[Tom Vaught]

Originally Posted by 455firebird1969

Stressing? There is some good information is being shared in this thread, especially why a larger return line is the correct way to plumb a fuel system . Additionally, one will understand that fuel hose & tube measured by the AN system is not necessarily ID.

Agree.

Tubing as was mentioned be it 2.0" tubing or .5" tubing is advertised in a generic size based on the outside diameter. You can buy tubing in different wall thicknesses.

A 316 stainless tube might be 1/4" (assumes .250" outside diameter) and have a wall thickness of .035" and a inside actual diameter of .180"
Example: .250" - (.035" x 2) = .180"

So if you ordered 1/2" outside diameter 316 stainless tube with a wall thickness of .035" your number would be .500" - (.035" x 2) = .430" inside diameter. Rarely can you buy tubing with less than a .035" wall thickness.

You can also get seamless aluminum tubing in the same wall thickness for .500" tube so the inside diameter would also be .430"

ZORO sells a 5/8" aluminum tube see link below that has a .555" inside diameter.
It is expensive though at $15 per foot of tubing.

https://www.zoro.com/value-brand-tub...03/i/G1781123/

Zoro sells a 6 foot length of 316 stainless 5/8" outside diameter .035" wall thickness for $8.25 per 6 foot stick that you could bend to your frame contour and add connection fittings. That might be the best deal out there. $1.37 per foot plus shipping.

https://www.zoro.com/value-brand-tub...B&gclsrc=aw.ds

Tom V.

[RobbMc]

I probably answer a question about return line size at least four times a week. There is a lot of confusion. Maybe I can make the waters a little less murky and reduce the phone calls I get.

There are at least three basic types of return line. Each returns fuel back to the fuel tank but for different reasons so the required minimum size varies depending on the intended use of the return line. (Keep in mind, a return line cannot be too big unless it is rubbing on a tire or dragging on the ground. But there is a point where making it bigger doesn't gain you anything.)

The first type of return line is the "vapor" return line or "bleed". The name "vapor return line" is a misnomer. A vapor return line returns liquid fuel as well as vapor back to the tank. It is called a vapor return because it is primarily intended to reduce the chance of vapor lock and keep the fuel and fuel pump cooler by circulating a small amount of fuel back to the tank. The amount of fuel circulated is restricted by a small orifice, maybe .030" to .040" in diameter (if you don't use a restriction you will lose too much pressure).
Some factory mechanical fuel pumps came with a small vapor return nipple on the pump. On other cars, the vapor return comes off the factory fuel filter. In either case, because a restriction is used, only a small (usually 1/4") return line is necessary. We offer our RobbMc 550HP pumps with or without a "vapor" return. I usually recommend it on street cars that do a lot of idling in traffic where the engine is using very little fuel and the fuel will get very hot if a vapor return is not used. A vapor return will also quickly bleed off any residual pressure in the line between the pump and the carb whenever the pump is not running. This alone can often improve hot restarts.
A "bleed" is the same as a vapor return except that the restriction and return line is connected to the outlet side of a dead-head (non-return) pressure regulator. This DOES NOT convert the regulator into a bypass/return regulator (where ALL of the unused fuel is returned to the tank). It is simply a bleed/vapor return that comes off a dead-head regulator instead of a mechanical pump or fuel filter. It still requires a small restriction to avoid a large pressure drop. But not only does it help keep things cooler, but it also eliminates the dreaded "pressure creep" that often occurs at idle when using a dead-head regulator. We offer a restricted fitting for use with our dead-head regulators to be used for this purpose. It can be used with a mechanical or an electric pump.

The second type of return line is the type connected to some larger electric fuel pumps when used with a dead-head regulator. Lower GPH electric pumps intended for dead-head regulators have internal pressure limiters that circulate the fuel internally to limit the pressure the pump produces (so that the pump doesn't make too much pressure and burn up). But once the pump GPH gets to a certain size, limiting the pump pressure internally would make the pump external dimensions HUGE and possibly overheat the fuel. So, the pressure produced by the pump is limited by sending excess fuel back to the tank via a return line. The required size depends on the pump GPH but generally can be a little smaller than a return line used with a bypass/return style regulator using the same size pump because the pump is near the fuel tank and because the pump pressure is usually limited to something between 12 and 30 psi (rather than the 3 to 9 psi the regulator for the carb needs to maintain). Still, for pumps over about 175 gph, a 1/2" (-8AN) or larger return line is usually recommended.

