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Fuel cut vs Ignition cut

EFI Tuning Fundamentals

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Which is more safe for a turbocharged engine Fuel cut or ignition cut ? it is for a drift car so the engine is going to spend some time on the limiter. Another question, I have noticed some drag cars as soon as they hit rev limit the radiator cap puke some engine coolant, why this happens ?

Consensus may be ignition cut as a fuel cut could cause a lean transition with the risk of engine damage.

You may be mis-understanding the reason coolant is "puked" out the radiator cap - this is when combustion gases leak into the cooling system and isn't directly related to the rpm limiter. If it occurs it's usually down to a leaking headgasket, or a failure of the head or block.

Really depends on the engine and valvetrain type. I don't have enough experience to be any kind of authority on tuning rev limiters but in general I believe ignition cuts are harder on the engine. Fuel cut doesn't have to be 100% on/off, Andre has spoken to this many times in podcasts and interviews.

My philosophy setting up drift cars is to try and grip the thing up before limiter. You really don't want to sit on limiter, you'd rather the power fall off before limiter and the engine not have enough power to spin the tire faster. This can be a bit more predictable to drive too.

Either method for a cut should be "soft' rather than "hard", to reduce the potential for bouncing of the limiter which, as Tyson said, can be an expensive problem for some engines.

My logic is, the thing about injection cuts is that for port injection there may be a problem with the transition as an injection 'squirt' isn't 100% going into the cylinder - there may be fuel pick-up from the walls and the back of the valves, and some stand-off within the port that's left from the prior cycle, and is in turn supplied fuel from the current to contribute to the next. This could mean two, or more, lean charges during the transition from working to shut off and back to working. Shouldn't be a problem with DI and may not with PI, might be me being overly cautious?

With ignition cut, whether soft or hard, there may be a richer mixture for the initial firing, or two, after the shut off, and slightly higher level of bore-wash, but that may be preferable with a boosted engine.

Maybe initially soft ignition progressing to hard ignition then fuel? Many modern ECUs will move the 'soft' cutout around the cylinders to even out the conditions for each cylinder, which should help either way?

100% agree that having the torque/power drop off is a better idea as it will always be powering the vehicle and should also be more stable in the drift. It may be advantageous to do that at least in part, by running more ignition retad at the top end - it's harder on the valves, but will help keep boost up and the turbine spinning, which may help in response?

OP, don't be in a hurry to make the call on the method - get as many views on it as you can - if it goes wrong, it's you that will be paying the bills.

Ahmad I've seen what you're describing about the radiator cap. It's possible those cars are not handling the limiting in a manner that's controlled enough to avoid cylinder pressure spikes, causing the heads to lift and push cylinder pressure into the coolant system.

Your question is a simple one, but I end up deciding based on lots of project specific factors.

For example, how well your ECU platform handles wall wetting/fuel film compensation may determine whether I suggest using fuel cut or not. When fuel delivered to the cylinder is exactly as intended, fuel cut can work really well in most applications, but in many cases because the stripping of fuel from the walls during the cut isn't handled precisely when fuel is reintroduced, you can end up with a temporary lean condition which can be harmful.

On the ignition side of things, particularly in a turbocharged application I find I need to be careful as this can generate a massive amount of exhaust pressure and heat, and potentially unwanted excess turbine shaft speed, despite the reduction in torque output.

As the other folks said, progressive cutting, only cutting as many cycles as you need, can really soften things up.

Oh, forgot - as the rpm rises the pressure in the cooling system will increase as the pump is trying to force more coolant through the resistance of the radiator, and I wouldn't be shocked if this can be high enough to crack open a radiator cap that doesn't have a high enough pressure setting?

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