fuel pressure

[Pappu1]

what are rthe advantages (if any)

of running lower fuel pressure with bigger injectors and mederate duty cycles

as opposed to

lowe dc on higher fuel pressure

been discussing some upgrades with a mate and

we have option of running the stock injectors at hi dc

or get bigger inj and run low dc with stock pressure

or run the bigger inj with moderate dc but low fuel pressure

current injectors = 565 pinks

been looking at a set of 720 at 30psi (decap pinks) - or something like this

current pinks hitting almost 100percent at full boost

still no afr issues but thinking or injector valve float and such..

anyway whats are the thoughts out there

what sorta dc etc will we be expecting

this interests me a lot as im in a similar boat and wud be cool to help out someone else do this and once its done right i can copy the same setup

[Wiretap]

Injectors should be run at their specified fuel pressure. Subaru standard fuel pressure is 44psi MRP (manifold relative pressure)

The most major effect of lowering the fuel pressure will be a change in latency. Other effects include a poorer atomisation of the fuel and the potential for greater differential pressure across the injectors.

The other thing to be aware of is the fuel pump's optimum flow range. Too low pressure and it will not perform, too high pressure and you're strangling it.

Decapped injectors have a notoriously poor dispersion pattern and will probably require quite a bit more warmup enrichment because of this.

[Guest]

Is fpr set with vacuum off to 44psi? i looked around for fuel pressure when i set my reg everone seemed agreed on 43.5psi wot.

[Wiretap]

 evowrx said:

Is fpr set with vacuum off to 44psi? i looked around for fuel pressure when i set my reg everone seemed agreed on 43.5psi wot.

As per the Subaru manuals it should be 44PSI MRP. However 43.5 is probably close enough.

[Guest]

set with the vacuum pulled off?

[Wiretap]

 evowrx said:

set with the vacuum pulled off?

MRP means 'relative to manifold pressure'. Essentially if you're pulling the hose off at idle you're fooling the FPR into seeing that MAP = BAP aka '0psi boost'. So yes, however you like to call it, if you pull the vacuum hose off you are setting it to MRP.

[Pappu1]

 Wiretap said:

Injectors should be run at their specified fuel pressure. Subaru standard fuel pressure is 44psi MRP (manifold relative pressure)

The most major effect of lowering the fuel pressure will be a change in latency. Other effects include a poorer atomisation of the fuel and the potential for greater differential pressure across the injectors.

The other thing to be aware of is the fuel pump's optimum flow range. Too low pressure and it will not perform, too high pressure and you're strangling it.

Decapped injectors have a notoriously poor dispersion pattern and will probably require quite a bit more warmup enrichment because of this.

yea man thats the reason we trying to see options

i have been told decap pinks have good flow pattern and atomisation

maybe for decap pinks stock pressure maybe the best pressure ay?

just to confirm - walbro 255lph pumps - relief pressure = 75psi??

[Stoffa]

Lower pressure will have worse atomisation on an injector that already has bad atomisation from being decapped. i'd imagine you may end up with idle dramas and maybe bad off boost drivability

[pl0x]

Don't people Re-cap injectors to get better atomisation? (different caps still providing higher flow)

[Pappu1]

hmm good point...

yea dont want bad atomisation..

whats top feed 720cc + sold for these days (straight drop for v7+ rails)

[Stoffa]

Kind of. ive got a set of brand new caps in my tool box but they only work on sidefed. they have 2 holes which help a little but still have a crap spray pattern

[Koom]

 pappu said:

just to confirm - walbro 255lph pumps - relief pressure = 75psi??

Another thing to remember along these lines, is if the pumps ultimate pressure is 75psi (I think its somewhere around there for Walbro's). And you take the base pressure + boost pressure (say 20psi) so then the pump will be putting out at least 65psi. You will also need to account for pressure losses through the fuel system i.e. will be a drop from travelling through the lines from the rear to the front, then through the filter and also through the fuel rails. I don't know how much of a pressure drop there will be due to these factors, but I'd guess that 20psi boost would be getting near the Walbro's limits?

[Guest]

there was a thread that touched on the walbro + psi the other week. like to know from someone whos done it though as i cant go for anything that isnt in tank.

[Koom]

They are fine in most situations. There's other intank options these days (names escape me right now though). Have also seen Bosch 044's made to fit intank.

