Chris White

If you have an original fuel rail that is over 20 years old and has been vibrated quite a bit for its whole life than you are only asking for more trouble running high pressures. Remember that the FPR adjusts the actual fuel pressure for manifold corrected pressures. So if you are running 20 psi of boost on a 70 psi base setting then you are running 90 psi when boosted……I have seen enough leaky fuel lines and split fuel rails in my time….fuel leaks at 90 psi make a very nice plume of fuel ‘fog’, just about the best thing for sudden and violent ignition!

944CS

You can say all the theory you want, but I believe if anyone has done the R&D as to what works best, it is John Milledge.

There are many cars that have 5 bar fuel systems stock...this was not the norm though back in the 80's.

Chris White

Cars burning to the ground is not a theory, it happens more often that you would like!
I have had 2 fuel line/fuel rail failures at the track with the stock fuel rails.
The stock fuel rail is folded sheet metal with brazed connections / seams. Ok for stock applications but it does not age well. Doubling the fuel pressure will only stress it more. If you run into Jon again you can ask him about fuel rails.

BTW – the stock Bosch injectors are a very uncommon impedance, neither high or low. They are not very good in comparison to the newer designs (when driven by a modern EMS).

NZ951

If its such a good idea, why didnt Porsche run smaller injectors with 70psi fuel pressure?

Its not theory, its fact.

Chris White

Many of the 80’s technology injectors cannot open under high fuel pressures. The pintle style injector valve has to open against the fuel pressure. I have never tested the stock Bosch injector to see if it can open against 90psi of pressure – I know of many injectors that can’t be reliable at the pressure.

BTW, interesting to note that MS word really wants to change pintle to pantie or penile….somebody at MS has a fixation!

toddk911

Fuel pressure, panties and penile?? Best thread ever!! Lol

I have always read you want to keep fuel pressure as low as possible. Volume over pressure.

Chris White

It depends on what you are trying to optimize – the fuel pump curve (generally they work better at high volume/low pressure) or fuel atomization (higher pressure yields better atomization). Somewhere in the middle is optimizing for the injector – too high and it may not be able to overcome the fuel pressure and it won’t open, too low and it will ‘dribble’ the fuel in instead of spraying it.

With our cars we have the problem of injector dynamic range – on a batch fired system you may exceed the dynamic range of the injector – the injector has to be able to deliver the right amount of fuel for idle as well as the right amount of fuel for full boost at max RPM. If you are running a batch fired system (stock) the injector fires once per revelation (twice per cylinder firing event). At 6500 rpm the engine makes one revolution in 9.23ms. so at 100% duty cycle the injector could, in theory, be open for 18.46ms per cycle (2 revolutions). That only happens in theory since there is a lag time in injector opening and closing. When you get close to 100% duty cycle some injectors get nonlinear since they cannot close and open fast enough to keep up with the signal from the EMS. In the real world the minimum time for an injector to cycle is somewhere between .7 and 2.0 ms and this depends on injector design, battery voltage and your ECU injector driver. If you have an injector dead time of 1ms you will only be able to run about 8.23ms per firing at 6500rpm, so you can get a usable 16.36ms of fuel injected in to the cylinder per cycle.

So you have to be able to deliver the minimum amount of fuel to get an acceptable idle and the max fuel to keep it from leaning out a high PRM – and in this example you have a total range of about 16ms. If you need to support 125 hp per cylinder at max power that is doable but difficult with that range limit. Even at 100hp per cylinder you may end up running at minimum injector time on at idle and over 90% duty cycle at full boost/max rpm. The one variable you get to play with is the fuel pressure.

So, typically you aren’t adjusting fuel pressure for better spray or better pump performance, you are playing with the pressure to get the dynamic range of the injector into the sweet spot so that you can have an acceptable idle and keep the top end under 90% duty cycle.

944CS

Cars today use extruded tubing with brazed connections....

We have learned through racing that the fuel rails are a maintenance item on these cars.

I'm not saying it is the ideal fuel rail.

What I am saying is that John has done the math and been on the dyno more than anyone - since these cars were new. He understands the Motronic, know where the maps are, knows what the math is - while I've never asked him this question I think he uses the stock injectors to keep the DME healthy.

944CS

Why would they need to run 70psi fuel pressure for a 250bhp car? Not to mention the higher stress on all components - this is a street car designed to last 200K + miles.

mr965

I have a Powerhaus mod 89 951.90k miles. I added every bolt on part they developed.
I never got into the internals but it has held up well with the K27 turbo from 2002. I even got the vented nose panel off Gillies car when he was parting it out

ehall

Cool thread.

Jay Wellwood

Interesting...

FWIW - I have the modest light-pressure turbocharger conversion from Anderson Motorworks mentioned in the February 2004 Issue.

Bought it from Trevor shortly after that article was published. The 928S4 just to the left was ours as well. Moved on to greener pastures now. Still have the 968 - current project.