fuel pressure mod (with modified inlet systems)

[vocky]

thanx go to JohnnyN for bringing up this subject cheers..

The question was what does a modified inlet system do to the fuel pressure notsure.gif



I measured the vacuum for the fuel pressure regulator using my digital manometer, range 0 - 20 mBar

The Results

standard z22se [precat removed only]

idle = 0.2 mBar suction to fpr (fuel pressure regulator)
4k rpm = 6.0 mBar suction to fpr

modified inlet system

idle = 0.2 mBar suction to fpr
4k rpm = 0.2 mBar suction to fpr

so because there is less restriction from a modified inlet system the fuel pressure will not rise with the engine revs (vacuum).

the solution is quite easy, move the vacuum pipe from the inlet pipework to the side of the inlet manifold

if the fpr vacuum pipe is close to the egr pipe get a longer piece, or it could melt :roll:

use the blanking plug from the inlet manifold to blank off the inlet pipe 'nipple'

 

[Daz]

Guess wot i'm doin 2morow...

 

[JohnnyN]

Thanks Neil, that's a job for me too next time I've got time.

;-)

 

[Daz]

prob wont make much diff to ures john...as it parked in the garage most of the time...

On a more serious note tho...is that 0.4 of a bar really gonna make much of a difference...

 

[vocky]

On a more serious note tho...is that 0.4 of a bar really gonna make much of a difference...

don't you mean 5.8 mBar difference

 

[Daz]

i got man flu neil so i'm excused for reading things wrong lol

 

[vocky]

I will let you off

 

[JohnnyN]

Am I right in thinking that the other outlet on the inlet manifold is the one with a rubber cap on it?

:roll:

 

[vocky]

Am I right in thinking that the other outlet on the inlet manifold is the one with a rubber cap on it?

:roll:

sure is

both the small outlets are the same by the way (identical vacuum take offs)

 

[blue se2]

prob wont make much diff to ures john...as it parked in the garage most of the time...

 

[JohnnyN]

I've made this a 'Sticky' topic as its a good bit of information.

 

[Daz]

Swapped my pipe over today...is there any differnece,as i cudnt really tell on the way home from work..

 

[vocky]

Swapped my pipe over today...is there any difference,as i cudnt really tell on the way home from work..

you will be getting the correct fuel pressure now, might take the ecu a bit to get used to it dunno..

 

[JohnnyN]

Done a bit of tidying up today. Moved the pressure regulator pipe to the branch on the inlet manifold, blanked off the hole left in the induction pipe, put the air temp sensor back round the right way because it was 90 DEG. out to the air filter and put my ported T/B back on.

8)

 

[vocky]

you have been busy Johnny cheers..

 

[Daz]

Since doin mine last week,i have noticed that my round town Mpg has gone up from 22-23 to 26-27...

 

[rabidh]

Thanks for measuring this. Its really good to know... It would be interesting to see what effect even changing the panel filter for an upmarket one has (or removing airbox restrictions)...

Seems to produce good results, but i'm not certain I get what the vacuum pipe is doing... More vacuum = more fuel rail pressure = more fuel? or is it the other way around?

Its just it seems to be that in the std engine, the vacuum is pretty much related to the volume of air going through the inlet pipe. So WOT=lots of air at higher revs=higher vacuum=more fuel. When you move the take-off to the engine side of the butterfly the opposite effect happens surely? You get lower vacuum on WOT, so less fuel - but when the butterly closes (overrun mainly) you get loads of vacuum and hence high fuel pressure.

...or is this not an issue because on overrun the injectors shouldn't be opening at all.

It would be really interesting to see how much the vacuum effects the fuel pressure. Must be a PITA to do, but it would be great if there was some kind of datasheet for the regulator.

edit: if its a lot of difference I could see it might be possible to make some kind of widget that changed the vacuum depending on engine revs so we could all 'remap' the stock ECU to some extent.

 

[vocky]

some more info


the inlet manifold vacuum is 144 mBar or 2 psi, at tickover or higher revs, that's on both a standard z22se and my 2.4 inlet manifold.

so originally it was 6 mBar or 0.1 psi at 4k rpm.

the idea on the usa sites was to increase fuel pressure at lower revs to compensate for a modified inlet system.

This may be the reason why a remap is required for a viper etc, more air but less fuel pressure :?

I used the inlet manifold vacuum for over two years on my vectra with zero issues

 

[rabidh]

Thanks. Thats quite a vacuum difference Under load it is lower though surely? I remember I used to have a inlet manifold vacuum meter in my car

Must admit i'm still not sure about it... It seems as if a modified inlet system (while it will sod up the fpr's vacuum) would let about the same amount of air in at lower revs (where the lambda sensor is being used) but more at higher revs. The ECU would adjust to be right at lower revs (most driving) so the end result would be lean burning at higher revs surely?

Maybe the vacuum is just so high that the fuel pressure regulator stays maxed out all the time though? I guess that would be better than the incorrect (or tiny) adjustment from a modified inlet system... As you say its obviously proved to work, so I'll have to give it a go. I'd just love to get straight in my head *why* it works

 

[vocky]

the vacuum is stable to the fpr with this mod, so keeps the fuel pressure at max (4 Bar) which is the same as fitting an adjustable fuel pressure regulator (set at 4 Bar) and hundreds of modified cars have adjustable fpr's fitted

the ecu compensates for the increased fuel pressure at lower revs, so you won't notice any difference driving dunno..