fuel cut vs. spark cut on MSPNP - Miata Turbo Forum - Boost cars, acquire cats.

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Old 11-16-2009, 11:50 PM   #1
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Default fuel cut vs. spark cut on MSPNP

So i hope i'm not bringing up a question thats been beaten to death but i'm just wondering what the deal is with spark cut limiters? I know that factory limiters use fuel cut and i've only seen spark cut in drag cars and very few street cars. Megasquirt has both so there could possibly be a reason for me to use spark cut instead of fuel.......?
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Old 11-17-2009, 12:59 AM   #2
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Use fuel cut. I learned the hard way. Spark may be fine for N/A cars, but not so much for turbos. Spark cut can cause detonation ON the turbine and destroy the blades.

Lots of raw gas + hot blades = sad turbo

Post about it on dsmtuners.com
evo3 16g turbine wheel - DSM Forums

Post about my turbine being messed up which I later found out was a cause of using spark cut.
https://www.miataturbo.net/forum/t39714/#post461862
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Old 11-17-2009, 03:48 PM   #3
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Thats what i thought. i was just wondering cus i saw and STi using a spark cut launch control at the drag strip and wondered if it really was bad. also, when i'm throwing the miata into a corner really hard and i've done a 3-2 downshift while on the brakes really hard, i sometimes get a flame poping out of my exhaust. is that bad? cus it seems to me like that is what happens in a spark cut situation.
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Old 11-17-2009, 05:13 PM   #4
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If you have a catalytic converter, it will not like seeing a bunch of raw fuel either. I'm pretty sure there is a warning to that effect on the screen in Megatune where you set your rev limit type (spark/fuel cut).
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Old 11-17-2009, 05:15 PM   #5
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spark sounds cool. thats why i run it!
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Old 11-17-2009, 05:52 PM   #6
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Brain, how long have you been running spark cut? i'm assuming you have no cat like me. and you havent had any problems?
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Old 11-17-2009, 06:47 PM   #7
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Spark cut reacts way way way faster than fuel cut.

Use both, in this order:

1. First step should be cut spark
2. Second step should be cut fuel, say 50-100 rpm later

Third step is cut the throttle cable...
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Old 11-17-2009, 06:52 PM   #8
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Quote:
Originally Posted by mx5autoxer View Post
Brain, how long have you been running spark cut? i'm assuming you have no cat like me. and you havent had any problems?

I'm not, I'm using spark and fuel cut.
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Old 11-17-2009, 08:18 PM   #9
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fuel cut does react awfully slow at times... mine is set to 7k but i've had datalogs with 7400rpm spikes :(
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Old 11-17-2009, 08:35 PM   #10
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Also be wary that spark cut can destroy mufflers and catalytic converters by having large pressure spikes in the exhaust system caused by fuel igniting in it.

That said, my buddy's L28ET T61 280z gets 7psi standing still and 12psi at launch by using the spark cut launch control MS feature.

Quote:
Originally Posted by JKav View Post
Spark cut reacts way way way faster than fuel cut.

Use both, in this order:

1. First step should be cut spark
2. Second step should be cut fuel, say 50-100 rpm later

Third step is cut the throttle cable...
I think it should be:
1. Absolute spark limit (I think MS calls this soft limit). It pulls timing to slow the engine's inertia
2. Then hard cut Spark+Fuel Cut after "x" time or "x" RPM

The launch and flat shift on MS if freakin sweet once you figure out settings that work with your car.
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Old 11-17-2009, 10:18 PM   #11
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I've been running spark cut for about 6 months (maybe more) havent had any problems yet.

It gets used maybe once a week if that though
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Old 11-17-2009, 11:59 PM   #12
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Quote:
Originally Posted by Braineack View Post
I'm not, I'm using spark and fuel cut.
This.
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Old 11-20-2009, 04:30 PM   #13
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Quote:
Originally Posted by JKav View Post
Spark cut reacts way way way faster than fuel cut.

Use both, in this order:

1. First step should be cut spark
2. Second step should be cut fuel, say 50-100 rpm later

Third step is cut the throttle cable...
Yeah, MS lets you do both. I'm not sure which comes first. I've run spark retard soft limit, fuel cut hard limit or something. Spark cut is supposed to be all around smoother, less violent which is good when power comes back on in a turn.
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