dwell reducer circuit for toyota COPs
 

Attached is the schematic and PCB layout.
IMPORTANT: Use a TS914 opamp instead of an LM324.

The values shown are for reducing the dwell by 1.5 mS, which is appropriate for OBD2 NA cars and '99/'00 cars. It will also help 99/00 cars because they can't drive 2 COPs at the same time. The Toyota COPs want ~2.1mS dwell, and the 95.5-00 cars have 3.6mS dwell, at 14.4V.

Circuit has been tested on a 99 and on a 97.

You can get all the parts on Digikey.com.

I could have used a PIC to do the same thing but I can knock together an analog circuit quicker than a PIC program. If you want to make a PIC program you should use a Schmitt input with 1.2V logic low and 3-4V logic high threshold, and the output must drive a 180Ω grounded load to >1.5V, with <0.8V logic low. The input and output are both logic high for dwell, fall to fire.

I also found out that because the COPs draw 8-9A each peak, (total 16-18 A in waste spark mode), compared to 5.5A for the 99/00 ferinstance, the factory wiring has a lot of voltage drop (like 3.5V!). It would help lots to put a 10,000 uF / 16V / 105°C (or 2x4,700 uF) electrolytic cap across the 12V and GND lines that feed power to your COPs.

The PCB layout is 1.1" x 2.4".

You guys will have to do a group buy for PCBs. There are suppliers that will do PCB layouts for something like $30 for 4x4". I can provide the Gerbers, "panellized" so several PCBs can fit in one standard size panel (dimensions depend on the vendor).

Very cool.

Quick question. You say that the 95.5+ cars have a 3.6ms dwell, is that factory dwell?

Hi Jason,

you have written, that putting a 0,000 uF / 16V / 105°C (or 2x4,700 uF) electrolytic cap might already help a lot. Do you think, just puting in that cap would already help to get them fired up (because my problem was just at startup with the EMU (if running - the COPs were fine).

Thanks

Dayum, thanks for the tip. I'm going to hopefully stop at Fry's on my way home.

Jason, would you care if I got that board design printed? I'm certain I have a source that can do them.



edit: read...looks like it's ok.

Holy shit. I couldn't even understand this thread. I'm a :noob:

Im a bit confused. Is this board designed to be used with the factory ECU to lower dwell? Who is this targeting?

I understand the addition of a cap across the gnd and +12V lines, but is that necessary for people with MS's?

I ask because if there is in fact such a huge voltage drop in the OBD2 cars, it will explain why I need to run twice the dwell on the MS for the COPS as people with the OBD1 cars.

it's targeting people running Toyota COPs on the stock ECU.

AH. very good.

The cap across the gnd and 12v as well? The way Ben posted, makes me think otherwise.

above my head....

How was what I posted confuse anything? Lay a freakin cap across +12V and GND.

If you have to run excessive dwell, I'd bet you have shitty +12v or GND connections.

Its confusing because you run a MS and own a 91, but this board and from what I understand this CAP mod is for people running the stock ECU. Based on the paragraph where the CAP mod is mentioned it seems its for people with the 99/00 cars, but im uncertain if thats just used as an example.

So my question is, why would you want to do the CAP mod on your car?(based on your post)

As for my excessive dwell, I dont know whats causing it but its def not my GND or +12v because those are connected to the factory harness with fat wires and I have tried everything short of new coils to solve the dwell issue.

From what I can tell, the CAP is for all miatas running COPs on stock wiring because of the large voltage drop. The large capacitor is supposed to allow the COPs to fire at full amperage instead of letting the coils just deal with the drop.

Chad is correct. the Vdrop is caused by insufficient wire size in the factory harness.

OHHHH.

Sweet. I cant wait to try this then.

I still wonder though.
How is everyone else running cops before this discovery?

Edit: man how awesome it would be if this solves my issue. Not really an issue, just requires a higher dwell but I dont know yet how it will affect in boost running.

The NA 1.6 factory ECU does not suffer from this issue (the one needing the circuit)?

I installed my COPS yesterday and they appear to be working very well. I'm using a link ECU but I would assume that has the same dwell settings as the factory ECU.

I checked the voltage drop on a 99, didn't on a 97. I wouldn't expect it to be any better though.

I don't know what the factory dwell is on the OBD1 NA's nor the 1.6's.

As for those not running this circuit at all, at redline you will be forcing the COPs to dissipate 5W or more extra in each COP - that is a lot of power in that small space. If the COPs don't overheat and fail outright, you will be shortening their life. However, the factory wiring voltage drop is so huge it's actually helping you by reducing the current and heat in the COPs. :D

Said voltage drop must be appearing elsewhere in the wiring harness and whatever is connected to it will be seeing some of it, as well as the EMI (electrical interference) generated by it. Automotive electronics in general is very robust and is designed to take it (this circuit for example will work down to 7V), but the COP current could interfere with sensor outputs. You are better off grounding the COPs at the chassis or block, just not on a piece of wire that is used to ground something else, like a MAP sensor :)). It would also be a good idea to get your switched 12V from the main relay instead of the wire from the ECU. Despite this wiring change, the 10,000 uF capacitor will still reduce EMI. And don't power the COPs from the battery, that's just stupid. :giggle:

Not on a 99/00 - the problem there is that the factory ECU coil outputs don't have enough capability to drive the COPs in parallel (180Ω to > ~1.2V).

For kicks you could try mounting 2 COPs on non-paired cylinders (like 1 & 2) and grounding the unused coilpack outputs with some thin wire. :)

The round things beside the wire terminations on the PCB are holes. A good trick to connect wires to a PCB while provideing stress relief is to have a hole right next to where you solder the wire in. The hole has to be slightly bigger than the wire insulation diameter. I will post a pic later describing it.

Not an issue for those of us running MS standalone style right Jason?