air temp sensor location

[quickstrike]

First off sorry to keep posting so many times here, but I find this site very useful in my search for a faster timeslip. I hope someone has some questions I can help with in the future.

A previous post of mine was discussing my high VE numbers(120).
I just read something in the archives that may help. My ACT or IAT sensor is installed on the bottom side of the intake in the plenum area. If this was seeing hotter than normal temps because of heat rising all around the lower intake, and not reading the actual air temp inside, could this cause the ECU to try and lower the pulse width of the injectors, thereby causing me to have to add higher VE #s to compensate?

My temps are around 180 and up during a run, and I am only making half of my 14psi normal boost.
Maybe moving it to somewhere in the supercharger discharge tube would be more of an accurate representation of input air temp. Could this be causing my high VE#s?

Randy

 

[JDEstill]

Randy -

1. At 7 psi boost, assuming an 80 F day, 180 F air temperature in the intake manifold sounds spot on. I think that is a very believable number.

2. With your VE needing to be so high, this implies that the air flow being calculated is higher than what is realistic. Higher air flow means higher air density, which means colder air temperature than what is being measured. To make a 25% difference in air flow (the difference between 120% VE and 90% VE) would require a 25% drop in absolute air temperature. With 180F + 460 = 640 R temperature in the intake, to make that much of a difference would mean that the actual air temperature would have to be 480 R - 460 = 20 F. That doesn't sound feasible I doubt that you have a 20 F air temperature in the intake.

While placement of the air temp sensor is important, it doesn't sound to me like this is the root cause of your VE being so high.

John Estill

 

[quickstrike]

I Dont quite undertsand the 25% more airflow equating to 25% temp difference. For 25% more air to be in the engine and fooling the VE formula, what temp difference would it take to change the density of the air that much? if for instance the temps are reading about 20 degrees F off from what they really are, is this enough to change the ECU thinking about how much air is in the engine? Which in turn would trickle down to needing more fuel or VE to keep it at 12-1. HHHHmmmmmmmm...............I am not sure.

Either way if you and others think I should move the temp sensor it wouldnt be that hard. Just wondered if anyone else has run into this.

Thanks

Randy

 

[TurboJim]

Its both my understanding and what I've been told that you want the IAT to sample charge air temp, NOT ambient temp in a speed density calculation.

Where you have it now is optimal. If the air entering the cylinder is 180 or whatever, the ECU will calculate the prolper fuel amount for the AIR DENSITY in the cylinder..

If you'll notice, most speed density systems in OE aplications have the air temp sensor in the intake plenum. Cars that use speed density to COMPARE to a MAF reading may have the air temp sensor in the air intake (GM, Caddy, Toyota, Niissan etc)

 

[quickstrike]

Sounds reasonable to me.

I was just trying to figure out some high VE numbers and thought of that as a possiblity. Thinking maybe the heat soak from the eng itself was cusing them to be high.
Tonight at the track iat was 145 in staging lanes at idle and climbed to 210 at the finish line. Car is non intercooled and blower is getting out of its efficiency range tho.

Thanks for the help guys
I dont always get this much help from the FORD guys on other websites!

Randy

 

[Craig Smith]

From what I have seen, I believe an air temp sensor in the plenum will read too hot. I think it heat soaks in that location. At least sometimes...

We had one customer who had a sensor in his plenum and was seeing VERY high air temps. Backing the sensor up about 12" in the intake tract reduced the air temp reading about 70 degrees. I can't believe that while running, the air actually picks up that much temp from the front of the plenum to the back.

Just my opinion... bear in mind that this opinion is from a naturally aspirated idiot.

 

[TurboJim]

Craig:

Please understand I'm not trying to be smart, but. Dont we really want to fuel for the air thats entering the cylinder, and not the coming into the intake. Given that, wouldnt it be better to sample real world charge air, in an intake runner or the plenum? As for heat soak, I guess thats possible, but the heat transferring to the sensor from the metal is also transferring to the air flowing by isnt it?

