Spark timing - Low Octane, how does the car know?

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Re: Spark timing - Low Octane, how does the car know?

Post by Charlescrown »

Had a little read of my EVO info and it says that the car runs on the high octane map and if the engine pings it registers the level (called knocksum) and will stay on the high map if the knocksum is at a low level of around 2-3. Obviously the higher the voltage from the knock sensor the higher the knocksum. After that if the level rises it then interpolated between the high and low maps till the knocksum drops to an acceptable level then it starts raising the timing back up to the high octane map whilst monitoring the knocksum.
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Re: Spark timing - Low Octane, how does the car know?

Post by JohnDee68 »

Charlescrown wrote:Had a little read of my EVO info and it says that the car runs on the high octane map and if the engine pings it registers the level (called knocksum) and will stay on the high map if the knocksum is at a low level of around 2-3. Obviously the higher the voltage from the knock sensor the higher the knocksum. After that if the level rises it then interpolated between the high and low maps till the knocksum drops to an acceptable level then it starts raising the timing back up to the high octane map whilst monitoring the knocksum.
Thanks. Now that makes logical sense to use the severity of the sensor readings to adjust the timing accordingly.
I do now wonder what would, if it has it, trigger a "limp mode" in the Vx 2002 series car.
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The1
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Re: Spark timing - Low Octane, how does the car know?

Post by The1 »

after VS models holden has high/low spark tables in action, basically copying the high map and make the low map the same but drop X degrees, then there is a setting to say if knock occurs and more than X degrees timing is pulled then switch to low map which would be more timing pulled out again. Then opposite to switch back to high map. There is also settings to switch it on from X rpm or some even have cylair, so knock sensors causing it to pull to the low map while idle or off idle, especially say with a manual where there is more chance of false knock.

On standard ecotecs for say running 98ron i would give 5 degrees between high and low maps then set it to more than 3 degrees knock then switch to low map. But it depends on the vehicle and setup, if you have larger cam then even the knock settings themself like ignore time may need adjusting to compensate for extra noise from the valvetrain.
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Re: Spark timing - Low Octane, how does the car know?

Post by antus »

In the VX/VY PCMs there is a multiplier which is offset between the high and low spark tables. The max spark you'll see is the high octane table, the lowest spark you'll see is the low octane table, and what spark its calculating might be anywhere between the two tables depending on what knock the ecu is seeing and for how long its been seeing it. It does change while the engine is running.
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immortality
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Re: Spark timing - Low Octane, how does the car know?

Post by immortality »

The1 wrote:after VS models holden has high/low spark tables in action, basically copying the high map and make the low map the same but drop X degrees, then there is a setting to say if knock occurs and more than X degrees timing is pulled then switch to low map which would be more timing pulled out again. Then opposite to switch back to high map. There is also settings to switch it on from X rpm or some even have cylair, so knock sensors causing it to pull to the low map while idle or off idle, especially say with a manual where there is more chance of false knock.

On standard ecotecs for say running 98ron i would give 5 degrees between high and low maps then set it to more than 3 degrees knock then switch to low map. But it depends on the vehicle and setup, if you have larger cam then even the knock settings themself like ignore time may need adjusting to compensate for extra noise from the valvetrain.
It's a shame though that they didn't actually make use of it. The ecotec was only ever tuned on 91. If I'm not mistaken it wasn't till the VZ that Holden made use of the feature and tuned the cars on high octane fuel with the option to run low octane fuel and loose some power.
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Re: Spark timing - Low Octane, how does the car know?

Post by Charlescrown »

I don't think the intent is to allow differing tunes for different levels of Octane. It's for engine preservation because it still has to ping and continue to ping to switch down from the timing in the high Octane map.
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Re: Spark timing - Low Octane, how does the car know?

Post by immortality »

Seems like a convoluted way to set up your knock strategy if that is the case.
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Re: Spark timing - Low Octane, how does the car know?

Post by The1 »

the series 1 VS V6 one of the first memcal revisions BPYR had high spark timing which was great for 98 octane, they revised it quickly after because of complaints of pinging and hesitation, obviously knock sensors were retarding the timing horribly, the low and high tables were the same so it was just taking max timing in and out.
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Re: Spark timing - Low Octane, how does the car know?

Post by immortality »

I had a mate pull a similar stunt on me. We were tuning his VS L67 Calais but said it was feeling very lazy. Then he comes over and I noticed he'd dropped a couple of pulley sizes on the blower. Took it for a spin with the laptop hooked up and found it was running on the low spark map (using your enhanced tune).
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Re: Spark timing - Low Octane, how does the car know?

Post by delcowizzid »

Some subarus just rely on the knock sensors fully they just take timing out and readd it over and over to keep it in the max timing zone lol
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