ignition cut possible? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 11-06-2007, 10:44 AM   #1
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Default ignition cut possible?

i just blew my engine. i was full throttle and my gas pedal got stuck under the carpet and hit rev limit set at fuel cut for like 2 seconds. fried piston #1 and signs of all cylinders went lean. car was tuned correct on dyno 11.5 afr super reserved on timing advance. but it did lean out, i think and a couple good sources say hit fuel cut to long will give a lean mixture for sure. now hear me out i know you guys are gonna say well dont hit redline. well i drift the car a good amount so its gonna happen here and there. but my thought was to run ignition cut and save my next engine.....

whats your thought guys?
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Old 11-06-2007, 11:08 AM   #2
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correct me if I'm wrong....if you cut fuel, nothing ignites, so you can't blow the engine from going lean, because its simply not burning anything.

but yes, you can have MS cut either/or, or both.

Last edited by Braineack; 11-06-2007 at 11:58 AM.
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Old 11-06-2007, 11:51 AM   #3
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Pretty sure that's part of the reason most drifters use spark cut only, that and the cool popping exhaust noises it produces.
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Old 11-06-2007, 12:27 PM   #4
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Quote:
Originally Posted by Braineack View Post
correct me if I'm wrong....if you cut fuel, nothing ignites, so you can't blow the engine from going lean, because its simply not burning anything.
That was my understanding too, However, this thread has made me think about it a bit more, and maybe there will be fuel injected.....

The more I think about it the more I think that there will be fuel abeit small amounts as the MS is batch injection.

On my car I have the fuel cut set to 3 in every 5 cycles, (IIRC)

Normal firing cycle 4 stroke * = bang

firing order 1,3,2,4

- - - * - - - * - - - * - - - * - - - * - - - * - - - *

fuel cut = _

- - - _ - - - _ - - - _ - - - * - - - * - - -_ - - -_

at those two points where the fuel is squirted again could fall on any of the injectors. if for example your injectors are batched 1 & 3 and 2 & 4 and those two spark events are on cylinders 1 & 4 you're effectively squirting fuel into every cylinder, because the injectors are batched.

with only 4 cylinders and 2 injector drivers, you will always have amounts of fuel being injected into every cylinder.......

I'm finding it hard to describe how I have it in my head, does that make sense to anyone?
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Old 11-06-2007, 12:41 PM   #5
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but so what? it's always sparking on each cylinder, then is fails to ignite nothing, then it ignites your random squirt of fuel on the cylinder, then nothing.

Sparking into the chamber isn't harmful. You can't detonate, there's no explosions, no pre-igntion...

you could probably pull #2 and #3 injectors and still idle the car. you wont be lean because you're still injecting the proper mixture to of fuel to burn, just on 2-cyl.

Tons of cars come will fuel cuts from overboosting. The 626 turbo for example, wont let you boost over 15psi on the stock ecu and it beeps and enters a fuel cut mode.

the mr2 turbo wont let you past 13psi, when it enter fuel cut mode it actually cuts fuel anytime you enter boost at all, until you restart the ECU.
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Old 11-06-2007, 12:48 PM   #6
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So why not cut both at the same time?
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Old 11-06-2007, 12:57 PM   #7
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Quote:
Originally Posted by Braineack View Post
but so what? it's always sparking on each cylinder, then is fails to ignite nothing, then it ignites your random squirt of fuel on the cylinder, then nothing.

Sparking into the chamber isn't harmful. You can't detonate, there's no explosions, no pre-igntion...
I know, but as in my example there wouldn't be 'nothing' in the cylinder, there'd be fuel due to the batch fire.

I'm not saying it's gonna cause my engine to go pop, but if you were a drifter and have the engine on the limiter for a good portion of it's life. it may be a contributing factor.

sure you can idle on two cylinders by pulling 2 injectors, but the signal is still goint go those injectors..... pulling injectors is not the same as telling the ECU to cut 2 squirt events. those events can occur anywhere in the firing sequence, if they occur across the two banks of injectors you'll still get fuel squirted in...not as much as normal but still....

the MR2 turbo is sequential injection so it can cut fuel to indiviual cylinders, which the MegaSquirt *can't* do as it's batch fire.
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Old 11-06-2007, 12:59 PM   #8
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Quote:
Originally Posted by magnamx-5 View Post
So why not cut both at the same time?
aye both would do it
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Old 11-06-2007, 01:15 PM   #9
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Quote:
Originally Posted by richyvrlimited View Post
I know, but as in my example there wouldn't be 'nothing' in the cylinder, there'd be fuel due to the batch fire.
and it would ignited like normal because you aren't cutting spark.

spark:- - - * - - - * - - - * - - - * - - - * - - - * - - - *
fuel__:- - - - - - - - - - - - - - - * - - - * - - - - - - - -
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Old 11-06-2007, 01:22 PM   #10
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Quote:
Originally Posted by Braineack View Post
and it would ignited like normal because you aren't cutting spark.

spark:- - - * - - - * - - - * - - - * - - - * - - - * - - - *
fuel__:- - - - - - - - - - - - - - - * - - - * - - - - - - - -

but potentially it won't be *enough* fuel.

those two points when there are injection events, it won't just squirt 1 injector, it'll squirt two, as it's batch.

