NOTE: Articles in this series were written in an order that might make "skipping around" confusing. If you didn't start at the beginning, you might want to CLICK HERE.If you are less interested in the history and more interested in what the differences are in Todays modern 7.3L/6.0L "Regulated Return" fuel systems, this is the article you want. Each section below will discuss a different style of AFTERMARKET fuel system layout, with pros & cons and associated commentary. I will cover "Alternative" fuel systems first, and then cover our fuel system designs at the end. Since I've covered the OEM systems in other articles, those will be left out of this discussion.
FUEL RAIL CROSSOVER KITS
SYSTEM TYPE: REGULATED SUPPLY (like OEM)
Fuel Rail Crossovers have been configured a few different ways over the years, but they all amount to the same basic thing. By connecting the ends of the rails together, the crossover essentially extends the length of the rail and moves the end point of each rail outside the head. Without an orifice, a crossover is effectively still a "Dead Headed" system with no path to allow air to escape the rails or maintain "Constant Fuel Flow" through the rails. Crossovers that include an orifice and a connection to the return are no longer "Dead Headed", but they have introduced a new unregulated drain on the system (the fixed orifice bleeding fuel to the return). Since they still rely on the OEM fuel pressure regulator (regardless of whether the regulator is upgraded for higher pressure), they remain a REGULATED SUPPLY type system, which is managing the fuel pressure in the fuel filter bowl, before all of the sources of pressure drop have been accounted for (regulator leakage, fuel filter condition, fittings, hoses/tubes and fuel injectors).
The single biggest advantage to a Fuel Rail Crossover is its low cost. Unfortunately, the low cost advantage is significantly outweighed by the fact that the crossover design doesn't properly address fuel pressure management (regulator location and adjustability) and proper fuel flow through the heads (the leaky OEM style regulator can still bleed fuel volume back to the tank instead of sending it to the fuel rails).
FUEL RAIL CROSSOVER PROS:
• Simple / Minimal Change to OEM Configuration
• Very Inexpensive due to Minimal Parts Involved
• Somewhat Beneficial IF Orifice Air Bleed Incorporated
FUEL RAIL CROSSOVER CONS:
• Fuel System Remains "Regulated Supply" (pressure management in the wrong place)
• Orifices are Fixed/Constant Drain on Fuel Volume/Pressure (outside of regulator control)
• Maintains OEM Fuel Pressure Regulator Design (leaky/poor performance, even with different springs)
PERSONAL OPINION: This would be my absolute last choice for a fuel system modification, and only on a nearly stock truck. There isn't enough bang for the buck, particularly when so many people that had done a fuel rail crossover have later needed to finish the upgrade and get into a full Regulated Return. The biggest reason for fuel system modifications is improper fuel pressure regulator location, poor OEM fuel pressure regulator design and increasing fuel flow through the heads. A crossover simply doesn't do any of these. Higher pressure springs in the OEM regulator are a bandaid that neither changes the regulator location or fixes the design flaws, and orifice fittings on the ends of the rails only add a minor air removal benefit but can't provide the increased flow benefits of a system that requires ALL of the fuel to pass through the fuel rails before any of it can be dumped back to the tank for pressure management purposes.
SINGLE CIRCUIT REGULATED RETURN
SYSTEM TYPE: REGULATED RETURN
The Single Circuit fuel system design is a true REGULATED RETURN, in that it does properly relocate the regulator to a point AFTER all of the rest of the fuel system pressure drop and the fuel usage from the injectors. In our "Cackle" testing, we did find that this layout performed equally to the dual circuit fuel system design seen in most REGULATED RETURN systems. We couldn't find any compelling difference to consider this layout superior to a dual circuit configuration, but we did have some concerns.
