View Full Version : Twin Fuel Pressure Regulators

16th May 2000, 11:50
Being new to this site, has this topic been discussed before?

by getting an extra fuel rail to the injector and an additional fuel pressure regulator will give better fuel flow to the right side of the engine( the side with infamous lean burn problems). With this modification, surely all 4 cylinders will be getting a more even flow of fuel and thus decrease the chances of detonation. Due also to a shorter fuel passage and less kniks in the piping.

Any comments whether this mod will increase HP's and guzzle more gas with no change in settings on a link ecu. Many have ungraded to larger fuel pumps, but would this solve the lean burn cyclinder problems as the fuel pipes downstream still has restrictions to the fuel flow.

Mention is had to Bob who has changed bigger injectors and gained HP's just by that. Is changing the fuel lines to the injectors and adding its own regulator to the left side be anywhere comparable to an injector or fuel pump change.

Having more balanced fuel flow to all cylinders. Matching the weight on all 4 pistons have been said to gives more hp's due to a more balanced combustion?

16th May 2000, 16:58

this mod has been used in the past (Anders is running two regulators at the mo, for example). With regard to the effectiveness of such a modification, I'm not aware of any data gathered on an engine dyno which would show whether the mod does actually stabilise the fuel pressure as suggested.... perhaps more effective is the use of a swirl pot; this way there is a second fuel pump, very close to the injector rails; because the length of the pip between the pressurising element (the pump) and the pressure stabilising element (the regulator), it stands to reason that any fluctuations would be smaller, and of shorter duration.

Balancing combustion is more than just getting the fuelmix spot on in every cylinder. You must also consider the cylinder head and piston crown design, as well as the cooling arrangements [no local hotspots etc].

If you are thinking of this type of mod, then it may also be worth testing the injectors on a flow bench. Theoretically they should be no more than a 3% difference in static flow rate, but it doesn't hurt to test them.... no point in having a "perfect" fuel system if the injectors aren't matched! :)



24th May 2000, 11:10
To be honest, I don't think it is a good idea to split the rail like this...The Japanese who make 500+hp use the stock rail and just upgrade the injectors and fuel delivery system...If you don't want to go to having an external swirl pot than a Nissan GTR fuel pump will fit with some mods in place of the stock pump and flow more than twice the volume at 45psi (WRX rail pressures are set between 45 and 47 psi, or at least 4 that I have seen here in OZ are..) that the stock pump does..YThe reson that the right side leans out is not the fuel rail but the inability of the stock fuel pump to keep up with proceedings....

Bob Rawle
24th May 2000, 13:07
I agree with Glenns on this, my STi 2 uses larger injectors with the standard fuel reg and pump, it makes 2.4 bar pressure at idle rising to 4.2 bar pressure at 1.35 bar. Twin regs are just a "gimick" IMHO ... standard rails are plenty large enough to flow the fuel. Twin regs can in themselves cause problems, how do you match them up, bet you need to go through a lot of regs to find two that have exactly the same rising rate in them, if they are different then each rail will run at different pressure dependant on conditions. So then you have two cylinders with a different flow of fuel to the other two.

I'm about to fit swirl pot and separate pump to my car to improve fuel flow ... I will be leaving the fuel pipework/reg standard with just the uprated injectors.


24th May 2000, 13:40

I was thinking two std new Subaru regulators and 1 new rail where the regulator fits mounted upside down on the other side of the engine. This would need the inlet manifold to be removed. Having more peven presure over the std one as the current one regulator set up has the left side with extra lengths of rails before reaching the injectors.
This will invaribably cause a pressure drop on the left side. By doing this, A perfectly even pressure to both sides can't be gauranteed , at least you will be getting higer pressures to the left injectors than the stock set up.(an analogy with equal length extractors).

Do the std rail restrict flow? especially to the left side of the engine, given also a longer travel path and kniks in all the extra bends.


Bob Rawle
24th May 2000, 17:51
The pressure drop back thro the rails is negligable, don't think more pressure equals more flow, it doesn't once a point has been passed and could equal less flow so be careful. So its not all about pressure but about a pumps ability to deliver a volume at the pressure, re leaning on the right side its said that no 2 runs leanest, this makes nonsense of the pipe run theory as no 4 is actually the furthest from the reg. The pipe bends are not going to slow things down too much either and so can be discounted as having much effect. The injectors nearest the reg will be at the highest pressure as that is the point of control.

If you want you need equal lemgth runs of pipe from the filter to the fuel rails which should be connected in parallel and controled by one reg which is equidistant from each. that way you get the same rising rate and the same pressure drops across both banks, not going to do too much for you though, injectors themselves have bigger variations than anything the rails are likely to induce. If the fuel system is that close and sensitive then its time to upgrade the pump and injectors.


25th May 2000, 11:58

You mention 480 injectors, are subaru paets or modified one. How do I get a set and how much are they. Would upping the injector sizes solve the lean cyclinder problem. Or is an uprated pump be in order as well.

So what you are saying is that once the rails are presurrised, irregardless of length, the same fuel flow will be acheived. so leaning out in any particular boils down to an inadequate pump or variances in injector flow.

Thank for that bit.


Bob Rawle
25th May 2000, 16:19
Injectors are a Subaru part and are fitted to a Twin Turbo Legacy. I'll mail you the part number when I can get at it.

re flow ... its not quite that simple because air flow through the inlet manifold, exhaust header effects, individual cylinder temps etc all play a part in wether a cylinder is lean or not. The point I am really making I guess is that the rails are good for quite alot of fuel flow over and above whats needed before they need changing and that the first limiting factors are injectors and fuel pump.