High ignition advance at high vacuum / RPM

[Wildcard]

During the tuning process I have noticed that there is sometimes a transient knock condition when going from boost [WOT] to vaccum at high RPM on my engine. I note the powerfc map runs the highest advance points on the map from about 4000rpm-7000rpm at max vaccum. This in itself seems correct when compared to other maps I have seen but the peak advance in this area is 52 which seems a little excessive when compared to other maps.

I have read a few other 3SGTE tuning stories where this same issue was identified and they fixed it by lowering the advance at this point - is what I'm seeing in terms of knock actually a real problem and if so why is the default advance on the standard PFC map so high in this area?

[fivebob]

Timing at Max vacuum shouldn't be an issue . Max vacuum at those points should imply that the throttle is closed and throttle overrun fuel cut should be in effect so no fuel = no combustion=no knock.

A more likely cause is the ignition timing in the 50-120KPa region which seems to be higher than it should be.

[Wildcard]

Timing at Max vacuum shouldn't be an issue . Max vacuum at those points should imply that the throttle is closed and throttle overrun fuel cut should be in effect so no fuel = no combustion=no knock.

A more likely cause is the ignition timing in the 50-120KPa region which seems to be higher than it should be.

Thanks for the response fivebob, 50kpa-120Kpa would be 375-900 mmHg correct? If this is the case I'm a little confused as the bottom of the map is about 600mmHg by my calculations. Here is the vaccum part of my map with my rough notes [Column A] on each vaccum point, could you comment on which rows you believe have unnecessary advance at high rpm?



Thanks

[matt dunn]

Maybe related, maybe not,

but I have had a scantool showing live data on my 100% standard blacktop 20V,

and under similar conditions, med to high rpm and low throttle angle,
and the live data has shown ign advance in the high 40's BTDC.

Matt

[fivebob]

Thanks for the response fivebob, 50kpa-120Kpa would be 375-900 mmHg correct? If this is the case I'm a little confused as the bottom of the map is about 600mmHg by my calculations.

Err, no it gets a bit confusing using mmHg Vacuum and converting it to kPa Pressure IIRC you have to subtract the values that the pressure conversion gives you from 780, so that would be 405 -> -120mmHg. Which is why I can't be arsed using that measurement system kPa is soooo much easier to understand.

Here is the vaccum part of my map with my rough notes [Column A] on each vaccum point, could you comment on which rows you believe have unnecessary advance at high rpm?

Ok for clarity I'll post the standard PowerFC map with those areas highlighted as seen when using the Datalogit software, figures at the left are kPa * 100 so it starts at 10kPa and goes to 260kPa.



Contrast this with my (not dyno optimised) M800 Map (read from bottom to top as it's the other way around in the software)



This map is street tuned using a steep hill, doing repeated runs over the same section starting at the same rpm/speed and measuring the rate of acceleration to check for changes in Torque. I also monitored knock and I know if I add 2° in the 6000-6500rpm range it will knock on '96.

NB this is posted for comparison only, I'm in no way suggesting that this is the timing you should use!!! Also it's not been tested above 220kPa as the stock CT20B won't sustain those levels above 6000rpm in the gears used for street tuning

[Wildcard]

Thats great, thanks for the detailed feedback. I can now see the differences in advance you are talking about - since fuel economy is not important to me at this point in the map I may just blanketly reduce the 0-100kPa timing to similar amounts as seen in your map as an experiment.

What surprises me is the differing advance between the stock map I posted (obtained via the gumby hand controller) and the datalogit interpretation you posted. Is it possible you have a newer powerfc with a refined map or is the difference likely attributed to the different rpm points or interpolation allocated to the 20x20 cells via the dataloggit?

Also, if you don't mind could you post up the rest of your map? I'm interested to compare the ~5800-6200rpm points you had previously identified as knock prone - we were able to find some problems with the stock pfc map in this area.

[fivebob]

What surprises me is the differing advance between the stock map I posted (obtained via the gumby hand controller) and the datalogit interpretation you posted. Is it possible you have a newer powerfc with a refined map or is the difference likely attributed to the different rpm points or interpolation allocated to the 20x20 cells via the dataloggit?

No that's just the map that came with the datalogit software, I believe that's from a different version, or someone has played with it as I've got the same map as you posted, except it's labelled as Apexi1.61 Must check mine PFC and see what map is loaded in that.

Also, if you don't mind could you post up the rest of your map? I'm interested to compare the ~5800-6200rpm points you had previously identified as knock prone - we were able to find some problems with the stock pfc map in this area.

It's not really valid to compare the two of them, the Motec has a lot of other tables that alter the base timing, which don't have an equivalent on the PFC, and it also has individual cylinder trims to retard the centre cylinders. Besides which, your timings and mine may have a different base point. I set the Motec up using a dial gauge to detemine TDC and it was about 3° different to the stock markings.

All I really did was to retard the area from 5750-6500 by a 1-2 degrees, it has a similar shape to the PFC map I posted, i.e. advance generally increases from peak torque, but is retarded by 1-2 degrees from 150kPa in that rev range.