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Fuel Pressure Regulators.

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Preface

Occasionally the topic of Fuel Pressure Regulators gets raised.

Not everyone requires an aftermarket or adjustable FPR to meet their engine build, so this article aims to help folks decide the appropriate replacement strategy.

Consider this a starting point to get you on the path of investigating your options.

 

Is it broken?

First thing to do is buy a liquid-filled or electronic fuel pressure gauge and confirm there's a pressure issue.

There's no point in spending $100+ replacing something unless you know it's broken. The issue could be the FPR or the pump.

The money isn't wasted either, as you'll need a guage to dial in the correct pressure with an aftermarket FPR anyway.

 

What is the OEM Fuel Pressure Regulator on an SR20DET?

The OEM FPR for the SR20DET is a non-adjustable rising-rate fuel pressure regulator.

Base pressure is 43psi, which is lowered at idle to roughly 36.3psi, depending on the amount of vacuum the engine draws.

 

What does Rising Rate mean?

Rising Rate Fuel Pressure Regulators alter fuel pressure to maintain a level of equilibrium with the manifold pressure/vacuum.

The idea is that if you increase manifold pressure by 1psi, the RRFPR raises fuel pressure is raised by 1psi. Decrease manifold pressure by 1psi and fuel descreases by 1psi.

In reality, it won't be exactly linear like this.

If you get an adjustable RRFPR, they normally allow you to adjust the base pressure.

 

Why are rising rate FPRs a sensible choice for turbo cars?

When you add air pressure to the manifold, you effectively add resistance to the injector nozzles.

Increasing resistance means that the fuel has to work harder to pass through the injector holes, the result being less fuel escapes the injector.

A RRFPR increases the fuel pressure to match the manifold pressure, and this offsets the resistance.

 

Example:

- OEM injectors are 370cc/min

- OEM base pressure is 43psi (with vacuum at idle it's 36.3psi)

- Duration of testing is 1 minute held constant (try this and you'll probably fry your injectors)

 

43psi (fuel pressure) - 0psi (boost) = 370cc fuel

43psi (fuel pressure) - 9psi (boost) = 336cc fuel

52psi (fuel pressure) - 9psi (boost) = 370cc fuel

48psi (fuel pressure) - 5psi (boost) = 370cc fuel

 

Do I require an aftermarket FPR?

The first step is to determine the size of injector you must run. This will determine the bore of the fuel line needed, and whether an OEM FPR is simply too small.

The conceptual order of upgrade requirement is:

 

a. Really big injectors need big rails

b. Big rails use big barbs

c. Big barbs use big lines

d. Big lines need big pumps to flow enough fuel to achieve stock pressure (at a greater flow rate of course)

e. More fuel flow needs a larger-bore fuel pressure regulator, so as not to bottleneck the system

 

In most cases you won't need bigger lines. People run 750cc with the standard line size, so an FPR of OEM dimensions should be fine for most SR20 setups.

 

My injectors are reaching full duty-cycle. Should I get an AFPR?

Increasing fuel pressure to use undersized injectors is like using a condom that's too small. Sure it can work out ok, but there's also a risk that things can fail in a steaming mess.

In most NS situations, you should instead seek to upgrade the injectors.

 

Where a simple base pressure increase can work quite well is on an old car with a slight cylinder bore. It stops it running lean, and avoids the expense of installing an aftermarket ECU and having it tuned.

 

Is the OEM FPR stable?

A brand new OEM FPR should be fine in most cases, but some people lean their tune way out, increase timing as much as possible, or run injectors at nearly maximum duty-cycle.

In any of these scenarios, the OEM FPR may fluctuate too much, and an aftermarket FPR is considered insurance.

 

Playing hide-the-truncheon on a Saturday night

If you have issues with the police, then it would be sensible to build your car so it can pass a visual inspection by a six-year-old.

Bright coloured blobs of alloy with chrome gauges fail in this respect, because they're shiny, and shiny things mean you're a dangerous driver. You also possess drugs.

The solution is to select an FPR that looks stock. OEM, Nismo and Tomei are good options, and worth consideration.

