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My quest for a fuel cut free car has finally come to pass

To start I acquired a VRG FCD, but after installing this I still had fuel cut at 13-14psi. I attribute this to the 5500ft of altitude I live at, as I learned that the ECU recalculates the Fuel cut psi from "base" atmosphere and goes from there. Others at altitude have had similar isssues with this unit. So the VRG came out, no big deal - it was cheap.

Then I was reading about all the bleed valves, small holes, MBC, etc... ways of fooling the MAP. These work, but I want the MAP sensor to see boost - just not over the threshold of fuel cut (I guess the MBC method would do this too, but I wanted a set-it and forget-it option)

Enter McMaster-Carr (www.mcmaster.com) . I'm an engineer so I deal with McMaster frequently and know they have unimaginable amounts of weird devices/components available off-the-shelf. What I found was a pressure relief valve that has a set pressure that it releases at.
I ordered the 10psi version, PN 4320T13 ($27). It is a push-in style miniature valve with 1/4 ID inlets. (you can get all kinds of release pressures, from 0.5psi-10psi)

I fitted this into the line between the solenoid and the MAP using push in plugs, like PN 5111K504, and some 8mm ID tubing I had to bridge the gap (wire tied to create a good seal).
I drilled out the plug on the inlet side with a 0.050" bit to provide some decoupling from the rest of the system, and slow the flow to the valve so it could keep up with the pressure increase. The other side (from the valve to the MAP) I drilled out to a 3/16" or so.

The result is a fuel cut free 14psi of boost. I ramp boost in 5th gear at 3000rpm and stand on it - No cut up to 100mph.

I'll get some pics up, but it is really easy to do and cheap (cheaper than an HKS electronic or something - and I like mechanical things anyway)

-Chad

(edited for correct ID on tubing)

11-1 EDIT -> I DID get fuel cut this AM. It was hanging around 32F, and my boost was up a bit due to the cold air. I hit just about 15psi in 3rd gear from about 3000rpm - and hit fuel cut just as I was about to lift off. It probably made it past 3sec, but.... I think the little valve just can't flow much. I'll be trying a smaller hole in the inlet plug - maybe more like .030" (or a 1/32, close enough at .0312"); if that doesn't fix it, then I'll go to a #75 (.021").

So what happens when the temp. drops below 26 degrees (out of it's temp range)? Does it no longer seal and give you more boost pressure?

I'm not entirely sure, but I would think that the seal inside the valve would possibly unseat and let pressure out prematurely. That would mean the MAP sees less pressure, not too big of a deal.
The MBC will still hold whatever manifold pressure it is set to.

That said, I suppose if you live in the great white north this particular piece may have varying results. Although the engine bay is generally quite a bit warmer than ambient once the car is warm. And the pressurized air flowing to/through the device is warm as well (just stay on boost - haha).

I'm not worried here in CO - even in driving up the Mnt in winter where it WILL be lower than 26F.

It would be nice to have an actual technical data sheet for the valve... I'll look into that.

When things slow down a bit here at work I'll pull the valve out and test it (pressure release at ambient, 25F, 10F, 0F, etc..) with a calibrated in-line pressure gauge. Then I'll know the actual limits.

MMC bad, WWG good. That's all I have to say.

Very cool, I was going to build an adjustable one of these (I'm an engineer and hobby machinist), but for $27... done deal.

Gary

Can you seeee it?



closer.

Still working like a charm

wow that is so clean. great lookn setup. if you ever want to try a electrical FCD try the HKS FCD for the wrx. it works so great. and its totally adjustable

what turbo are you running? looks like the stock one.

it does look stock from the top.... but it's a VF12

Update, I finally need to install the pressure release valve I bought from McMaster, but was unhappy with the result. I couldn’t get it to work consistently, even bench testing with a regulator and gauge I couldn’t get a repeatable pressure setting. Maybe I just got a bad part? I had it installed with a 0.040 orifice on the inlet which seemed both too big and too small. Big enough that it seemed to bleed a good bit of air, but small enough that it seemed to delay/affect the reading of the MAP sensor. In the end I build one of the adjustable electronic FCD in the sticky thread… works great (and was cheaper).

Just though I’d share.

Gary

Yeah, I should change the title of this thread to "...sort-of works"

My setup, using a 1/32" hole will still hit fuel cut when it's cold out, but it only does it the first time you get into boost (then I presume it is warmed up and works properly). It runs 14psi now and almost never hits cut when >freezing (even in 5th gear staying at full boost for much longer than 3sec). So for me it has been helpfull - although not a permanent solution.

However, I will be revamping the VRG FCD I have with info from Vikash on making it more altitude friendly. Hopefully I'll get on it soon.... then EM of some kind soon to come

I was under the impression that the FCD is supposed to go in between the #1 intake manifold runner and the pressure exchange solenoid, not in between the MAP sensor and the pressure exchange solenoid. Maybe that's why you're still hitting fuel cut...