The Answers: DeatschWerks’ Responses To Your Fuel System Questions

Last month, we posted an article calling for your questions to be answered by David Deatsch and the crew at DeatschWerks. Being a full-line fuel system manufacturer, the range of questions submitted was broad and quite interesting. Ranging from ideal fuel pressures and temperatures to the ins and outs of converting to return fuel systems, there were solid questions posed. We even had a question about a fuel system for a supercar that most of us have only seen in video games.

Read on as DeatschWerks answers questions and drops knowledge bombs on a wide range of fuel system subjects.

Is there an optimal injector duty cycle you should target for the upper-RPM limits of a given motor? If there is a, do different fuels affect the optimal duty cycle? – Cory L.

In EFI port-injected engines, it is ideal to avoid exceeding 90-percent duty cycle. In the 90- to 95-percent duty cycle range, the injector will begin to go “static.” A static injector can’t meter the fuel and is at the limit of its flow rate; you risk running lean and detonating. At Deatschwerks, we recommend 80-percent duty cycle as an optimum upper limit. The headroom can save your engine if your fuel pressure drops and it also allows a little room for future power upgrades.

Different fuels do not affect the optimal duty cycle of an injector, but switching from gasoline to E85 will increase your BSFC by about 30- to 35-percent, thus requiring an equivalent increase in the duty cycle to achieve the same power.

How important is fuel temperature? Is there an ideal fuel temperature, and how is it relative to making or losing power? – Anthony W.

High fuel temperatures can decrease power slightly, but the more common issue associated with excessive fuel temperature is vapor lock. Overheating fuel can cause it to boil which introduces air into the fuel system. Air in the fuel can create a lack of fuel flow causing the car to misfire, die, hard-start, or no-start. Returnless fuel systems are more susceptible to vapor lock because they do not circulate the fuel back to the tank, the only place for the air to escape is out the injectors, causing hot-start issues.

“Higher fuel pressures create better atomization; however, it is a situation of diminishing returns. — David Deatsch”

Is there an ideal base fuel pressure? Some engines run 43.5psi base-pressure from the factory and some run 58.0psi. Is one better than the other for fuel atomization or overall flow? – Barry R.

Higher fuel pressures create better atomization; however, it is a situation of diminishing returns.  Little is to be gained from raising your delta pressure (rail to manifold) above 60psi. In general, OE return-style fuel systems operate at 43psi with a static delta, and returnless fuel systems operate at 58psi with a variable delta.

As for flow, pressure affects injectors and pumps inversely. Higher pressure increases injector flow rates but decreases pump output. Lower pressure increases pump output but decreases injector flow rates. You can use this knowledge to your advantage and max out your fuel system potential by increasing fuel pressure when you are injector-limited and decreasing fuel pressure when you are pump-limited. However, we still recommend keeping your delta pressure between 40 and 60psi.

If you use a fuel pressure gauge will there be different readings depending on where you install the gauge sending unit? For example, if you install the gauge sending unit right after the fuel pump, will it read higher or lower than if you install it two inches from the fuel rail? Where would you recommend installing the fuel pressure sending unit on a fuel system? – Joe B.

Fuel pressure does read higher at the pump and lower at the rail. In a properly engineered system, this difference is negligible. However, if your fuel system is overly restrictive, relative to your required fuel flow, the fuel pressure difference can be 10psi or greater. Having appropriately sized fuel lines, filters, and rails results in less restriction, and yields less pressure drop. In any system, the best place to read fuel pressure is at the fuel rails. The pressure at the injector is what matters most.

I currently have an Ultima GTR with twin tanks (one on each side of the car). What would be the proper way to have both tanks act as one (crossfeed and vent)? Both tanks need to supply a swirl tank that contains the high-pressure pump that supplies the fuel rails on an LS7. – Bob S.

There are several ways to handle this setup. One option would be to choose one tank to be the primary, with a high-pressure pump to feed the swirl tank; the other tank would use a small low-pressure lift pump, like the DW Micro, to pump fuel across to the primary tank to keep it full. A second option would be to run a high-pressure pump out of both tanks and Y them together into the swirl tank. In both instances, it’s vital to have a crossover hose so that if one tank becomes full, it can spill over to the opposite tank.

If running a multi-pump surge tank, do I need to run multiple in-tank pumps of equivalent feed rate to keep the surge tank full? Does the feed rate to the surge tank need to be 1:1 to the surge tank’s output? – Raymond T.

You do not need to run the same size feeder pumps as surge tank pumps. The feeder pump will see almost zero pressure, which results in a significant increase in output volume. In addition, the fuel return from the rails will be routed to the surge tank, which will supplement the feeder pump output.

Using a bit of math, you can estimate feeder pump flow requirements.  You would need to know pump voltage, rail pressure at WOT, surge-tank volume, max time at WOT, and fuel consumption at WOT. DW provides pump-characterization data (flow vs. pressure vs. voltage) on all in-tank pumps to help with these types of calculations.