 

Price Point

Most FPRs on the market are not of OEM design. The cheaper [legitimate] ones generally never look OEM. Great for a track car, but on the street it encourages an impromptue body-cavity search.

Fun.

So, if you're on a budget and require adjustability, then it will be overtly aftermarket. If you want an OEM-esque upgrade, you will simply have to spend more.

 

I saw these SARD FPRs on AliBaba.com...

It's not always true to say that cheaper is inferior, however established brands are more likely to be reliable. The company's size usually allows them to run a bank of R&D and QA tests, the cost of which they gracefully pass onto the end-user.

In contrast, Chinese manufacturers seem to test their products by throwing them at a wall. Anything that survives is ready for shipping.

 

Further to this, there are some good brands out there that don't circulate through the import world, as they cater to other markets, such as hot rodders.

All you can do here is research.

Edited by pmod

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No,

 

The stock item (if its not broken) does a fantastic job.

 

I was going to install a basic adjustable GTR item on my 25det and was cautioned against it by my tuner

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No,

 

The stock item (if its not broken) does a fantastic job.

 

I was going to install a basic adjustable GTR item on my 25det and was cautioned against it by my tuner

 

Not sure what the 'no' was in reply to, since this is an article and not a question, but it's good that we agree.

For most situations oem is fine. Not all, but certainly most.

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Because you edited your post.

 

 

.....

 

 

 

Or I was too tired to read the whole thing.

 

 

Most likely the latter :lol:

Edited by r33cruiser

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Haha, makes sense. I hit enter at the wrong point and posted it too soon.

Still no questions though. :thumbsup:

 

TBH, I wouldn't bother reading the whole thing either. Not unless you actually needed info about FPRs.

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good write up dude! my car has a turbosmart fpr, black, kinda stealthy, it has a gauge on it too which sort of sticks out but i kinda dont really know how they work, sort of do now :) thanks

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good write up dude! my car has a turbosmart fpr, black, kinda stealthy, it has a gauge on it too which sort of sticks out but i kinda dont really know how they work, sort of do now :) thanks

 

You're welcome. Glad it could be of use :)

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oem fpr are good for how much power, some say 300rwk? true?

 

For the most part, the OEM reg does an acceptable job regulating pressure.

However the amount of fuel they can flow is limited, due to their barb size. Running 300rkw will very likely require dual pumps and larger diameter lines. Can't run an OEM reg in that scenario.

Edited by pmod

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oem fpr are good for how much power, some say 300rwk? true?

 

For the most part, the OEM reg does an acceptable job regulating pressure.

However the amount of fuel they can flow is limited, due to their barb size. Running 300rkw will very likely require dual pumps and larger diameter lines. Can't run an OEM reg in that scenario.

 

when does it become "limited"

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oem fpr are good for how much power, some say 300rwk? true?

 

For the most part, the OEM reg does an acceptable job regulating pressure.

However the amount of fuel they can flow is limited, due to their barb size. Running 300rkw will very likely require dual pumps and larger diameter lines. Can't run an OEM reg in that scenario.

 

when does it become "limited"

 

Imo, when your fuel flow requirement exceeds 250/300LPH at 80% injector duty cycle. Above that will need dual pumps and new lines, hence an fpr with bigger barbs.

300rwkw is around the border of 300LPH flow requirements on 98 octane, whereas ethanol requires even more flow.

 

You could probably get away with an oem reg and keep a single pump at 300kw, but realistically people upgrade the complete fuel system at those power levels anyway:

 

A. Safety net to avoid leaning-out

B. Option to run ethanol, which needs more fuel flow

 

You would change the reg if upgrading the complete setup.

Edited by pmod

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i know this will sound noob but meh flame away.

 

can the stock fuel rail flow both ways? like it wont f**k around with the injectors at all?

 

reason i ask this is because in my 180sx sr20det i set my fuel lines up to enter where the oem fpr usually mounts then run out the other end to where i put an aftermarket fpr. fuel filter n everything still on the incoming line before fuel rail its just literally feeding the fuel in backwards.

 

only reaon i did this was coz i could hide the fpr away so it wont be seen.

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