Lastly, there is the type of return line used with a bypass/return style pressure regulator. This is the type of return line where you really want to error on the large side. A bypass style regulator limits the fuel pressure sent to the carb by sending ALL of the unused fuel back to the fuel tank. Since most people use a pump that produces more GPH than their engine will ever need (which is usually a good idea), the return line will be sending fuel back to the tank, even during full throttle. It is not uncommon to need a larger return line than feed line. The feed line between the pump and the carb is under pressure (it better be!) but the return line needs to see as little pressure as possible for the bypass regulator to work really well. If you have two fuel lines of the same size, and one has more pressure, that fuel line can move more volume. So, to make sure there is little or no pressure in the return line, it will normally need to be bigger since, at idle, it must flow 99% as much as the feed line. In fact, if a bypass fuel system works great, even though the return line is the same size as the feed line, then the feed line is bigger than it needs to be.
So why does a lower pressure in the return line make the bypass regulator work better?
Try this mental experiment: Imagine your engine is at idle with an electric pump and bypass regulator installed. Nearly the entire volume of fuel delivered by the pump is being sent back to the tank by the regulator. The regulator works by using a plunger connected to a diaphragm with a spring on top. When the fuel pressure on the diaphragm gets high enough, the spring is compressed which lifts the plunger, allowing the fuel to go back to the tank via the return line. You can adjust the pressure at which this happens by turning the adjustment screw that is on top of the spring which compresses the spring, putting more force on the plunger. Let's say your return line is a bit too small but still big enough so that you can adjust the pressure down to a minimum of 5 psi. Let's also say that you want to have 7 psi delivered to the carb. So you screw the adjuster down until you have 7 psi at the carb. The restriction in the return line is producing 5 psi due to the large amount of fuel trying to get back to the tank. The other 2 psi is being produced by the regulator. So, now what happens when you go to full throttle? Suddenly, a lot less fuel is trying to return to the tank, so there is now less back pressure in the return line. Let's say the back pressure in the return line now drops to 2 psi because your engine is using a lot of fuel so a lot less is going back to the tank. What happens to the fuel pressure delivered to the carb? Remember, you adjusted the regulator so that the spring puts enough force on the plunger to provide only 2 psi. And now there is only 2 psi remaining in the return line. So, the carb now only gets 4 psi. You have a 3 psi pressure drop at the carb at full throttle, even though you are still not using all the fuel the pump can provide.
To summarize; Assuming the pump can provide adequate volume and pressure, the amount of pressure drop at full throttle decreases as you reduce the amount of back pressure in the return line at idle. (To test the back pressure in the return line, turn the pump on with the engine off. Now see how low you can adjust the pressure. If you can't adjust the pressure down to 3 psi or less (less is better), I recommend you reduce the restriction in the return line with a larger line and/or less restrictive fittings.

I hope that makes the waters a little clearer.

[johnta1]

Thanks, Robb!

[BruceWilkie]

Excellent info Robb Mc!!!

[RobbMc]

You're welcome!

PS. Skip, check the instructions that came with the regulator. The size of the return line depends on the GPH of the pump, not the feed line size. (For that 250 gph pump, you are going to need a 5/8" (-10AN) or larger for the regulator to work really well.)

Link to instructions, just in case:
http://www.robbmcperformance.com/ins.../Regulator.doc

[Tom Vaught]

The Power Surge set-up is a very nice unit based on the info I have read on Robb's website. The system should work great as there has been similar Surge Tank systems out there even in the mid 90s time frame. Typically these were fabricated systems like the one Marty Palbykin had on his daughter's Grand Prix.

Marty used a mechanical Pontiac fuel pump to feed the surge tank, a return line (factory) to return fuel to the tank, and a EFI fuel pump to fuel the efi system.
Do not remember which bypass regulator he used to return fuel to his rectangular surge tank he mounted on the inner fender.

The thing I like about the Power Surge deal is that there is minimal fabrication required to use the system and the plumbing is straight forward. I see that he uses the Walbro Fuel Pumps which are very high quality. Walbro is owned by TI. I personally think the Aeromotive Pumps are made for them by Walbro/TI.

This beats the heck out of running even more lines under the car.

