Fuel Cut vs Ignition Cut
 

OK, I may be asking for it with this question, but hey, I wanna learn...

What I don't understand is that if I have a clogged fuel filter, the engine goes lean and I pop it. Or if the gas sloshes around I go lean and boom. Seems like everything makes the engine go lean and then go bang. So why doesn't fuel cut do the same thing? And what's it feel like? I've never had the nerve to take it that far into the redline on my own car. Is it as vicious as ignition cut? Pro's and cons of each method?

Just curious why Mazda went this way instead of cutting spark like everyone else seems to do.

Ignition cut for the rotory engine at high rpm will
cause the engine to break down. Withou fail.

These days in Japan, the top tuners recommend
ignition system replacement for higher quality ones.

MSD or B&M, for example.
I use B&M New bolt coil unit.

i had a problem with my 93 a few months ago......my car would hit ignition cut once it went into boost about 2-3 psi. They replaced the ignition coil thinking that was the problem but it wasn't. I had to take the car to a mazda dealer who has an experienced 7 mechanic, turns out to be the injectors ,,,they werent working at all., replaced then now the car is fine....( no wonder every now and then the check egine light went on )

Seems like everything makes the engine go lean and then go bang. So why doesn't fuel cut do the same thing?

To answer your question, When the ECU shuts off the injectors there is no fuel to preignite unless one of your injectors is leaking. When you run lean because of the reasons you mentioned you still have some fuel comming from the injectors and yadda yadda yadda your engines blown.

This is from the Racelogic website...

Fuel Cut
The idea of cutting fuel to an engine sets alarm bells ringing in engine builders, as they all know of the potential disaster of a high revving race engine running lean. Running in a lean combustion mode will elevate in-cylinder temperatures very rapidly, the denser the air/fuel charge, the more heat the lean burn can generate. Therefore it is vital that a fuel cut system will not cause a lean burn.

The simplest way of preventing a lean burn is to remove more than 50% of the fuel from the pulsed delivery. A mixture will only ignite if the air/fuel ratio is within a tightly defined window, look at the efforts being put into making lean burn engines fire on very low air/fuel ratios (1:20 or more). Removing more than 50% of the fuel will cause an air fuel ratio of over 1:25 and will result in a complete miss-fire, with the unburned fuel passing out through the exhaust valve. Even if a high air/fuel ratio did manage to ignite, the energy available from the amount of petrol injected wouldn't be enough to elevate temperatures significantly. Of course the ideal system will remove 100% of the pulsed fuel delivery, allowing the cylinder to take a gulp of fresh air, and the in-cylinder temperature would remain virtually unaffected.

Prolonged fuel cut on one particular cylinder would cause scavenging of the petrol lining the inlet tracts, and when the next full fuel pulse arrived, it would be partially reduced in quantity by the re-wetting of these tracts. Therefore it is often important to manage a rotation of the cylinder cutting to prevent this situation from occurring.

Spark cut
Cutting the spark to an engine will stop any chances of a weak mixture occurring, but it carries it's own potential problems due to a large quantity of unburned fuel travelling through the cylinder and out of the exhaust. This petrol can remove some of the oil lining the inside of the cylinder, and pass it thorough the exhaust, again this only becomes a problem if the fuel to one particular cylinder is cut for an extended time. The best way to overcome this is to rotate the order in which the cylinders are cut.

The unburned fuel in the exhaust will have a catastrophic affect if there is a catalytic converter in the exhaust, as it will try to convert the unburned fuel to harmless elements, effectively burning the mixture. This causes the catalytic converter to heat up very rapidly, reaching temperatures in excess of 1000°C, and possibly melting down completely. Thus prolonged spark cut is not recommended for catalytic equipped cars.


©1998 Julian Thomas (Managing Director, Racelogic Ltd.julian@racelogic.co.uk)

Thanks for the very informative post, mmaragos. That all makes sense now!

I'm still curious how harsh the fuel cut is. I've seen friends hit ignition cut and it looks like it has to be causing damage. But I had a bike one time that just seemed like it lost all of it's power at redline and didn't feel damaging at all (no vicious surging). Thought maybe it used fuel cut...