Rev Limit, Fuel vs Ign cut

Dweezle · Postby **Dweezle** » Mon Mar 02, 2015 9:46 am

I have a question about what I should be using for my Rev limit?

I have always used Fuel cut for the hard limiter and Timing for the soft limiter.

I have been thinking about it lately and wondering if I should be using Ignition?
The main reason being a lean condition as the injectors shut down.

Though I am also worried that raw fuel will damage my Cat especially if it starts going bang inside if I cut ignition.

I can cut both and am unsure of how this will affect anything.

Any advice much appreciated.

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Postby **mrpham** » Mon Mar 02, 2015 11:02 am

I'm no expert, but I was told to cut fuel and ignition when setting up my Megasquirt.

Postby **beavis** » Mon Mar 02, 2015 11:29 am

I have no cat, i just run spark cut.

Dweezle · Postby **Dweezle** » Mon Mar 02, 2015 12:33 pm

Thanks guys.

Beavis, if I had no cat I think i would totally just go Spark Cut.

Having a cat though and raw fuel just pouring in and potentially pooling in a hot exhaust worries me about things going bang.

Maybe cutting both is ideal.

I assume factory cars usually always just use fuel cut.

hks_kansei · Postby **hks_kansei** » Mon Mar 02, 2015 3:19 pm

Fuel + cat will kill the cat.

The fuel will likely ignite in the cat, but will probably just end up making popping and banging noises from the exhaust, maybne a few flames from the tailpipe.
That's of course, before the substrate gets cooked and comes flying out the back....

Personally I'd do fuel cut.
Lean is only a problem when it's still got enough fuel to ignite (detonate etc)
Since fuel cut will stop the fuel all together the engine will just pump air, which wont ignite or anything.

Dweezle · Postby **Dweezle** » Mon Mar 02, 2015 3:37 pm

Thanks mate.
I regards to the fuel cut creating a lean Condition.
I am mainly worried about injector timing not being exact and having a lean final and lean first stroke.
I might be over thinking this though ;)

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project.r.racing · Postby **project.r.racing** » Mon Mar 02, 2015 5:25 pm

hks_kansei wrote:Fuel + cat will kill the cat.

No/low fuel + ignition will kill pistons.

Oh decissions decissions.

Dweezle · Postby **Dweezle** » Mon Mar 02, 2015 5:48 pm

Exactly

I still can not work out why cutting both would be bad?
Or why it doesn't seem to be the popular way?

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project.r.racing · Postby **project.r.racing** » Mon Mar 02, 2015 5:59 pm

Dweezle wrote:Exactly

I still can not work out why cutting both would be bad?

Just do ignition cut. Everybody loves flames...

hks_kansei · Postby **hks_kansei** » Mon Mar 02, 2015 6:18 pm

project.r.racing wrote:
hks_kansei wrote:Fuel + cat will kill the cat.
No/low fuel + ignition will kill pistons.

Oh decissions decissions.

How will it kill pistons?

I mean, running lean causes lots of heat, pinging, detonation, all that sh*t.
But if there's no fuel (ie: fuel is cut completely) then there's nothing to ignite, the engine will just suck a mouthful of air, compress it, spark (with no ignition) and then push it out the exhaust valve.

Dweezle · Postby **Dweezle** » Mon Mar 02, 2015 6:31 pm

The issue I've read about is that at high rpm and not 100% accurate Injector timings, that you can get a real lean condition happening on the final stroke and first stroke after the RPM has dropped and it fires back up.

I doubt this is much of an issue on a standard engine but maybe when you crank up compression and Rpm it raises it head as a real danger.

I hardly ever touch my limiter at 7800rpm but I'd like to do it as safe as possible.

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