Fuel Cut vs. Spark Cut

[Toph6888]

Interested to hear others opinions on using fuel cut or spark cut as a 2 step/launch control.

 

First off, definitions because some people seem to get these confused:

 

Anti-lag: Retarding the spark timing so far past top dead center that you are burning the air/fuel in the exhaust to create full boost and flames out the tailpipes.

 

2Step/Launch control: creating a 2nd rev limiter to launch from that does not change ignition timing. This just allows the rpms to be held at a certain point and the throttle to be held to the floor.

 

While Anti-lag is a form of launch control, it is not what I am interested in discussing.

 

So how to create the 2 step. There are two main methods, either fuel cut or spark cut.

 

Fuel Cut: The ecu will only inject enough fueling during certain pulses to maintain a set rpm. Ignition still tries to ignite, but if there isn't fuel (or extremely low levels of fuel, far beyond what we would consider a "lean burn" condition, look up lower explosive limit).

 

Spark cut: all the fuel is still injected, but the ecu will just not command a spark during some combustion timings so as to control the rpm at a set point.

 

Most "launch control/flat foot shift" options from OEM's use fuel cut due to ease of control. On my mustang I have a N2MB box that uses spark cut because I didn't want to use fuel cut because I was concerned about "lean burn" conditions leading to knocking/detonation. The issue with the spark cut is that there is a lot of fuel in the system that can lead to backfires and as well as wet the cylinder walls with fuel lowering oiling performance of the piston rings.

 

However, I personally still lean towards spark cut because I worry about the transition time from fuel cut to the actual movement of the car and the time it takes the engine to go back to full operation, potentially causing knocking/detonation during this transition due to lean burn.

 

Interested in CR's thoughts (constructive please, don't flame) on spark vs. fuel cut for a 2 step/launch control feature.

[cruizin01]

I have nothing useful to add other than I loved the N2MB box on my GTI. Gawd is was loud. Full 3" exhaust no cats. Do it in a parking garage and it sounded like gun shots.

[Rally Pat]

Anti-lag is not launch control. Launch control may be used before, but anti-lag is used while the vehicle is in motion already. Its also worth mentioning that there are lots of different types of anti-lag, it isn't all necessarily the ECU retarding the timing and adding fuel off throttle.

 

See: Any rally car ever. Note the pops and bangs off power and between gears.

 

[Toph6888]

Antilag has been used for launching, thats the way all the world rallycross cars start off the line. Not saying that antilag isn't used between shifting either.

 

My main point was that this thread wasn't supposed to be about antilag, but using spark cut or fuel cut as a 2 step rev limiter for launching.

[Nate1647545505]

Hi Chris,

 

Just my .02 - I think a fair statement would be, "why not both?" Keep in mind OE's have to worry about emissions, or melting emission control systems. This is kind of an interesting dilemma - you wan't RPM's up to have torque available for the launch but not to much to completely overcome the available traction.

[craig71188]

I believe the OE's use fuel cut due to emissions - there's not much difference in controlling (killing a cylinder with) spark or fuel via the ECU. As to the "lean burn" concerns, if the injector is working properly (no leaking) and does not fire (inject), there is no fuel to burn, and when activated on another cycle, with full fuel pressure at the rail, will deliver the proper amount of fuel (again - no lean condition). I suppose a case can be made for "some" fuel in the air coming into the cylinder via reversion in the intake, but I would think it too miniscule to be an issue.

 

Using spark cut (as you mentioned) there is still fuel going into an unfired cylinder and the potential for wash down is there, but with a random cylinder kill it is unlikely. Again, I think the OE concern is the more likely harm to O2's and catalysts in particular.

[Cordell]

I believe the OE's use fuel cut due to emissions - there's not much difference in controlling (killing a cylinder with) spark or fuel via the ECU. As to the "lean burn" concerns, if the injector is working properly (no leaking) and does not fire (inject), there is no fuel to burn, and when activated on another cycle, with full fuel pressure at the rail, will deliver the proper amount of fuel (again - no lean condition). I suppose a case can be made for "some" fuel in the air coming into the cylinder via reversion in the intake, but I would think it too miniscule to be an issue.

 

Using spark cut (as you mentioned) there is still fuel going into an unfired cylinder and the potential for wash down is there, but with a random cylinder kill it is unlikely. Again, I think the OE concern is the more likely harm to O2's and catalysts in particular.

 

I totally agree with this, couldn't have said it better.

 

My personal thoughts is that if either way is used for minimal time, not standing on the 2 step in a parking lot like a dickhead, it's not a big deal either way.

[Nate1647545505]

I decided to see if my SAE membership was still active and found by some strange reason, it is. If you look up http://papers.sae.org/942475 "Traction Control for a Formula 1 Race Car: Conceptual Design, Algorithm Development, and Calibration Methodology" , Bosch and Chrysler used a rotating fuel cut to manage torque reduction. I know it doesn't exactly apply to the situation at hand but still has some relevancy.

[vonrottes]

I don't like fuel cut off....because when I set it up on my S10 for the "abuse limiter" at 3,500 activate 3,450 resume it would NOT hold the rpm like I wanted and at WOT it was reaching 5,000rpm....Violently shaking the whole truck...but still revved past the set limit.

I dunno maybe I had a leaking injector or something...

 

IMO the safest and best way to get launch control is deffinitly Electronic throttle body..

Super easy to set it up...and safe because it literally just keep the throttle blade from opening all the way at a set rpm.

[Nate1647545505]

I don't like fuel cut off....because when I set it up on my S10 for the "abuse limiter" at 3,500 activate 3,450 resume it would NOT hold the rpm like I wanted and at WOT it was reaching 5,000rpm....Violently shaking the whole truck...but still revved past the set limit.

I dunno maybe I had a leaking injector or something...

 

IMO the safest and best way to get launch control is deffinitly Electronic throttle body..

Super easy to set it up...and safe because it literally just keep the throttle blade from opening all the way at a set rpm.

 

What system was controlling the engine?

[Geeesammy]

What system was controlling the engine?

 

Sounds like stock ecu.

 

I would never attempt any form of 2 step while using a stock ecu, and even at that I would only trust 5 or so brands with their strategy on the aftermarket.

[vonrottes]

What system was controlling the engine?

 

Sounds like stock ecu.

 

I would never attempt any form of 2 step while using a stock ecu, and even at that I would only trust 5 or so brands with their strategy on the aftermarket.

 

 

Yup Stock GM PCM.

wasn't really setting it up for anything other than just playing around with it since it only activates in Park & Neutral

[Cordell]

Yup Stock GM PCM.

wasn't really setting it up for anything other than just playing around with it since it only activates in Park & Neutral

 

Not the same thing. It's a feature to keep idiots from over revving shit, and barely works for that. If you want a 2step on an LS based vehicle MSD makes a box to interupt the coils. Otherwise aftermarket ECm.