I just can't leave well-enough alone!
My car runs very well with the Z32 MAF, but I felt like there was some area for improvement. I noticed that the signal would bounce around a lot (a lot only being 0.05 or 0.1 V, but that is significant in MAF terms). This wasn't much of an issue except that I had to turn up the MAF Averaging setting to smooth it out a bit, and this setting dampens response time which means you need more acceleration enrichment correction. I also think that the Z32 design suffers a bit from reversion because the wire is sitting in the open. So air can bounce back and swirl in the pipe and the Z32 design might be more susceptible to noise. Lastly, there was some noise in the system even with the car off which leads me to believe the sensor itself is a bit finicky.
I decided to give the R35 MAF a try. It seems like there are plenty of variations on this sensor design (the slot style) and also lots of people using them in standalone applications. The sensor is designed to minimize reversion and also includes an integrated IAT sensor. I thought that it would be easy to find data for this sensor as well, although I failed to account for the fact that the different variety of slot style sensors makes it difficult to find the correct voltage curve, and also that Hitachi doesn't actually make the datasheet available for this sensor. But those are solvable issues.
I picked up a sensor (Hitachi MAF0031) and wired it up to test:
0.34V at zero airflow. Blowing on the sensor changed the reading which confirmed I had the wiring correct.
I found a helpful document that showed the pinout, but the IAT sensor had some confusing nomenclature. I expected it to be a simple thermistor,but the phrasing in the document called one pin "IAT Signal" and the other "IAT Ground". This had me wondering if it was a 0-5V sensor powered from the same 12V supply as the MAF sensor, but it turns out that the document was just awkwardly phrased. It is, in fact, a plain old thermistor:
Before the sensor arrived I had actually started designing the housing. I needed a mounting boss for the sensor and didn't find a free 3D design for the boss on it's own. Either it was a paid design or it was already part of a tube. So I found this diagram and made my own.
Step one was to import the diagram into Fusion 360 as a canvas:
There's a very convenient function in Fusion that lets you pick two points and provide the distance between them, then the software will automatically scale for you. In this case I used the two bolt holes.
Then I spent a few minutes drawing all of the lines:
And a few more extruding:
Done. I'm glad I started learning Fusion; Blender would have taken ages to achieve the same result. Not that Blender is bad, but it isn't really designed for CAD. I fell into the trap of becoming comfortable with the software I knew rather than trying to learn something more appropriate for the job. Using Blender for CAD is like deciding to cut down a tree with a hand-saw when a chainsaw is available that will do the job in 1/10 the time (Fusion).
I imported an existing adapter tube from Thingiverse. I've used this tube in the past when I was running a speed-density tune, and it works really well (as long as it's printed in something like polycarbonate that can withstand the heat). The only tricky thing is that since it's an STL mesh and not a solid, it's not as easy to modify in Fusion.
I located the sensor boss as far back in the tube as possible, and made an educated guess with a set of calipers to try and locate the opening for the hot-wire right in the center of the tube:
Then used Fusion's excellent "To Surface" setting in the Extrude tool:
Lastly, Extrude-Cut the opening for the slot:
I printed one with my remaining PC filament, but unfortunately ran out just before the end of the print. It was certainly good enough for testing though:
The fit on the sensor is about 90% of the way there. I moved the sealing surface on the inside of the boss up 0.5mm, and the flats the screws tighten up against also up 0.5mm.
I did a pretty good job centering the hot-wire too. I later revised the design and moved it down a few more millimeters.
To be continued
While waiting for another role of PC filament I decided to test out the sensor using the first tube I printed. I took out the airbox (a literal 2 minute job, I love working on FCs) and realized I had completely forgotten to account for clearance when designing the tube:
It's a really, really tight fit due to the shape of the airbox:
But happily my new design still fits:
The next one (printing as I type this) moves the sensor down about 5mm, so clearance is even better. Changing the sensor will still be an airbox-out job, but like I said before that isn't exactly an arduous task.
Lastly, I wired up the sensor by just connecting the exposed pigtail wires to the Z32 connector I added to the harness last time:
That's obviously for testing purposes only, and to shake out any possible issues without making a permanent change to the harness. The IAT wires are just extended over to the stock connector on the dynamic chamber.
I couldn't find the calibration for the thermistor, so I took a resistance reading at room temperature and compared it to a plain old mercury thermometer. Then I did the same comparison at 45C with a heater, and also at -20C in the freezer. This lets me input the three points into the MS3X and it will generate a curve. I kind of don't like how this feature is implemented (it would be nice to see the curve) but it does work.
