Recently I've been developing the ignition system for this car.
Ever since I put a brand new starter motor in, I was experiencing interference with the ignition system during cranking so I decided to finish building up the EFI distributor that started life in a 7K-E.
The 7K-E distributor differs from a 4K distributor in that it has no advance mechanisms, is about 10 mm longer, has a bigger cap diameter and different rotor. Usually a 7K-E distributor cap is of the type that you clip the leads onto instead of pushing them on. I found a 22R distributor cap at repco that pretty much fits onto the 7K-E distributor, only needing the mounting holes drilled out slightly. This allowed me to use the original push on 4K leads. I also had to make up a spacer plate to put between the distributor and the engine block, this was relatively simple to just make out of some steel.
The 7K-E distributor uses the same rotor as a 4A-FE and I'd assume 22R. This rotor worked perfectly with the cap.
Having only 4 points for the VR pickup, the distributor wasn't exactly suited to a megasquirt based EFI system. Toyota used a system known as VAST, which basically used the distributor for base timing and would fire a spark when a signal came from the distributor unless the ECU overrides the module. I had no VAST module, so this setup wasn't exactly ideal as the megasquirt would just have to guess at the base timing (10 deg BTDC) based on the average speed of the engine. If using a basic 4 point distributor trigger setup you need to phase the distributor so the trigger comes in earlier than the most advanced timing you are going to use (ie 50 deg BTDC).
Anyway, I did try to use the distributor with the factory 4 point setup (this was before I changed to the new starter), but I had problems with getting the engine to start. If the engine started, it worked somewhat well but the timing as no very precise. It was pretty clear that more data points (teeth) were needed.
After that failed attempt I changed back to fuel only EFI and just kept using the car. Then the original starter motor stopped, the solenoid contacts were a bit burnt. I rebuilt the solenoid but I didn't trust it to work again so I bought a new aftermarket starter. The new starter was interfering with the ignition during cranking, causing tach spikes. The starter would also crank at a randomly selected speed every time I turned the key. This was making it somewhat annoying to start the car as it wouldn't always start on the first try. So I decided to go back to ECU controlled ignition and see if I could get it to work properly.
I was considering using the flywheel of the engine as a VR trigger wheel, but I think they have 105 teeth, which isn't divisible by 4 or 2 so it wouldn't have worked with the megasquirt.
My Barry lathe came complete with a broken quick change gearbox, and it just so happened to have a 20 tooth gear with an outside diameter of 35 mm - perfect for the distributor. I clocked up the distributor shaft in the lathe and turned the toothed portion down to 19 mm so the gear would fit over it. I welded the gear on, removed some of the thickness and knocked a tooth off to make it a 20-1 wheel.
This first attempt actually worked quite well, the car ran great and the timing was accurate but I still had issues getting sync while cranking. I figured it was due to the small diameter of 35 mm not giving enough tip velocity at cranking speed to get a good signal from the VR pickup and decided to redesign the system. I used this setup on the dyno, and tuned in a good ignition map.
The next attempt was to make a trigger wheel that went over and around the VR sensor, almost doubling the diameter of the wheel (so almost double the signal). The VR sensor needed to be modified slightly to sense from the outside. This modification consisted of welding a bit of gauge plate to the existing sensor so it would stick out more and removing some material from the VR sensor housing. I epoxied the sensor together after, to reduce any risks of bits falling out over time.
I made a 30-1 wheel for this, but as soon as I put the details into TunerStudio, the ECU said "no". I somehow missed the fact that 30 isn't divisible by 4 and the ECU wasn't having a bar of it. The hole in the wheel is mainly to allow for screw driver access for assembling/disassembling the distributor, but it should also help (or make worse) the balancing from the missing tooth.
In a bit of a huff, I ripped it all out and went back to fuel only mode for a few days before I built up the motivation to cut all of the pretty teeth off the wheel and try again. I used my Barry lathe to remove all of the teeth and then used a mill with a DRO to put 23 holes on a 61 mm PCD (set to 24 equispaced holes). These holes were 2.4 mm. I then cut up a 2.4 mm diameter mild steel TIG rod to use as teeth and welded them all in.
After making the wheel, it was a simple case of heating it up, slipping it over the distributor shaft and hoping it would stay put - it did! Then I faced off the welded surface and reassembled the distributor.
At this point, the new starter motor started to smoke and quite often not even start turning. I put the original starter back in the car and it worked perfectly (turning twice as fast with half the power). The car started up pretty much first try. After adjusting the distributor phasing in TunerStudio it now works perfectly and starts first time, every time. Looking back, the 20-1 setup probably would have worked fine using the original starter motor but this new wheel should be slightly more robust to signal noise.
The car just runs generally slightly better all around. It now uses about 15% less fuel at idle, about 3.5 L/100 km at 50 km/h and 5 L/100 km at 85 km/h so I can't really complain.
I also bodged up a cover for the J121 igniter module, so it looks a little more factory. Hopefully I will get around to making it a little more permanent one day but this will probably last 20 years.
So basically I achieved my goal of having an OEM looking ignition setup that uses the ECU to control the timing.