Ignition maps

[woollymoof]

Hi all,

Consider for a given RPM. A cylinder is on the limit of knock due to the advance and the pressure ratio (~compression ratio) of that cylinder.
If you increase the pressure ratio, with compression ratio increases, you are likely to have to take out advance in order to prevent knock. Likewise, if you reduce the pressure ratio, with compression ratio decreases, you might be able to add advance to bring you up to the knock limit again.
Thottling the engine also reduces the pressure ratio meaning more advance can be added, but I have never seen this on an ignition map.
Why is this? Am I missing something completely?

[woollymoof]

Hi all,

Consider for a given RPM. A cylinder is on the limit of knock due to the advance and the pressure ratio (~compression ratio) of that cylinder.
If you increase the pressure ratio, with compression ratio increases, you are likely to have to take out advance in order to prevent knock. Likewise, if you reduce the pressure ratio, with compression ratio decreases, you might be able to add advance to bring you up to the knock limit again.
Thottling the engine also reduces the pressure ratio meaning more advance can be added, but I have never seen this on an ignition map.
Why is this? Am I missing something completely?

[Charlie]

You've never seen an advance map with more advance at low loads? I have... Every one I've ever seen.

Though knock may be sometimes a limiting factor, it isn't always. Sometimes (often with our engines) torque decreases before the knock limit. I wouldn't assume that you would always be against a knock wall.

[Chris Boyden]

The general trend that we use on a MAP based setup has more advance with higher vacuum and more rpm.

Starting from the upper left hand corner of the ignition table, the advance generally increases with rpm and less load.

Chris

[B Lewis @ PE Engine Management]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Thottling the engine also reduces the pressure ratio meaning more advance can be added, but I have never seen this on an ignition map.
<HR></BLOCKQUOTE>

Many ignition maps tend to increase timing at low loads. For the mechanical types out there, think of how the vacuum advance on a ditributed engine works. more vacuum == more advance with this setup.

[Trans Am]

As far as I've seen, there usually is a seperate map in the controller that contains either a raw number or a multiplier. The map is dependent on load (using either TPS or MAP) and engine RPM. This figure is then added or subtracted to the ignition map.

[Mark Peugeot]

<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by woollymoof:
Hi all,

Consider for a given RPM. A cylinder is on the limit of knock due to the advance and the pressure ratio (~compression ratio) of that cylinder.
If you increase the pressure ratio, with compression ratio increases, you are likely to have to take out advance in order to prevent knock. Likewise, if you reduce the pressure ratio, with compression ratio decreases, you might be able to add advance to bring you up to the knock limit again.
Thottling the engine also reduces the pressure ratio meaning more advance can be added, but I have never seen this on an ignition map.
Why is this? Am I missing something completely? <HR></BLOCKQUOTE>

Well you are missing some pretty important things. First MBT is definately not always knock limited especially with 100 Octane fuel or E85.

Increasing static compression does not always increase dynamic compression in a linear fashion at all RPM's which in our restricted motors is an issue.

As noted by charlie, less load usually allows timing to be advanced which will often help light load fuel economy.

The dyno is so important for proper tuning that there is no way that you can simulate all the needed conditions otherwise to really build a top notch map.

Chris Boyden noted that at higher RPM that more timing can be run, often times even in some lower RPM areas additional timing can be run, as you approach where the motor makes peak torque, this is where it is most likely to encounter detonation and the most likely spot where timing may need to be reduced by an additional amount if you are timing limited and not at MBT.

Other times you'll run rich to avoid detonation to get closer to the timing required for MBT. It's a complex balancing act and the best advice is not to assume that you know what the engine wants, but rather let the engine tell you what it wants via the dyno.

Mark

PS - Investing in some EGT gauges and a Wide Band AF meter is important too.

[woollymoof]

Thanks all,

Mark,
Thanks for your reply. I'm aware that MBT isn't always knock limited, I was just trying to get a feel for what happens at a single RPM when you are knock limited.

My question has certainly been answered though.

[BryanH]

Kirk, you have "missed something completely".
I'm assuming your have been looking at Motec influenced timing maps which is why you didnt see any light load advance. The mapping priorities of a race engine are different to a street engine & very different to an emissions engine. What the driver wants is linear torque modulation........
btw the safety margin between best power and knocking with Shell ultra is 5-7deg.
It is vital for consistant lap times to keep combustion temps low, I take 2-3deg out of timing and/or increase fuel 3 to 5% after mapping on dyno to achieve this.
Bryan Hester