With a single circuit design, 100% of the fuel pump flow is passed through the first fuel rail, then crosses over to the second fuel rail, then exits to the fuel pressure regulator. Compared to a dual circuit design, this represents 2x the fuel volume being pushed through each cylinder head fuel rail, creating a larger pressure drop at each restriction point in the system (typically the entry and exit points of the fuel rails). This means that the fuel stream will see (4) entry/exit points as well as (8) fuel injectors as sources of pressure drop along the way to the regulator, creating a possibly noticeable pressure imbalance between the (2) different fuel rails. It also means that the same fuel stream will pass through both cylinder heads before returning to the tank, doubling the opportunity for the fuel to pick up significant heat from the heads.
SINGLE CIRCUIT REGULATED RETURN PROS:
• Fuel System Converted to "Regulated Return" (pressure managed in correct location)
• 100% of Fuel Passed Through Fuel Rails (no "Leaks" before the injectors)
• "Dead Head" Eliminated - Impossible to Trap Air in Fuel Rails
• Fuel Pressure is Adjustable
SINGLE CIRCUIT REGULATED RETURN CONS:
• Aftermarket Fuel Pressure Regulator and Additional Plumbing is More Costly
• Additional Hoses and Components on the Engine
• Fuel May Spend More Time in Rails and Take More Heat Back to Fuel Tank
• Possible Pressure Imbalance Between 1st Injector and 8th Injector to Receive Fuel
• Less Ideal for Supporting Larger Injectors that Require More Pump Volume
PERSONAL OPINION: If I'm doing a REGULATED RETURN, I can't see any reason why I would consider doing a Single Circuit layout over a Dual Circuit layout. I don't particularly care for doubling the fuel volume through the restriction points when there isn't a need, since this can actually cause entrained air to be released from the fuel stream. Since you are still supplying (8) injectors with the same overall volume (GPH) of fuel, you have not increased the size of injector you can support, you've just increased the effect of the pressure drop points on the fuel, and potentially increased the amount of fuel rail heat that is being taken back to the fuel tank. If I've got the expensive regulator and hose and fittings already, it just makes more sense to run a dual circuit system.
4-CORNER FEED FUEL SYSTEM
SYSTEM TYPE: REGULATED SUPPLY (like OEM)
The most recent fuel system layout to hit the market is the 4-Corner Feed. This system uses a 4-way distribution manifold to supply pressurized fuel to all 4 fuel rail ports in the cylinder heads at the same time. Since this design relies on a remote mounted fuel pump with a regulator (typically an AirDog 4G/5G or FASS), and fuel pressure is managed well before the fuel rails, this is technically a REGULATED SUPPLY type system. Proponents of this style of system make 2 main claims of superiority:
- First, they claim that the 4-corner feed type system can supply more fuel for larger injectors than a typical REGULATED RETURN...this claim is TRUE. Technically speaking, by supplying fuel to both ends of the fuel rails, you can in fact double the volume of fuel that can be pumped to the injectors. In effect, each port in the head is supplying fuel for (2) injectors instead of all 4. Unfortunately they leave out any discussion of whether "Double The Volume" is actually needed, or what benefits are lost from this design.
- Second, they claim that the 4-corner feed type system doesn't heat the fuel like a typical REGULATED RETURN...this is also true, but in a way that I feel is misleading.
Something else I never see mentioned is how to purge the air from a 4-corner feed system (either after installation, or after a filter change). Since this system doesn't have a return off the regulator at the engine, there is no path for air to escape the rails and return to the fuel tank. This leaves 2 options, either start the engine and let the air purge through the injectors (not ideal, even if short lived, as air is neither a lubricant nor a cushion for the moving injector parts), or crack the fuel lines at the distribution block in the valley (which won't do anything for air already in the fuel rails and will certainly make a mess). Not really a great set of options.