Just my opinion.

Tom V.

[WARPed]

Maybe this should be a sticky?

[Tom Vaught]

Originally Posted by WARPed

Maybe this should be a sticky?

X2 on that deal.

Tom V.

[Skip Fix]

Been reading the instructions! I understand the theory but as I said -10 or 5/8 is pretty big line to run especially back by 3" tailpipes-probably have to make a heat shield for them. But I see a lot of guys, so here, using -8 for return also. Going -10 and maybe a section of 5/8 back by the tank.

Got the 5/8 aluminum in from Summit yesterday-not really that much smaller than -10 braided! Not sure I can bend it tighter than a -10 braided without kinking it either but Harbor Freight has a 5/8 bender I'll see.

So on a test run plugging output port to carb give a rough setting for idle pressure?

Going to try the 550 mechanical and a bleed return on the '81's new 455 also.

[RobbMc]

"So on a test run plugging output port to carb give a rough setting for idle pressure?"

Yes.

[grandville455]

Skip, what did u ever figure out on this? Did u go with a -10 feed and return? Are u using a bypass style regulator?

[68WarDog]

Grandville, after reading this post several months ago I tested my -8/-8 system, but I'm only using the Mallory 140 pump. Was able to turn regulator down to 2.5 psi. So maybe guys not running bigger pumps,175 GPH or higher should be fine. But like the idea by Robbmc to test our system. What size is your system, and what were your test results?Roger

[grandville455]

Roger, My current setup is 2 holley Blacks, fed with a -8 to each pump and then -6 from each to a tee before a big holley reg, then a -8 to each bowl... I also only have a -6 vent, which i think is also hurting me, i'm going to use a mallory 250 fed with a -10 and the -10 to if i can afford it lol. areomotive 17248 fuel log with bypass reg back to my tank with a -10, i will be either adding a return and bigger vent to my current cell or i will buy a new one already setup, prob what ill do.. My pressure is solid, but i just think the volume isnt there either cause of the -6's or the too small of vent, i haven't did the gallon test yet with old setup, with the mallory 250 i tried the other day and with a -10 on each side, just short hose , i was able to pump a 2 gallon can 3/4 full in 14.40 seconds. I know that will change once i get it all setup in the car possibly, but it really sounds like the bypass regs are the way to go, u get full pressure from pump at all times and reg does what its suppose to and regulate.

[Skip Fix]

I ended up with a -10 return line. First trial was Aeroquip regular line. Started oozing some oily crap through the braid when pressured up for a fuel test! Help here pointed me to Fragola having -10 PTFE(teflon) hose I could not find elsewhere. It is smaller OD and turns are about as tight fit a little better snaking through where exhaust will be and subframe connectors are. They even have a -12 teflon.

Plain SS braided rubber lines have a life span especially with any of today's ethanol fuels. They can start leaking like a sieve with pressure! The PTFE is not supposed to have that issue and is rates higher PSI. I took some pictures I'll post later.Mallory 250 pump McRobb bypass regulator set at 6.5

[grandville455]

Thanks Skip!! Look forward to pics

[Skip Fix]

I thought it was always best to have the bypass as close to the carb as possible for cooler fuel.
Here is it with the -10 rubber lined SS return,vent w/ hard line and filter, pump, out, return and vent, supply and retuen tucked away form where exhaust will go can see sway bar link there on left, and where it will be once the Pontiac motor goes in.

[Skip Fix]

And that is a RobbMC mechanical on the 383 SBC there .

[grandville455]

Originally Posted by Skip Fix

I thought it was always best to have the bypass as close to the carb as possible for cooler fuel.
Here is it with the -10 rubber lined SS return,vent w/ hard line and filter, pump, out, return and vent, supply and retuen tucked away form where exhaust will go can see sway bar link there on left, and where it will be once the Pontiac motor goes in.

Skip, How do u have that pump mounted? The orientation in pics is are to understand. Do U have it mounted vertical? I want to mount mine vertical and not have to use any type of a 90 coming into it. I picked up a aeromotive 17248 setup used for a great price! Wasn't going to pass up on that.

[Skip Fix]

No it is almost sideways slight angle vertical

[grandville455]

Yea, I'm wondering if I'll have to cut a hole in trunk pan so I can mount it vertical, haven't looked at car much since I put it away for winter lol