Learn me - adjustable rate FPRs

I just finished up the installation of a turbo kit on my s13 240sx with a KA24E. The kit is supposed to work with stock management and injectors when kept to the 7lbs of boost it was designed for.

Eventually I want to go for more boost, but that will entail bigger injectors, e85, and a dyno tune.

I was hoping to bump fuel flow, especially under boost, for a bit of a safety margin on fuel trims. The few test runs I've done have revealed a misfire/stutter when first getting in to boost. It eventually smooths out but I don't want to push it too far and melt pistons.

Some research shows that the rising rate/adjustable fuel pressure regulators help on the (apparently) common dead spot between 4" vacuum and 3 psi when boosting stock motors.

I have found lots of options on ebay, ranging from <$20 to $180+. Some are adjustable, some are listed as rising rate with boost, while others seem to just multiply (6/1 - 12/1).

Being GRM, I want cheap of course, but I was hoping for some personal feedback. Anyone here use them for boosted applications?

Not for years, but they were the hot ticket a couple of decades ago. We kept finding them around our shop for years after discontinuing them.

First: make sure the pump can handle it.

They all work on a ratio - the ratio of fuel pressure gain vs manifold pressure gain once you get into positive pressure. The adjustable ones just let you dial in that ratio. Obviously, that's much preferred as it turns a random blunt instrument into a less random blunt instrument.

Aren't they fairly popular for nitrous as well, or is that just a standard FPR with the vacuum port feeding off the nitrous controller?

Keith Tanner wrote: Not for years, but they were the hot ticket a couple of decades ago. We kept finding them around our shop for years after discontinuing them. First: make sure the pump can handle it. They all work on a ratio - the ratio of fuel pressure gain vs manifold pressure gain once you get into positive pressure. The adjustable ones just let you dial in that ratio. Obviously, that's much preferred as it turns a random blunt instrument into a less random blunt instrument.

I was planning on upgrading to a Walbro 255 too. I figure this will give me room to grow when I want more boost and fuel.

Any feedback on the fixed ratio units? I noticed some go as low as 6 to 1 and as high as 12 to 1. I have no idea where to start.

Do you know what is causing the misfire? Are you sure it's a lean problem, and not a rich problem? Or an ignition problem?

In reply to alfadriver: I don't think ignition, just because it was running great before hand and I haven't touched that side of the system.

Rich/lean is unknown until I get my wideband installed. I can only go by the lack of black smoke and assume lean. Unfortunately I only have a MIG, so I need to farm out my stainless downpipe to get a 2nd O2 bung installed (which is in the works).

In reply to The Hoff:

It may not be pretty, but I've welded exhaust stainless with MIG....

As for the rich/lean part- the other key question- is the FRP the root of the lean or rich problem? With that vintage of car, you should also consider a fuel pressure sensor. Record it the same time you are recording the exhaust.

The Hoff wrote:

Keith Tanner wrote: Not for years, but they were the hot ticket a couple of decades ago. We kept finding them around our shop for years after discontinuing them. First: make sure the pump can handle it. They all work on a ratio - the ratio of fuel pressure gain vs manifold pressure gain once you get into positive pressure. The adjustable ones just let you dial in that ratio. Obviously, that's much preferred as it turns a random blunt instrument into a less random blunt instrument.

I was planning on upgrading to a Walbro 255 too. I figure this will give me room to grow when I want more boost and fuel. Any feedback on the fixed ratio units? I noticed some go as low as 6 to 1 and as high as 12 to 1. I have no idea where to start.

That depends on what your wideband is telling you. Get an adjustable FPR. They're cheaper than engines.

In reply to alfadriver:

I couldn't find a dump tube for my external wastegate that had the correct configuration. I just bought a $30 stainless piece from eBay, cut off the flange and re-clocked it. Not only did I use my MIG, but it was completely out of gas, so it was UGLY. That was just a temporary piece out of sight, so I didn't care much.

I need to bring the downpipe in to have the wastegate dump back in to the exhaust anyway, so I'll just have the bung installed then.

I was planning on upgrading to a Walbro 255 too.

Rising Rate Regulators can easily push fuel pressure up to the point where a 500hp fuel pump becomes a 200hp fuel pump. One of the things you have to worry about is that Walbros for in-tank use have a built-in pressure relief valve that can kill your motor if you don't know it's there with a RRR setup. I defeated one of mine with a PC case fan screw, other people epoxy the outlet shut.

I think pretty much all RRRs have adjustable base pressure, but you really want adjustable Rate-Of-Gain as well. That's what alters the ratio at which the RRR multiplies your fuel pressure by your manifold pressure. It's just an air bleed screw. Also, just know going in that to properly tune a RRR with adjustable base and adjustable rate of gain you need several things including at minimum a wideband, fuel pressure gauge (that is visible while driving), and vac/boost gauge. PREFERABLY you would also have a hand vac/pressure pump (good ones are $60) and a way to monitor your fuel trims in your factory computer if you plan to use the RRR as a way to 'rescale' for larger injectors.

You can diddly around with bandaids like rrfpr and a bad for turbo stock ignition map or do it right once and be prepared for the future with one of these http://www.nistune.com/equipment-products-type4.php

In reply to chiodos:

Unfortunately they don't list an s13 with KA24E as being applicable.

I am running a chipped ECU, so I have the capability to dyno tune at a later date. My ultimate goal is more boost and e85, but I'm trying to keep it to low boost and (near) stock management for now.