The stock ECU has this thing where it will, momentarily, cut fuel injection to one rotor.

From the service highlights:


Understanding: there's a special decel fuel cut mode when one rotor still has combustion where the other doesn't. This is for a brief moment of time and only happens as you lift off the throttle at small RPMs, small load. The rotor that is still alive gets more fuel poured in.

(conspiracy) theory: Since there is a phase in the engine cycle when both rotors are in the exhaust phase, it is possible that crossflow of exhaust gas is possible between the 2 of them, if one rotor has combustion(and exhaust gas) and the other one doesn't. This leads to excessive heat being packed into the rotor that is doing the fuel cut. I suspect that this accelerates wear on the rear rotor as that's the one I've most often see to fail catastrophically, with seals flying out the exhaust and trashing 2/3 of the engine.

Relevant tables:


Unfortunately I couldn't fix exactly which rotor it is that gets this fuel cut, but I might look into it further. For now, I've decided to 0 out the time durations and move the throttle threshold so high that there will never be a 1 rotor fuel cut ever again.

 

Thank you for your continued work on these cars. I just bought my 3rd one and intend to keep driving them as long as I can.

I appreciate people like you who have the knowledge for this work, and do it, and then post it for all of us to benefit from.

 

I have a setup on my car that allows me to watch instantaneous fuel consumption (among many other parameters). I also live in a mountainous area, and have MANY opportunities to coast. If I let off the throttle in gear and just coast, it's definitely still injecting fuel. I get better fuel economy if I coast in neutral. Mine is a Series II, and we all know there were many many changes made to the Series II engine for reliability purposes. I don't know if that's a tick in the pro column, or completely unrelated. If we had the same literature for Series II, and that mode were lacking that would firm it up a bit. Similarly, the same datapoint in a Series I would firm it up. The constellation of all those datapoints would be a strong argument in favor of your theory.

 

I presume you have a generic gauge. It's useless what you are told by it since generic scanners don't have access to the guts of every car. They can only compute from low hanging fruit that works on all cars: MAF, AFR and vehicle speed. All common parameters on all OBD2 compliant cars. Yours is still reading some fuel being used because the leanest the stock O2 wideband reads is ~20 AFR. Let's say your MAF during coasting is picking up 8g/s, divided by 20, you get 0.4 grams of fuel injected per second(that's what your scanner believes!). Thus 0.4 grams a second * 60 times a second * 60 times an hour = ~ 1.4kg of fuel injected an hour during coasting in fuel cut. This is obviously bullshit as true fuel cut injects nothing - injectors are turned off entirely - this I checked myself using dedicated hardware plugged straight into the injectors. I confirmed it while studying the sequential fuel delivery system some months ago. More on top of this is that there are timers in place that set the duration of 1 rotor fuel cut, this is clear cut, and no way is this mode operating for more than maybe half a second or so.

Moving back to more important business it seems that I have touched some hot stuff that I shoudn't have: the 2 DBW RPM/IAT tables are used 4 times in various places, and one shall not meddle with those tables at all to delete 1 rotor fuel cut. Doing so upsets the DBW system, causing a limp mode, no accelerator pedal, no DBW control. Pretty nasty.

I however 0'ed out the timer tables - meaning that now 1 rotor fuel cut has a duration of 0 [time units] - meaning no 1 rotor fuel cut at all. This was instantly felt as a slightly harsher fuel cut at lower RPMs and in gear. To a veteran RX8 owner the distinction is clear, so I know that there is a thing I did there.
The sad part is that I will most likely never drive the car enough to come to see if this effort provided any fruits, given the expected tiny benefit already... but still a thing I felt that needed to be done at little effort.

I'm dropping here the 1 rotor fuel cut logic as I understand it now from decompiling, in case you touch computers indecently for a living.

 

You know what they say when you assume... It's pulling directly off the CANbus.

If it's just looking at AFR readings, it would read idle (whatever borderline arbitrary fueling they could keep it ticking over at across a range of assumed conditions) as a LOWER AFR than a fuel cut at high RPM (which would read whatever highest AFR the system will put out). What I described is the opposite of what you assume.

I'm not disputing anything you've said, and likely supporting it. You're talking Series I, I'm talking Series II. Many many changes were made in the interest of reliability. Possibly even the very change you describe making.

 

I've explained once how your gauge 99% probably works. I extremely doubt they pool primary injector duty cycle, which is not useful in all cases since there is another injector pair(or 2) to inject fuel, and it requires OEM level of diagnostics.
So they pool AFR instead, which during fuel cut is measured as ~20 when in reality it's infinite. At idle it's 14.7(or should be). It doesnt matter how much or little AFR it reads, if it isn't infinite or MAF reading isn't 0, then it will read a fuel qty being used.