Typically referred to as an RS4 rail limiter valve. As most know the OEM rail valves are limited to 106 or 116 bar. Why VW and Audi changed them I have no idea, but let me tell you why you need one.
If you are running stock tune, you will not need one unless you have one that is worn out. If you are running a S1 tune or any tune that commands over 115 bar, you need a PRV (Performance Rail Valve).
The rail limiter valve is used as a safety device, when you go over the target "crack pressure" (this is when the rail valve opens) it bleeds of fuel from the fuel rail/injectors. This additional fuel gets dumped back into the low pressure side of the fuel system and gets compressed by the high pressure fuel pump. Several people have upgraded to the RS4 valve, but this is not the best solution for your application.
The RS4 rail valve is designed around dumping excessive fuel supply, so when it opens it dumps a larger volume than the stock or our PRV. This large dump of fuel will show a huge drop in fuel pressure, then the valve closes, the system builds pressure, then valve opens and repeats the cycle. This is not the optimal solution for a tuned car/engine. The RS4 valve had its time and place, now there are better solutions on the market.
The tune will depend on what PRV you buy. The RS4 valve had a crack pressure of 136 bar, and it worked much better than the OEM units (see crack pressures above) on a tuned car. Problem being, it was not rated high enough for the S2 + tunes. Some S2+ tunes would target 130 to 135 bar for fuel pressure, this is only allowing 1 to 6 bar of opening protection, this is not enough. Rule of thumb when buying a PRV, go 10 to 12 bar over your target fuel pressure. So if you are commanding 130 bar, you should have a 140 to 142 bar rail limiting valve. 125 bar tune should have a 135 to 137 bar rail limiting valve.
Why don't want to run your stock or OEM rail limiting valve? The rail limiting valve is used for over pressure control, not as a return line to your high pressure fuel pump. If you are running a 125 bar tune but have a 106 bar rail valve all that fuel is getting dumped back into the low pressure side. This can create a huge problem with heat and wear on the rail valve. When you compress the fuel over and over again it will start to build heat, that heat will then start to work its way into the surrounding items; lines, HPFP, injectors, fuel rail, ect. All this additional heat is getting put into the fuel, excessive heat in your fuel = poor cooling during injector spray event = loss in power. Not only this, but the constant opening and closing of the OEM rail limiter valve will cause irregular tune or power delivery to the drive train ( have seen this and dyno results from customers have confirmed this). Remember, the OEM valve is used for "safety" not to control your fuel system pressure. The OEM valve has a cycle life, if its consistently held open, that cycle life drops very quick.
OK, we have talked about the RS4 valve, the OEM valve, and why you don't want to run either one of these. Now why should you run one of ours?
1. OEM supplied part that is built for the 2.0 FSI engine (we are under contract to have the OEM MFG build these for us)
2. Target crack pressure levels at 138 (new pressure we are adding first of the year) 142, 145 and 150 bar
3. Proper flow control when the valve "cracks"/opens
4. Testing, we have a bench tester that allows us to test one out of every 10 units to verify proper operation
5. More power, better tune, direct fitment, and affordable.
We are making this post to let everyone know that after the first of the year the pricing will be going up to $169 on every valve that we offer (MFG says that they cost more to make now, damn them!)
Current pricing is:
142 PRV $125
145 PRV $150
150 PRV $150
You can find these listed on our website at www.hpfpupgrade.com
The first of the year will have our new 138 bar PRV listed on the site, that will be when the prices go up. We are currently ordering a 100 units per crack pressure at this time in order to bring you the best pricing possible.
John with HPFPUPGRADE