The bypass will help keep your fuel cool so you do not encounter vapor lock, which is a huge thing on these cars..

 

Bypassing an electric is really the only way to go. It keeps cool fuel running through the pump and the pump is not "loaded" up......
Now on a mechanical.....it really depends on how much it can flow........I have a 120gph Holley on my car dead headed and it works fine......but a lot of folks use a bypass of some type.
The point is that bypasses work well......but sometimes not easy to accomplish.....if you have the return line....I say do it.

Jebby

 

I'll be running a Holley mechanical pump with a inline billet filter, Quick Fuel billet pressure regulator, set to 7 psi and a Quick Fuel Q-850 carb.
Just basic and no frills.
Thanks

 

On a mechanical pump, I would probably try it without the return first. The reason I say that is, running it with a bypass style regulator and returning to the tank, the fuel pump may not have enough volume to keep up 6-7 psi at the carb at WOT, especially on a 600+ HP motor. You would likely have to restrict the return line to minimize the return flow or something. Also, pay attention to the plumbing on the mech pump inlet. On my old Carter 172 gph race pump (mechanical), It used a -10 AN inlet, but the way our frame wraps around the fuel pump, I could only squeeze a tight 90* elbow and -6 AN. I ran it like that for years and fuel pressure would fall off like a stone. It would never lean out, but fuel pressure was 0 psi in the traps. No matter how I clocked the fuel pump, I couldn't make room for the proper size inlet fitting. Ultimately, this is why I went to an electric pump. I did run the mech pump with low fuel pressure for years, I had a wideband O2 meter and kept tabs on the AFR, but I always knew it wasn't "right".

The Holley Ultra 200 mech pump has clocking available with 180* placement from the inlet to outlet, its also -10an inlet. I THINK it will fit and clear the frame, but the pump has mixed reviews and for $400....it was too much of a gamble for me.

These are just some tips for you as you move forward and things to watch out for.

 

Some mfrs recommend a regulator with some models others dont.
172 Carter mech pump 9which looking back is overkill) no regulator, runs between 7-8 lbs no problems. May be better with one but cant say theres been a problem to justify it.
Seen some 110gph that do, not sure why others the same size or larger dont, must be the design . No internal bypass?

 

Here is what I had on the Carter 172 pump, and the -10an on the right is what the pump actually required to hit that flow rate. Needless to say, I was not getting anywhere near that flow rate...Not even half. This pump is self regulated at 7 psi so it doesn't need a regulator..... Plumbing it correctly is a nightmare in a vette frame though.

 

This is the fuel pump I purchased: Holley 12-454-11
Features:

• High output fuel flow
• Flows 110 GPH free flow
• Shutoff pressure pre-set from 6.5 to 8 psi
• Fuel pressure regulator is not required
• Heavy duty construction for continuous high rpm operation
• Fuel body casting can be rotated to accommodate various plumbing situations
• Inlet & outlet tapped for 3/8" NPT

 

 

I have a 79 and it came with a ac Delco pump and the stock return line.

830 demon carb 434 doing 7500 rpm at WOT for the longest straights like at the 3000 foot Miller road racing track. My headers must have been glowing because I melted the Energy Suspension red poly motor mounts. Never missed a heart beat and only about 5 psi because I used the anti airation big titanium needle and seats for alky fuels.

I went to solid engine mounts when the driver side broke.

but anyway people seem to over think fuel pumps. The vapor lock happens at the pump is why GM went to the pump with the return line. 5 psi and big diameter needle and seats keeps the floats from continues cycling

some motor doing a drag racing is only at WOT for maybe 10 or even 12 seconds not 20 or even 30 on long straight road racing or even longer like trying to get to 200 mph at Bonneville salt flats

 

Original equipment on my '74 included a return line. During a rebuild I used a fuel pump that didn't support the return line. (At the time I didn't know any better.) This car isn't driven all that much but on two occasions I was left clogging the road when the car wouldn't run. After the second instance I realized it was vapor lock. I replaced the fuel pump with a return line and have had no problems since.

 

Quadrajet's needed a return line and Holley's did not. That's why they are there on some cars GM built, but not all of them.

 

And then there is the 71 LS6 which has Holley and a return line as built in St. Louis.

 

Like I said it has nothing to do with the carb type. It was when GM engineers figured out that the return line cured vapor lock problems The carb type was year related. My 61 dual 4s were Carter carbs. Then GM must have gotten a better price on Holley carbs through the rest of the 60s until they transition to all Quadrajet.late 60s through the end of the c3 when they went to crude fuel injection.

I don't know all the NCRS trivia

 

A good thread, but I have been wondering for a while if there is another way. Could you plumb a FPR off a fuel injected car into the fuel line near the carby? These are vacuum operated and if mounted near the carb, would send petrol back to the tank under light throttle/ high vacuum conditions, avoiding a dead head situation. Vacuum would be sourced from the manifold nearby. And when you put the pedal to the metal, the vacuum disappears, the return port in the FPR closes and no petrol is sent back to the tank, allowing full pump pressure into the carby. If this would work, then vapour lock could become a thing of the past on a hot summer's day, like we are experiencing in Melbourne right now. It got to 45*C one day last week and I did not want to drive the 'Vette that day. Food for thought???

Regards from Down Under.

aussiejohn

 

I'll be running dual Spal electric fans on my Dewitt's radiator.
Would it help to wire a circuit into the fans so they will continue to operate and circulate air through the engine compartment for a couple of minutes after the engine is shut down?

 

You can do a very simple return simply by tee'ing the fuel line at the carb and having a small orifice leading to the return line. A number of people have done this quite successfully. I don't remember the size range offhand, but it's not hard to find if it's something you want to try.


Air flow over engine components just doesn't do much useful since they weren't designed for air cooling (would needs fins added). The heat energy in the block is enough that once the minute or 2 of airflow stops the temperatures reached are still about the same. Also, you can end up with a dead battery if you walk away and the fans stay on for some reason.

 

I just cured my vapor lock issue by reconnecting the return line.
Previous owner bypassed and used a cheap plastic filter

Ran fine for 5 miles and them stalled and sputtered badly.
Had to buy a fuel filter with the built in return on my 68 but it was worth it.

Good luck

 

My 73 doesn’t have an OEM bypass line and I haven’t had any problems, but I’ll install one while the body is off and if I have any issues then I’ll set it up.