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Compensation Tables for air temperature

EFI Tuning Fundamentals

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Hi all,

I have just watched the air density vs. air temperature video. Andre recommends adding or subtracting 3 percent fuel every 10 degrees from a certain "zero point". So far it is understandable. But what happens at the same moment with my ignition table? If I subtract 5% fuel, should I take the ignition back? And if so, how much? I am running a rotary engine and I do not want to have too lean a lambda. I have a FuelTech FT500 and here I can also set the ignition for intake air and engine temperature. Thanks for your input.

While I’m learning too, I feel I can give some answers to your questions and also in a sense check on my knowledge and see if I’m understanding it as well.

So to answer your question, I personally wouldn’t touch the ignition unless there was an issue with knock. If you made it leaner by the 3% and it didn’t introduce knock then personally I’d leave it. Now if it did, I would just retard 2 degrees, retest and check the datalog.

All while if you added 3% you may be able to add another degree or two of timing if you’re not at MBT already and there’s no signs of knock.

hopefully this helps,


Hi Damen, thanks for your answer. I think similar to you and just wanted to have a confirmation. I can also imagine that my engine runs too rich again if I reduce the ignition in the same amount as the fuel.

In preparation for my day on the Dyno, I've set my zero point to 20 and 30 degrees as the car runs good at these points regarding our logs.

I only changed the -10 degrees and the 80 degrees cells and let the ecu interpolate. So it is more or less 1,5% of fuel. This should be safe i hope.

We are switching from E85 to 100% methanol and I'm looking for every input I can get.

Thank you !

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Hey guys.

You're on the money pretty much Damen, but you want to be careful with the term 'made it leaner'. We're pulling a bit of fuel out as the IAT rises to compensate for the lower density of the air at this temperature, this actually keeps the air to fuel mass ratio the same, letting us hit out AFR targets. We're putting in less fuel, but there are also less air molecules to go with it, so overall our AFR stays the same.

Marc, letting the ECU interpolate is fine, as for our working range, air density vs temperature is reasonably linear :-).

With the ignition timing, its common to have a compensation table setup that will pull some timing as the air temp rises. Detonation becomes much more likely with high intake temps, the charge has more energy overall, so it more likely to combust in an uncontrolled way.

You find you have to mess with IAT vs Fuel compensation much less when using an ecu that uses a modeled fueling equation setup, as its all built into the background math. Can get awesome results with a millisecond based ecu too though, just requires a good understanding of whats going on, which it seems like you've both got!

Hey Zac, thanks for your input. Detonation is always a big problem with a rotary engine. We tried a knock sensor ( with headphones ) but we could not tell if the engine knocked or not. Unfortunately our ECU has no way to record the knock or i just have not found it yet :-).

I will reduce a bit of ignition and then test it on the dyno when I finally have access.

Zac, thank you for clarifying that even more. Now I understand why he said to pull or add 3% fuel for every 10 degrees, is to keep the measured AFR the same SO it doesn’t go lean or rich due to higher IAT (or lower). Now it’s starting to come together. I just started the lessons three weeks ago never having tuning knowledge before and just got done with the practical reflashing tuning course. I think what this means is, I’m going to go back through everything again that I’ve learned so far to make 100% sure I have a true understanding. Thank you again for sharing your knowledge!

Best Regards,


Damen, That's what we're here for! :-) And the courses are yours now, forever, so watch them over, and over, and over. Pro-tip; install the Vimeo Speed and Repeat plug in for chrome and re-watch them at 1.5x speed. Saves time, and since you've already seen it once, you wont miss anything :-).

Marc - Its the curse of rotary tuning. You need to stay a reasonable level below the knock threthold, as even a couple of knock events can seriously damage the engine... But you need to make it knock to find the level to stay below.... Makes tuning a rotary out at the pointy end of the field (like yours!) pretty tricky.

Re-reading my above post, I should clarify that you absolutely should NOT make the engine knock to find the limit... as it'll end badly.

I thought with higher intake temps we wanted a little more fuel for cooling ?

Higher intake temps you may want a richer target AFR. However, to achieve a given target AFR you need less fuel when the air is hot, because the air is less dense.

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