Introduction ------------ Since I removed the airbox and fitted the Ram Air system with a big K&N cone filter, I have been wondering about air intake temperatures and their impact on performance. I have plans to build a heat shield, but before I do this I wanted to find out what sort of temp the air under the bonnet is as opposed to outside air. I also wanted to see how much cool air is actually being ducted to the air filter via the cold air vent just behind the kidney grill, i.e. the place that the snorkel used to attach to off the M42 airbox. I have suspected that this vent probably delivers quite a bit of cold air. Most people I have spoken to have disagreed with me, saying that the open air filter suffers due to underhood temps.

The New Gauge ------------- So, on Saturday, I bought myself a temperature gauge that has a sensor with a long amount of wire on it. The sensor is a nice, small, very very light little unit with some peel off adhesive on its back. The control unit has a little switch on it that lets you toggle between indoor and outdoor temp. The control unit is also nice and small and has a large LCD readout in Centigrade or Farenheight. It only cost AUD$19.95 too which is about US$14.40. So now, I have three temperature readouts in the car, interior, exterior and air intake. All I need now is a rain gauge... and possibly a Spear & Jackson shovel to keep in the boot ;-) (OK, that joke is obscure but some of you may get it :-))

Installation ------------ The only challenge involved getting the sensor under the bonnet while the control unit sits in the car. This is probably easier for me as my steering wheel is on the right side of the car.

To do this, I removed a panel underneath the glove box and found a small hole that leads to the fuse box. Unfortunately the sensor was slightly too big to fit through the hole, so I ended up cutting the wire to it. As the wires were not colour coded, I split them then I cut just one of them and stripped each end of this wire. I then cut the other wire but left it unstripped so that I knew which wires to reattach to one another. I then got a coat hanger, straightened it and made a small loop on its end. I threaded the wire through this and poked it through the hole. I then opened the bonnet and took the lid off the fuse box and tried to see where the end of the coathanger ended up. This was frustrating, but finally, with the help of my wife we spotted where the end of the coat hanger appeared. It is really much easier with two people as the coat hanger is easy to spot when it is moving. So, she grabbed the wire off the end of the coat hanger and that was the hard part over with. I then reattached the wires and threaded the wire along through the clips that are on the left hand side of the car where all the wires are held in place for the headlights etc. I then removed the backing paper for the adhesive on the sensor and attached it to the front of the cone filter, this was a nice fit as the cone filter has a large circular piece of aluminium on the front of it with a hole in the middle where more air can get through. Overall, a neat job and the sensor is in an ideal location.

Back to the interior, I threaded the wire neatly around the upper back part of the footwell and reattached the panel. The wire currently exits through the gear shift boot. I haven't decided exactly where I want to mount the control unit yet so this is currently acceptable.

The Readings ------------ Now for the most interesting part. It was a cool day on Saturday, outside temp was 17 degrees C. Air intake temp was about 18 degrees C when I set off. The dashboard temperature gauge was fully cold, in the blue area.

Once the dashboard temp gauge was in its usual spot, bang in the middle, I

kept watching the air intake temp and comparing it to the BMW outside temp.

While waiting at traffic lights for a couple of minutes, I saw the air intake temp creep to about 23 degrees C. That's six degrees C warmer in only couple of minutes. And that's without the engine being REALLY hot! Hmmm. I then drove away from the traffic lights travelling at about 70km/h.

Let's just say that the cold air vent next to the kidney grill does a very very good job. Better than I could have imagined. In about three minutes, the air intake temp was down to the same as the outdoor temp. At some stages, it was lower. Wow! I guess this is due to the BMW exterior sensor being closer to the road which will always be warmer than a few feet above it as the road retains some heat from the sun that was out sporadically on Saturday.

So, I parked the car at home and went in for half an hour then headed back out to the gym. This is more of a stop start drive.

As you all know, the alloy head on the M42/44 retains a lot of heat and after half an hour the temp gauge on the dash will still be bang in the middle.

Well, I checked the air intake temp before setting off and it was 43 degrees C. A full 26 degrees C above the outdoor temp. And yes, the idle hunted up and down quite a bit before settling on 800rpm.

After about a kilometre or so of driving the temp was down to 23 degrees C or six degrees above outdoor temp. The temp didn't get much chance to go below 20 as the trip to the gym is quite stop start.

Summary ------- So, is there an impact on performance when the air is hot under the bonnet. Bloody right there is. The M42 is such a highly strung little bastard, especially with new EMS, exhaust, cone filter and cams that the difference is really rather noticeable. I have always suspected that something funny was going on and now I have some good data to back it up.

Now I know why I get the hesitation after a long wait at traffic lights. When the engine is warm and there are a few sets of lights this can account for 20 degrees C over the outside temp!! A definate downside to the cone filter.

It has certainly convinced me that I should make a heat shield and what's more, it will let me see how effective my heat shield design is. Thankfully as the aluminium is quite cheap I should be able to try a few different designs.

David Walsh

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