Up The Boost A Bit And Fuel Cut

[tommyb]

hey guys i up the boost a little bit the other day and my car cut the fuel just wondering if there is a way around it with out gettin a fcd

thanks

[Mr_RS4]

Not as far as I know but to hit the fuel cut you must be pushing 15psi or more as this is where the ecu will cut the fuel afaik, and if your running the standard turbo you don't want to be running this much boost. I've had mine at 12 before and had it spiking in cold weather to almost 14 (before I turned it back to 10 which its at now) and haven't hit the fuel cut yet. But you will need some sort of voltage clamp on the afm to fool the ecu so a fcd is the way to go.

[pixel8r]

I've heard the fuel cut defender isn't that good anyway. The better option that most people here have chosen is the apexi SAFC or SAFC2 piggyback ecu.

In short - there is no such thing as "fuel cut" in stageas/skylines etc. - what it is is when the ECU detects too much airflow (via the AFM) it goes into what we call R&R mapping (rich & retard) which is some kind of "safe" setting programmed in to save your engine when nissan thinks there's a problem - such as the possibility of running too lean, as in this case.

These SAFC units modify the signal from the AFM to the ECU, thus fooling the ECU into thinking there is less airflow than there really is, and hence supply the appropriate amount of fuel without a problem. Its a little more complicated than that in reality but basically it allows you to accurately tune your Air/Fuel Ratio (AFR) on a dyno, which is not possible with just the stock ECU.

Have a look at others' mods lists - a lot of people have gotten over 200awkw with the help of an SAFC or similar piggyback ecu. It should only cost you around $300-400 for the unit plus installation and tune, or a bit cheaper if you get one 2nd hand. Very good value for money for what it can do.

[Mr_RS4]

There is a fuel cut believe me. When we installed a bleed valve to a friends 4 door R33 it was accidently (got too dark on the street ) wound the wrong way and once it hit 15psi (according to his boost guage) the ecu hit a fuel cut. Feels a lot different to a rev limiter but same sort of thing, just the cut pulses are a lot longer on a fuel cut. Wound it back down again and didn't cut. But R&R will also occur as well but that (to me anyway) just feels like a massive power loss from retarded timing and richer fuel mixes. But there are also downsides to fitting FCD's as they will affect your timing as well in most cases advancing it and make the engine more prone to pinging.

[Stinky Rooster]

But wouldn't cutting the fuel in an overboost just cause massive lean running and thus the expected holed pistons/collapsed ring lands etc?

I find it a little hard to believe Nissan would have something like that intentionally built in, would it be a lot more likely that its a spark cut, like the rev limiter?

[mitchy_]

the 'fuel cut' you speak of was probably missfiring from the coilpacks under higher boost, rb25 coilpacks are notorious for it.

its definately R&R they use when its airflow/boost is too high.

you will need some form of air flow controller to get around it,safc is one of the best.

[tommyb]

the 'fuel cut' you speak of was probably missfiring from the coilpacks under higher boost, rb25 coilpacks are notorious for it.

its definately R&R they use when its airflow/boost is too high.

you will need some form of air flow controller to get around it,safc is one of the best.

well i do have a bit of a miss fire but thats not the problem

when i up the boost and give it in second gear at about 4000rpm

the just does a big surge then goes then a big surge then goes

so thats the fuel cut

i do not have the factory turbo i have a bigger one and a external wastegate

[pixel8r]

A few people have reported being thrown forwards when R&R sets in, and it has often been described as "almost sending people through the windscreen" (most likely an exaggeration but gives you the idea).

I have experienced the same in my car with a bigger turbo and 3" exhaust but no safc. It would hit R&R every time even when only boosting to 10psi (stock boost solenoid). It wasn't until it was tuned properly with a SAFC that this behaviour went away.

SAFC2 is your best bet. It will advance your timing a bit but get it tuned by a reliable tuner and you wont have any trouble.

[Mr_RS4]

SAFC2 is your best bet. It will advance your timing a bit but get it tuned by a reliable tuner and you wont have any trouble.

