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Does an "overlay" fuel table extrapolate outside its bounds?


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I've been struggling to get a steady idle with only 850rpm, 1000rpm and 1500rpm rows in the fuel map.

So decided that I would make an overlay dual fuel table to see what's going on, zoned in very finely just around the idle area.

So I ran a few iterations of mixture map on this, and its made a good difference to my idle.

However.... since this Dual Fuel "overlay" table only goes to 1200rpm... Is this second fuel table trying to extrapolate it's 1200rpm values across the rest of my map?

Or does it just cut off at 1200rpm.

Same question for 4D and 5D tables, do they extrapolate out if you're using them to just zoom in on an area that needs some more resolution.

In this case I've used a virtual aux as a switch to only switch this map on below 1300rpm, so its not a problem but just curious for if I use 4d and 5d maps later on.



Edited by Davidv
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Hi Davidv,

Is the reason you are using the table because it simply will not hold a quality AFR around your idle? Posting a log would be very useful. In 4d and 5d mode you can switch this as table activation or always on. You will need to setup the x and y axis of the conditions you would want this to be active. When in the overlay setting 4d / 5d. You add a 6th dimension and the table becomes much larger.

To answer your question it will not extrapolate if you have your axis maxed out for 1200 rpm. So in overlay it will simply add a percentage to the main fuel table. So at 1100 rpm at 16.8 kpa, you may have a value of 32. With a number like 10 in the table. This would be 10% + 32 = 35.2 would now be the calculated value for that table. I would assume you will have better luck in interpolate mode. This will allow a ratio table that you can control via what x and y axis conditions you would like. So a percentage of how much of table 1 to 2 is being used. Easily after 1200 rpm you could set this to just table 1. 

Hope this helps and I answered your questions.


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Hi Rich,

I'm running staged injection and my intake manifold has a lot of resonant peaks and troughs. Engine goes to ~8200rpm so to adequately address areas that need a little more or less fuel I've chopped the bottom end off my fuel table and setup a seperate one. (Just being picky really)

Idle is now a lot more steady after using mixture map to develop a very high res idle map, which after some more testing I'll downsize to fewer rows and columns. So all is well with that.

The reason I ask about 4d and 5d fuel, is that mixture map is a very powerful tool for making an overlay compensation table that can spot trends on a table that has axes different to your main fuel map.

For example if your main fuel map is based on RPM and MGP.
Let say I fitted a surface mount thermistor to the intake manifold, and wanted to see what effect heat soak has.
I can setup a 4d table with the axes of thermistor temp and effective injector pulsewidth, and then run mixture map and it'll populate the table to show me if there are any trends visible by which it would make sense to establish a compensation table.
If not, I can change the axes on the 4d table and rerun mixture map and see if anything else shows a clear trend.
(This is a good method for finding slow changes like manifold temp which are hard to quantify otherwise)

However I want to be confident that if I set a more specific type of 4d or 5d fuel map that just addresses a particular zone or load area or whatever, it isnt extrapolating out unpredictably across the rest of the map...
Which you have answered for me. Thanks :)

So far I've seen some interesting results by disabling IAT compensation, and then setting a 4D table based on injector pulsewidth and ECT.
As the required compensation varies a lot based on pulsewidth as well as ECT.
I will post some results up later if interested.

Edited by Davidv
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I would agree with you in every way. Getting into factory engine calibration type equations, without the crazy algorithms we all wonder about. LOL so from what I am understanding is you want to better control pulsed width in certain conditions? Sounds like using a injector EQ table dependent upon heat soak. At which point I would put a temp sensor for fuel at the rail to try and determine injector tip heat soak. You could very well set another table using 5d and add this. Would be interesting to use as well as what you are testing.

I really like the way you are going about this. Post up some data or email me logs. [email protected] I would be very interested to see what you are coming up with.

Well done!



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What are you injectors like at low pulse widths? Could they be working in a non linear area?

Are you using a idle ignition table?

Also have you tried adjusting the charge temp compensation table instead of disabling IAT compensation?

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I built an injector test bench and worked out deadtimes, CC rating, minimum pulsewidth, and built a short pulsewidth adder table to compensate for the non linear section of flow. Then did the same again for outer injectors (its very important to how well staged injection works)

However it well idles above the non linear region.

Yes I've got an idle ignition table, this has helped.

I've turned off charge temp compensation, the reason being that all of these other compensation tables - you cant use mixture map on them.

However if I use a 2nd fuel table or 4d/5d map with the same axes as the normal comp tables, I can use mixturemap to figure out the values (Then I could transfer this back to the normal comp tables if I wanted)

Edited by Davidv
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If the numbers in the Fuel cells achieve the lambda target valves at normal operating temp i would of thought the issue lies with your idle solenoid and idle ignition advance settings. What idle speed and lambda are you trying to reach?

Also are you using closed loop idle or open loop? 



Edited by Ducie54
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I'm running e-throttle as idle control, and also have idle ignition timing target. Which I've just spent some time optimising over the last few days. Running closed loop.

I recently switched to modelled fuel so I've just been having a bit of a review of other things like idle etc too.

Since making the changes my idle is really good now, when you're deceleerating and there's higher vaccum than normal is when I used to have problems as the car would stall. But now that I've got a much finer map it was easy to see where the fuel was incorrect.

Fuel was the problem but now sorted. Now it idles at ~850rpm nicely only fluctuating by ~20rpm each way.

Here's the idle fuel map that I came up with.


Although, my wideband has just crapped itself AGAIN so I cant finish it off for now.

4 sensors, two controllers and two positions for wideband later and I'm still having issues with sensor life... This one (Bosh 4.9 with Aim LCU one) lasted just over two weeks.

Argh! Getting to wits end with the bloody things.

The only thing I can think of now, is that its getting thermal shock when the car first starts. Or the staged injection makes the exhaust sooty or something.



Edited by Davidv
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