I'd ASSume, if in the intake at X degrees, air density is a certain amount, (John Estil or Carl Ijames would haveta do the math, I'm doing more theoretics) if we fuel for that amount, when the air actually entering the cylinder is hotter and of a different density, our calc would be at LEAST a little off.

Is it enough to worry about? Probably not, but if were looking for that last bit of accuracy, isnt it important that we sample everything correctly?

If you insist its better to sample a bit away from the intake, I may rethink my placement. BTW, no, I'm not talking about a 7 sec race car, I'm talking about a street car.

Let me know your thoughts please.

 

[Craig Smith]

My thought is that the sensor is always there, and will get hot after a little while. The air is only there for a split second, and I don't know for sure, but I wouldn't guess that the air is THAT much hotter than it is 15 inches up the pipe. Just a theory.

 

[TurboJim]

OK, a good point. You think the intake metal gets hotter than the air in a turbocharged app (as in mine) ? I'm seeing people speak of 180*+ charge air temps. Think the metal gets hotter than that (assuming a 160 stat) and could cause problems with the calc?

 

[JDEstill]

Originally posted by quickstrike
I Dont quite undertsand the 25% more airflow equating to 25% temp difference. For 25% more air to be in the engine and fooling the VE formula, what temp difference would it take to change the density of the air that much? if for instance the temps are reading about 20 degrees F off from what they really are, is this enough to change the ECU thinking about how much air is in the engine?

Randy - sorry my explanation was a little obtuse...

Hopefully this will be a little clearer:
FAST calculates the air flow by the displacement of the engine, the rpm, the charge air density, and the VE. The VE is just a fudge factor.

Assuming you have the displacement entered correctly, and the rpm reading is correct, then to get fueled properly (which it must be if the O2 correction is close to zero) if the VE is 25% too high then the calculated air density must be 25% too low so the errors cancel each other out.

For the air density to be 25% off due to a bad temperature measurement, then the measured air temperature has to be 25% off, on an absolute basis (ie deg Rankine or Kelvin, NOT Fahrenheit or Celsius).

If you are reading 180F now, which is 640R, then 25% off would be 480R, or 20 F, and it is doubtful that your intake is at 20 deg F. But that is what it would take to force the VE to go from 90% to 120% so that your fueling is correct.

In other words, the air flow is the same with VE = 90% and air temp = 20 F as it is with VE = 120% and air temp = 180 F (everything else being the same).

You know the old saying about 10F = 1% hp? This is where that comes from. Going from 180F to 170F raises your air density by about (180+460)/(170+460) = 1.016 = 1.6%

That 1.6% increase in air density means a 1.6% in air flow, which should translate into about 1.6% more power, all else being equal. (Since all else usually isn't equal, rounding down to 1% more hp works pretty good.) Looking at another way, from the FAST view point, if your air temp reading was 10F off, then your VE would only have to increase by 1.6% to balance things back out and get your O2 correction to go to zero.

So, while you should try to get the best air temp reading you can, the VE "fudge factor" will cover up a fair bit of error (esp if you are running closed loop), and it's going to take a heck of an error in the reading to force your VE's to go off by 25%.

John

 

[bruce]

You want the sensor's tip in the actual airflow.
If you mount it in a stagnant flow air them it might be SLOW in responding to actual temp changes.

The Thermistor in the sensor is well isolated from the body and not prone to heat soaking problems. I've tried several sifferent ways of mounting it, and the results were consistant.

Close to the intake valve, and in a high flow area.

Oil temp play a min role (contrary to rumors to the otherwise), I've played the game with oil, air gaps etc...

On a hot soak or in stop and go traffic you can get into the 130s for MAT. That's the primary function of the I/C fan, is to pump enough cool air thru the system to drop the MAT. That way you also have a larger reserve of heat sinking available from the I/C.

 

[TurboTR]

re:

Bravo John, bravo!

