Relationship between AFR and IAT

[trakkies]

Is there a rule of thumb relationship between air fuel ratio and inlet air temperature? Like for every 10C increase in IAT, the AFR should increase by 0.5:1 or whatever? Just to clarify the last bit, say at 20C my mixture is 14.1:1 at 30C it would be 14.6:1. And so on.

[Matt Cramer]

When correctly tuned, the AFR should be the same at any normal IAT.

[DonTZ125]

Dave - I wonder if you're remembering carb tuning rules. Certainly, as you say, for my 2-stroke race bike I have to change main jet size every 3-4deg C in order to maintain the correct AFR. The entire point however behind the IAT sensor is that the MS corrects the amount of fuel needed to the air temp; for a given MAP, a higher IAT = fewer pounds of air => fewer pounds of fuel = same AFR.

[trakkies]

When correctly tuned, the AFR should be the same at any normal IAT.

As shown by a wideband lambda sensor? If so I have a problem. ;-)
I've never altered the IAT correction - it's still the default.

[Matt Cramer]

As shown by a wideband lambda sensor? If so I have a problem. ;-)
I've never altered the IAT correction - it's still the default.

That is correct. If your AFR is changing with IATs, something is wrong with the IAT correction.

[grippo]

The default afr is based on the air density vs temperature equation. It is correct as long as the air temperature at the IAT sensor is equal to the temperature of the air going into the cylinder on the intake stroke. Since a sensor for each cylinder, placed in the cylinder port, would be required to get this equality, there can be some error in using a single sensor. Generally what happens is that the sensor sucks up the heat from the manifold and is not representative of the air going into the cylinder, which is moving too fast to soak up manifold heat. To correct for this, use the IAT correction table which adds a correction to the base air density equation.

[trakkies]

My RV8 originally had flapper injection with the IAT sensor in the flapper unit. I've replaced the flapper with a bit of plain ally tube, and mounted the IAT sensor in that. When idling in a traffic jam etc, the IAT sensor reports a large rise in temperature - way above ambient. The inlet trunking draws air from the bottom front of the engine bay.

My 'normal' idle AFR with the engine at normal temp is the factory spec - 14.1:1 But as the temperature increases, this goes to about 15.3:1 at worst. So do I just alter the IAT correction to get back to 14.1:1?

It's only a minor problem - what happens is a tendency to stall when going from drive to reverse on the auto box when it's very hot. There is a factory extra air valve (solenoid) operated by the gearbox selector which increases the idle speed over neutral when the box is in drive or reverse. This will close and open when going through neutral and seems to confuse something. It's not helped by my liking a slow idle - I've got it to be reliable at 500 rpm rather than the factory 800.

['66fb]

My 'normal' idle AFR with the engine at normal temp is the factory spec - 14.1:1 But as the temperature increases, this goes to about 15.3:1 at worst. So do I just alter the IAT correction to get back to 14.1:1?

trakkies,

I've been working on adjusting the IAT correction also. What I've come up with so far (and I may be way out in left field) is to use the stimulator and adjust the IAT correction at the the different temp points so GammaE is zero, that is there is no correction for IAT. Then I'll run the engine (out of WUE) at idle and adjust the VE at that point to get the AFR required. This will essentially be my "zero" point temp. Thereafter as the IAT changes at idle (causing AFR to change) I'll adjust the IAT correction table to give the correct AFR.

Am I making a mountain out of a mole hill?

I'll only be making changes at idle and AFAIK this should correct the entire table at that temp.