High Fuel Pressure??

[edhorne]

Hi guys

I plumbed a pressure gauge into my fuel rail today to check the fuel pressure was up to scratch.

The gauge read 3.5 bar. I thought It was supposed to be at 3 bar. I realise the gauge may not be reading correctly but it seems like a big error if it isn't.

Would this affect the fuelling in anyway?

Any thoughts appreciated.

Ed

[GreenV8S]

edhorne said:

Would this affect the fuelling in anyway?

If it's wrong, yes. I can't tell you whether it's wrong though. If it's spot on a nice round number that would seem to suggest that it's been set there deliberately, but you need to know what the pressure is supposed to be set to to be sure.

[optimax sniffer]

I would rather it would be higher than lower. You need to know what it is meant to be...

[edhorne]

Can anyone tell me what the fuel pressure should be?

I thought it was 3 bar. I stand to be corrected.

Cheers


Ed

[SXS]

edhorne said:

Can anyone tell me what the fuel pressure should be?

I thought it was 3 bar. I stand to be corrected.

Cheers


Ed

It varies, as far as I know, over the years different pressures were used. For the cerbie your pressure shouldnt really need to be higher than 40psi, thats about 2.7bar, I'd be more worried if it was running .5 bar lower...

If your fuel pressure is too low, you will have bad tip-in detonation and will possibly run lean at WOT. If pressure is too high, you will unnecessarily stress your fuel system and run too lean at WOT... there has been a past concern on the cerbs that there has been too high fuel pressure which has resulted in hoses running at their limit and when running too hot has burst hoses that caused the car to burst into flames...

At 40PSI fuel pressure, and 85% duty cycle injectors, you're looking at around 65hp per injector, that works out nicely. I'd suggest firstly have a look at your injectors, you may have different injectors in there that someone decided to change to in its history which resulted in a need for higher fuel pressure.

Edited by SXS on Monday 17th September 02:11

[julian64]

Before indulging in a flight of fantasy involving previous owners preparing for nascar, take a look at the fuel pressure regulator on the back of the rail.

IIRR the serial number punched on it should tell you what pressure its supposed to be keeping the rail at.

The way the fuel pressure regulator works means its more likely to lower the pressure rather than raise it, but worth check the serial number to make sure you have the right part.

Mostly Cerbs don't have the bottom of the fuel pressure regulator connected to manifold vaccuum. In other words the fuel pressure is running higher than it should at low engine revs, high vaccuum.

[SXS]

julian64 said:

Before indulging in a flight of fantasy involving previous owners preparing for nascar, take a look at the fuel pressure regulator on the back of the rail.

IIRR the serial number punched on it should tell you what pressure its supposed to be keeping the rail at.

The way the fuel pressure regulator works means its more likely to lower the pressure rather than raise it, but worth check the serial number to make sure you have the right part.

Mostly Cerbs don't have the bottom of the fuel pressure regulator connected to manifold vaccuum. In other words the fuel pressure is running higher than it should at low engine revs, high vaccuum.

The vaccum differential across the manifold on the normally aspirated cerb is marginal to say the least.... IIRC the connection is ported pre-TB, not post (cant remember now)... thus it see's no where near the level of vacuum compared to a post-tb setup... leaving this port disconnected is IMO neither a disadvantage or an advantage...

But I am compelled to believe this car may just be one of those 'injector modded cars with raised fuel pressure' back from the whey days.... who knows.... if the fuel pressure regulator was starting to fail then it would lose pressure, not gain, due to the way the internal actuator operates...

but if someone has stuck on a reg that wasnt within spec to what the cerberas fuelling system requires then the company who did this modification should be sourced and their name posted up here... if mechanics are replacing parts with parts which are out of spec, this could lead to potentially dangerous outcomes.... obviously.

[julian64]

SXS said:

julian64 said:

Before indulging in a flight of fantasy involving previous owners preparing for nascar, take a look at the fuel pressure regulator on the back of the rail.

IIRR the serial number punched on it should tell you what pressure its supposed to be keeping the rail at.

The way the fuel pressure regulator works means its more likely to lower the pressure rather than raise it, but worth check the serial number to make sure you have the right part.

Mostly Cerbs don't have the bottom of the fuel pressure regulator connected to manifold vaccuum. In other words the fuel pressure is running higher than it should at low engine revs, high vaccuum.

The vaccum differential across the manifold on the normally aspirated cerb is marginal to say the least.... IIRC the connection is ported pre-TB, not post (cant remember now)... thus it see's no where near the level of vacuum compared to a post-tb setup... leaving this port disconnected is IMO neither a disadvantage or an advantage...

????

[cossiemetro]

if the pressure is too high watch the fuel lines might not hold and your have a pressure fed fire not good i would check the fuel lines anyway to be on the safe side IMHO

[edhorne]

Thanks guys

The fuel lines seem fine and all the injectors are the standard Bosch -744 beige injectors. I must admit I haven't actually driven the car with the pressure gauge connected so it was only the reading at idle that I posted. I will try giving it some beans and see if it makes any difference.

I will have a close look at the regulator as well. See what's stamped on it.

Cheers

Ed

[julian64]

edhorne said:

Thanks guys

The fuel lines seem fine and all the injectors are the standard Bosch -744 beige injectors. I must admit I haven't actually driven the car with the pressure gauge connected so it was only the reading at idle that I posted. I will try giving it some beans and see if it makes any difference.

I will have a close look at the regulator as well. See what's stamped on it.

Cheers

Ed

Giving it beans shouldn't make any difference at all, unless you have a problem with the fuel pump.

If you had the vacuum hose connected to the bottom of the regulator, and I've only ever seen two cars that did. Then giving it beans should actually increase the pressure a bit as the vacuum would decrease.

[edhorne]

So does anyone know what the pressure is supposed to be?