I also couldn't find a flow-curve for the stock sensor. There are plenty out there for the PMAS HPX sensors but I couldn't find a stock R35 curve. I decided to just fire up the car, and I was able to get a (crappy) idle by multiplying my prior curve by 1.5 or so. I found that the reading wanted to jump around a lot (more so than the Z32 sensor even) but that can also happen when the engine is running poorly and vacuum is inconsistent so I don't know whether it's a problem yet.
I was also operating under the assumption that the two curves had a similar slope. Simply multiplying the curve by some number (in my case 1.5) assumes that both curves are increasing at the same rate as the line moves rightward. Meanwhile if the Z32 flow increased by 15g/sec from 1V to 2V and 100g/sec from 2V to 3V (arbitrary numbers I chose), there's nothing to say the R35 MAF couldn't increase 15g/sec from 1V to 2V and 45g/sec from 2V to 3V. I'm probably not explaining that well, but that's the best I can do.
With my current flow-curve obviously insufficient, I played with the PMAS HPX transfer function generator to try and get a better curve. The generator tool is an Excel spreadsheet that allows you to input the diameter of the pipe and then returns a flow-curve. This is what it gave me:
I'm not using all of the resolution the MS3X gives me, since I haven't rescaled the curve. But assuming it's in the ballpark this should be good enough for testing until I rescale and actually spend time tuning it. I should have the new tube printed by sometime tomorrow, so if all goes well I can take it for a drive and see what happens.
Until next time 
I was able to fix the 3D printer for about $40, but I ended up returning the MAF I bought. One thing I noticed was that when idling the signal would jump around (a lot, much more than the Z32 sensor). I also noticed that there were no part numbers on the supposedly "new OEM" Hitachi sensor, and some scratches from it being installed previously. I assume the Amazon seller was selling used aftermarket parts instead of OEM, and I didn't really feel like investing the time to tune a sensor of unknown quality and origin. I will pick that project up again sometime with a proper OEM MAF.
I ended up spending a bit of time and money on something entirely unrelated - the steering wheel. But first here's a refresher on my adventures with the prior steering wheel (feel free to skip to the photos if you aren't interested).
When I bought the car it had the stock 380mm wheel. This is a slightly larger size than normal, and like most people I kind of wanted a 350mm wheel but didn't really feel like buying one since the stock wheel worked fine. I did however have one strange issue which is that the horn would go off randomly. I had already resoldered the body CPU more than once and failed to resolve this issue, and I noticed that it would only happen when turning the wheel (and only sometimes).
This came to a head when the horn started blaring in a Tim Horton's parking lot and wouldn't go off (terrifying some poor old man trying to park his car nearby). I just ended up lifting the hood and pulling the negative cable off the battery with my hands (yes, I've replaced the terminal since then) and then reaching my hands through the grill and unplugging the horns.
Driving around without a horn isn't safe, so I ended up buying a "genuine" Momo wheel on eBay and an NRG short hub. Then when they arrived I realized that the wheel hit the turn-signal paddle and I needed a spacer to push it away from the dash. The spacer I picked was a shiny one that I kind of liked at the time but would later be a bit too "busy" for my taste.
Oh, and the problem with the original wheel? Well there was no obvious issue that would cause the horn to go off, but eventually I shook the wheel in just the right way and a tiny little screw fell out. It wasn't an OEM screw and I have no idea how it found it's way into my steering wheel. I assume that every so often it would land in just the right place and bridge the contacts for the horn button before falling back out again.
The new wheel worked fine for a few years, but I carelessly bent it last year (or the year before? I don't know anymore). A shop that had done an alignment for me a few years back cranked the NRG hub down on the column so hard that it started to damage the splines on the hub, and bottomed out the hub slipring against the sprung contact for the horn (if that doesn't make sense there will be photos shortly). I was able to later pull the hub off the column directly, but not before putting a small (but noticeable) bend into the wheel itself.
I could live with a bent wheel (I have better ways to spend my money) but then a few weeks ago someone cut me off, and when I pressed on the horn button it just blew apart in my hand. Gee, I sure showed that guy! Turns out the plastic in the horn button was very brittle after almost a decade and it finally gave out. So now I was in the position of needing to replace the button, as well as wanting a non-bent steering wheel and a less eye-catching spacer.
I grabbed another "genuine" Momo wheel off eBay, and supplemented it with a "genuine" Momo quick-release. It does the same job as the spacer, except all black and with the added safety benefit of being able to remove the steering wheel quickly in an accident. It's fairly unlikely I'd survive an accident since most of the cars on the road nowadays are trucks with a hood-line higher than my head or 5000lbs SUVs that could drive up my car like a ramp, but every little bit helps.
