Cutting ignition or injection?

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Cutting ignition or injection?

Postby Mobne_s13 » Wed Nov 21, 2018 10:03 pm

Is there any real downsides cutting injection instead of ignition for revlimiting functions?

Ignition cuts often make exhaust bangs and create high pressure spikes in the exhaust manifold.
This is a real problem with engines with shims on the valves because the valves are being pushed down from the pressure and the shims fall off the valve.

Common on m3 e46.

Please give inputs.

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Re: Cutting ignition or injection?

Postby Drwaffles » Wed Nov 21, 2018 10:21 pm

I'd say that every vehicle is different. If you exhaust valves aren't particularly strong or you've got weak springs then there's always a chance of damage.. Do remember though that the expanding gas is going to take the easiest exit path. Unless you've got a restrictive exhaust or turbine housing, I'd be fairly confident in saying that the pressure would be fairly manageable in the manifold. Get an E-MAP sensor and monitor, see if much different to pushing a small turbo to the limits.

That being said, OEM's use ignition cut or retard to reduce load on gearboxes during a change.
Ever listen to an A45 in sports mode SPAM through the gears? Like an assault rifle! And they've got a tiny twin scroll turbine.

Injection cut is a safer option IMO, on launch control or flat shift it just doesn't hold as much boost. Application dependent/Personal preference.

Posts: 370
Joined: Sun May 25, 2008 8:56 pm

Re: Cutting ignition or injection?

Postby Mobne_s13 » Thu Nov 22, 2018 4:38 am

I have had a valve shim come loose on an n/a m3 engine with 3.5” exhaust system all way back when it’s banging on the rev limiter.

As said those engines are famous for this issue. I can’t imagine bmw ecu cut the ignition because m3 engines don’t bang on the limiter standard.

I would expect it to require many bars of over pressure to force the valves down on the m3 head.

It’s double valve springs on it so the pressure created from loud exhaust bangs could possible be over 5bar peaks. I have never logged this and I don’t expect to do because you can really damage the cylinder head when the shims come loose.

If someone have back pressure logged maybe they just can look in thier logs and see how high the spikes is.

Anyhow I like the inputs why not allways cut the injector when you run sequential injection.
If no fuel is injected how could there create a situation with detonation? Even on a launch control.

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