Does the location of the fuel pressure regulator matter at all in a return-style system? Does it make a difference if I run the regulator before or after the fuel rails? – Robert C.

A true return-style fuel system places the regulator after the fuel rails. A dead-head fuel system places the regulator before the fuel rails, but on the engine-side of the firewall. As with everything, there are pros and cons to the different systems.

A return-style system provides the most stable fuel pressure and the highest flow potential. This is the most common set-up for aftermarket performance fuel systems and is the best option for builds with really big fuel demands.

Dead-head fuel systems are usually used when the customer has OE rails that have no provisions for a return line, and they don’t want the added expense of aftermarket rails and additional lines and fittings. They also provide a cleaner looking engine bay by eliminating some of the “unnecessary” plumbing.

The only downside with this style versus return-style is they are more susceptible to fuel rail hammer and fuel pressure spikes in rapid transitions from high to low fuel demand. If I were building a 1,000-horsepower or bigger system, I would go with full return-style. In anything else, it doesn’t really matter.

How critical is fuel-feed-line sizing? Is there such a thing as too big? Also, are there any benefits or drawbacks to running hardlines for feed and/or return lines? – Jeff C.

“Oversizing” injectors and pumps can cause issues, but oversizing the fuel-feed line is only going to hurt your wallet and possibly make installation more difficult. Undersizing fuel lines will cause excessive restriction resulting in a pressure drop between the pump and injectors, which will require the fuel pump to work harder to hit the target fuel pressure at the injectors.

This essentially downsizes your fuel pump and will also decrease its life expectancy. An easy way to test the restriction of your fuel system plumbing is to measure fuel pressure at the pump and at the injectors under full load. If there is only a 1 or 2psi pressure drop there is no need to upsize your lines.

As for hardlines, there are no drawbacks other than OE hardlines are limited in diameter. Many OE systems utilize 5/16-inch hardlines, but some are 3/8-inch. A 3/8-inch hardline will actually outflow a -6AN line.

What are the benefits of converting a returnless system to a return-style system? Are there any drawbacks? – Cliff C.

There are pros and cons to both systems. Modern EFI returnless systems are highly engineered to be efficient, durable, and cost-effective.  They use fuel pump pulse-width modulation and fuel pressure sensors to control flow and pressure. They rely on a proper understanding of the system and proper pump-module tuning to remain effective in high-horsepower applications.

When upgrading returnless systems, be sure to take time to learn how the entire system works and then perform upgrades that work with the system instead of against it. Much can be gained by simply upgrading the in-tank pump(s) and EFI injectors.

The biggest mistake we see customers make is increasing rail pressure beyond the set-point of the in-tank pressure relief valve. To achieve rail pressures beyond OE, you need to upgrade the in-tank pressure relief valve. Otherwise, a considerable amount of the pump output will be dumping right back into the tank instead of feeding the injectors.

However, even when upgraded properly, at some point (usually around 700-900 whp), a return fuel system will be a better option. The biggest drawback to a return system is heat (especially in multi-pump systems used in street applications). Big pumps operating at full capacity, and looping fuel through the hot engine bay repeatedly, can kill a fuel pump within days of installation. To avoid overheated fuel, stage the fuel pumps so only one is running while fuel demand is low and the others turn on as demand increases. Another good idea is to install a fuel cooler on the low-pressure return side of the fuel system.

Could I use one of your surge tanks as a standalone fuel tank/system for my direct-port nitrous system? Would there be any drawbacks? – Kyle A.

You can — with some modifications — but it probably wouldn’t be worth the effort. DW surge tanks are sealed systems and would need a fuel filler port and a vent to convert them into fuel tanks. Surge tanks are also not designed to hold pressure; keep that in mind while designing the standalone nitrous fuel system.

In addition to the modifications needed, capacity needs to be considered. DW offers tanks in 2.5 liters, 3.5 liters, and a soon-to-be-released 5.5-liter dual-stage triple 400 surge tank. There are several companies, such as Nitrous Outlet, that have off the shelf solutions for nitrous fuel tanks.

Do aftermarket fuel rails offer any performance benefits over factory pieces? What do they offer over a factory fuel rail? Aaron W.

The benefits of aftermarket fuel rails are very dependent on your particular application and build. Higher flow is often the least important benefit of aftermarket rails. Aesthetics is the benefit that everyone always gets to enjoy. These days, I would say the most important benefit that aftermarket rails provide is the ability to convert from an OE returnless system to a return-style system. Also, some aftermarket rails (such as DW) include options for plumbing kits specific to your vehicle and include 1/8-inch NPT and 3/8-inch NPT ports for pressure gauges, Schrader valves, nitrous feed, and fuel pulsation dampers.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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