Thread: Rev Limiter Discussion

So now that we see how the stock rev limiter behaves, what is the best way to implement a rev limiter? Spark cut, timing retard, fuel cut?

I'm not so sure about a fuel cut. I would personally think that having only a fuel cut would cause lean/detonation issues where they're most importantly needed to be kept at bay.

I like the idea of spark cut, since cutting the combustion cycle by not giving it fire would be better than having a bit of fuel compressed and ignited.

By spark cut, do you mean all 4 plugs are cut off at the rev limit?

Dave

I think a spark and fuel cut at the same time (or spark cut right before fuel) would be the best compromise. However a soft limiter using spark retard would be a sweet option as well.

Like a lot of people I've never liked a fuel cut on aggressive turbo engines and I think the science backs me up there. From the video they found as much as a 1/3 of an injection worth of fuel just chillin' in a puddle in the intake runner. This puddle is kept liquid by the boost pressure and sheer amount of fuel. When the boost pressure drops (like when you lift the throttle) all of that fuel instantly evaporates in the low pressure air and is ingested by the engine giving natural deceleration enrichment (which is nice) but during a WOT rev limit hit where the fuel is cut I believe the fuel is also able to be evaporated into the dry air (which now is not saturated with fuel) and also ingested by the engine causing potentially lethal lean ignition events.


As for cutting things cylinder-by-cylinder instead of all at once. I think if you can do that easily and cut each cylinder in a safe way I don't see why not to do that.

Dave,

I need to do a bit more reading/reSearching on my own to determine how rev limiter events are handled. I like the information you've posted here, however I still need more in depth reading material to determine the timing of the events.

For instance, all four plugs being cut off simultaneously at the rev limiter that you impose would be a little jarring, at least in my mind? If fuel is cut per cylinder bank, per cycle, can the spark not be cut in the same fashion?

I'm leaning towards what Ben is suggesting here, in that there's no need for a fuel event, if there's no spark event.



Agreed. I tend to bounce off of my rev limiter at 7800rpm when playing around, and both soft and hard rev limiters are the same. Apparently this is the same thing as having no hard rev limiter, according to Dave's posts above?

Great! Details on how this should work are exactly what I'm looking for.



Yes, both fuel and spark can be cut synchronously. One of the details we need to decide is how to "soften" the cut, which means something starts happening at some point before the actual rev limit set point is reached. This could be either timing retard or selected cylinder cut.



Yes, the soft cut limiter has cut all 4 injectors by the time the rpm reaches the set point. Are both limiters set to 7800?

Dave

Yes, both limiters are set to 7800. I had previously set them both to 8500, and when I received the new ECU I didn't set them as high. I probably should, though. Doesn't make sense to keep making power at redline

This is similar to the option that my Link G4 standalone ECU gives. The soft limit is set based on coolant temp, and it cuts alternate ignition events. The hard limit is a set "hard limit addition" past the soft limit and it cuts both ignition and fuel entirely. In the pic below this is set to 300rpm.



You also have the options of using just a soft cut, or just a hard cut. Dunno why though.


For launch control, Link gives 3 options with the G4: Speed-based, speed-based but with an on/off switch, and clutch switch-based (used in this example).
For some reason, Link only offer two limit modes for launch control - soft & hard limiting, or hard limit (in this case fuel cut) only. In this example, soft limit is at 4500rpm and the hard limit is 200rpm later.
With this setup I can build up to 10psi on the start line quite rapidly.




Either limiter doesn't feel overly harsh, but you certainly know it's there.


Hope this helps

Yes, that helps a lot. I'll download PCLink and have a closer look.

On another topic, the stock ECU code also contains an overboost limiter, which cuts fuel when the load (measured by Q/N) exceeds a limit based on RPM. I'm planning to remove this limiter, unless someone tells me it should be kept.

Thanks,
Dave

The overboost function is related to the map/baro pressure switch? Or no?

This would come in handy for those of us running DET setups, in case something should go wrong with the gate, and creep/spike are sensed.