TurboTR

 

[TurboJim]

SO, this means it really doesnt matter where you put the MAT, so long as you cover up the faulty reading with the VE trims? If you sample outside air, just cover it up with the VE trim? If you sample air from the actual intake charge, you just gotta do less "fudging"?

I kinda would like a definitive suggestion before I hack a hole in my plenum.... because now I'm confused.

 

[bruce]

Originally posted by TurboJim
SO, this means it really doesnt matter where you put the MAT, so long as you cover up the faulty reading with the VE trims? If you sample outside air, just cover it up with the VE trim? If you sample air from the actual intake charge, you just gotta do less "fudging"?
I kinda would like a definitive suggestion before I hack a hole in my plenum.... because now I'm confused.

Put it in the upper plenum, just downstream of the TB. There aint alot of meat to tap so be careful.

The most critical part of this is for a timing correction. Note clever word useage....

Might just take a look at the MAT timng and CTS in the GN code.

The CTS is used at airflows below LV8 128, and the MAT from LV8 128 to 255. Looking at a N/A 148'd car will let you see the air temp bias, and what they do with the timing. Better yet look at the bin for a late model S/C pontiac and see what they are doing.

You want MAT for accurate Air computations, and for TIMING CORRECTIONS.

Putting the sensor anywhere but as close as possible to the intake valve, will generate problems. Remember the I/C can heat soak, and also be artificially raising the MAT.

 

[JDEstill]

Originally posted by TurboJim
SO, this means it really doesnt matter where you put the MAT, so long as you cover up the faulty reading with the VE trims? If you sample outside air, just cover it up with the VE trim? If you sample air from the actual intake charge, you just gotta do less "fudging"?

I kinda would like a definitive suggestion before I hack a hole in my plenum.... because now I'm confused.

Jim -

I'm not suggesting that at all! Definitely put it downstream of the IC, either in the piping or in the intake manifold. You still want it to be as accurate as possible, no doubt about that. If you left the MAT in the stock location you COULD cover up the problem by tuning the VE table, esp if you were running in closed loop all the time, but that is not the way you would want to run unless you just had no other choice.

I know of some local FAST installs that were done that way (and by people that should have known better), with the MAT in the stock location. They seemed to cope with it okay, but still, that is NOT the Right Way to do it.

I'm just saying that some error can be tolerated by tuning the VE, and I don't think the air temp measurement is the source of Randy's problem. But you still want to minimize the errors, and get the air density calculated as accurately as you can, and that means measuring MAP and MAT in the intake manifold.

John

 

[TurboTR]

A good compromise spot for it IMO is in the up pipe, you can just put a bung in the pipe and screw it in.

TurboTR

 

[quickstrike]

Thanks everyone for the responses. I may move it to the neck of the intake next time car is apart, if for no other reason than to see if there is a noticeable difference in air charge temps.

JDEstill
Thanks for the clarification I understand what you mean about the 25%.
Thank you.

Just wanting to learn as much as I can about this system(I am very anal like that). I am a logic person, and it is hard to accept that to identical size motors use different VE numbers. Doesnt porting of heads, combustion chamber size, and even dished pistons effect the volume of air that a given motor can ingest? Maybe this is an explanation to the desparity in VE numbers.

Oh well
Thanks again for the replys all.

Randy

 

[TurboJim]

Originally posted by bruce


Put it in the upper plenum, just downstream of the TB. There aint alot of meat to tap so be careful.

Gonna weld in a 3/4" aluminum bung

The most critical part of this is for a timing correction. Note clever word useage....

Might just take a look at the MAT timng and CTS in the GN code.

You want MAT for accurate Air computations, and for TIMING CORRECTIONS.

Is there timing correction built into the FAST? Or is it only used for the Speed Density calc?

Putting the sensor anywhere but as close as possible to the intake valve, will generate problems. Remember the I/C can heat soak, and also be artificially raising the MAT.

Thats kinda what I thought. You want the ECU to fuel for what is entering the CYLINDER, not the intake.