Now one of those that squirted will be be due an ignition event next, but because the previous round of 'squirts didn't happen there's now not enough fuel waiting behind the valves as there is normally, thus creating a lean condition in the cylinder.
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Old 11-06-2007, 01:45 PM   #11
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I don't see why it wouldn't inject the same amount of fuel. It's all timed, when it finally squirts again, it will be exactly when it would normally squirt.

how do you start your car each day?

what about overrun? you cut fuel on every decel.
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Old 11-06-2007, 01:58 PM   #12
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but when you bounce off you fuel cut dont you go back into your set values for a split second and throw fuel? and would creatate a lean mix..... but its a good topic to cover
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Old 11-06-2007, 02:08 PM   #13
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intake valve opens • pure air enters • piston reaches TDC • spark • exhaust valves open • air exits.

intake valve opens • predetermined air/fuel mixture enters • piston reaches TDC compresses mixture • spark ignites • torque forces piston down • exhaust valves open • exhaust exits.

the cylinder that ignites wont be lean, the ones that done won't have ignition to worry about.


spark only cut causes some ridiculously nasty backfires when the fuel is finally ignited....
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Old 11-06-2007, 02:10 PM   #14
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<--- Sticking to spark/fuel cut.
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Old 11-06-2007, 05:01 PM   #15
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Quote:
Originally Posted by Braineack View Post
I don't see why it wouldn't inject the same amount of fuel. It's all timed, when it finally squirts again, it will be exactly when it would normally squirt.

how do you start your car each day?

what about overrun? you cut fuel on every decel.
overrun is all of the injectors being cut though.

My point is that as it's batch injection a single injection pusle on say cylinder1 won't be the complete amount of fuel required as there's another injection going to happen after cylinder no 2 gets a dash of fuel, (assuming 1&3 are paired etc).

the firing order is 1 3 2 4. as 1 & 3 and 2& 4 are paired for every 4 cycles you get 2 squirts per cylinder.

Now if one of those squirts was missing because of hitting the rev limiter you have a lean condition in that cylinder?.....

EDIT: All this said, i'm sticking with fuel cut, I don't see it ever causing *my* car any harm, it's just the more i thought about it the more it makes sense in *my* head
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Old 11-06-2007, 05:06 PM   #16
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just cut both and call it a day
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Old 11-06-2007, 09:50 PM   #17
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I think there's some misunderstanding in this whole thread on the fuel cut--- it's not a 'cut x out of x cycles' thing. It's an absolute fuel cut when you're at or above the rev limit. 0 PW.

The 'cut x out of x cycles' is for the spark cut rev limiter, it will cut ignition events, 3 out of 5 in this example. The unburnt fuel goes out the exhaust and makes pretty flames if it ignites on the way out on cars without a cat . (or instead, melting your cat if you do have one)

So that said... there's no 'rolling cylinder fuel cut' that could allow only part of the fuel needed to get to a given cylinder. It's on or off.
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Old 11-07-2007, 04:37 AM   #18
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Quote:
Originally Posted by FoundSoul View Post
I think there's some misunderstanding in this whole thread on the fuel cut--- it's not a 'cut x out of x cycles' thing. It's an absolute fuel cut when you're at or above the rev limit. 0 PW.

The 'cut x out of x cycles' is for the spark cut rev limiter, it will cut ignition events, 3 out of 5 in this example. The unburnt fuel goes out the exhaust and makes pretty flames if it ignites on the way out on cars without a cat . (or instead, melting your cat if you do have one)

So that said... there's no 'rolling cylinder fuel cut' that could allow only part of the fuel needed to get to a given cylinder. It's on or off.
cheers Jerry, as ever the fount of knowledge
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Old 11-07-2007, 01:10 PM   #19
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Sweet so you are telling me i can shoot flames like F&F cool
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Old 11-07-2007, 01:11 PM   #20
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turn off your overrun, change fuel cut to spark cut.... have fun.
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