Monthly Archives: November 2012

Do I need to upgrade my fuel pressure regulator, for Euro/Asian “tuner” cars?

I saw this on Flickr and couldn’t stop myself from replying.

POTD - Do I need a fuel pressure regulator?

POTD – Do I need a fuel pressure regulator?

Do you really need an aftermarket fuel pressure regulator?

We see people fitting these when they really could save their money. That is not to say they don’t have an important place, but often the stock regulator will be up to the task. To give you some perspective, we have seen 500 kW at the wheels from an EVO 9 that still uses the factory FPR!

STM | Speedtech Motorsport Ltd
Performance Vehicle Specialists

Nice picture, clear thought. I’m most familiar with the VW/Bosch set-up, with three pipes at the fuel tank. Supply pipe (1) goes from the tank to the pump, pump output goes to the regulator, bypassed fuel from the regulator goes back to the tank (2), unused fuel from the injector rail(s) go back to the tank (3).

Therefore the pump is moving buckets more fuel than is being used, all of which flows through the pressure regulator. Also, the filter is filtering the fuel over and over. So the size of all the fuel system components upstream of the injector spritzers is much larger than the volume of fuel consumed would require. Compare to an engine oil-pump, where its more about volume, not the precise pressure at the furthest point. Thrashing the oil through the oil pump consumes a modest amount of engine power, at idle, but the engine’s speed range varies by a factor of 5 to 10, so you have to do something to limit pressure at the high end, or live with pretty low pressure at the low end. Nothing is perfect and its all tradeoffs.

The fuel injection system varies the time(s) the injector spritzer is on, and constant pressure with variable timing make variable, controllable, fuel volume. For perfect combustion, you want to regulate the weight of fuel (number of molecules) to match weight of oxygen in the air (number of molecules). That’s why the injection system air-sensors are “mass sensors” and not *volume* sensors. A modern engine’s computer probably makes a density correction for fuel temperature.

The wholesalers who sell gasoline to your neighborhood station do a temperature correction when they fill the station’s tanks, because they charge a fixed price per weight of fuel. Volume is easy to measure, but the energy in the fuel is a direct function of weight. All the “barrels” of crude oil and refined products are interpreted as being at a standard temperature. Only the unwashed and uneducated end user pays money for fuel by volume. And that’s why the British Thermal Unit (BTU) is the amount of energy it takes to change the temperature of a *weight* of water (1 lb) by 1 degree F (59.5 to 60.5). And Air Conditioners are specified by “Tons”, which refers to the cooling power of a weight of ice over a 24 hour period. But I digress.

So, bottom line, you need a fuel pressure regulator and it needs to be big enough to accept the flow rate from your fuel pump. They work as pair, providing a variable volume of fuel at a constant pressure. Note that the volume/time varies directly by engine speed – make the engine go 5 times faster? You’ll need 5 times as much fuel. 10 times faster? (Honda S2000, 9000 rpm?) 10 times as much fuel. HOWEVER. The fuel consumed is not only less than the pump and regulator handle, its less than the regulator sends to the injectors. Because the point where the constant pressure matters is where the injectors plug into the fuel rail(s).

So, just as the pump has to move more fuel through the filter and regulator to make the desired volume at the regulated pressure, the regulated pressure side of the system has more fuel flowing through it than it actually sprays into the engine at any given time.

If you make 3 or 4 times stock output power, thus flowing at least 3 or 4 times the weight of fuel and oxygen through the engine, you might start to need a bigger fuel pump, and regulator, and rails, and injectors. Unless the extra oxygen comes from NOx, or an oxygen tank, a 4X increase in power probably means a turbo or mechanical supercharger. Porting, polishing, blueprint balancing, big valves, savage cams, even higher compression pistons are likely to work just fine with the stock fuel pump and regulator. And if you upgrade to a larger volume regulator, you’re wasting your money unless you upgrade the pump and filter, and possibly the hose size as well.