This is the 6th thread and hopefully final thread in a series of discovery:
- I was so wrong for so long
- Thoughts on Cold Air Intakes and temp sensor relocation
- Instrumented and ready to test
- Day 1 Heat Tests - Fascinating
- Day 2 Heat Tests - Some confirmation
- Days 3&4 - Final Heat Tests, Conclusions and Recommendations
This is the last of my reports on the temp testing. I wish I could have left the sensors in and measured cold day data, intake plenum temps, and much more. But what I've done has been very illuminating to me, and hopefully to you. Importantly, I've reversed myself on a few previous positions - first, I always thought the OEM intake setup drew cold air, it doesn't. Second, I questioned the value of insulation of the intake plenum back when it was discussed a year or more ago. I now believe insulation of the entire intake tract is a worthy endeavor.
My lasts tests were conducted on a 2-day trip up to the wine country. It was 103 in Healdsburg on Saturday, about 97 on Sunday. In other words, HOT!
What's new in these tests?
In addition to testing in even hotter temperatures:
1) I used a video camera to log the results. On Test Day 3 I hand-held it - not a great idea.
2) On Test Day 4 I moved Probe 1 from outside the MAF to a pre-radiator location very close to where the UF Motorsport sensor goes. This gave me a known, calibrated outside air temp reference. I also moved the displays up on to the dash and mounted the video camera. I had hoped for you to be able to see the thermometers and an out-the-window view, but the exposure control was impossible, so I am not planning to post much video.
I took a pic of the relocated sensor, and I grabbed a series of frames from both days' video to show some "typical" temps - the page is here.
3) On Day 4 I went to our famed private race track, Stewarts Point Road. I wanted to see how sustained hard driving impacted temps. It WAS kinda fun - so I did put a little video (12Mb) of some driving (and me taking to myself out loud, as usual) together for you here. (right click and save.)
Unfortunately the glare and exposure issues make it impossible for you to read the thermometers in the video. The only usable video log data was collected when I aimed the camera down and zoomed in on the displays.
4) When I was done I removed all the probes and re-checked their calibration at several stable temperatures. They tracked each other amazingly well from 32 degrees to 212 - amazing for such cheap little devices. P1 and P3 matched at every stable temp and tracked up and down within 1 degree. P2 tracked as well, but in the ranges we care about (80-120 or so) it read 1-2 degrees lower than P1 and P3. Because P2-P3= heat rise from intake to MAF, this means the heat rise was actually a little more than I reported earlier. The thermometers seem to sample every second or two, so they react reasonably quickly. The thermal mass of the probe tip does create some lag, so instantaneous readings aren't possible. Still, you can see the airbox temp begin to drop, for example, during a full throttle run through any gear but 1st. All in all, I think that this data is very trustworthy.
Data and Conclusions:
There are two distinct issues under discussion, but they have been masquerading as one. I want to separate them.
Problem One - Hot Air Intake
1) There is some warming of the engine's air on its way up and through the intake duct, primary airbox, filter, MAF and intake plenum (aka secondary airbox aka bundle of snakes box.) I did not measure rise beyond the MAF location). Both road speed and power setting affect how quickly things cool down, but normal cruising, even at 60, will take many minutes - like 10-15, before the system reaches an equilibrium. At that point the temp in the upper engine compartment was 155 +/- 10 - and this heat transfers to upper intake ducting at a constant rate. These parts are, of course, cooled from the inside at a variable rate, which is a function of the inlet air temp and the volume of air flowing through the system. Temperature in the airbox can be very high - as much as 40 degrees warmer than the inlet temp, after a 10 minute stop and soak. With any opportunity to use power in anything other than first, though, it will cool quickly to about a 20 degree differential, and a freeway cruise will lower this to about 10-15 degrees above inlet temp over a period of several minutes. Sustained aggressive driving will cool it further - to within about 10 degrees of inlet temp. Driving hard and high speed also seem to cool the air near the hood - I noticed upon slowing down or going down a hill during my aggressive driving that the airbox temp didn't climb as fast as I was used to seeing. This could be because the supplemental fan had come on, or the increased activity simply moved more air through the area helping to cool it.
2) Temperatures at the air inlet are much better managed - and above 35-40mph are typically no more than 5 degrees above ambient, dropping to 2 degrees under moderate (80mph cruise) to hard (8/10- Stewarts Point) driving. At speeds below 35 or so and with low power settings (i.e., getting out of town), the inlet temp climbs, which in turn heats up the inside of the upper ducting, which is then slower to cool back off once you're moving. However the inlet air is quite quick to cool to within 2 degrees (f) of outside air once you're up to speed again.
So: There is
a heat-soak problem, both of the intake components and the post-radiator air during slow driving - that delivers warmed air to the engine at all times. The amount of temperature increase is minimized with increased road speed and throttle settings. In other words, the problem is at its worst when you care the least, and is the least severe when you care the most.
