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Ummm, before you all shout "search the forum you knobhead!" I have - so there!

Anyway, to the point: anyone ever swapped out the stock 4bar FPR on a B5 S4/RS4 for an aftermarket model?

Searching the ol' interweb thingy I found that Eurospec do direct replacement FPRs for VW/Audi ... they just slot right into the mounting on the fuel rail in place of the stock FPR...

(link: http://www.eurospecsport.com/fuel.htm)

Was looking at a fixed 4.5bar FPR .... OR ... an adjustable 3-5bar FPR ...

As i'm an enormous idiot when it comes to these things can anyone shed any wisdom on whether upping the pressure in the rail is a good/bad/potentially-catastrophic thing to do?

Searching on the forum i see that this topic has come up a few times before ... but there was no real definitive statement from anyone as to the results, or indeed, if they actually did it at all...

TIA

If you change it you'll need a remap.

Even if the stock 4bar FPR was swapped out for a 4.5bar FPR?

I'm guessing this is because the extra pressure in the rail would change the amount of fuel offered by the injectors during their cycle...and make the car run rich?

Yep.

I'd be interested to see results for a car with a higher pressure regulator and properly mapped. In theory it might give better MPG.

It's the US boys how mainly go for higher FPR when going homebrew

I'm also interested in the hysteresis (lag) in the throttle response .... been reading about pressure regulators that can respond quicker, here's a quote:

"The regulator maintains a constant fuel pressure under constant load but there is an inevitable delay in the regulator response when you put your foot down on the accelerator. This results in a momentary dip in fuel pressure as the engine sucks more fuel out of it and tends to result in a lean mixture .... faster FPRs can compensate for this by maintaining a higher pressure in the fuel rail and hence richer mixture when you initially put your foot down .... throttle response is improved"

Wouldn't this also be good for engine temps as a richer mixture is "cooler" than a lean mixture ...?

Not heard about that before, interesting though. I wonder if it is fuel pressure dipping though?

Been pointed to this the other day though http://www.autops.co.uk/html/sprint_booster.html

Hmmm ... that looks good .... and probably more what i'm after than a new FPR as it's throttle response that i'm interested in improving (i do love a fast throttle after having had many motorbikes!)

30 day trial - money back guarantee - free installation .... and just down the road from me at APS ...

Reckon I'll give 'em a call and see what they say about sticking this on a re-mapped B5 S4 ....

What I would say though... couldn't Doug adjust your map for more aggressive throttle response?

Be interested to hear how you get on if you do try one.

Maybe ... but isn't this device some kind of signal amplifier that alters the signal direct from the electronic throttle prior to the ecu, therefore "tricking" the ecu into thinking the throttle is it a different angle than it actually is (potentiometer) ... ?

Not sure if re-maps change any of the electronic throttle angle parameters .... do they?

I thought remaps did, my AmD map is very sharp for the first few mm pressed then eases off. I've not driven an MRC mapped car so can't compare.

You may be right. We need a "tuna" to input on this ...

I'll ask Dougal when i go to get my broken horn fixed later today.

Read and heard a lot about the Sprint Boosters. The Mercedes forum I used to take part on loves them as MB designed the throttle response for an 80 year old man. Had I still my C230K I would get one.

I just don't feel much of a DBW lag on the S4. All it can do is emphasise the throttle input signal.

Meant to ask Doug if the MRC map altered the stock throttle sensitivity but try not to hound him with too many dumb questions when I'm there!

Would be very interested in what Doug says Julian.

Hmmm ... see, the thing is I DO feel a bit of throttle lag on mine, and i DO like an aggressive and sharp throttle (it's that motorbike thing again!)

As i said earlier: i'm sure it's a signal amp (probably non-linear though - got a bit of fuzzy-logic maybe?)

I think you're spot on with what it is and may do just as you want. Waiting to hear what Doug says...

The cheapest you'll get one of the Sprint Booster things is about £150inc so will be interesting to know what map options their are and if the stock remap already alters some of these parameters. Wish I had asked now!