Or you could search for MONTHS for a SITC like I did and not only adjust the timing where the safc affects it but also add some more to come on boost earlier. Still need to get my combo tuned yet. Hopefully next week!

What someone described and what seems to be happening to your stagea IS just like a surge on and off similar to a rev limiter but much slower. I always thought it was a hard fuel cut by the ecu and R & R was just a massive amount of timing retard and rich mixtures which could be felt as a sudden power loss but not as sudden as a fuel cut. I've experienced both in skylines and other nissans. And it definately wasn't missfiring from coils etc. SK care to comment?

[Sydneykid]

OK, firstly let’s define “boost cut”.

Firstly the ECU has no idea what boost a Stagea S1 is running, so there can be no such thing as “boost cut”. That’s what inferior cars with MAP sensors do, not Stageas with AFM’s.

Next let’s define “fuel cut”.

Does the ECU turn off the fuel pump? NO. That would be pretty stupid, while the pressure was dropping (over a second or two), the engine would run lean. Nissan ain’t that silly, especially since the idea is to protect the engine (when the ECU sees unusual things) not blow it up.

We need to keep in mind that ZERO fuel means that the engine is NOT running at all. That happens (injector duty cycle = zero) on normal overrun, to save fuel. It is NOT lean running, sure the engine is rotating, but no combustion is taking place. So it’s not lean in a combustion sense.

What the standard ECU does (when it sees unusual things) is to add some fuel and retard the ignition to protect the engine. The more “unusual” the readings (eg; lots of airflow) the more fuel and retard the ECU adds. Eventually there is so much fuel and the ignition is so retarded, that the engine simply refuses to run properly. You can feel this as a decrease in power output, which then returns as the airflow drops. At the same time you can get backfiring in the exhaust, that’s the unburnt (rich) fuel burning.

Does the ECU cut the ignition?

Nope, the tacho would drop to zero if it did, and it doesn’t.

There is one more circumstance to discuss, that is knock. What happens when the knock sensors hear lots of pre-ignition. This is totally different to excessive airflow as sensed by the AFM. This is when the boost is so high that the knock sensors detect detonation and the ECU reacts to protect the engine. At low knock levels the ECU simply uses the Rich & Retard mapping. But when the knock gets high the intervention is both swift and savage, the ECU closes the injectors (injector duty cycle = zero) and stops the combustion process. This feels like a sharp stab on the brakes. Since this stops the knock, the power quickly returns. If the knock returns, then the process happens all over again.

In summary (simplified);

Excessive airflow = R&R mapping and reduced power

Knock = turn off the engine and zero power

cheers

[Sydneykid]

I would add that the boost level is completely irrelevant. It’s all about airflow, the standard ECU couldn’t care less about boost. An example, my Stagea ran just fine at 10 psi with the standard turbo, but when I stuck the high flow on it hit savage R&R mapping, but still at 10 psi. Why? Because the GCG Ball Bearing High Flow has more AIRFLOW at 10 psi than the standard turbo.

So repeat after me

BOOST IS IRRELEVANT

cheers

[joe26f]

i was macking along from stop to say 80kph really quick n my intercooler pipe came off i nearly hit the windscreen it was like stomping the brakes hard.

[WhatBrake]

I would add that the boost level is completely irrelevant. It’s all about airflow, the standard ECU couldn’t care less about boost. An example, my Stagea ran just fine at 10 psi with the standard turbo, but when I stuck the high flow on it hit savage R&R mapping, but still at 10 psi. Why? Because the GCG Ball Bearing High Flow has more AIRFLOW at 10 psi than the standard turbo.

So repeat after me

BOOST IS IRRELEVANT

cheers

More boost, more volume of air going past your afm = very relevant =P whether that be a bigger turbo or more boost..

[Duncan]

lol welcome back thread!

for what it's worth my car was TERRIBLE! when it hit the cut...and fuel cut defender did the job perfectly

[KiwiRS4T]

More boost, more volume of air going past your afm = very relevant =P whether that be a bigger turbo or more boost..

Not necessarily. Boost is just pressure. If you stick a crappy intercooler on which doesn't flow as well as the one it replaced then you will need more boost (=more pressure) to move the same amount of air(flow) through it