Maximum advisable fuel pressure?

12sec1/4 · Post by **12sec1/4** » Wed Feb 16, 2011 11:06 am

I'm just about to install a Walbro 255 high pressure pump as my OEM is maxxed out along with the RC550's. Currently I'm at around 48psi fuel at 10psi boost. Once the 255 is in place what would you suggest to be the highest fuel pressure I could raise it to without having injector or idle problems? Cheers, Paul.

relic1 · Post by **relic1** » Wed Feb 16, 2011 3:03 pm

are you trying to raise the pressure to compensate for the injectors being too small? I've run 54psi @ WOT before... but only as a temp patch till larger injectors could be fitted. More PSI means shorter life for the pump and more wear on the entire fuel system.

12sec1/4 · Post by **12sec1/4** » Wed Feb 16, 2011 6:16 pm

relic1 wrote:are you trying to raise the pressure to compensate for the injectors being too small? I've run 54psi @ WOT before... but only as a temp patch till larger injectors could be fitted. More PSI means shorter life for the pump and more wear on the entire fuel system.

In a word. Yes... I know it's a short cut that's why I'm asking opinions as to what the maximum recommended pressure is before I am forced to buy injectors. short cuts are not good I know and I'm not looking to run dangerous pressures... Just want to know the general limit. Paul.

Bindegal · Post by **Bindegal** » Fri Feb 18, 2011 4:29 pm

If the fuel pump was the limiting factor, you wonÂ´t need to up the fuel pressure. Try to run it without any mechanical changes first. Remember the rule of thumb is: Double fuel pressure results only in a 50% increase in fuel flow. So if the actual problem is that the injectors canÂ´t flow enough... do the math

Another thing to consider: Fuel pressure is offset by the boost pressure (this is why thereÂ´s a vacuum line attached to the fuel pressure regulator). So if your fuel pressure at idle is 43PSI, well then 14 PSI boost equals 57 PSI fuel pressure relative to atmospheric. So ensure the fuel lines, fittings etc. can cope with the added fuel pressure in case you decide to dial it up. /Allan

12sec1/4 · Post by **12sec1/4** » Fri Mar 18, 2011 7:25 pm

Thank you Allan, Great response. And very informative... Well after weighing up the pro's and con's (and being helped by BillB on here by way of instant RC750's [true gent]) I fitted a Walbro 255 and installed some RC750's. The injectors were installed at the tuning shop and the car was remapped by Piv on here. Before the injectors and pump were installed, the car was re-tuned by Piv (my tuner) at 10psi. With the high fuel pressure and 100% duty RC550's the maximum timing value was something like 17* at MBT. Did not matter if any more was put in, it would not make anymore power. We came to the conclusion that it was down to the fuel getting hot due the the pressure and injectors! After fitting both injectors and pump, it went back onto the Dyno... Was re-tuned at 15psi and Piv managed to get a shed load more timing in! Now fuel pressure is set at around 48-50psi and the injectors are at 80% duty. At 15psi on my modest turbo I'm making 343bhp and 250ftlb. The car is transformed and 100% drivable! (2nd gear is still abit useless at the moment even at wastegate pressure... Will have to wait for the warm summer to really test it) Basically, as I found out... High fuel pressures are not good! Another interesting thing I noticed... RC550's have one large hole (or jet) on the bottom and the 750's had 3 jets on the bottom... Could it be that the 750's allow for the fuel to atomise better than the 550's?? Paul.

TopMountGSR · Post by **TopMountGSR** » Mon Mar 21, 2011 12:59 am

You cant really crank your fuel pressure up too much on your setup, but I run 75psi base threw some pencil injectors