The QFT Fuel Line/By-Pass Regulator kit works for most 4150-style carbs and requires a fuel return line. Installing the QFT by-pass regulator helps prevent vapor lock and erratic throttle response during stop-and-go driving. The return line attaches to the bottom of the fuel regulator and should return to the top of the fuel tank. The return line needs to be the same size as the feed line.

These types of regulators are also referred to a “loop style” system as well, in that a return line is required to exit unused fuel back to the fuel tank. These regulators start to “open” when the pressure is built up to the calibrated or adjusted level. Excess fuel is routed back to the fuel tank or fuel cell. A more proper name to call these types of regulators would be “relief valves”. The advantage of these types of regulators, and why they are used in modern EFI vehicles today, is its accurate fuel pressures, especially at low engine operating conditions. Another advantage to these types of regulators is that a higher pressure source to the regulator is not required, and thus not requiring an additional bypass or relief system to prevent “dead-heading” the fuel pump. One disadvantage of a bypass style regulator is that the regulator regulates all of the fuel system, typically preventing multiple pressure levels (such as the use of Nitrous Oxide).
Capacity of the regulator is related directly to the ability to “open” enough to send fuel back to the tank fast enough. If the return line is too restricted or if the regulator capacity is too low, the fuel pressure regulator will not operate. This will result in an over-pressure condition. If the fuel pressure is too high (higher than the desired amount and yet still supposed to be within the range of the regulator), chances are you have too much restriction in the return line, or have too “small” of a regulator.

Many people base fuel line sizing based on common misconceptions and customs and as such a fair amount of fuel installations may be fine for a race only application they are unsuited for street applications due to the street duty cycle being on average 20%-30% with idle, city cruise of 15%-20% of the engine power and if the fuel system has reserve these duty cycles would be 5%-10% of the fuel system capacity.

Fuel pressure supply lines are usually adequate for maximum performance, with a preference of 125%-150% fuel line delivery of 100% power to ensure adequate fuel in the event of an unforeseen problem with fuel delivery, i.e. pump speed/voltage/dirt. Fuel pressure lines should be sized to expected capacity and fluid velocity below 4 fps to reduce the Reynolds Number which signifies excess turbulence hence reduced performance.

With such low volume use of the fuel system capacity return fuel lines area usually inadequately sized. The purpose of the return line is for the unused fuel delivered to the engine to be bypassed back to the tank as all constant volume pumps cannot be dead headed. When this fuel is bypassed it experiences a pressure drop and any air dissolved in the fuel or entrained in the fuel boils out forming air bubbles as well as any fuel components vaporizing during this pressure drop. Ideally we want this activity to occur in the fuel line and not in the tank. When the pressure drop in the line is not sufficiently close to ambient the gasses do not condense back into the fuel and the air does not form complete bubbles in the line to escape the fuel in the tank. What does occur with an insufficiently sized return line is that when the bypassed fuel returns to the tank there has not been a sufficient pressure drop in the line and the remaining pressure is discharged in the tank and with it air and gasses discharging forming foam. Return line velocity should preferably be less than pressure line fluid velocity to accomplish the reduction of foam and fuel rail pressure rise.

Fuel line bypasses are commonly called a regulator which is incorrect. A regulator controls pressure on the outlet of the regulator, regardless of the input pressure to the regulator. A fuel bypass function is to bleed off a preset pressure to the outlet; the outlet pressure should be atmospheric and is determined by any restriction downstream. The bypass outlet pressure is ideally ambient atmospheric although return line sizing if too small will increase outlet pressure and added rising fuel pressure line/rail pressures. Another problem with fuel bypasses is the under sizing of the bypass orifice resulting in low flow causing low fuel consumption pressures higher than high fuel consumption pressures causing erratic fuel rail pressures. A larger bypass would be required or a series of bypasses to bypass the total fuel not used at low fuel consumption conditions.

Return line example - If the engine requires 600 lb/hr of fuel at maximum power and is used in low speed/power applications then the return line and bypass needs to return 90% of the fuel with ideally no pressure loss. That would mean a return line sizing equal or larger than the pressure line.

Return line example - If the engine requires 600 lb/hr of fuel at maximum power and is used in low speed/power applications then the return line and bypass needs to return 90% of the fuel with ideally no pressure loss. That would mean a return line sizing equal or larger than the pressure line.

OP .. bypass fuel system requires same sized return and feed lines else it will have a fuel pressure creep issue when at low fuel demands

.. im not reinventing the wheel.. this is well established fact and has been for eons

Lets apply a little common sense to your "five seconds of contemplation"... -6 line is not too restrictive. And it's not going to create fuel pressure creep, if it was than I would have noticed it while I was flow testing my 044 (several times) last week. When I set it to 43psi it held dead even at 43psi for the duration of multiple (five gallon) tests. When I set it to 70psi it held dead even at 70 psi for multiple tests, all at 0% IDC. Sorry, that says to me that a -8 feed and a -6 return is perfectly fine.

Fuel line bypasses are commonly called a regulator which is incorrect. A regulator controls pressure on the outlet of the regulator, regardless of the input pressure to the regulator. A fuel bypass function is to bleed off a preset pressure to the outlet; the outlet pressure should be atmospheric and is determined by any restriction downstream. The bypass outlet pressure is ideally ambient atmospheric although return line sizing if too small will increase outlet pressure and added rising fuel pressure line/rail pressures. Another problem with fuel bypasses is the under sizing of the bypass orifice resulting in low flow causing low fuel consumption pressures higher than high fuel consumption pressures causing erratic fuel rail pressures. A larger bypass would be required or a series of bypasses to bypass the total fuel not used at low fuel consumption conditions.

Return line example - If the engine requires 600 lb/hr of fuel at maximum power and is used in low speed/power applications then the return line and bypass needs to return 90% of the fuel with ideally no pressure loss. That would mean a return line sizing equal or larger than the pressure line.

return line must be same size as supply in a bypass type fuel system .. single 044 will be at its limits

EFI conversions will require a larger-than-stock return line for proper low pressure control.

Another problem with fuel bypasses is the under sizing of the bypass orifice resulting in low flow causing low fuel consumption pressures higher than high fuel consumption pressures causing erratic fuel rail pressures. A larger bypass would be required or a series of bypasses to bypass the total fuel not used at low fuel consumption conditions.