Rising Rate AFPR with OBDI S52 and 24# Injectors

[ACHTUNG]

I have a Bavauto rising rate AFPR sitting around and was wondering if I should use it in my S52 swap. I'm running the OBD1 conversion, a 3.5" MAF with 24# injectors and the TRM tune. I also have a pressure gauge that I can mount inline to see what pressure I'm at.

I called Bavauto and they say that the max flow rate has been tested to be ~60# without vacuum so I think it should be fine with 24# injectors.

Would it be beneficial to install this unit? If so, would I have to change the TRM tune? Please discuss all thoughts of using a rising rate AFPR vs. not using one.

[BigD]

Unless you are going to boost the car, what do you hope to accomplish with a rah-rah-fpr?

[ACHTUNG]

Unless you are going to boost the car, what do you hope to accomplish with a rah-rah-fpr?

I'm not sure. That's why I'm asking if it's worth installing it.

Here's the Bavauto description of it:

"This adjustable rising rate fuel pressure regulator enriches the fuel mixture as you accelerate to increase power and improve mid-range throttle response. Maintains stock fuel mixtures at idle."

[BigD]

I'm not sure. That's why I'm asking if it's worth installing it.

Here's the Bavauto description of it:

"This adjustable rising rate fuel pressure regulator enriches the fuel mixture as you accelerate to increase power and improve mid-range throttle response. Maintains stock fuel mixtures at idle."

It helps to know the reason why you need a variable-rate FPR to begin with. Say you want 40psi. Great, get a valve that opens at 40psi, done, right? Well what if your engine is drawing vacuum on the intake manifold, as will happen at sub-WOT levels? Then you have 40psi relative to atmospheric on one side of the injector, and, say 10psi of vacuum on the other (idle). This means that you effectively have 50 psi of pressure in the fuel rail, and it'll make the ECU give WAY too much fuel. So the traditional FPRs will bleed fuel pressure under vacuum (in this case, the actual FP will be 30psi).

But when you introduce boost, the problem reverses. You have 40psi of fuel in the rail, but you have 10psi of boost in the intake manifold. So the injector will actually only be opening to 30psi of fuel, not delivering enough. So the RRFPR will not only decrease pressure under vacuum, but also increase it under boost.

So unless you plan to have greater than atmospheric pressure in your intake, I doubt there's any benefit to the RRFPR. I mean it's possible, that with the right conditions, you'll exceed atmospheric pressure (due to ram air, or tuned intake charge pulses - eg the velocity of gases at the cylinder head horn of an F1 engine is supersonic), but I bet it's a waste of $.

[ACHTUNG]

It helps to know the reason why you need a variable-rate FPR to begin with. Say you want 40psi. Great, get a valve that opens at 40psi, done, right? Well what if your engine is drawing vacuum on the intake manifold, as will happen at sub-WOT levels? Then you have 40psi relative to atmospheric on one side of the injector, and, say 10psi of vacuum on the other (idle). This means that you effectively have 50 psi of pressure in the fuel rail, and it'll make the ECU give WAY too much fuel. So the traditional FPRs will bleed fuel pressure under vacuum (in this case, the actual FP will be 30psi).

But when you introduce boost, the problem reverses. You have 40psi of fuel in the rail, but you have 10psi of boost in the intake manifold. So the injector will actually only be opening to 30psi of fuel, not delivering enough. So the RRFPR will not only decrease pressure under vacuum, but also increase it under boost.

So unless you plan to have greater than atmospheric pressure in your intake, I doubt there's any benefit to the RRFPR. I mean it's possible, that with the right conditions, you'll exceed atmospheric pressure (due to ram air, or tuned intake charge pulses - eg the velocity of gases at the cylinder head horn of an F1 engine is supersonic), but I bet it's a waste of $.

Okay. Well all this makes sense. I guess I'll sell it since I don't plan to introduce boost.