them from heat soaking. Could some one (Tony) confirm this, provide picture and/or a discription on how this is done.

[josh817]

I'm not using one right now. I was curious about it too but I just went ahead and hooked it up how I figured it would work.

 

No fire, yet, at least!

[PanzerAce]

The fuel return through the cooling bodies I *think* is only a possibility on mikunis (or, at the very least, that's the only carb I've seen mentioned for it). And I'd say run a fuel return anyways, not like you're losing out on anything by having one, and it's pretty easy to set one up. Just have every carb feed be off of a T in the lines, and just daisy chain them along. Last T goes to the fuel return line.

[turboHLS30]

I'm also curious because I'm running blowthru and was wondering if it is any different with the fuel.

[MREDDLE]

This all makes sense. I have a Carter 4psi fuel pump. So would I have to just run a line back to my fuel cell or use a fuel pressure regulator? If/then would the FPR go before or after the carbs? I know just enough about Webers to make me dangerous.

[PanzerAce]

If the pump will only push 4psi, in theory you're probably ok, but I'd say run the return anyways. Not like it can *hurt* anything.

[josh817]

This all makes sense. I have a Carter 4psi fuel pump. So would I have to just run a line back to my fuel cell or use a fuel pressure regulator? If/then would the FPR go before or after the carbs? I know just enough about Webers to make me dangerous.

Run it before and just forget about the fuel return if I were you. Like I said, it works without a return, so if a return is in question, just don't do it.

[yetterben]

Heat soak is the issue with no return line

[MREDDLE]

Heat soak is the issue with no return line

 

This is part of the question that hasn't been addressed yet. I am really hoping someone can post up pictures of the fuel return line bing used to cool the carbs.

[PanzerAce]

This is part of the question that hasn't been addressed yet. I am really hoping someone can post up pictures of the fuel return line bing used to cool the carbs.

TBH, I've never actually seen pictures of anyone who's actually used the mikuni cooling bodies. Best bet is to hope that Tony has some pictures of them being used (and that he sees this thread)

[josh817]

There should already be a return line in there for the SU's or EFI I think... My car has 3 lines, carbon canister, into carbs, and then return. I capped the return off I think. Maybe it was the canister since I don't have one. Either way, if you want to prevent heat you could go with a ceramic coat on the headers and then wrap them, along with a heat shield. Such an effort just to keep "heat soak" away seems... I don't know... Silly to me. I'm in Texas and I don't have any problem with that stuff. Running Webers, no return, and ceramic coated headers.

[Tommy]

I'm also running triple 40 DCOE 18's with the 4-5psi Carter 4070. No return line. L28 cannon manifold, mild engine work done.

 

I have a pressure regulator before the carbs, though it is not needed with this fuel pump, reference http://www.racetep.com/webfuelspark.html#webfuel

 

This past summer, I drove the 240 from Vancouver to Toronto. These were relatively hot days, and the car was ran hard through much of the rockies. On the parries, I put in a couple 10+hour driving days, in the heat, occasionally ran hard. No fires to report.

[MREDDLE]

I'm also running triple 40 DCOE 18's with the 4-5psi Carter 4070. No return line. L28 cannon manifold, mild engine work done.

 

I have a pressure regulator before the carbs, though it is not needed with this fuel pump, reference http://www.racetep.com/webfuelspark.html#webfuel

 

This past summer, I drove the 240 from Vancouver to Toronto. These were relatively hot days, and the car was ran hard through much of the rockies. On the parries, I put in a couple 10+hour driving days, in the heat, occasionally ran hard. No fires to report.

 

Sounds like wee have the same setup. Its good to hear this. Although I would still like to clear up my original question, and have someone that is/has used a fuel return line to cool their carbs. Also, I do believe the webers can be used this way.

[Tony D]

Weber has no provision for cooling bodies, Mikuini, being an OEM fittment in hot climates, has them.

 

Advantage Mikuini.

 

I don't think a dead-headed fuel pressure feed scenario can properly be regulated without a Pulsewidth Modulated Variable Speed Fuel Pump---correct me if I'm wrong but somewhere in the system is a check valve, regardless of the e-cheaps dial a regulator installed. When the engine is shut off, fuel gets hot, expands, and sinks the floats off the inlet jet making for a high float bowl level (and possible hot restart issues).

 

With a return line and proper bypass FPR, this can never happen if components are working.

 

With a returnless fuel system, it happens every time the car is shut off.

 

Backpressure regulation of the fuel rail to tripples is the most efficient and precise way to meter fuel equally to all carb inlets. With a restrictor style 'regulator' all you do is overwork your fuel pump and run the chance that the last carb in the line is the first to run lean (nice for #5/6 which run hottest anyway...)

 

If you have Webers, you have to insulate against heat, there is no way to run a cooling body. Wayne Calder had Mikuinis on his 3.2, and would percolate like crazy in the summer heat in SoCal. The addition of cooling bodies made it like night and day, afterwards the car ran in hot stop and go traffic like an EFI vehicle. No more stumbles, gas smell, bucking surging, etc...

 

If it gets hot, Mikuinis are the only way to go, if you're stuck on carbs. Most of the area north of AZ,NM, and SoCal have enough aerosols in the atmosphere to really cut the UV radiation to the tarmac, and consequently the air going through the radiator and into the engine bay is likely very close to ambient. In the perviously mentioned states, on an 85 degree F day, the thermal layer into the engine bay, BEFORE passing through the radiator is close to 130F. For all those who pooh-pooh a 160F thermostat, keep in mind that air coming off the radiator will likely be within +10F of your thermostat rating. Nothing I like more than blowing 200F air onto the carbs and components in the engine bay...

 

It's a system, people put bliners on and look at one thing at a time, have the best components imaginable, and can't run worth a hill of beans when it gets hot because they didn't look at the package as a whole.

[PanzerAce]

....yah, that reminds me that I really need to run my fuel return to the cooling bodies *before* it starts hitting 110+ ambient this summer

[MREDDLE]

Thank you very much Tony. Although I may not live in the mentioned states, but I live in Las Vegas NV. Ambient average temps hover around 100deg for a few mounths here and many nights so hot the temps dont drop below 90deg. Im not going to change over to the Mikuinis. So, I will have to keep an eye on my over all design with a fuel return line and before carb regulator.

 

So this does bring up another question as to were to mount my heat shield. I'm looking at attaching it to my manifold. I have seen peapole attach it to the carbs, then I have seen some new ones that attached to the bottom of each carb?

[ZR8ED]

I will second Tony's response.

 

I ran without a return line for a little while just like several of my friends Z's. Then when I sought a solution to occasional hot hard starting issues, I ran a return line (my 280 had one conveniently located) and those issues disappeared. Even on smokin hot days at the track. No issues. Return hot fuel to the tank and replace with cooler fuel. No brainer really.

 

Scott.

[cygnusx1]

In my poor-man college days, I had a high pressure FI pump feeding the webers, with a return line. I wrapped electrical tape around the return line, put a clamp over the tape on the hose and tightened the clamp until the Webers saw 3psi. Worked like a charm.

[josh817]

I saw in some Datsun a book a picture of the oil spray bar that had fins on it with a caption that said "The SCCA counted this as an oil cooler" or something. Get were I'm going? Why not make an aluminum fuel rail with fins?

 

Like this but make it a fuel rail...:

[BluDestiny]

If you're using the stock fuel rail, why wouldn't you hook up the return line.