Talk to me about Fuel cut
 

is the correct term Fuel cut or boost cut?

Anyway im not having much luck finding info about the fuel cut on FD's so far i found there is a debate to weather it helps save engines from damage or makes it worse. what i would like to know is how it works, and at what psi it kicks on, also were is it located? im having some trouble with cutting out while slowly adding throttle in the 6k rpm range around 60mph sometimes.

Fuel cut is "located" in the ECU programming.

I could never get my head around people leaning the engine out under boost and blowing it, which is essentially how I understand fuel cut to work- fuel is cut off under high boost, apparently "saving" the engine. Which to me sounds rather debatable. Modern cars use ignition cut presumably because it is superior, and doesnt result in a lean cut condition?

If it feels like its cutting at 6000rpm, there's a few things to consider. If it is running the stock ECU, it will not be 'Fuel Cut' as it doesnt activate until an upper boost/rpm limit is reached, which is definitely not at 6000rpm. It sounds more like a limp mode thing as I read it. But I'd wait for someone more knowledgeable to chime in.


Where's ArghX when you need him? :p

Stock ECU fuel cut numbers:

https://www.rx7club.com/3rd-generati...0/#post8369595

Fuel cut > ignition cut. If there's no fuel to burn then there's no ignition. No ignition means no lean condition. This has been beaten to death over and over.

Here's the short answer: as ^ pointed out, no fuel at all = no combustion. No combustion = no knock. Gazillions of cars use fuel cut as a failsafe for one reason or another (overboost, limp mode, etc).

Now let me get a little more detailed.

There are three basic functions of fuel cut:

1) Fuel Economy
2) Emissions
3) Failsafe/Protection

In the [bad] old days of the carburetor, there was strictly speaking no fuel cut for anything unless you used some rather creative means. The 12A engine had a shutter valve to cut the mixture to one rotor under certain circumstances. With electronic fuel injection, that's all determined by software.

https://www.rx7club.com/attachment.p...1&d=1357499788

https://www.rx7club.com/attachment.p...1&d=1357499788

The FD and basically most/all standalones are crude by the standards of modern production cars. They just have a bunch of lookup tables and very few actual calculations. In the case of a stock FD, somebody set the fuel cut parameters on an engine dyno over 20 years ago and then some other engineer probably tweaked the settings a little bit based on some chassis dyno and real-world testing.

Now let me talk about fuel cut in the full context of engine control. On most newer OEM software, the control is all (or mostly) torque-based. There's a realtime calculation of engine output torque, possibly drivetrain loss calculations, etc. A torque reduction request or torque limiting function is one way that engine protection is implemented.

Now here's the key. On your basic port injected engine, there are three ways of reducing engine output torque:

1) air
2) spark
3) fuel

Air is the slowest way to reduce engine torque. You have a delay in closing the throttle, reducing the boost, etc. Spark is faster, but it still requires combustion cycle(s) to take its effect. Cutting fuel injection is basically instant. It's harsh (driver will notice it easily), but stops combustion immediately. No combustion = no end gas to spontaneously detonate, no high combustion temperatures to melt things, etc. Here's an example of torque reduction determination for basically any vehicle running a Bosch Motronic 7 system (late 90s VW/Audi, BMW, etc).

https://www.rx7club.com/attachment.p...1&d=1357500515

What happens is that,

1) first the ECU calculates how much the engine torque needs to drop

2) Then it calculates how fast it needs to drop

3) Then it calclulates how fast the torque will drop by closing reducing air to the combustion chamber (closing throttle, changing boost, whatever). That's the "settling time."

4) If the settling time is too long, the ECU judges whether/how to cut or retard spark. This would be like when the stability control system activates and closing the throttle would take too long.

5) If it's basically an "oh shit" situation like exceeding maximum allowable torque or boost pressure, fuel cut can occur.

https://www.rx7club.com/attachment.p...1&d=1357500837

I've had an issue with this, I had a rebuild that ran great, normal driving and the occasional hard pull. At about 17,000 miles I took a friend for a drive, and did a pretty hard pull up and through the gears, and then it started putting and died, with the ignition still on I checked the peek hold on my PFC and it was 8,430 rpm at 18 psi. My rev limit was set to 8,000 and my fuel map only read to 8,000. After limping the car home, did a comp. test and found all apex seals on the front rotor completely gone.
Now I'm not sure if it was actually the map fuel cut that caused it or not. as the heat in the combustion chambers was probably pretty hot, mixed with 18 psi of boost and the end of an injector dribbling out the last bit of fuel before the cut (causing a lean condition) could had been a leaky injector or many other sorts, I find it kinda coincidental that it happened like that. Just a thought.

Sounds like you didn't hit fuel cut, but your rpms spiked ( unsurprisingly) when you popped in the clutch between shifts.

Problems occur when fuel cut kicks in too late to save the engine. I've hit overrev and overboost many times.