IAT and outside air temp

[Noddle]

Hi,

 

If I tune my car at a IAT of 17 to 19 degree C (62.6F to 66.2F),

what happens to the AFR if the air temperature goes up, i.e., does it run richer because the air is less dense, thus less oxygen, or leaner ?

 

this is in a N/A application

 

Nigel

[RTz]

If no air temp compensation were being used, an increase in air temp will richen the mixture.

[Zmanco]

MS calculates how much fuel is required before every squirt using the ideal gas law which takes into account the temperate of the incoming air. Hence in theory it will lean out the mixture as the air temp goes up, all else being equal.

 

In practice there are a host of issues that can cause the IAT sensor to heat soak to some degree so the system tends to be less accurate at low air speeds (light throttle) and more accurate at WOT (where the air flow is greatest). If you're interested, search on IAT in the MS section and you'll find many discussions on the best place to mount the sensor etc.

[Noddle]

MS calculates how much fuel is required before every squirt using the ideal gas law which takes into account the temperate of the incoming air. Hence in theory it will lean out the mixture as the air temp goes up, all else being equal.

 

This is the opposite to what rontyler said

 

If no air temp compensation were being used, an increase in air temp will richen the mixture.

 

In practice there are a host of issues that can cause the IAT sensor to heat soak to some degree so the system tends to be less accurate at low air speeds (light throttle) and more accurate at WOT (where the air flow is greatest). If you're interested, search on IAT in the MS section and you'll find many discussions on the best place to mount the sensor etc.

I'm not concerned with heat soak, the issue I have, is that in morning the air is colder, afternoons it's hotter, and evenings it's colder again, the temperature can change 15 degrees C, in a day, and it's coming up to summer..

I have turned my MS early in the day, and will use IAT fuel correction to adjust for the warmer temperature.

 

Nigel

[RTz]

This is the opposite to what rontyler said

 

No, we're agreeing with each other. If there were no correction, the AFR would go rich. Thats what I said. What Zmanco is saying is that the system, through correction, reduces pulse widths so that it doesn't go richer. The goal is to keep the same AFR.

[Noddle]

I think this should work,

 

as the IAT temperature goes up, I reduce the the fuel correction, thus leaning it out..

 

I assume this is correct ?

 

Nigel

[Zmanco]

Noddle, please re-read Ron's and my previous posts. MS will take the change of temperature from morning to afternoon (and summer to winter as well) into account and automatically adjust the amount of fuel delivered without needing the IAT correction table.

 

The formula it uses to calculate how long to fire the injectors is the ideal gas law: PV = nRT where T = IAT and n = the amount of air in the cylinder. Check out wikipedia to learn more: http://en.wikipedia.org/wiki/Ideal_gas_law. You should also read some of the MS manual as they do a good job of explaining this as well.

 

Note to the MS purists: I'm leaving off the affect of VE and other inefficiencies here on purpose.

 

You can see that as T goes up n must go down if all else remains unchanged. You do not need to use the IAT correction table to achieve this. As Ron and I said earlier, this is already built into the way MS calculates how long to fire the injectors before each squirt.

 

My engine is NA like yours and I do use the IAT correction table to take into account heat soak of the IAT sensor when idling in summer traffic. But it doesn't even begin to kick in until 110F at which point it's only +1%, and then it grows slowly from there. And when I tune, I set those values back to zero to make sure they're not having any effect. Otherwise you'll find yourself tuning AROUND the IAT table which is NOT what you want.

[cygnusx1]

Maybe unrelated but a tidbit of info...

 

When you hot start a Z (even in colder weather) typically it will run lean for a few minutes if the fuel rail has heat soaked. Using the IAT tables. What I do is set about a 15% correction enrichment on the IAT for IAT temps of 180F and then set 0% correction at around 70F. This helps smooth out the transition from heat soaked hot start and "somewhat" compensates for a hot fuel rail until the temps stabilize to normal.

[Noddle]

Thank you for your help,

 

and I'm sorry to sound like a dick..

 

Nigel

[Zmanco]

No problem Nigel, just wanted to make sure you weren't headed in the wrong direction with your tuning strategy.

 

Dave, I've noticed the lean condition at start when the engine is warm and your explanation of warm fuel in the rail makes sense. With the MSII/extra code there's pretty fine grain control of ASE so I give it about 5 seconds (assuming an idle of 850 rpm) of +10% which seems to help.