I ran my car up and down the driveway yesterday. Temperature was in the upper 30's F. When I started driving it the coolant temp was 175*F and the manifold temp was 71*F. It idled great with a MAP of 39 and AFR of 12.4, with a VE of 46.

By the time I finished the coolant temp was 184*F, manifold temp was 132* F, the idle MAP was 45, and AFR was over 13, even after I raised the VE to 50. Idled like crap and stalled periodically.

I'm using the stock MAT sensor on an S5 intake housing. I'm really confidant that the air flowing through the intake manifold wasn't 132*. It seems to me that the engine heat makes that stock MAT sensor pretty worthless.

My use of this car will very likely be limited to operating in ambient air temperatures between 50* and 95*.

Here are the options I'm considering:

1. Disconnect the MAT sensor - should give a constant 100% air density factor.
2. Change the air density inc file to all 100% entries. Lets me see and log the MAT but not screw up the mixture with it.
3. Convert to use the air temp sensor in the Air Flow Meter. Seems like that would be far less subject to heat soak.

Any wisdom out there?
Thanks,

 

The only time I have ever been able to get heatsoak like that is when I've been sitting in one place for long periods of time. Driving up and down the driveway seems like a good example of that kind of driving.

There is some heat-soak compensation in the later megasquirt codes, but I've never used it, so I couldn't tell you if it works well or not. You may want to try it though.

I wouldn't suggest moving the air temperature sensor out further as you want the temperature to be as close to the temperature of the air going into the MS as possible.

How did you regenerate the inc files for the air temp sensor? Did you remember to use them with easytherm (or move them to the firmware directory and recompile, and upload the recompiled version)? I've heard of this happening too. MT will still show the right temp, but the megasquirt itself has the wrong air density at the ADC value that the air temperature sensor ADC is at.

Ken

 

Can you confirm one assumption I have - that the MAT temperature is only used to select the air density factor.

WRT the inc files, I generated them with EasyTherm, then moved them into the 029i src directory when I migrated to that version, and rebuilt it. I did miss the air density inc file in the process, but when I compare the standard inc file with the one from ET, they have the same air density factors at the ADCs that would have been in effect at those temps. I'll correct that, but I don't think it changes the issue.

I think that heatsoak is likely to be a real problem because this is a dedicated race car. It runs wide open 98% of the time, and cooling is a very big issue. Requires a large aftermarket radiator, multiple oil coolers, and special air flow management to keep the coolers cool. It gets HOT in that engine compartment. Gets hot in the passenger compartment, too. I think you could burn your hand on the transmission tunnel (exposed metal).

I agree that I want to measure the air temp as close as possible to where it enters the combustion chamber, but if the stock MAT sensor gives a false reading then its not helping me. I've also got rules issues - I can't modify or relocate it or add a different sensor. Since the AFM temp sensor is already in place I'm legal to use it. Since it's in the airstream that I want to measure, it will only be off by two factors:

1. Whatever heatsoak I get in the AFM - should be much less then sitting on top of the engine, especially if I put an insulating box around it and keep most of the air from the radiator away from it (this is legal).
2. The heating of the air on its way from the AFM to the combustion chamber. This may be somewhat compensated for by #1, the heatsoak in the AFM, and in any case should be far less then the error I'm seeing with the MAT sensor.

I've got a test day at the track coming up April 1. I'll find out then whether this needs fixing. In the mean time I'll test with the MAT disconnected to confirm that this will actually produce a constant 100% air density. If it gets hot at the track I'll pull the MAT plug.

Thanks,

 

Possibly, but the driveway is long enough that I could hit 8K RPM in 1st gear, so I had a lot of air movement.
I'll look for that. Is it in the o29i code?
The Gair values I see in the log correlate properly (in the inc file) with the temp values in MT and in the log.

 

You may try the Coolant corrected Air Density Correction feature in MSNSE.

You can set the air density correction for seven temperatures based on either CLT or MAT. I used MAT to help compensate for heatsoak. Make sure your last temp is real hi like 200f, there is a bug in the code that if the temp goes beyond the table, the Gair parameter gets set to zero (bad). By setting corrections you can make the fuel mixture leaner or richer as you need to.

