What Would Your Fuel Pressure Be With The Fuel Pressure Regulator Deleted?

[sonicz]

Like a de cat, If you just ran a straight through hose where the FPR normally sits, does anyone know what the fuel pressure would be in the stock fuel system running a 340lph pump?. Just at idle, you can negate injectors here.

I'm trying to bring my fuel pressure down and no matter what I do its always too high. I have checked there are no pinches anywhere. Stock fuel lines.

Before I spend money on a high end FPR I want to know if its even possible to go as low as I need, or I I need to change out the pump for flowing too much just the stock fuel line is enough of a restriction to raise the pressure above what I need.

Edited September 28, 2015 by sonicz

[scotty nm35]

What car Sonicz?

The only cars I have had an issue with so far are the Evo's and S15's as there are restrictions in the return. Anything smaller than 4mm or so will bump the pressure up too much.

[sonicz]

Its a 1998 R33 GTST. Do you have a picture of the restriction hose? Can you feel anything inside the hose if you squeese it? I have done everything I can to drop the fuel pressure, Nismo FPR is not enough, my only remaining option is to throw a lower flow pump and Its such as shame as this one was installed so well those years ago. Deep down in the hole and proper. I don't think I have the patience to do it again sitting in the boot like a monkey feeling around inside a dark hole.

Edited September 28, 2015 by sonicz

[scotty nm35]

Never had an issue with the Skylines, only the s15 had a restrictor in the return hose off the reg. The Evo had a small return line in the tank.

[KiwiRS4T]

Spit it out man - what pressure are you reading? What pump do you have? Have you tried pushing air though your return hose? If your pump was installed years ago why should it be too big now? What have you changed?

[33SOM]

I was reading 100+psi on my 32gtr. Turned out to be blocked @ the very end of the return line in the tank.

Disconnect the hose after the reg. Simply blowing into it will tell if it's blocked anywhere..

[blind_elk]

I'm guessing that if you did away with the FPR, you would have next to no fuel pressure at the injectors.

But it begs the question - why do you think you need to drop the fuel pressure? What pressure do you have now? And what do you think it is doing to the overall running of the engine?

[sonicz]

Spit it out man - what pressure are you reading? What pump do you have? Have you tried pushing air though your return hose? If your pump was installed years ago why should it be too big now? What have you changed?

There's not much to spit out but I'll tell you what I know. My pressure was around 65psi at idle with the vacuum hose connected when I measured it. The pump I just got of ebay. It was a hi flow walbro type pump, possibly even a 400lph one. It was always too big, I just never knew it. Car originally felt ok after install but as the weeks went on it started acting weird and I guess I just never put two and two together or realised new the fuel pressure was so high.

No i have not tried blowing the return hose, but but its a great idea! So it should just blow with little no resistance? Do I need the fuel cap open for this? Should I wear lipstick?

33SOM, thx for the same idea as well.

Blind elk. Same as above.

Well It depends, the simple size of the stock fuel hoses would offer a certain pressure at a certain flow level. I was just curious what the pressure would be without an FPR with say for example at 340-400lph. Surely it wont be 0. Maybe it will be 2 or 20psi but it wont be 0.

Because I don't understand how a fuel pressure regulator can claim to have a certain range of adjust-ability without know the flow rate. For example if an FPR has a range of adjust-ability from 35-70psi, then if you flow twice that amount of whatever flow rate was used to get that reading, not matter how low you set the FPR you will never get get 35psi.

Edited September 28, 2015 by sonicz

[KiwiRS4T]

Flow and pressure are two different things. Walbro should be fine. Check for blocked lines (yes take the petrol cap off)!

[GTSBoy]

Because I don't understand how a fuel pressure regulator can claim to have a certain range of adjust-ability without know the flow rate. For example if an FPR has a range of adjust-ability from 35-70psi, then if you flow twice that amount of whatever flow rate was used to get that reading, not matter how low you set the FPR you will never get get 35psi.

That's why a pressure reg IS a pressure reg. They are a variable orifice where the size of the hole is changed to maintain the pressure where the setpoint is. If there is low throiugh-flow, the hole gets smaller. The only times regs are a problem size are when the minimum hole size in it is too big (hence you can't maintain high enough pressure at the lowest through-flow) or the maximum hole size is too small and then you can't keep the pressure down when the is high through-flow).

[sonicz]

That's why a pressure reg IS a pressure reg. They are a variable orifice where the size of the hole is changed to maintain the pressure where the setpoint is. If there is low throiugh-flow, the hole gets smaller. The only times regs are a problem size are when the minimum hole size in it is too big (hence you can't maintain high enough pressure at the lowest through-flow) or the maximum hole size is too small and then you can't keep the pressure down when the is high through-flow).

Exactly that last part I am talking about. If the maximum hole size is too small for a certain amount of flow, it wont be able to keep the pressure down below a certain amount no matter what its set to. Therefore the range of adjustability an FPR has is totally dependent of the flow rate.

