This article covers what each cut does inside the cylinder, and the downstream effects. We will also cover off why you'd choose one over the other or both, the difference between a hard and a soft limiter, and the negative consequences that can occur if we don't choose wisely.

Both cuts stop torque, but they're not the same

A cylinder needs air, fuel, and a spark to make torque, and a limiter only has to take one of them away to stop that cylinder contributing to engine torque production on that cycle. The air can't be removed quickly, because the throttle is typically open and the engine is still pumping, so the logical choice is between the fuel and the spark which can both be cut on a per-engine-cycle basis, and across as many or as few cycles as necessary.

Take the fuel away and the cylinder still goes through all four strokes. It draws in air, compresses it, has nothing to burn, and pushes that air straight out into the exhaust on the exhaust stroke. No fuel means no torque from that cylinder that cycle, and the exhaust it sends downstream is mostly fresh air, carrying all of the oxygen with it.

Take the spark away instead and the injector still delivers its fuel as normal. The cylinder fills with a full air-fuel charge, compresses it, never ignites it, and pushes the whole lot out of the exhaust valve unburned. That cylinder makes no torque either, but what it sends downstream is a combustible mixture rather than air, heading into an exhaust manifold that is already full of hot gas from the cylinders that did fire.

Each cut event is one cylinder making no torque for one cycle, and that is true of both fuel and ignition cuts. How many cylinders get cut at once, and in what order, is a separate choice, and we'll look at that in more detail later.

Both cuts stop the cylinder making torque. The difference is whether air, or air with the fuel still in it, is leaving the cylinder that has been cut.

A fuel cut sends air into the exhaust

The big advantage of a fuel cut is that nothing combustible goes downstream. A cut cylinder pumps air, the exhaust gets leaner and cooler rather than hotter, and the turbo, the manifold, and any catalytic converter in the system have a much easier time. That makes it the cleaner of the two cuts as far as everything after the exhaust valve is concerned.

The trade-off is how coarse it is as a control. Fuel can't be trimmed back a little at a time to bring torque down gradually, so each cylinder either gets its fuel or doesn't, even when the cut is applied one cylinder at a time. Ignition has a step in between, because the ECU can pull timing to bring torque down gradually before any cut comes in.

There's also a delay in both directions with a fuel cut in a port injected engine, because not all of the fuel a cylinder receives comes straight from the injector. Some of it sits as a film of liquid fuel on the port wall, constantly evaporating off and being topped up by the injector. Stop injecting and that film keeps evaporating into the cylinder for a couple of cycles, so the fuel doesn't disappear the instant the injector stops. Start injecting again and some of the fuel goes into rebuilding the film rather than into the cylinder, so the first cycles back run slightly lean of the target. How lean depends on how long that cylinder went without fuel. A cylinder that misses one or two injections has only lost a small part of its film, so the shortfall is small, while a cylinder that's been cut for longer has more film to rebuild. A direct injected engine sprays its fuel straight into the cylinder, so there's no port wall film to rebuild in this case and therefore the fuel cut limiter strategy doesn't have this to consider.

There are ECU strategies built to deal with this, including on the limiter. One approach adds extra fuel to each cylinder as its injection comes back after a cut. The extra amount starts from a value set by how many injections that cylinder missed, then steps down to zero over the injections that follow, so the film builds back quickly. The same compensation covers a limiter cut, a traction control cut, and the overrun fuel cut on deceleration. Fuel film recovery is often described and tuned around overrun fuel cut, though, so check what your ECU does when fuel comes back after a limiter cut rather than assuming it's covered. The longer the cut lasts, the more that compensation matters.

On the data side, expect the wideband to read lean while a fuel cut is active. The sensor is only measuring oxygen, and a cut cylinder pushes out air full of it.

A spark cut sends unburned mixture into a hot exhaust

A spark cut keeps the fuel flowing, so there's no fuel film to rebuild and no lean recovery when the spark comes back. What it does instead is move the combustion out of the cylinder and into the exhaust.

The unburned mixture a cut cylinder pushes out lands in a manifold full of hot gas from the cylinders that fired, and it reacts almost immediately. That reaction happens in the exhaust manifold and the turbine housing, releases heat, and expands rapidly enough to build pressure. On a turbo engine that pressure drives the turbine, which is exactly why a spark cut two-step is used to build boost on the line. On the limiter, though, the same process is putting extra heat and extra pressure into hardware that wasn't designed to have combustion happening inside it.

The turbine is the first thing downstream. Turbine inlet temperature is already one of the limits a boosted engine is calibrated around, and a spark cut adds heat right at the turbine inlet. Further downstream, a catalytic converter exposed to unburned fuel, whether from a misfire, an ignition failure, or a spark cut, can overheat severely and be damaged beyond recovery. A converter can handle the occasional misfire. It isn't designed to have raw fuel burned inside it for the length of time an engine sits on the limiter.

The valvetrain is affected too. Exhaust valves are held closed by their springs, and pressure in the exhaust port acts on the back of the valve head in the direction that pushes it open. Raise the pressure in the manifold and you take force away from the spring holding the valve on its seat. I've seen broken rockers caused by exactly this on an engine running an ignition cut, where the pressure from combustion in the exhaust lifted an exhaust valve off its seat, which unloaded the rocker. The hydraulic lash adjuster pumped up to take up the slack, the rocker came loose, and the valvetrain broke it. Pops and bangs maps work the same way, because they're deliberately putting fuel into the exhaust and lighting it there. Personally I'm not a fan of pops and bangs for that reason, but each to their own.