Still, it is significant enough that it shouldn't be disregarded, especially among autocrossers and drag racers - people who need every last drop of power right away after getting moving, and who create the conditions that cause engine compartment temps to soar beforehand.
Problem Two - Factory Temp Sensor vs. After-market
1) The factory air temp sensor, co-located with the MAF, when read via the ODBII port, reports temperatures 15 degrees hotter than my P2 sensor did. There are many possible explanations for this. Maybe manufacturing changed thermocouple suppliers and didn't tell the engineers. Maybe the number we read at the ODB interface isn't even the same as the number the ECU actually uses. (It does, after all, get converted to degrees f and formatted for use by the industry-standard ODBII system.) But even if the number IS the same, it still cannot be reasonably concluded that this is a "culprit" in reducing the engine's performance. The engineers could easily have adjusted the map to compensate for it when they tuned it in all those heavily instrumented, logged and analyzed Arctic Circle and Sahara Desert testing marathons. If they didn't know it early on, they still could have also supplied a new map with any subsequent software upgrade for the car, and there have been several. I will allow that there is a possibility this simple issue has eluded BMW engineers for all these years. But I gotta tell you I find it a little hard to believe. Unfortunately, these are questions that most likely cannot be answered by anyone other than a BMW M engineer.
2) Replacing the temp sensor with a different unit almost certainly does the following:
- Sends different values than the stock sensor to the ECU
- Responds more slowly to changes in temperature (after-market sensor has much more thermal mass and is considerably less well insulated from its mount.)
- May or may not alter the shape of the temperature response curve.
3) Relocating the sensor to the beginning of the intake tract, or into ambient air creates a differential between the air it measures and the air the engine is actually receiving - a differential I have seen can be as great as 40 degrees.
4) The ECU's response to a colder temp is to advance the engine's timing, and this does make the engine more powerful. (This was a supposition of mine in an earlier thread - I had it confirmed today by an independent BMW mechanic.) But is the engine over-retarded in general? Consider this: If you've ever bought a tank of 100 octane, you know the engine will make more power. The adaptation program works, and it advances the timing until it reaches a maximum value or knock is detected. Did you ever notice that you still have good power for a while after switching back to 91? The timing doesn't get retarded right away. But it does get retarded.
The only explanation for this must be that knock was indeed detected under some conditions. Do you ever hear it? Of course not. But that doesn't mean it didn't happen. Thinking you would know if a substitute temperature sensor sent your engine into the knock zone on occasion is probably a dangerous assumption.
- Insulating the air intake tract will be more beneficial than a CAI - both will help a little more.
- Eliminating the OEM air filter box, relocating the air filter to the beginning of the chain, a la the Dinan CAI, also seems wise. This reduces surface area and thermal mass in the hottest part of the path, therefore reduces heat transfer.
- We seem to have more engine cooling capacity than we need - even driving hard in these temps I've never see the engine overheat, so removing a few of the baffles that otherwise guide all air from the grills to the radiator should help. If you don't have the brake duct mod, opening up those that false grill area would provide a lot of airflow into the engine compartment. However, with no place for the air to escape up top, I think this air will flow mostly low, and out the rear of the underbody panels, having less effect than we might wish, and reducing downforce as well. The combination of increased compartment airflow and hood vents makes a lot of sense to me.
- Ventilate the hood, perhaps even with an electric fan, to reduce upper area temps. It may also be possible to open a hot air exhaust port in the divider between the engine compartment and the passenger air intakes, allowing air to flow up, back and out, helping to cool this area down. It would need to be baffled from the passenger intakes to avoid heating the cockpit! Addendum: There is some risk that opening vents in the hood would rob a good deal of flow now helping to cool the exhaust headers. Experimentation in this area should be done gradually and with caution!
- Open the hood when you park! After driving almost 2 hours and parking in Healdsburg, I returned to the car less than 10 minutes later. Underhood temperature was 199 degrees!!! The airbox had soaked up to 165. The air down by the intake was "only" 135. Over the course of the rest of my time I experimented with opening the hood when I parked at the house - temps fall below 140 within minutes.
- If you do install the replacement sensor, I would mount it in the airbox to ensure it is seeing close to the same temp air that the factory sensor receives. Of course, once it is there, it may not result in as noticeable a difference if there is a difference at all.
I cannot assure you that a sensor substitution/relocation is safe under all operating conditions. Nor can I prove it will cause any harm whatsoever. I am not making a recommendation here - just providing you with the facts and my conclusions.
I will say this, though. All of this testing was educational, easy, and cheap. Anyone can do it. I hope others will - there is a lot more to learn.
Someone DID do it! Check out M5XTRM's Test Results with CAI, Hood vents and intake tract insulation!
Thanks for listening!