Best IAT sensor position.

[DaveEFI]

Have been reading quite a bit about hot start and heatsoak problems.

My IAT sensor is where it was on the original factory injection with AFM - about half way along the trunking between the TB and air filter. The trunking is about 2ft long. I tried one in the air filter housing - and that made matters worse.

So where would be the ideal position?

[whittlebeast]

When mounting a MAT, you have to keep in mind that the temperature of the intake air is really with respect to absolute 0. That works out to -460 degrees F. It we are talking about a 100 degree F MAT compared to a 130 degree MAT (460+100)/(460+130) or about a 5% change in fueling.

No matter where you mount the MAT sensor, there are down sides. Here is a pic of my NA race car. I elected to mount my MAT very near the air filter but on the firewall.

http://www.nbs-stl.com/CRX/CRX%20Engine.jpg

The positives is the sensor can never break and wind up in the motor. The sensor is not getting into the air flow of the intake track and causing turbulence. The sensor can not pickup heat from the intake mass. The down side is it will not pick up cooling from the fuel evaporation or PV=NRT issues with the pressure changes in the intake.

Keep in mind that that is not a valid option on turbo installs. There is simply too much temperature rise caused by the turbos. I have seen some really huge temps on a twin turbo Ford V6 in a Miata with no intercoolers.

Andy

[72bluetr6]

Good question. I mulled over this, too. The fueling calculation is based on grams of air to grams of fuel. So ideally, one would want the air temp sensor to measure air temp just before compression. Impractical, but some engine developers have installed sensors right next to the intake valve to get more precise data. But in general, you want the air temp sensor to be as close as practical for more precise measurements. In my case, I have the air filter, followed by about 3" of throttle body and supercharger spacers, then about 6" of air box to where the MAT is located. After this about 9" of intake manifold to get to the intake valve. I do not have a cross flow head, so the exhaust manifold is just below the intake manifold and the air box. Not a good setup for keeping intake air temps close to ambient air temperature. (Andy has a nice setup..cross flow head, too) My MATs mainly depend on whether or not the supercharger is providing boost, acceleration or deceleration (a big difference in exhaust gas temps), air flow into the engine and car speed (lots of heat build up at idle from the exhaust manifold, lots of cooling air while freeway driving coming into the engine compartment cooling down components). I have recorded air temps 50F higher than CLT temperatures and 70F over ambient (accelerating under boost on a 100F day). Some V-8s with intakes on top of the block between the V can have intake air temps around 180F! But also consider, if you do your tuning with the air temp sensor at the air filter, your VE table will take into account the average temperature rise - because you tune with respect to the O2 sensor. If you move your air temp sensor toward the intake valve - after another 15" of intake tubing, for example, the sensor will be reading higher temps and you will have to retune your VE to take this into account. (Using Andys example, in general, 5% less fuel for a 30F rise in air temps) If you have a cross flow head and a well ventilated engine compartment, you would expect much less air temp rise between the air filter and the intake valve as the air would stay closer to engine compartment temperature. I consider hot starts and heat soak as different, more temporary issues. Also, there are fast acting air temperature sensors, needed for forced induction installs if you are going this route, as the air temperatures can fluctuate rapidly. In any case, you will have a block/head heated to at least coolant temperature and this will warm up incoming air to a certain degree. And your radiator will pull in air into the engine compartment that has taken a lot of heat out of the coolant. My thoughts anyway .......

[Gazbon008]

This is an interesting subject

Seems to me that latter model cars have a plastic sensor for the IAT somewhere in the plastic early in the induction
as opposed to the brass style sensor screwed directly into the intake manifold.

Seeing as we are striving to bring older motors into the present best practices - or we would just be screwing on a carby - is that not why we are tinkering around with EFI
Should we not follow latest trends regarding IAT style and location

So the question needs to be asked "Why do car makers use plastic IAT sensors" - is it just cost or do they bring a better result?

The problem I see with having a brass IAT sensor screwed into the manifold is that at low revs (small airflow) they could be influenced by heat soak - giving a higher reading than the actual air being received by the cylinders.
This I see as bad - even if it is just a temporary event until the air flow increases - because as I understand it - Hotter air is thinner; so the MegaSquirt would Lean the mixture in response.
If the air is actually denser than the Ms thinks would result a Lean Mixture being delivered to the cylinders.

A plastic sensor reading the Air Temp in the early parts of the induction may read lower than the actual temperature arriving at the cylinders - again because of heat soak - the air picks up some heat from the induction manifold
But this would cause the MegaSquirt to make the mixture more rich.

So which is the lesser of two evils - I think it is safer to have a slightly richer mixture than to risk leaning out.

Please correct me if you can see a hole in my logic


Of course the above applies to Non-blown motors - It would be rather undesirable in a blown installation to pick up IAT before all the compressing (which raises Temperature) and any intercooling has happened