[Guest]

http://www.deatschwerks.com/products/fuel-pumps/dw300-in-tank-fuel-pump

they were in nzpc.

h8glos spelling? fitted one last week i believe

[Koom]

Who you calling a douchebag???

haha yeah I think thats the one.

[funkytown]

Fwiw i run only a walbro intank and 800cc sidefeeds plus 25psi boost for about 300wkw.. Its about the pumps limits i think lol. I use 2x factory fpr's

[Stoffa]

i now run an aeromotive 340lph intank pump, exactly the same physical shape as a walbro

[Pappu1]

 funkytown said:

Fwiw i run only a walbro intank and 800cc sidefeeds plus 25psi boost for about 300wkw.. Its about the pumps limits i think lol. I use 2x factory fpr's

Is Ur one. 255lph Walbro? Aka 500hp version...

So 45 base plus 25 boost making 70 max rail pressure at full boost plus line losses etc

Hmm either line losses are low as in few psi or the Walbro limit is higher than 75. .....

Anyway so consensus is that running stock pressure even with bigger injectors is a safer bet?

[Stoffa]

I run 45psi base pressure but I also run ID1000's which are ment to have their best atomisation at 80psi

[Pappu1]

 Stoffa said:

I run 45psi base pressure but I also run ID1000's which are ment to have their best atomisation at 80psi

Wow that's gotta be hardcore setup

[Stoffa]

Trying to crack 250kw atw. didnt want pinks stopping me. anyway no need to upgrade if we ever get e85 on tge pump in welli

[WRXONP]

allways better to keep the fuel pressure at 3 bar for atomisation as said

whats with the crazy confusion of words about setting a FPR - you pull the vac line off and set it to 43.5 (3 bar)

realistically - since a walbro flows more fuel than a stock pump - using a stock reg + walbro the fuel pressure will be higher - as all the FPR does is restrict the return to make pressure - more flow = more pressure (more fuel flow = higher rail pressure because its the same restriction on return) - how much i wouldnt know you would need to stick a fuel pressure gauge on it - which is why you normally run a little richer with a walbro compared to a stock pump (because rail pressure rises slightly)

http://www.google.co.nz/imgres?q=walbro+flow+chart&um=1&hl=en&sa=N&biw=1366&bih=662&tbm=isch&tbnid=-lvhcBYaP4MzsM:&imgrefurl=http://forums.pelicanparts.com/911-930-turbo-super-charging-forum/323855-fuel-pump-options.html&docid=qi9FBJTZS0cxtM&imgurl=http://www.jayracing.com/images/Bosch_044_vs_Walbro_255HP.JPG&w=709&h=482&ei=cg22T_nxGNSjiQf6qZWOCQ&zoom=1&iact=hc&vpx=317&vpy=158&dur=814&hovh=185&hovw=272&tx=169&ty=113&sig=104147622277968793972&page=1&tbnh=141&tbnw=208&start=0&ndsp=18&ved=1t:429,r:1,s:0,i:73

as you see here - the walbro hits the pressure releif valve at around 75 psi- it does more pressure - but since the releif is open the flow drops off from its linear shape that it had

you can do the pressure releif mod - and fuel flow stays linear - all your doing is blocking the releif valve off

[Wiretap]

 WRXONP said:

allways better to keep the fuel pressure at 3 bar for atomisation as said

whats with the crazy confusion of words about setting a FPR - you pull the vac line off and set it to 43.5 (3 bar)

realistically - since a walbro flows more fuel than a stock pump - using a stock reg + walbro the fuel pressure will be higher - as all the FPR does is restrict the return to make pressure - more flow = more pressure - how much i wouldnt know you would need to stick a fuel pressure gauge on it - which is why you normally run a little richer with a walbro compared to a stock pump (because rail pressure rises slightly)

as you see here - the walbro hits the pressure releif valve at around 75 psi- it does more pressure - but since the releif is open the flow drops off from its linear shape that it had

you can do the pressure releif mod - and fuel flow stays linear - all your doing is blocking the releif valve off

....

....

....

You don't actually understand how MRP works or why they do it do you?

It may be 43.5psi at atmospheric pressure, but if you are at, say 10psi vacuum... it becomes 53.5psi MRP. at 20psi boost it becomes 23.5psi... Your injectors are not affected by atmospheric pressure, they're affected by manifold pressure. Their best atomisation is at 43.5 PSI MRP. not ARP.