With the dual fuel pumps in the X300 XJR the second is only ran above 4k RPM to prevent saturating the pressure regulator at lower engine speeds.

In a situation with single a high-pressure external pump, can I use a universal fuel regulator to keep the pressure at 4 bar throughout the entire rev range? (am I correct that 4 bar is the correct pressure?)

Also, the pressure regulator is normally fitted to the return line, but the stock feed line in the Jag also has a similar device fitted to it. Is this also a regulator, or some kind of check valve?

Thanks! :-)

 

Hi Gabriel,

I'm not sure about the X300 XJR and the parts diagram at jaguarclassicparts.com don't show the feed line you describe, but on an XJ40 the fuel feed hose has a metal disc-shaped "damper" that helps dampen fuel pulsations that can lead to noise heard inside the passenger cabin. Perhaps that's what you're seeing that looks like a smaller regulator.

Cheers,

Don

 

Thanks Don, thats good to know. Can someone confirm if the fuel pressure should stay at a constant value throughout the rev range and if that value is indeed 4 bar?

 

Here's tidbit from the service manual.

"The fuel tank of AJ16 engined vehicles contains one fuel pump, supplying fuel to the normally aspirated engine and two fuel pumps, supplying fuel to the supercharged engine. They are regenerative turbine pumps supplied by Nippon-Denso. Nominal operating pressure is 3 bar (3.7 bar for supercharged engine) above the manifold depression and
pump delivery is 90 litredhour minimum at 13.2 volts, 3 bar outlet pressure. The pump(s) draw a nominal current of 7 amperes at 13 volts, 3 bar outlet pressure, ambient temperatures. Built in to the pump assembly is a over-pressure relief valve which blows at 4.5 - 8.5 bar."


The key point being ".....above the manifold depression....". So, regulated fuel pressure will vary somewhat, which is typical for a fuel injected engine, to maintain a 'constant pressure drop across the injectors'.

If you were thinking of a purely mechanical regulator which doesn't react to manifold pressure, it wouldn't work.


Cheers
DD

 

I was thinking of using a universal adjustable pressure regulator which is referenced to manifold pressure such as this one:



I have an external pump which can deliver up to 200l/h at 6 bar. What I was planning was to use this single pump, remove the factory regulator, and just set the aftermarket one at 3.7 bar.

 

I just measured the fuel pressure on my NA XJ6 the other day and I have about 2.5 bar (36 PSI) at idle (700rpm) rising to 3 bar (42PSI) as the engine revs up. This makes sense as manifold depression will decrease as rpms increase.

So if you set your pressure to 3.7 bar at idle you will have a higher (too high?) pressure at WOT.

There is no fuel damper at the rail on the AJ16 like there is on my AJ6 powered XJS