Regarding the claim about not heating the fuel like a typical REGULATED RETURN, this is ONLY true if we are talking about the fuel that is being returned to the fuel tank, but that clarification isn't usually made. What gets missed in the discussion of fuel temperatures is what is happening to the temperature of the fuel that is in the rail, about to be injected. Since the temperature of the injected fuel has a direct impact on the amount on power production and effiency, I'm more interested in fuel rail fuel temperatures than fuel tank temperatures. In both HEUI engine platforms, the fuel rails run horizontally over the top of the cylinders, in the middle of the cylinder heads, so it is safe to say that they are hot. Consider the following data regarding how much time fuel spends in the fuel rail before exiting the other end (REGULATED RETURN) or being consumed by the injectors (4-CORNER FEED) under a light to moderate throttle condition:
- REGULATED RETURN - 1.7-1.8 seconds for fuel to pass through a 7.3L cylinder head at STOCK PUMP FLOW RATE
- REGULATED RETURN - 0.9-1.0 seconds for fuel to pass through a 7.3L cylinder head at DOUBLE THE STOCK PUMP FLOW RATE
- 4-CORNER FEED - 6.5-7.5 seconds for 180cc-205cc injectors to consume the volume of the fuel rail at 25% of max injector flow
- 4-CORNER FEED - 5.5 seconds for 250cc injectors to consume the volume of the fuel rail at 25% of max injector flow
- 4-CORNER FEED - 3.5 seconds for 400cc injectors to consume the volume of the fuel rail at 25% of max injector flow
I chose 25% consumption for the 4-CORNER FEED simply because that is likely a more realistic range for constant, regular injector operation than WOT for 99% of truck owners. In reality, much light throttle cruise is even less than 25%, which means that the time to fully exchange the fuel in the rail with the 4-CORNER FEED is even longer than the numbers above. For example, if 10% is more a realistic light throttle cruise figure for a set of 205cc injectors, it will take almost 17 seconds to fully exchange the fuel in the rail. Tell me again how the 4-CORNER FEED doesn't increase fuel (injected) temperatures...
4-CORNER FEED PROS:
• Simple Plumbing With Only Fuel Supply To Engine (no return to tank)
• Lower Cost Due To Lack of Expensive Regulator (until you factor in AirDog/FASS purchase)
• Fuel Pressure May Be Adjustable (depends on pump/regulator setup chosen)
• Fuel Returning To Tank Not Heated By Engine Compartment / Fuel Rails
• Supports Highest Horsepower Levels
4-CORNER FEED CONS:
• Fuel System Remains "Dead Head" Design
• Absolutely No Place for Air in the Fuel Rails to Escape
• Relies On Other Aftermarket Pump/Regulator for Pressure Management
• Fuel Pressure Managed Too Far Before Fuel Rails / Fuel Injectors
• Injected Fuel Spends Far More Time Picking Up Heat In Fuel Rails
• Not Ideal for Engines Spending A Lot of Time at Less Than WOT
PERSONAL OPINION: Before you go off thinking that I'm trashing on this fuel system design...I'm NOT! This design absolutely has its place. Based on the loss of what a true REGULATED RETURN gains (flow "Through" the heads, purging of air, less heat in the injected fuel), I think that the 4-CORNER FEED design is best used in short distance, ultra-high performance competition applications only. I would consider this my #2 pick for fuel system design, behind a properly designed REGULATED RETURN, based entirely on the need for every last drop of fuel I can get to the injectors. In a balls to the wall application with huge injectors and tuning that is emptying them. the heat soak issues are negated and the double volume capability is beneficial! I will also add that it is hard to not really like just how clean the plumbing is in a 4-Corner Feed, since there is no need for a regulator or return plumbing on the engine...it looks great! I just tend to be "function before form", so I will take the extra plumbing and regulator and bracket, even if it doesn't look as tidy.
DUAL CIRCUIT REGULATED RETURN SYSTEMS
(Standard and Bowl Delete)
(Standard and Bowl Delete)
SYSTEM TYPE: REGULATED RETURN
This fuel system design is called "Dual Circuit" because the fuel flow is split (either at the filter bowl or at the bowl delete distribution block) to feed each cylinder head separately. This system design is what a majority of the fuel system market considers a REGULATED RETURN to be, it is the "Norm". As such, it will just be referred to as a REGULATED RETURN fuel system for the remainder of this article, and any others. Also, for the purposes of this discussion, even though I have provided both diagrams above, the term REGULATED RETURN can be taken to include either our "Standard" (retains the OEM filter bowl) or "Fuel Bowl Delete" designs, unless otherwise specified.
