claykos

We recently purchased a car with a 996 Cup engine in it and have been having some fuel pressure issues.

What is the proper spec for fuel pressure at the fuel rail?

Thanks in advance.

amondc

I will look it up tonight but you also want to check volume. We had a pump going bad but still had good pressure. The street GT3 should be at 3.8bar + or - .2 at idle. I think the cups are the same.

claykos

Thanks for the information.

The old setup was a small Fuelsafe lift pump in a surge tank in the cell feeding an OEM 993 turbo Bosch pump. I was seeing 55 psi at idle (3.8 bar) but intermittently losing pressure on track with the car stumbling and fuel pressure dropping down to 30 or worse.

First we changed the Bosch pump out to a new Bosch 44 and replaced all the fuel lines and filters (which were several years old). This improved the situation, but still seeing the occasional drops in fuel pressure.

Per some advice from Fuelsafe we have eliminated the lift pump and mounted the Bosch 44 inside the cell. Now it has 65 psi (4.48 bar) at idle but have not had it on the track yet.

Basically I'm wondering if 65 psi is too much or was 55 too little?

The joys of a new (used) racecar.... :-)

amondc

Sounds like 65 will be ok. was is stubling around 5k?

claykos

The car has a stack dash that logs fuel pressure and also has an alarm tied to it. I was getting a fuel pressure warning light followed by a stumble intermittently.

I just looked back at the data log and it seems like it was usually happening around 5.5-6k. Looks like a couple times at 7k also...

Is there something significant if it was doing it around 5k?

amondc

mine was doing the same thing at 5k ish and it was a bad pump. Sounds like you had the same probelm. New pump should fix it.

m42racer

Cup and RSR pressure is 5b or 72 PSI.

claykos

Thanks -

I wonder if I still have some yet to be discovered problem (seeing 65 psi at idle...)

Geoffrey

Since the Cup engine is a common plenum manifold, the engine will have vacuum at idle. If the fuel pressure regulator is manifold referenced, the fuel pressure will be reduced at idle.

claykos

I have to look at it again, but I don't think the sensor is referenced of the manifold. It is a standalone VDO sensor hooked up to the stack dash.

m42racer

Cup regulators are NOT manifold referenced. It is on the 4,5,6 side attached to the Fuel rail. Check the O ring is not leaking. There is a lock ring that can be unsnapped and the Regulator will come out of ot's holder. There is an O ring in there that needs to seal to hold pressure. Check the Filter as well. Pumps do go bad as well. Before you take the whole car apart, put another gauge on the Furl rails and check the sensor is operating correctly.

Rick DeMan

You will need to make sure the pressure in the rails does not exceed 3.8 bar...the fuel mapping is based on the factory pressure and increasing it will make it run rich...the ECU will be working overtime to lean the mixture under closed loop..

claykos

Ok....

So far votes are:
2 for 3.8 bar
1 for 5 bar

Fired the car up again yesterday and it was about 62 or 63 psi (4.3 bar) at idle (cold)....

There is a test day we are planning to attend this Saturday so I will see what is happening when the car is warmed up and on the track.

Thanks for the replies.

m42racer

Maybe the early Cup cars had a different FPR than my 08 engine has. I will look at the number on the FPR and post it. You could them compare. I know for sure that I run 72 PSI as I just had the engine dyno'ed and it was measured. I would have thought that if it was 43 PSI, then 62 PSI wpould seem to indicate that the FPR is stuck partially closed and not operating in it's correct range. In the past I have only ever seen them not making pressure as the diaphram usually leaks and pressure is not made at all.

Try fitting a mechanical gauge and see if it shows 62 PSI.

m42racer

I looked on my engine and the Bosch number uis on the bottom side of the Regulator body. Remove the clip and carefully turn the regulator so the numbers are facing up.

Mine is B 280 550 113/3 and has 5.0b also stamped.

Hope this helps.