Safe rev-limit with Wasted Spark

[uvii]

What is the correct, safe, way to introduce a rev-limit with a wasted spark ignition set-up? Also is it safe to turn off overrun fuel cut?

I am concerned about a cylinder being ignited out of sequence with the random cylinder cut from the RPM limit mainly...

[uvii]

Anyone?

[Holmz]

Do wasted spark ever alight out of sequence?
If they did I can not imagine then would be widely used.

[IDP]

The wasted spark would be on the exhaust stroke where no fuel will be present.

Never heard of it being a problem before.

[uvii]

What I was thinking is, if you have a random ignition cut as defined by certain RPM limit settings, then you could have a cylinder with fuel get left unburnt. That cylinder then gets 'accidentally' ignited out of sequence by an other wasted spark from the ignition process of another cylinder.

[yotamhks]

when you shut down wasted 4 cyl spark you shut down both of the cylinders that the coil runs.
the situation you thought of doesn't occour.

[Holmz]

The wasted spark would be on the exhaust stroke where no fuel will be present.

Never heard of it being a problem before.

With a cam with some overlap, then there is fuel and air present - just the pressure is low.

[uvii]

Are you sure? My application is 6cylinder, not that it should make any difference.

Why would the following not occur?

You hit limiter, and cut spark from Cylinder 2. This now has unburnt fuel in it. Some will be lost on exhaust stroke but not necessarily all.

You then fire Cylinder 5 (or whatever the matching wasted spark cylinder to 2 is) . Cylinder 5 will fire as expected but Cylinder 2 may also have its unburnt fuel from the limiter cut above ignited from the wasted spark.

[Holmz]

With a high overlap cam the exhaust leaves and the intake gets sucked in.
How can all that be happening during the period when the wasted spark happens and you get no bangs and pops?

[uvii]

^ My post was out of sync...

I'm trying to work out why my main bears were all loose after a teardown with only dyno time on the engine. We stripped the engine after each engine dyno session and everything was perfect. Then on the last strip down all the shells were loose; this was over a period of 4 dyno sessions and about a week.

Only thing that was different was we hit the limiter a few times on the last dyno session. The rev limit was low (6750) and the engine is a 9k rpm engine...