The Fuelab 54501 adjustable fuel pressure regulators are more compact in size compared to their 515/525 series, but they also weigh 50% less. They accept port style, and non-port style union fittings without interference. There are several configurations available supporting carbureted applications as low as 10 PSID to fuel injected applications up to 125PSID. The 535 Series Fuel Pressure Regulator has a -6 AN return port located on the bottom. The 545 Series has its return port in-line with the inlet port (not on the bottom) which allows for easy installation in really tight locations.