Hot restart issues, solved.

[bartmanftw]

From what I understand the OEM ECU doesn't use the air temperature for fueling in the same way since it uses a AFM/MAF to calculate mass air flow into the engine (air density is accounted for).

I definitely think fuel temperature is a big factor with hot restarts as well. I'm using a fuel temperature sensor and adjusting my fueling based on fuel density vs temperature as well as an air temperature correction curve that closely follows the ideal gas law curve. Hot starts are quite good even with the air temperature sensor located inside the engine bay just before the throttle body.

I did find that when using the suggested air temp correction curve on miataturbo as well as fuel temperature correction my AFRs would go quite rich at high (+40) air temps.

[NitroDann]

I believe that the typical IAT curve is rich at high temps to mitigate detonation.

Dann

[Red_Bullet]

I believe that the typical IAT curve is rich at high temps to mitigate detonation.

Dann

Are you are referring to the miata turbo style curve rather than the ideal gas law Dann? Detonation mitigation is something the "gurus" never mentioned but it does make good sense.

[bartmanftw]

I believe that the typical IAT curve is rich at high temps to mitigate detonation.

Dann

Yeah that's true however I think it would be better if there was a setting to richen the AFR targets at certain points vs IATs. I'm reducing timing a bit with high IATs anyway and am using the "high power time enrichment" to richen AFR targets under sustained load as well.
Doing it with the IAT curve would throw out your VE table if you made adjustments based on the AFRs you're seeing with high IATs where the curve flattens out.

[NitroDann]

Do you mean using long term trim?

Dann

[bartmanftw]

Nah, there's a feature in ms3 that allows you to lower your AFR target vs time spent above 'x' manifold pressure and 'x' rpm for 'x' seconds.
Not something you can do with ms2 so you'd be stuck having to modify the IAT density curve to do richen it up at high air temps although not quite the same thing.

[NitroDann]

"Doing it with the IAT curve would throw out your VE table if you made adjustments based on the AFRs you're seeing with high IATs where the curve flattens out."

When you say "made adjustments" do you mean with long term trim?

Dann

[bartmanftw]

Ah sorry my mistake. Yeah that would be one example. Would also happen if using the autotune feature or even if you adjust the VE table by hand to hit AFR targets unless you account for the extra fuel added by IAT.

Anyway back on topic. Moving the air temp sensor outside of he engine bay is a great idea to reduce sensor body heat soak. I wonder if there is an open element plastic body air temp sensor that would be suitable to use since it should be even less likely to heat soak.

[bartmanftw]

This Motorsport air temp sensor would be a bit better than the standard GM sensor since it's smaller in size. I think you mentioned preferring these Dann.



There's also this Bosch one that has a plastic body however it looks like it would be a pain to mount in comparison.

[NitroDann]

An intercooler which is not heat soaked is very good at keeping IAT within 10*c of ambient. Heat soak sensor readings are typically 20-30*c higher and therefor the IAT curve matching ideal gas law tapering to rich at the very high end is a good workaround in my opinion.

Dann

[NitroDann]

An intercooler which is not heat soaked is very good at keeping IAT within 10*c of ambient. Heat soak sensor readings are typically 20-30*c higher and therefor the IAT curve matching ideal gas law tapering to rich at the very high end is a good workaround in my opinion.

Dann

[Red_Bullet]

Here is another theory from Miata Turbo. It's to do with injector heatsoak. At 30 degrees C the injector compared to 100 degrees C. At 100 it takes 100uS longer to open than at 30 degrees C. This makes the actual deadtime longer hence cutting total fuel supply by around 10% at idle. Opening time increases because of the increased resistance of the injector coil. Apparently it goes from 15 ohms at 30C to 19 ohms at 100C.

This might explain at least some part of the hot start issues.

https://www.miataturbo.net/ecus-tuning-54/injector-current-trace-shows-slower-opening-when-hot-67534/

[NitroDann]

No shadow of a doubt but we cant solve that any easy way.

Dann

[NitroDann]

Well, 95% of us anyway, most ecus we run here probably cant deal with it.

Dann

[Red_Bullet]

Maybe just try to ASE our way out of it, more fuel and longer taper maybe. No doubt though you've already tried that.