Edited September 28, 2015 by sonicz

[GTSBoy]

Yuh, but in practice, in the narrow world of RB engines, it's not an issue. The stock FPR is large enough to flow shit tonnes of fuel. Any aftermarket adjustable reg is sufficiently larger than that to flow even more shit tonnes.

[KiwiRS4T]

Exactly that last part I am talking about. If the maximum hole size is too small for a certain amount of flow, it wont be able to keep the pressure down below a certain amount no matter what its set to. Therefore the range of adjustability an FPR has is totally dependent of the flow rate.

Stop trying to find an abstruse technical problem and just clear out your fuel line.. If by some chance it is not in fact blocked I can sell you an OEM GTR fpr for $10 plus postage.

[sonicz]

Thanks for the offer, but I already have a few stock ones around and I find little value to non adjustable FPRs.

[scotty nm35]

They are still working 20 years on, and do a very good job of controlling the fuel pressure. Millions of oem cars can't be wrong.

[Ben C34]

Thanks for the offer, but I already have a few stock ones around and I find little value to non adjustable FPRs.

What value do you find an adjustable reg has? Today. Not 15 years ago.

[sonicz]

What value do you find an adjustable reg has? Today. Not 15 years ago.

I don't really get why you are asking this question? What does the time frame of 15 years ago vs now have to do with anything? Not everyone has access to the latest and greatest tunes/computers injectors etc if its what you are implying.

The value of an adjustable reg depends on the circumstances, and in my case it has infinitely more value than a stock reg because it allows me to lower the fuel pressure to a more reasonable level, whereas a stock one does not, but even if we were generally speaking, what value do you not see they have?

They perform exactly the same function (if not better at it) as the stock reg, with the added bonus that they give you the ability to adjust the fuel pressure, should the need arise, or simply if you want to run a little richer or a little leaner without changing the tune. Or just to get that fuel pressure bang on. They have no downsides. If the need does arise, as in my case, its an easy tool you can use to lower or raise the fuel pressure.

I also think there is a misunderstanding somewhere. Are you guys under the impression that a properly working stock reg will keep a proper pressure in my case? I tried a few stock regs, they all perform identically, too high. Even with a perfectly functioning stock regulator, my pressure will be too high. I reason that its simply because the pump is flowing too high, not because there is anything wrong with the stock regs or my setup. The lines seem ok. So no stock reg will be able to do the job in this case. And I don't see why you guys keep bringing up stock regs?

It is this same line of thinking that the stock regulator somehow is able to maintain proper pressure (even if you install a high flowing pump) that got me into trouble by installing a higher flowing pump in the first place without being prepared for it. It doesn't do that based on what I have seen. If you fit a 400lph pump, the fuel pressure will be too high running the stock reg on stock fuel lines no matter what. Maybe there is something wrong with my setup, but I suspect that is how it would be in any car running the the stock reg.

Who is running a 400lph pump and has the stock rb25det reg keeping 43psi? I don't think thats possible. If anyone has evidence to the contrary I'm curious.

A user called kingtube69 recently installed a 400lph pump, and he said his fuel pressure was too high afterwards. Which makes sense because he is now flowing more, so he needed an adjustable reg to lower the pressure. The stock reg cant keep 43pis anymore. Same as in my case. I don't think its a matter of stock regs going bad by coincidence when you fit a new pump, and I don't think you can say to him the stock regs are fine. They suddenly become useless. You need an adjustable reg if you have a much higher flowing pump than stock, unless you want to tune for and run whatever higher fuel pressures your stock FPR decides to give, which I am not in that position. I need it at 43psi, so a stock FPR is a paper weight to me.

http://www.sau.com.au/forums/topic/459719-wiring-questions-for-intank-fuel-pump-upgrade/

Edited September 29, 2015 by sonicz

[GTSBoy]

Have you blown back through the return line yet?

[GTRPSI]

By pass your reg as a test, or in other words take it off the rail and use a fitting to connect the return line directly to the fuel rail.

Whats your fuel pressure now?

If low you need a bigger reg as the stocky cant flow enough out.

But if your smarts, you would try to blow through the return line first as its the most probable logical issue you may have.

[KiwiRS4T]

"It is this same line of thinking that the stock regulator somehow is able to maintain proper pressure (even if you install a high flowing pump) that got me into trouble by installing a higher flowing pump in the first place without being prepared for it. It doesn't do that based on what I have seen. If you fit a 400lph pump, the fuel pressure will be too high running the stock reg on stock fuel lines no matter what. Maybe there is something wrong with my setup, but I suspect that is how it would be in any car running the the stock reg."

The "somehow" is by means of a spring. I have a Walbro fed surge tank with a Bosch 044 pushing more than stock flow through my still stock FPR.

Do yourself a favour and do the simple no cost exercise of blowing though your return line before you go to the trouble of fitting a new pump.