Steve Baur
Former Editor, Modified Mustangs & Fords
October 12, 2006
The recirculating race valve pictured here is for the Mustang making big power but needing to keep the noise down a bit. The single vent can be connected to the supercharger inlet, hence the "recirculating" name.

But this may not be the optimum place should you be employing a blow-through mass air meter, so consult with the manufacturer for the best place. Comeau also says there is no such thing as having too much bypass valve. It's better to have the extra air out than in, so it mostly comes down to performance and price.

If your ride frequently blows off the induction tubes, you may not have enough bypass valve, or it may not be working properly. "We sometimes get tech calls complaining of poor driveability or tuning issues. Oftentimes it is a result of the bypass valve, whether it is too small for the application, or it is not working because there is some sort of interference with the linkage, or it is not getting a good vacuum signal.

How many bypass valves do you need? Well, one should be enough, but you can double up the smaller Pro Flo to get race-valve performance with a quieter sound.

Rumors, Myths, and the Truth
With the Internet, you can find a lot of opinions about things, but your best resource is most likely the manufacturers, as they spend far more time and money in research and develop-ment testing, and know their products far better than anyone else.

That being said, we queried the ATI guys for some of the most common misconceptions they hear. "People often think the blowers' limitations are a result of the impeller becoming inefficient at a certain rpm," Comeau says. "This is not the case, as it has more to do with reaching the limit of the transmission and bearings."

Evidently, a lot of people believe the adjust-ment on the bypass valves control boost pressure, but in fact it only controls the tension on the release spring. ATI sets the tension from the factory, so you shouldn't have to touch it at all.

"People think engine oil cools blowers better, but the fact of the matter is that engine oil temperatures are actually hotter, and increase far quicker than the self-contained oil temps do," Comeau claims.

Fueling the Fire
Failing to provide the appropriate amount of fuel to an engine modified to breathe more air is a sure recipe to turn expensive parts into a mess of molten metal. Ironically, even though it's the fuel that contains all the heat that makes horsepower, most do not think of their fuel systems as a performance modification. Of course, without a fuel system, you wouldn't have any performance at all.

The classic 5-liter Mustang was factory equipped with a return-style fuel system. In this setup, a pump pushes fuel up to the engine and through the fuel rails, then out into a regulator or relief valve designed to restrict fuel flow just enough to maintain a certain pressure for the injectors. Excess fuel is constantly flowing through the fuel rail, whether it is needed or not, and it relies on the regulator to return it to the fuel tank by way of the "return line."

Aeromotive is one of the only companies that sells a complete system designed specifically for 5-liter Mustangs. All of the measuring, cutting, and component choosing has been done for you. All you have to do is install it, and Aeromotive gives you everything you need.

"A return-style system is superior, providing the most effective fuel flow and pressure control to the injectors" Clow says. "It creates a constant flow of fuel over the injector inlet, at the desired pressure." The major drawback to this system is it can heat the fuel by circulating it through the hot engine compartment, increasing chances of vapor lock and creating more evaporative emissions. "Evaporative emission control is why most new cars built after 1999 are required to use a returnless system instead," says Clow.

He continues, "Today's modern returnless fuel systems are now part of the EEC or evaporative emission control system, minimizing heat transfer into the fuel tank, producing less evaporation, and providing new car makers with a cost-effective solution to comply with the latest EPA standards. Ford was the first to completely eliminate the traditional bypass regulator, replacing it with variable pump speed in order to control fuel pressure. Unfortunately, these new fuel systems leave a lot of performance on the table, providing poor pressure control and flowing fuel only to replace what is lost."

The Upgrade Path
"Whether it's the original return-style fuel system of old, or today's returnless system, the 'stock' fuel system can't be pushed much beyond 500 flywheel horsepower, even with modified components," says Clow. "You can beef up the in-tank pump, add a second pusher pump, and install an adjustable regulator, tweaking it to make 500 hp pretty easily. Some have gone as high as 600 hp, but this is the gray zone, where fuel supply can become unreliable."

To make over 500 hp reliably, the whole system should be reengineered. In order to accurately determine what your fuel system needs are, you'll need to take into consideration your total horsepower goal, if it is drag race only or subjected to continuous duty (street driving or open track), and if you are using a power adder and what type.

Boosted engines have different requirements than naturally aspirated engines. This is because it takes horsepower to turn the compressor wheel of a blower or turbo, meaning a supercharged or turbocharged motor making 500 hp at the flywheel makes a good bit more power in the cylinder than an equally rated engine that's naturally aspirated.