I'm looking forward to more sports car racing this year (alas, just as a participant) and so I thought I'd look over the rules in detail. Specifically, I was reading the 2016 IMSA Technical Regulations for the GTLM class in the IMSA Weathertech Sportscar Championship. For the crankshaft, and mostly consistent with last year's rules, it says:

Free design. Its weight must not be more than 10% lower than the original (titanium-based alloy forbidden). The firing order is free.

It's obvious to me why they'd restrict the material and weight (since lighter is often faster in a racecar), but I was curious about the possible ramifications of the firing order being free.

By my reading of the rules, the engine control unit is also free, which suggests that a non-fixed firing order might be employed. I know that in the current F1 cars, it's allowed to have the engine control unit "skip" cylinders (e.g. at idle to avoid overheating the engine on the grid).

My question: is a dynamic firing order ever used in racing?

If not currently, it seems likely that somebody must at least have consider using it in the history of racing, but I can't find much to indicate one way or the other.

To be clear when I say dynamic firing order, the simplest case would be to simply skip cylinders as the old Northstar engine did to save fuel when cruising. My understanding of the rules (which may be flawed) is that variable cam timing is allowed if the original engine had it, so I'm imagining a scenario in which cam timing, injector and spark plug firing might all be controlled (as it is in some production engines) to the extreme that the firing order might be changed as the engine is running.

Even Koenigsegg, doesn't quite yet have an entirely cam-less engine (yet), but I was wondering if a more conventional cam-based engine might still be able to employ such tricks.

  • 1
    Depends on the motor if changing the original firing order will help or not. This would require a cam change also. Define Dynamic firing order, I assume this is cylinder skipping.
    – Moab
    Commented Apr 28, 2016 at 22:01
  • I've added a clarification to my question to try to better explain what I'm asking.
    – Edward
    Commented Apr 28, 2016 at 22:21
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    Firing order is tied to the camshaft , so you cannot do that. The cam lifts the valves in the same order as the spark firing order, so would have to change that on the fly also, which cannot be done yet, it could be done with electronic valve actuators and a computer.
    – Moab
    Commented Apr 28, 2016 at 22:55
  • I've not heard of it, but I've not heard of everything (obviously). The question I'd ask is, to what end? As @Moab stated, the firing order is tied to the crank/cam. If you want to interrupt the firing order, you'd have to skip cylinder firing events to do so, which means you'd have a loss of power/torque to complete the action. If you are racing the vehicle, this would do nothing but hurt the vehicles performance. There is no real reason to change the firing order during a competition. Also, you cannot change the firing order (unless you skip cylinders) without changing the camshaft(s). Commented Apr 30, 2016 at 4:25
  • Skipping cylinders could save fuel which can be decisive in many forms of racing, but yes, I hadn't really thought through the fact that the cam is necessarily tied to the firing order.
    – Edward
    Commented Apr 30, 2016 at 11:42

2 Answers 2


If you are trying to change the cylinder firing order, I'd recommend against it, since the firing order helps prevent the engine from vibrating excessively.

Consider the Inline-6, which has primary balance due to the firing sequence (in terms of crankshaft angle when a cylinder fires) matching the movement of the pistons themselves.

Similarly, V6 engines tend to use 60° bank angles to match piston movements with crankshaft firing angles.

Both situations reduce the need for counterbalances and dampers along the crankshaft, which means less rotational inertia than a different firing order would necessitate.

Also, imagine if all the cylinders fired at once. Especially at lower RPMs, you would have very inconsistent torque generation, leading to poor launch performance and exceptionally harsh vibrations in the engine bay. Conversely, spacing the cylinder firing order evenly helps to maintain consistent torque throughout the crankshaft rotation.

Disabling or skipping cylinders isn't terribly new, and many typical road vehicles employ it to reduce fuel consumption. If you simply don't use a couple cylinders, you can open the throttle more and lower losses on the remaining cylinders. Beyond saving fuel, I doubt you could get much advantage from attempting to modify the firing order, especially when it comes to power generation.


The only thing I can think of is if you skipped cylinders which would "change" the firing order lets say from 12345678 to 10203040 where the 0's were the skipped cylinders. This would be paired with making sure those injectors didn't fire and that the compression stroke would no longer compress anything.

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