john banks

Fuel cuts seem a little unpredicatable. My gauge does not have any restrictors in its line, but it does not see the peaks like the MAP sensor does. When trying to engineer in some overboost, it was very difficult to get it consistent from one run to another, for obvious reasons - it is chaotic, and control works much better with a small peak of about 0.5 PSI on the gauge, which is perhaps a lot more in reality as it can spool up so fast I don't think the gauge keeps up.

I saw some fuel cuts at 17 - 18 PSI held on PPP sometimes but not others. This was where the gauge was showing peaks of 1-2 PSI over held. There was also not the usual four second or so pause before fuel cut - came within 1 second, so this leads me to believe the ECU does fuel cut based on peaks if they are high enough. Note that you can hold 19 PSI without a fuel cut on PPP if it is not peaky.

This could explain the apparently unpredictable nature of fuel cut with people setting up Dawes etc?

After loosening my actuator a bit and dialling out the peak to within 0.5 PSI on the gauge I no longer get fuel cuts despite running the same held boost.

ScoobyJawa

I hit the cut twice over the w/e having been running fine for a while. Indicated peak on the gauge was 1.25 (its a defi reading in kg/cm so a bit less than bar). Mine and scotts gauges seem to read similar and he hit the cut setting up at 1.35 on the gauge.

I hit the cut in 3rd gear - didn't see what the boost was. Turned down a 1/4 and on the way home again hit the cut in 5th. Again, didn't see the gauge - but each occasion I reset the peak indicator and both were 1.25 on the gauge. Turned down another 1/4 and seems ok now.

nom

How about based on the rate of increase of boost? This would also fit (I think ) what you describe. What I am thinking of as well is what is the fuel-cut trying to protect against - or rather, why is it needed in the first place? Presumably in case something falls off - in which case the boost would rise very quickly (or not at all ), so would this be what it checks for?

Just as a follow-through to that - what if the cut is based on the value of the integral past a set boost level? So... (trying to make myself clear here ), without drawing a picture which would admittedly be easier , if cut is at, say 18psi, and you go to 18.5 psi, after 5 seconds the 'area past' the cut would be 5seconds * 0.5 psi & therefore '2.5psi seconds' (of all units ), at which point you get fuel cut. Also, the same result could be achieved by going to 19psi for 2.5s, or 20psi for 1.25s, and so on. Seems to fit the results to me, although I may be completely noo-noos here.

[Edited by nom - 3/25/2002 6:00:52 PM]

john banks

Sounds plausible.

EvoRSX

John,
Yet again, I'm only speaking from an Evo point of view but fuel cuts on the Evo are triggered by the MAF reading a certain Karmann Vortech value (1700 Hz in the case of the Evo).

After detecting a certain mass of air it triggers the cut therefore you will get more cuts when its colder and the air is more dense than when its warmer and you're running the same boost level. As a result of air temperature differences during the day, you can get cuts at pressures ranging from 1.2-1.3 bar.

Is this the same as on the scoobies?

Andy

Cosie Convert

Andy

The fuel cut on the 93 - 97 Subaru is triggered instantly by MAP signal.
Appears similar on later models too although there may be some time delay in certain cases.

tweenierob

What is the fuel cut pressure on a 93 car, thanks people...
Rob