A lot has been learned since we figured out that fuel system modifications could make a noticeable difference in the dreaded "Cackle", as well as improve throttle response and fuel economy, back in 2002. At first, we just wanted to quiet that stupid noise. Now, 20 years later, it's all about making these trucks run their best, keeping injectors alive, supporting the power mods (even mild daily/tow stuff), etc. Our REGULATED RETURN fuel system design (Standard or Bowl Delete) takes fuel coming from the pump, splits it to supply each of the heads separately, then recombines the fuel streams after the fuel rails at the fuel pressure regulator. There are no additional "Drains" (places for fuel to leave the closed system) before the fuel rails, 100% of the fuel is passed through the fuel rails (accessible to the injectors) and out to the fuel pressure regualtor. Because the fuel pressure regulator is located at the end of the system (on the outlet of the rails), the pressure it is managing has truly accounted for every bit of pressure drop possible in the system, most importantly including the pressure drop created by (8) injectors taking fuel from the rails at varying rates based on their size and what the throttle and programming are telling them to do. Whether your truck is still running stock injectors, or you've upgraded to something larger, if you are regularly driving the truck in conditions other than Wide Open Throttle, our REGULATED RETURN is still the most effective layout for ensuring correct fuel pressure, constant fuel flow, continuous purging of air from the fuel rails and the lowest possible injected fuel temperature.
As I pointed out earlier, our REGULATED RETURN systems have been successfully used to supply fuel to a number of competition vehicles with injectors in the 400cc range. As you exceed 400cc in injector size, or try to drive your 7.3L/6.0L to engine speeds beyond 3200/3800rpm (respectively), you may find that a 4-Corner Feed setup will do a better job for you. Simply put, there is a really big difference between variable throttle street driving (even with huge injectors) and all-out high RPM competition with huge injectors.
For the record, it does not matter which direction the fuel is flowed in the fuel rails, despite the fact that some drawings make it look like they are "directional". We choose to flow front to rear simply because we believe that components at the rear of the engine bay are subject to more exhaust/turbo heat soak, and we don't want to pre-heat the fuel entering the rails any more than necessary.
DUAL CIRCUIT REGULATED RETURN PROS:
• Fuel System Converted to "Regulated Return" (pressure managed in correct location)
• 100% of Fuel Passed Through Fuel Rails (no "Leaks" before the injectors)
• "Dead Head" Eliminated - Impossible to Trap Air in Fuel Rails
• Fuel Rail Pressure is Adjustable and Rock Steady (assumes good/properly sized fuel pump)
• Supports Up To 400cc Injectors (with Adequate Fuel Delivery System)
DUAL CIRCUIT REGULATED RETURN CONS:
• Aftermarket Fuel Pressure Regulator and Additional Plumbing is More Costly
• Additional Hoses and Components on the Engine
• Less Capable than 4-Corner Feed in Huge Injector, High RPM, All Out Competition Situations
PERSONAL OPINION: See above...
WRAPPING UP...
I'm sorry for the length of this, but I wanted to make sure that I got as much useful information down as possible...and like the "Soup, Salad and Breadsticks" at an unnamed Italian chain restaurant, the words just kept coming.
DRIVEN DIESEL products that are related to this article:
7.3L and 6.0L Regulated Return Kits
Next up, I will be covering the famous "Hutch Mod", what it really is, where it came from and how it got its name.
Author: Dennis Schroeder - Co-Owner of Strictly Diesel
Dennis has been Designing, Building and Supporting Aftermarket Fuel Systems for 7.3L and 6.0L Powerstrokes since 2001.