I am setting up antilag and I have no ability to do an ignition cut. So to emulate an ignition cut I am setting ignition timing extremely negative to induce a misfire.

Where would the valve exotherm temperature peak? 0, -45, -90 degrees timing?

I'm imagining if you set it too late and the exhaust valve is closing and you ignite the fuel, it will cause a very hot edge on the valve. But if you ignite it even later most of the fuel should be out of the cylinder already, so you will only get a very small amount burning as it passes the valve edge. This will obviously depend on valve/cam timing, but I'm after a general answer.

Thoughts? Edit: By -90 degrees timing, I am referring to 90 degrees AFTER TDC. Eg on the down stroke of the combustion cycle.

  • What do you mean by "negative timing"? Timing is expressed as either before, or after, TDC.
    – Solar Mike
    May 2, 2019 at 7:05
  • I normally refer to ignition timing as "ignition timing advance". Eg degrees before TDC. So a negative value would be degrees after TDC. Eg -90 would fire 90 degrees after TDC.
    – rollsch
    May 2, 2019 at 8:38
  • So TDC + 90 is now negative? The negative is usually taken as the anticlockwise values ie TDC - 90 is 90 degrees before...
    – Solar Mike
    May 2, 2019 at 8:46
  • Every single manufacturer refers to timing as timing advance that I am aware of. Eg if you run 40 degrees timing advance you fire the ignition 40 degrees before TDC. Every single software package I've used that has timing maps uses negative numbers to indicate degrees after TDC. I can show you 10+ software packages that do this if you want. Do you have any comment on the exotherm temperature?
    – rollsch
    May 2, 2019 at 9:05

1 Answer 1


If you want to simulate ignition cut, you could run at + (or -) 180 (plus or minus your normal advance) so you'd fire the spark when the cylinder was at the top of it's exhaust stroke before the inlet valve re-opens (this is where the Alfa twin-spark competition cars run their secondary spark as any fuel remnants will be burned here). Of course that's assuming you aren't running a wasted spark setup because if you are, you're already effectively firing the spark at this time.

However, in order to implement anti-lag, surely all you want to do is fire your spark late (i.e. when the exhaust valve is open to ignite fuel in the exhaust system). I don't know what type of ignition system you're running but surely a relay to make the LT side live could be used to cut the ignition by simply switching off the power to the relay when you don't want the plugs to fire?

Very high temperatures are a byproduct of this type of anti-lag implementation I'm afraid. Use sodium filled exhaust valves, ceramic coat as much as you can and make sure your system suspends anti-lag when the coolant begins to boil.

  • Merlin engines used sodium filled valves...
    – Solar Mike
    May 2, 2019 at 10:09
  • Lancia used sodium filled valves on their Integrale WRC (and 16v road) cars. They're about five times the price of standard valves. It's one of the only differences between the Thema 16v Turbo and Delta 16v Turbo engine. May 2, 2019 at 10:13
  • So, depending on the engine, the OP may have to provide better head cooling as well...
    – Solar Mike
    May 2, 2019 at 10:15
  • Absolutely, I'd suggest as much as possible to take head away. Bigger oil cooler, alloy finned sump, alloy finned rocker cover, cut a slat in the bonnet near the windscreen, incant the trailing edge of the bonnet upwards, etc, etc. May 2, 2019 at 10:19
  • This is only used to spool a turbo off the line. So its under load and for a few seconds at most. By firing the ignition very late I was hoping we would avoid killing the valves as the fuel would combust in the turbine opposed to on the edge of the valve.
    – rollsch
    May 3, 2019 at 0:05

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