fuel cut/ boost differences at varying altitudes?

SubaruNation · Post by **SubaruNation** » Sat Mar 29, 2008 5:53 am

google says im at roughly:

3 188.97638 feet

Basically, I want to know how this will affect where/ when ( at what pressure) I hit fuel cut and how it will affect my boost.

Will fuel cut be lower than 12.75psi for me because I'm at roughly 4k feet?

or

Will I hit fuel cut at a higher lever of boost than 12.75 psi?

or does it even matter, and it's the same no matter what b/c the pressure is regulated by my stock ecu...

any ideas or recommendations?

thanks for looking,

SubaruNation · Post by **SubaruNation** » Sat Mar 29, 2008 7:54 pm

somebody know this...

Please?

and a bump

Arctic Assassian · Post by **Arctic Assassian** » Sun Mar 30, 2008 10:09 pm

what you need to do is measure your atmospheric pressure with a barometer. Take your fuel cut - atmospheric pressure= amount of boost you can run before hitting fuel cut.

SubaruNation · Post by **SubaruNation** » Sun Mar 30, 2008 10:38 pm

Barometer: 29.63 in. <---- does that make sense?
fuel cut = 12.75psi right?

29.63-12.75=16.88 or (-16.88 ).....uh i don't think that's right.

and that comes from this page... http://www.wrh.noaa.gov/forecast/MapCli ... -113.99306

vrg3 · Post by **vrg3** » Sun Mar 30, 2008 11:01 pm

I'll tell you what I know if you remove the image from your signature and promise to reread the forum rules.

SubaruNation · Post by **SubaruNation** » Sun Mar 30, 2008 11:11 pm

vrg3 wrote:I'll tell you what I know if you remove the image from your signature and promise to reread the forum rules.

.........there isn't one & read again

SubaruNation · Post by **SubaruNation** » Sun Mar 30, 2008 11:30 pm

le bmpu for VRG3

vrg3 · Post by **vrg3** » Sun Mar 30, 2008 11:37 pm

Hehehe... Thank you.

So, from what I can tell, overboost fuel cut seems to happen at about 13 psig -- that is, 13 psi above atmospheric pressure. So, fuel cut will happen at a lower absolute pressure for you than for me, since I'm less than a thousand feet above sea level.

What your boost gauge displays is gauge pressure (measured in psig), so if you see more than about 13 psi (or about 12.75 psi or whatever) for more than about three seconds, you'll get fuel cut.

If you're using stock boost control, none of this matters of course, since the ECU controls boost and regulates it to a level that's below the overboost threshold anyway.

If you would like to run more boost than the ECU normally allows, you can use a fuel cut defender. I of course like my design:

http://www.graphics.cornell.edu/~v/fcd/

But there are some errors on that web page... At that point I was under the impression that the overboost threshold was an absolute pressure rather than a gauge pressure.

You're about a kilometer above sea level, so atmospheric pressure for you is about 1.5 psi lower than at sea level.

If you were to use my electronic FCD, I'd recommend reducing the clamp voltage to about 4 volts by replacing the 10K resistor with a 12K resistor.

So what exactly prompted your question?

SubaruNation · Post by **SubaruNation** » Sun Mar 30, 2008 11:44 pm

i dont have a FCD and i need one...badly, and i was wondering...

1. how boost (and fuel cut) was affected by altitude

2. and how the boost was regulated in higher elevations, by the car itself and how that would in turn affect me.

i don't know why i thought of this stuff but i did. mostly b/c i thought i could only run like 11psi before i blow something.

i'm @ 10psi of boost right now, and would like to go to 12 or 13 so i need a good FCD..somehow.

SubaruNation · Post by **SubaruNation** » Sun Mar 30, 2008 11:49 pm

i have a MBC installed...

and i say somehow b/c the thought of me building your FCD scares me.. i know i would ruin something very badly. not too confident in my soldering skills...

is there a way i could entice you to build one for me?

i can paypal

vrg3 · Post by **vrg3** » Sun Mar 30, 2008 11:56 pm

If you can't find someone local to help you, maybe I can, but it'll take me a little while to get the parts and find the time.

Have you thought about dropping by a local college or university's Electrical Engineering department and offering to buy someone a six-pack in exchange for building you the device?

SubaruNation · Post by **SubaruNation** » Mon Mar 31, 2008 12:03 am

we don't have and electrical engineers here

otherwise i would indeed do that, hell i'd buy a 30.

i also don't know of anyone that can solder, or has the equipment to do this.

my bet is that i'm hitting fuel cut @ 10 gauge psi, where i'm at right now.

i have the MBC installed with my gauge and i don't really know what to look for in fuel cut but my guess is that i'm hitting it

vrg3 · Post by **vrg3** » Mon Mar 31, 2008 12:09 am

You're not. Fuel cut is very distinctive. It's been described as feeling like you hit a brick wall.

SubaruNation · Post by **SubaruNation** » Mon Mar 31, 2008 12:11 am

hmmm......ok cool,
still worried anyway!

Post by **Legacy777** » Mon Mar 31, 2008 6:02 pm

Yeah.....you'll know if you hit fuel cut.

All_talk · Post by **All_talk** » Mon Mar 31, 2008 6:34 pm

It feels alot like hitting the Rev limiter... not fun.

Gary

asc_up · Post by **asc_up** » Mon Mar 31, 2008 8:58 pm

Hahaha turn up your boost a bit and gas it. That way you can understand exactly what fuel cut is.

I hit it when I was doing a 3rd gear pull up a hill. Since I was going pretty fast, it actually kind of hurt my rib cage lol.

SubaruNation · Post by **SubaruNation** » Mon Mar 31, 2008 9:40 pm

honestly?
ok, i'll do that but i still don't want to just blow my engine trying it out...
thats my only worry

asc_up · Post by **asc_up** » Mon Mar 31, 2008 9:42 pm

Seriously. It's not going to blow your engine. Don't run a ridiculous amount, just 13 PSI.

That way if it ever does happen to you, you'll know what it is. Lol.

SubaruNation · Post by **SubaruNation** » Mon Mar 31, 2008 9:49 pm

ok cool, 13 is ok without a FCD?
even @ my altitude?

ugh

asc_up · Post by **asc_up** » Mon Mar 31, 2008 9:51 pm

Lol you'll hit fuel cut. I'm just saying you should do it so you can hit it and actually know what it feels like.

SubaruNation · Post by **SubaruNation** » Mon Mar 31, 2008 9:52 pm

ok cool

biggreen96 · Post by **biggreen96** » Tue Apr 01, 2008 7:04 am

Yeah hitting fuel cut once or twice to feel what its like will not harm anything. I am at 19psi now and not exploding my stock internals.

Arctic Assassian · Post by **Arctic Assassian** » Wed Apr 02, 2008 3:49 am

so vrg... does the computer account for atmospheric pressure when calculating fuel cut? I would love to come up with a correct equation, my first was a guess.

SubaruNation · Post by **SubaruNation** » Wed Apr 02, 2008 5:33 am

and +12 to that!

and my 3 sources for building your FCD wouldn't do it b/c it sounded too risky to them.

if anyone build's Vrg's FCD i will pay cash for a legit working one that won't blow up my car.

think you could swing it maybe Vikash?
not now of course...

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fuel cut/ boost differences at varying altitudes?

fuel cut/ boost differences at varying altitudes?