TheRealLefty

"Is that why most chips expect you to run a 3-bar FPR - not a lot of headroom on the injectors to add more fuel?"

I'm thinkin' that's a match, bud

User 52121

It would certainly make sense.

My LBE should be here tomorrow. I'll hook it up, see how the car runs, and go from there. Worst case, I'll call Lindsey up and see what it'll cost me to do their full "270hp kit", but without the exhaust as I already have one on the car.

sweanders

You have it wrong. Here is a very simplified description.

The injectors are in a simplified way of seeing it valves with a certain inner diameter pipe. They open and close according to what the chips tell them to. Longer opening times give more fuel than shorter opening times. The fuel pressure regulator does not care how much fuel the pump sends it, it just makes sure that the injectors get a certain pressure. Since the FPR is not a rising rate typ it stays at the same rate no matter what boost you run on the intake side. So it is a 1:1 no matter if it is 2,5 or 3 bar.

What you do when you put a 3 bar FPR is simply adding the pressure to the injectors, so they flow more fuel on the same opening times. So if you increase the fuel pressure with 50% they will flow 50% more fuel - as long as the pump delivers enough. Rising rate regulators that are boost dependant are often used when hotrodding a naturally aspirated engine with a supercharger and is a shortcut to make sure that the fuel is always increased accordingly to the intake pressure. With electronic fuel injections it is much better/easier to program the injector open time with a static pressure and only control the fuel from the DME and not two sources.

If you run stock chips with 3 bar FPR you will always get to much fuel, the car will not run well and idle is lumpy. It's an efficient way to destroy an engine.

Now, on the stock 951 computer it run on a solid map and has to mapped according to the intended boost pressure. If you use a modern MAF/MAP-conversion that controls fuel from the actual measured air flow and pressure you can change the boost pressure and have the amount of fuel changed.

The stock AFM-meter does not really measure how much air that goes into the engine. It only measures how much the the intake flap is opened and calculates the air flow from this SET value and gives a certain amount of fuel.

Please correct me if I am wrong. But don't tune the car with a fpr - use electronics to do that.

Rogue_Ant

Raising fuel pressure a certain % does not equal the same change in fuel flow.
It is the sqrt of the change...
So changing from a 2.5bar to a 3bar is a 20% change in pressure. However actual fuel flow increase is only ~10% (sqrt of 3/2.5)
The stock AFM is a volume sensor. This data combine with AIT and baro, the DME derives Air Mass.


-Rogue

User 52121

Um... ok. I think at this point we're just talking AT each other with respect to how a FPR works and it almost feels like we're just trying to prove each other wrong. I'll just bow out of that part of this discussion now, as I got the answer I was looking for, rather than delve into the super detailed explanation of the differences between pressure and flow, etc.

Thanks for the help, guys.

choinga

just put one in - it ain't gonna hurt anything and then you know you don't have to worry about it.

sweanders

All you need to make sure is that you have an FPR that match the chips.

User 52121

Yeap. Got it.

Since my chip was not made for the 3-bar... I'll leave the stock 2.5 bar one in there. If this Vitesse datalogger thing ever materializes, then I'll run whatever FPR they recommend to go along with their software.