Twin plug - does it need CDI?

[CamB]

This is probably a bit premature, given I am still working on what I've got, but its bothering me.

I've got an SC-based 3.2 with Mahle 98mm P+C with RSR depth valve pockets and 10.3:1 c/r. An ancient Motec controls fuel and spark through a 964 distributor / two Bosch coils+igniters and ITBs (actually MFI stacks converted to use EFI injectors and adding a TPS).

I've had problems with plug fouling at idle (and generally have a crappy idle or high idle), so avoid driving in traffic (sometimes this just means "avoid driving it"). Very infrequent use contributes too (see "avoid driving it"), and it needs another trip to the tuner as while it runs great at 2k and up it is a little fussy below (eg bucks on closed throttle) and has a poor idle (can't tell if its rich, lean or wrong advance ... and can't really change any of those without a trip to the tuner with the special cable). I've been spending some time chasing intake leaks, equalising linkages, etc and have bought a synchrometer to make sure the TBs are balanced. Fingers crossed I am getting there. If I can get it "good enough" it'll be time to get it tuned some more.

But what's been bugging me is that cars which had high dome single plug pistons had (and needed) CDI and I don't. The factory twin plug cars had CDI - suggesting it mattered, but I have an igniter/coil setup that was good enough for (for instance) 964s. I can search here and find that wasted spark twin plug setups aren't necessarily as good at keeping plugs clean too - perhaps more evidence.

Would a pair of MSDs help, or is it just a band-aid? I don't want to just throw parts at it unnecessarily.

[Quicksilver]

It sounds like the idle mixture is off. Idle is the easiest time to fire a plug. Jumping a spark is easiest in full vacuum (eg vacuum tubes). The higher the pressure the higher resistance.
In other words: You don't have a spark problem.

You have a problem making a mixture that is burnable. Work on the problem you have instead of trying to over power it. Pick up an Innovate Motorsports LM-2 and find out what is actually happening.

[T77911S]

you may also look into the plug heat range.

[CamB]

Thanks guys. Plugs are NGK BP7ES and a 12mm NGK with a 7 heat range. I don't really want to go any hotter.

I think I'll keep on the path I'm on. If I can get it idling well and with a reasonable AFR, and still end up with fouling then I might see about the MSD option.

[LJ851]

Just for arguments sakes, i just talked to Richard at PMO to order some parts to carb my 3.2 Carrera motor and he suggested running BP5ES plugs, way hotter than stock, for street use.

[Steve@Rennsport]

Which Motec do you have? M48?

Whats your lambda at idle?

What cams are you using?

[CamB]

Hi Steve

- Cams are ground to an early S profile (at least that's what I was told when I bought them). It comes on cam like an S and made 245hp at the hubs on a Dynapak dyno
- Not sure on lambda - I guess I am getting to the stage where I need to buy an o2 setup to check. I should just bite the bullet they're not THAT expensive these days
- Motec is a "2 Group". Its really old. 20 years old. Motec don't offer the cable any more because there are parts required to make the cable that are no longer available!

Basically all I can control without going to the tuner is what's on an MFI stack (ie, idle and butterfly correlation) and stuff I shouldn't really touch (system fuel pressure and TPS position). I do plan to go to the tuner, but figure I should do as much as possible first.

[Steve@Rennsport]

Hoo boy,..that IS old. Those old EMS's didn't drive drive the coils very hard and don't have as much output as the M48, much less M84 & M600.

I think the cheapest solution would be the addition of a pair of MSD's and matching MSD coils. Naturally, you'll need to make certain your idle mixture isn't too fat.

Your plugs are OK for hard running, but may be cold for street use. You can also V-series NGK as they have a wider heat range and more foul resistant.

[CamB]

I actually have the MSDs - I bought them on impulse a few months ago when I found someone with a cheap price and cheap shipping to NZ. I had hoped my existing coils would be alright (had planned on researching).

(edit - note I've got a couple of Bosch ignition modules and transformer style coils - will this help given it means the ECU doesn't really do any work to drive the coil? Or do you mean it doesn't drive the ignition module hard?)

I'm glad I got a good reaction on the Motec. Its basically a museum piece but seems to work well.

[Steve@Rennsport]

Originally Posted by CamB

I've got a couple of Bosch ignition modules and transformer style coils - will this help given it means the ECU doesn't really do any work to drive the coil? Or do you mean it doesn't drive the ignition module hard?

Thats correct. I'd be using the MSD's as well as fine-tuning the idle mixture.

[CamB]

A mini-update - leaving me even further in two minds as to what to do...

Spent 30+ mins fiddling with throttle stops, linkages and air balance screws and a synchrometer. While its not perfect (or even great - its just better), the main thing is that it didn't foul a plug in 30 mins of idling (to be fair - often at 1400 rpm). Idle about 1100 now and more even, and they are reasonably well balanced at 3k rpm. Its possible I've gone as far as I can without a trip to the tuner.

So. 30 mins without a fouled plug makes me wonder if I do need those MSDs. Will see how it goes on the dyno - either the charge is being lit or it isn't.