IAT correction/abient temps. How are you dealing with it? - Miata Turbo Forum - Boost cars, acquire cats.

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Old 06-29-2010, 03:17 PM   #1
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Default IAT correction/abient temps. How are you dealing with it?

My MS1 VE table doesn't seem to like even a small change in ambient temps resulting in running lean when ambient temps drop.

At the moment my IAT sensor is just loose behind the headlamp so not exactly hot or prone to heatsoak, it pretty much just tells me ambient air temps.

The table on the right was tuned using autotune and dialled itself in over the course of an hour of mixed driving with IATs of ~25-30degC. The engine felt super responsive and target afrs were 99% perfect at the end of the tuning session.
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The next morning ambient temps have dropped to 18degC and the idle was really lean 16.5:1. I couldn't drive the car as afrs were too lean so I had to wait for the engine to warm up and start auto-tuning again which gave me the table on the left which also felt great.

IAT correction is a fudge. I can have great hot-starting by upping the warmer regions by a few percent but If I tune at a warm ambient temp region, I then need to up the colder regions of IAT correction to accommodate for dense air.

My injectors are RX8 425cc high imp. Dead time is set to 1.0ms. I cant for the life of me find correct listings for the dead time. I've tried to tune dead time by switching from 2 squirts to 4, and also 1.2ms dead time 2 squirts to 4 and each time the afrs at idle run completely lean and the engine stalls.

Could injector dead time be the sole reason for my temp issues? I've also been advised to try running a dead time of 1.6ms or higher and then switch to 4 squirts to see if I can get the engine to still run and tune the 2 to 4 squirts effectively but I haven't tried that yet.

Advice anyone?

Last edited by CRAIGO; 06-29-2010 at 03:32 PM.
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Old 06-29-2010, 03:59 PM   #2
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Could a contributing factor be that your not actually seeing the true IATs that the engine is seeing?

Think about it, the IAT is seeing cold air yet the engine is getting it x deg higher, then the ambient temps change which exasperate the problem.
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Old 06-29-2010, 04:35 PM   #3
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Good point simply put Rich, but equally, couldn't I say hit 3000rpm at 75kpa and ambient temps be 20degC lower for instance? I'd still have a similar situation on my hands wouldn't I?

I'm going to get the sensor fitted into the FMIC this weekend. It has to be only variable that differs from most set-ups. If it fixes things, then that's great. I've only really had issues with this since our Summer came along.
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Old 06-29-2010, 05:33 PM   #4
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Dood, if you have an intercooler then the IAT needs to be in the intake piping between the IC and the throttle body. Having it sit loose behind the headlamp is not accurate enough and that is almost certainly the cause of your problem. You can't just have your IAT sitting in what you think is ambient air (which it probably isn't anyway). How hot it is that day, how fast the car is moving, and other factors will affect the magnitude of the discrepancy between what the IAT sensor sees and what is actually going into the engine. Fix that ****, put the sensor in the right place and don't bother messing with your tune until that's done because at this point you're dealing with a moving target.

Also, 4 squirts isn't helping anything. With 4 squirts instead of 2, the injectors are opening & closing twice as often. At idle with larger injectors, that means your open time isn't very long relative to the dead time. Especially with the lower fueling and rpm resolution of MS1, this is going to affect fueling accuracy at those low pulse widths and your idle will suffer. So stay with 2 squirts until you go to sequential injection.
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Old 06-29-2010, 06:09 PM   #5
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I have the same injectors and use 0.92 dead time. Try switching from 1 simultaneous to 2 alternating, for me it works. When I switch to 4, I have the same result, idle just drops until dead. Maybe the squirts get too short to get decent idle. I'm running full sequential btw.
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Old 06-29-2010, 06:29 PM   #6
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I feel like a right **** end now...

I've got a 3/8npt tap coming my way but after seeing some of the IAT location threads, it seems the side of the intercooler outlet wall was the best trade off between not getting the sensor heatsoaked and accurate temps? I'm thinking there is also more "meat" in the wall of the TDR intercooler outlet than that of the thin TDR intercooler pipes. (on the intercooler outlet to intake manifold inlet)
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