Valve overlap adds another problem. During overlap the intake and exhaust valves are both open, and the flow at the exhaust valve can reverse, sending exhaust gas back into the cylinder and even into the intake. Normally that gas is just hot, sitting somewhere around 1000°C (1830°F). With a spark cut there can be combustion still going on in the manifold, and if that reaches a cylinder full of fresh mixture during its overlap, it can light the charge before the spark is due, or send a backfire up into the intake. The more overlap the cam has, the bigger the window for that to happen.

The wideband reads lean during a spark cut as well, even though there's fuel going through. The unburned mixture still carries its oxygen, and the sensor reads that oxygen without knowing the fuel next to it never burned. This was covered in our article on why your wideband reads lean when fuelling is fine, so don't trust the lambda reading while the limiter is active.

A spark cut doesn't remove combustion from the engine. It moves it into the exhaust manifold, the turbine, and the converter.

Cutting both gives you a staged limiter

The two cuts don't have to be used on their own. Some ECUs bring the ignition cut in first, then add fuel cut at an offset above it if engine speed keeps rising, so ignition retard and the spark cut do the fine control around the limit and the fuel cut acts as the backstop.

Combining them also changes what goes into the exhaust. A cylinder with both its fuel and its spark cut is back to pumping air, so once the fuel cut joins in, the cylinders stop sending unburned mixture downstream. You keep the finer control of ignition close to the limit, and limit how much raw fuel goes into a hot exhaust if the engine goes further past it.

A hard cut removes all the torque at once, a soft cut reduces it progressively

How many cylinders get cut at once is another choice we can make as tuners. A hard cut removes torque from every cylinder at once the moment the limit is reached, then restores it to every cylinder once speed drops back below the threshold. A soft cut starts by cutting a small proportion of the firing events, then cuts progressively more cylinders and more often as engine speed rises further past the limit, so power comes off gradually rather than all at once. Some ECUs also pull ignition advance on approach to the limiter, which softens the whole process further.

And don't forget how quickly this is all happening. At 7000 rpm the crank turns about 117 times a second, and each cylinder fires every second revolution in a 4-stroke engine. This means a four cylinder engine has about 233 firing events every second, one every 4.3 milliseconds. A V8 has about 467, one every 2.1 milliseconds. For every event, the ECU decides whether to cut, and with a hard cut every decision is all or nothing. Sit on a hard limiter and the torque on the crank is being switched between full and zero hundreds of times per second.

A crankshaft is already designed around torque arriving in pulses, one per firing event. Torsional vibration is enough of a danger that many engines carry a damper on the front of the crank to control it, and crankshafts are commonly finished to slow the growth of fatigue cracks. A hard cut adds a much larger swing in torque on top of those normal pulses. A soft cut cuts only some cylinders at a time, so each change in torque is a fraction of the size.

A hard cut switches the engine's torque on and off hundreds of times a second. A soft cut takes it away a few cylinders at a time.

Choosing between fuel cut and spark cut

There's no single right answer, because each cut moves the problem somewhere different, and the right one depends on what components are downstream and what you are trying to accomplish.

If the engine has a catalytic converter, a spark cut sends raw fuel into it every time the limiter is touched, so a fuel cut, or a combined cut with fuel cut joining in early, keeps raw fuel out of the converter. On a turbo engine, a spark cut puts combustion heat and pressure into the manifold and the turbine housing, which is useful on the line and hard on the hardware everywhere else. If it's a port injected engine running a fuel cut, and especially one that sits on the limiter for more than a few cycles, check that the fuel film compensation covers the limiter so the first cycles back come in on target. And on any engine, the more overlap the cam has, the more care a spark cut needs.

How long the engine sits on the limiter matters too. A limiter that's touched once at the top of a gear is a very different load case from one the engine sits on for seconds at a time, on a two-step, or in a drift car bouncing off it through a corner. Everything that heats the exhaust gets worse the longer it goes on, and every hard cut is another full swing in torque on the crank. Where the ECU allows it, a soft cut changes the torque on the crank in smaller steps than a hard cut, and I tend to lean towards this strategy as much as I can.

The limiter is there to protect the engine from overspeed. It shouldn't become the thing that breaks it, so choose the cut, and how hard it comes in, for the engine and components it's there to protect.

Key points

  • A rev limiter takes torque away by cutting either the fuel or the spark. Both stop the cut cylinder making torque, but a fuel cut sends air into the exhaust and a spark cut sends unburned mixture.
  • A fuel cut keeps raw fuel out of the exhaust, the turbo, and the converter. Each cylinder either gets its fuel or doesn't, though, with no in-between step like ignition retard, and on a port injected engine the fuel film on the port wall means fuel doesn't stop instantly and the first cycles back run slightly lean unless the ECU adds fuel to rebuild that film. The longer the cut, the bigger the shortfall, so check that your ECU does this after a limiter cut, not just after overrun fuel cut.
  • A spark cut has no lean recovery to worry about, but the unburned mixture reacts in the manifold, the turbine housing, and the converter, adding heat and pressure to all three.
  • Exhaust pressure from a spark cut acts against the exhaust valve springs and can lift a valve off its seat, which can break rockers. During valve overlap, combustion in the manifold can also light the charge in another cylinder early or cause a backfire.
  • Cutting both progressively can stage the limiter, with ignition retard and spark cut for fine control near the limit and fuel cut added further past it to limit sending raw fuel downstream.
  • A hard cut removes all torque at once, hundreds of times per second at high engine speed, on top of the torque pulses and torsional vibration the crank is already subjected to. A soft cut brings the cut in progressively, so each change in torque is smaller.
  • The wideband reads lean whichever cut is used, so don't trust the lambda reading while the limiter is active.
  • Choose the cut, and how aggressive it is, depending on the application, what components you have, and how long the engine will sit on the limiter.

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