Fitting an RX7 AFM

[adamjp]

I just read through STMs words on fitting the slightly faster crystal to his NA6.

Whilst I do not want to hijack that thread, I thought that I should correct some of the information about RX7 AFMs that are in there.

My car is not stock, quite some distance from it in fact. When I fitted the RX7 AFM, I gutted the MX5 electronics and fitted them to the RX7 one.
For the two AFMs I had on hand, the two sets of electronics are quite different in their values, and the spring in the RX7 AFM is definitely stiffer.

For the record there are 8 different resistance points in the electronics.

Code: Select all

Position    RX7    MX5
0            0      0
1           48     45
2           97     90
3          130    121
4          154    145
5          171    162
6          183    175
7          191    186
8          199    197

Additionally the difference in the spring thickness was
RX7 0.70mm
MX5 0.65mm

For my car, the RX7 AFM allowed me to easily reach redline, where before the engine had run out of puff at 6500 and struggled to redline. Monitoring of the WOT O2 readings indicated that the car was still very rich. This makes sense as everything I have read indicates that the ECU essentially ignores the O2 sensor for WOT conditions and only comes into play for the part throttle 'cruise' condition.

According to my GTECH Pro, the car made 74hp before and 80hp after. Are these real numbers? No. But they are as close to the same condition as I could make it, so they are comparitive. Seat of pants inclined me to believe them. The numbers were across several runs and were utterly repeatable.

[Boags]

Did you change the spring as well as the electronics?

Those are interesting findings, thanks for sharing!

Boags

[adamjp]

Spring, and electronics went complete.
All that was left was the post in the middle for it to all attach to.

Word of warning, a little WD40 or similar on the central post is your friend when getting the spring off the post.

[Uncle Arthur]

Adam - please - would love to know more - I am planning this exact same mod to support a turbo install.........

I have gutted the RX7 meter, but broke the ribbon solders in the process - would love to know what precautions you took while removing the MX5 guts so they went right back in without any breakages or repairs.

Cheers,

Mike

[adamjp]

Unlce, which solders?

I unsoldered the three legs from the plug socket to the circuit board, and pulled the socket off the case. Then undid the three screws and the board simply dropped out, followed by the air temp sensor (plastic thingy).

The hard part was getting the damn MX5 spring off the shaft. Be very careful and use some oil to grease the shaft. DO NOT pull the spring up from the outside, you will pull the spring off the plastic centre unit and you may not be able to fix it. Slowly and carefully lever the centre piece up off the shaft, try not to twist it, or twist the spring. Practice on the RX7 AFM first, lots!

Putting it back in was simply the reverse with a little more solder required to mate the socket back up with the circuit board.


For reference.
A - Where you adjust the spring tension. You can just see the blue pen mark of the original detent 3 teeth to the left of where the detent is now.

B1 to B3 - These must be unsoldered before you try to pull the socket off the case. If not you will stuff it and have to play around to get it fixed.

C - The fuel cut-out switch. Ensure that the spring tension is strong enough to open the switch when you let the flapper door go.

D - The air temperature sensor.

0 to 8 - the resistance pads I got the readings from in the table above.

[Uncle Arthur]

I think from memory I broke B1-3 on the RX7 one pulling it apart...

That's OK, cause I only wanted the outer casing anyway to house the MX5 internals - I'll be more careful with the MX5 one.

I actually found that to pull the spring etc off the shaft all I had to do was undo a small grub screw on the side of the black plastic upper component. Then it just slipped off.......

[adamjp]

The screw is for the arm on the shaft (7mm head bolt). The spring unforunately is a press fit.

My RX7 one came off with little fuss, the MX5 one was a real bitch. Of course, it was the one I cared about to remove so that figures.