BrianG

I have just checked the Fuel Pressure on my '87 S4 and find about 48 psi (3 Bar). A quick forum search reveals that this is a nominal value. I do not see a rise in that pressure on throttle tip-in however. Should I?

Can I elicit an appropriate fuel regulator response (increase in pressure) by simply pulling the vacuum source from the regulator?

If not, how can I test it?

Many have suggested raising the pressure to garner a few more ponies, especially with mods like Dave's new X-pipe (that's in the mail). In setting an adjustable regulator or a RRFPR what should I want for fuel pressure at 0-vacuum and at idle-vacuum?

dr bob

The 48 PSI is with NO vacuum. That is, engine off, fuel pump running. With the vacuum hose hooked up and the engine at idle, the fuel pressure should drop. On throttle with engine load (not just tip-in), the fuel pressure should rise toward the static 48 PSI, but will only get close to 48 under full load, at the point where the throttle is no longer restricting inlet flow. IIRC, the other two fuel pressure regulator/dampers will slow the response of the FPR somewhat, making a tip-in response in fuel pressure hard to identify.

This is from theory, not experience, so I can't tell you what the actual numbers should be under load. I have the RRFPR on the shelf, along with the gauge. Too busy or lazy (you choose...) to get the rail fitting and tap it for the gauge. One of these days...

HTH!

BrianG

with the stock equipment, I'm getting 48 psi at idle. Does this imply reguator failure?

or vacuum leak?

What pressure is expected/acceptable at idle?

m21sniper

Sounds like a vacuum leak or a bad diaphragm in the regulator.

John Speake

Hi Brian,
Handbook figures for S4/GT/GTS are 47psi at idle, 55psi with engine stopped, fuel pump bridged.

So your figure of 47psi is OK.

BrianG

Well, that's a load off............ thanks John!

So, on to the second part........ with an adjusatable fuel pressure regulator:

what pressure at idle"?

what pressure at 0 vacuum? (or engine off / relay bridge)

Dave??

John Speake

Hi Brian,
I don't know anything much about RRFPR. However, I belive some of them are adjustable for the setting at idle (max vacuum). A standard FPR, because it is controlled by manifold vacuum, always maintains the same effective pressure difference between inlet manifold pressure and and fuel injector pressure. (55psi)

So at idle, if you disconnect the vac hose you will see a rise in idle fuel pressure from 47psi to 55psi.

A RRFPR doesn't obey the rule of tracking manifold pressure and regulating fuel pressure directly. on a lb per lb basis. It allows the fuel pressure to effectively rise at the engine load rises (i.e as manifold vac drops).

Some RRFPR allow you to adjust this effective increase in fuel pressure (fuelling) at high engine loads, or sometimes at what point it starts deviating from the 1 to 1 law

That's the theory as I know it. I have no knowledge of specific makes of RRFPR.

BrianG

So, what 's the new target value for idle and for 0-vacuum fuel pressures with the X-pipe and no CATs?