There is a start and end correction based on RPM, I have mine pretty low at about 500 to 1200 rpm. This way it only affects idle and low speed driving.

It has seemed to work for me--especially when you park a hot car and it really heatsoaks bad. The only drawback I see is the table may need to be seasonally adusted since it gets really hot down here.

Here's what the menu looks like:



After work today I'll post up my settings, right now I don't have them.

Good Luck,

Scott

 

The air density inc file should be different from the standard one if you're using a non-standard air temperature sensor. I just checked my own inc files, one that's using the GM sensor, and one that's using the rx7 sensor, and the two files are significantly different.


I agree that heatsoak COULD be a real problem in your setup, but I'd suggest that before you change anything else, change the "easy" thing that we know is not set up right. If fixing that inc file doesn't change anything, then you can always switch over to the air temperature sensor in the air box (although I'm not sure what the scale is for that one).

You are correct in thinking that the air temperature sensor is only used for the air density factor. The temperature file is mostly so you can see it properly in megatune if I understand correctly.

Oh, and I know it's not doing you any good if it's incorrect, but if it's only getting "incorrect" on 8000 rpm wot blasts up your driveway, then I think you may be able to correct for it using the heat-soak compensation, as I doubt that the temperature it's reading is ALWAYS wrong... I still think that given a bit of time above the low revs it'll read correctly.

Ken

 

I would think during cruise conditions, your MAT would be close to ambient--assuming you're not boosting the hell out of it. Where are you getting the intake air from and do you have a CAI?

 

Well, the car never cruises - it only runs on the race track, and is usually at wide-open throttle.
No boost - NA.
The intake air comes through the stock snorkel with a K&N filter attached. The stock air box and the duct over the radiator are gone, so the air comes mostly from the engine compartment. I'll be working on improving that as time permits.

 

Very helpful info. Thank you all very much. I'll see what happens to the MAT on the track on the 1st.

 

For track applications I doubt you'll have any issues. In any case, you may want to try relocating the stock MAT sensor to the plastic elbow, or some other position further away from the heat of the engine. The goal is to measure the true air temperature as it enters the engine. The sensor in the intake manifold is likely more accurate for steady state, higher flow conditions, while the other MAT sensor in the AFM might be more suitable for hot-starts and idle. I'm just guessing here, but I'd suspect that's why there's two. It'd be interesting to know exactly what the stock ecu does with those two inputs...

 

Hope to find that out next week.
Cannot do that - those pesky rules.
If the MAT sensor is way off on the track, I'll see if I can get MS to use the AFM sensor. If that doesn't pan out, I'll use the Coolant Corrected Air Density.

Thanks,

 

yeah, I've had bad experience putting the sensor in the plastic elbow as well... since at least part of it sits right over the exhaust, I was getting heatsoak there that was even worse than any heatsoak I saw with the sensor in the stock location.

 

Good point! My n/a setup uses an adapted TII intake system that breaths from the driver's side, where it's away from the heat of the exhaust.

Eagle7, if you are up for some experimentation, you could try using the MAT sensor in the AFM. Unfortunately, the characteristic curve is quite different from the one in the dynamic chamber, so you'll need to swap back to the original bias resistor, and re-flash with updated thermistor tables. It'd be an interesting test, but not applicable to most users who switch to standalone in part so they can remove the AFM from the system.

 

Very nice of you to post the FSM info. Thanks. My plan is to wait until I can test on the track (April 1), and do the conversion if necessary after that.

 

I spent the day at the track, and for my setup heatsoak definitely is a problem. Ambient temp was in the upper 30's. I warmed up the car thoroughly before hitting the track, and the MAT read 110*F. After 20 minutes at speed it was down to 105*F. I pulled the cable off the sensor and stuck in a resistor that read 60*F and ran like that the rest of the day (with very good results). I'll be looking into converting to use the temp sensor in the AFM. If that also proves to be a problem I may end up with a rotary **** to select the ambient temperature for a session.