Here's the old wheel:
It actually felt really nice to drive with. The bulges at 10 and 2 made it really easy to grip. However, the whole wheel doesn't really "fit" the aesthetic of the car in my opinion. It's not terrible, just a little bit out of place.
Speaking of out of place, here's the spacer:
Too shiny. And speaking of out of place (again), here's the issue I mentioned with the horn slipring:
The hub isn't supposed to sit down that far on the column. Instead of the horn contact being held against the slipring by spring tension, it's compressed against the slipring. I had fixed this issue in the past, but due to some damage to the splines it doesn't require that much torque to compress it down this far. A new hub is probably a good idea, but I'll get to that later.
Looking at the hub, I also had to address a design flaw. The slipring and the plastic ring it fits onto can move a little bit independently of the hub itself. This causes the wire for the horn to bend a bit and then eventually it breaks. This is of course aggravated by the extra friction on the slipring from the over-compressed horn contact, which means when I turn the hub the slipring really does not want to turn with it (placing tension on the wire):
So I designed a little tube to take up the slack:
Resoldered a new wire (had to center the wire in the middle of the solder joint or the tube won't sit flat):
Dropped the tube in:
Put the whole assembly on the back of the hub:
And there we go. I put a dab of hot-glue on either side just for some strain-relief and to prevent any rattling.
With that done, I had to install the quick-release. It's a bolt-on install, but the height of the Rx7 column nut will compress the wires for the horn when installed and could wear through the insulation. So I had to use a file to grind a little channel for the wires to go into:
I also reattached the ground wire using that little threaded hole, but forgot to photograph it.
Then the wheel bolts onto the front part of the quick-release, and it clicks into place:
All black, and complete with speed-holes!
Clearance for the horn button is very tight, so I made some custom wires:
This still barely clears, but with the spring for the horn button installed (not in the photograph above) it fits securely.
To be continued
In reply to WondrousBread :
Nice work. Wheel looks good 👍
Light on photos this time, but I always try to log even minor repairs so I can refer back to them in a few years when I've forgotten them. I have a terrible memory. You wouldn't believe the number of times I google an Rx7 related question and find my own answers from years ago.
This actually took place a few weeks ago, but I was driving near my house and when I changed into second I got a really big "CLACK" noise from the transmission. I was on it pretty hard (around 7500rpm) but even so, that shouldn't happen.
When I got home my dad asked if I was sure I had my foot all the way down on the clutch, and I was. But it did raise the question of whether the clutch was properly adjusted. I remembered two things:
1. The clutch master and slave had both been replaced, but the fluid in the reservoir would still turn black and need to be replaced every few months. I had been putting off replacing the hose.
2. I have never adjusted the pedal freeplay in the clutch.
I figured I would try the freeplay first and then look at the hydraulics later. The freeplay is adjusted by loosening a 12mm lock-nut and then spinning the pushrod on the pedal in or out. You don't need to mess around with removing the c-clip and clevis pin - ask how I know. Adjusting the freeplay so that there was only about 1/2" of space at the top of the pedal seemed to work and I was pretty happy with it.
I thought about putting the hydraulics on the back-burner, but the decision was made for me when I was moving the car in the driveway and the clutch pedal stopped coming up. My dad helped me roll the Rx7 back into it's spot and I took out the old hydraulics:
Now the only original part here should be the hose, but as I was changing this out I got to thinking how foolish it was of me to continue using a 38 year old rubber hose. The fluid has been changed more than once and it still came out looking like the black goo from the X-Files. I can only assume this is from the inside of the rubber hose, although I thought they were PTFE lined so maybe it was something else.
About $80 on Rockauto got me some brand new parts:
I sprung for the Exedy cylinders, and I think the hose is a Dorman? They all seemed to be really good quality.
I didn't take any pictures of the process (I rarely do when hydraulic fluid is involved). Removal and re-installation took a cumulative 30 minutes. I think even the bleeding process only took like 10 minutes total, plus 5 minutes vigorously washing anywhere that the fluid may have dripped below the cylinder. After the replacement all is well.
I have been finding myself wanting a lighter flywheel and more aggressive clutch lately. The heavy stock flywheel is kind of nice when you're learning to drive (this was my first car 8 years ago) but once you're experienced driving manual, you can shift a lot faster than the stock flywheel will allow. Plus I notice that when my current clutch (Exedy stock replacement) is hot after a long drive it isn't as grippy as I would like. I can replace the clutch / flywheel in only a few hours, but considering that none of the parts involved (S4 counterweight, lightened flywheel, grippier clutch) would transfer to the Turbo II engine I'm not really sure whether I want to spend on it.
Until next time
