Timing motor with ITB's

[JoeMag]

In general on a race motor with ITB's does the ignition timing typically go up, down, or stay flat at WOT across the workable RPM range (say 4.5k to 7.8k). "up" meaning advance increases.

[Speedy Squirrel]

Well, the ITB's are mainly affecting the fuel distribution and induction restriction.

The ignition timing is a function of A/F, inlet air temperature, inlet air pressure, fuel octane, and RPM. So, the main affect from the ITB's is on the inlet air pressure, which should be slightly higher at WOT with ITB's.

The critical condition for timing is usually WOT. To answer your question, timing advance increases with RPM, until the detonation limit of the fuel is reached, and the timing is further modified (retarded) based on the inlet temperature and pressure measured by those sensors.

[JoeMag]

…as you gradually increase advance until torque does not increase, how many degrees should timing be backed off to be safe?

As a side note I saw an article that noted you should not consider the peak torque, but the point where increasing timing does not yield as much a gain as reducing timing. ...similar to example below where 24 degrees should be chosen as max because if you back off a degree you loose 10 ft-lbs where going up a degree only gains 4 ft-lbs.

torque timing
275 22
285 23
295 24
299 25
293 26

Does this ring true with others?

[JoeMag]

...just to be clear, we'd expect to see an ignition curve something like the blue line, not the orange.

[Speedy Squirrel]

Yes, I think to summarize:

1. Compared to what you had before ITB's will probably require less spark advance for the same safety margin, since the inlet pressure will probably be higher.

2. It is correct to say that you should stop advancing the timing when the improvement in torque is small (< 5 Nm).

3. I would leave a 3 degree margin from your max spark as a detonation safety factor, and that assumes you are able to correct timing for inlet air temperature.

4. The blue curve is the ideal behavior. It is not knock limited and is fairly aggressive. I have attached the 911chips.com (no affiliation) measurement of the 911 3.2 Carerra WOT timing for comparison. They are playing conservative for american pump fuel and flattening the ignition curve at the top end for extra safety.

If it was me, I would start relatively conservative, and add or subtract as I gained experience.