When some good condition E65 18″ wheels came up locally with nearly new tires it seemed a good bargain, and would hopefully cure some rear vibration where I suspected a problem with one of the rear wheels. They are ‘style 94′ and came as standard on many E65/E66 cars.
I knew plenty of other E31 owners run 18″ wheels, and I know the center bore (72.6mm) is identical between E31 and E65, as is the bolt pattern (PCD 5×120) so what could go wrong? Well, I completely forgot about the tire aspect ratio! The new wheels & tires simply did not fit in the space available under the mount for the spring. Where did I go wrong?
The old tires are 235 45 17 – that means 235mm width, 17″ inch wheel and 45% sidewall aspect ratio. In other words the side of the tire is 45% the width. So if we convert that 17″ to mm (431.8) and multiply the sidewall by 2 for top and bottom we end up with:
(235 * 0.45) * 2 + 431.8 = 643.3
The new wheels are 245 50 18 – so if we convert 18″ to mm (457.2) we get:
(245 * 0.5) * 2 + 457.2 = 702.2
So that’s a difference of almost 59mm – no wonder it didn’t fit! In fact the clearance available is more like 10mm. It’s worth noting even if it had fit running a vastly different size like this would have upset the speedometer as the rolling radius will be different, and could potentially confuse DSC/traction control as it too will find the electronically measured rotation is different from the physical one.
So this meant different tires, a good online calculator for working out sizes is http://www.rimsntires.com/specs.jsp
So by switching to 235 40 18 tires the overall height is now within a millimetre of the original and all is fine. It’s also worth mentioning the ‘offset’ of the wheel – this is the distance from the hub to the mounting surface of the wheel. You should always be able to find this stamped on the inside of the rim – for the E65 wheels this is 24mm – the E31 spec is 15mm-25mm so it’s just inside! If the offset is greater than 25mm then the wheel would not clear the strut body, but a spacer could be fitted to compensate for this.
Old and new pictures:
I ordered a set of 3 chips from Ebay – $90. Cheap for chips I know, but I assumed it was clearance of old stock as I doubt there is much demand for M70 engine stuff nowadays. In retrospect I suspect this is really a ‘pirate’ set consisting of a copy of eprom data developed by someone else. The set has 3 chips – 2 for DME (engine) and 1 for EML (gearbox).
But anyway.. being a nosy sort first thing I did was to read the chips on my PC as I have an eprom reader for old arcade stuff. I also found a stock ECU chip image floating on the internet, comparing the two revealed clear changes in the ‘map’ area of the chip, and no changes in the ‘program’ area. That’s good as the chip seems ‘legit’! However, problem suspected with the 2nd DME chip – I couldn’t get a consistent read from it – for the technical folks the top data bit seemed to fluctuate (and yes, pins were clean), which changes random bytes in both the program and maps. That’s real dangerous to put in an engine! I am 99.9% sure the seller sent me a bad chip, it’s a good I thing I had the capability to check & verify what was I doing.
Other sites have a good guide to removing the ECUs:
My car has the ’352′ DME which is different from the one mentioned in the links. Here you do NOT need to undo any torx screws – they only hold the pcb to the case, not the case to the lid. (I found this out afterwards of course). You only need to prey apart the metal clips.
But.. why is my DME held together with silver sticky tape!?
Seems a previous owner has been in here before!
And that hardly looks like a factory spec chip!
So seems my car was previously upgraded.. I read these chips too and it’s a different map to stock and the one I was sent.
Difficulty – pretty easy to remove ECU’s and swap chips, but be careful!
(Update : After getting a replacement for the bad chip I ultimately went with the new chips as they ‘felt’ a little better than the old).
This E38 740i was bought with some damage to the trim on the front bumper and a bit of a scrape. You can buy replacement trim from the dealer, but unfortunately if the car has headlight washers then the entire bumper must be removed as the washer screws into the trim on the inside. Likewise if you need to replace the washer itself.
Removal is fairly easy – just unscrew everything on the inside of the wheel wells, then remove the two large Torx T50 bolts at the front on the bottom. You can then gently pull the plastic connectors apart. This can be done by one person, though two would be easier.
All better! An all black grill, CCFL angel eyes, and clear indicators with silvered bulbs were also added in this photo.
Not sure why these pics are so bad quality! This is looking into the rear right wheel well of an E38 750iL with self levelling suspension. The self levelling suspension works via hydraulic fluid going directly into the special shock absorbers. A regulator for the fluid is attached to the rear anti-roll bar, so roll bar deflection leads to suspension height adjustment. The system works at very high pressure so do not mess with it without safety equipment! In this case.. rust had led to pinhole leaks on the pipe that attaches to the shock, so there was no pressure anyway. Replacement is essentially just unbolting old and bolting new, but new to age and rust the old pipe can be very hard to remove. Heat and a c shaped wrench to avoid any chance of rounding off the nuts are essential.
Once re-installed and the CHF fluid topped up the system should bleed automatically. Remember never to use ATF as it can destroy the accumulator sphere lining!