Twin plug what if question
 

I'm helping my friend commission his 930 on Megasquirt. We're having the usual teething problems but that is expected. He is running a twin plug setup with COP's on each. My question is: what if one coil on a twin plug setup goes south? It would still run but now just on single plug on that cylinder. I'm thinking on a boosted engine putting out lots of HP, the loss of one coil/plug would not be good. What are your thoughts of loss of one plug?
And for those that like to worry, short of testing each coil every day, you really have no way of knowing both coils are working. That would bug me.

You would potentially see a lean spike when that cylinder's exhaust made it to the oxygen sensor, losing a plug would result in less complete combustion and would indicate as lean on a wideband. That would be about the only way I would see being able to identify a dropped plug on a cylinder.

wouldn't a dropped plug show rich??
 

If the mixture isn't completely burned, wouldn't the o2 sensor show rich??

Do the ECU's have the equivalent of a "check engine light" that would store a code??

When I worked in a dealership at the onset of computer tailoring of the last bit of mixture, (GM CCC), we would joke about putting black tape over the check engine light - but never did.

Seriously, doesn't the ecu have a way of indicating a problem, short of hooking up a laptop??

It would seem helpful to the driver if the O2 sensor went out of spec, alerting her to a likely problem.
chris

Oxygen sensors measure oxygen content to determine air fuel mixture. An unburned mixture will have excess oxygen, thus reporting as lean. Some ECUs have fail-safes that can put the engine in limp mode (or completely shut the engine down depending on how you set it up) if the oxygen sensor reports too lean of a condition for a prescribed load. Others store codes, but really the best method is to look at the logged data to identify the problem. FWIW, the Infinity has the capability of triggering a MIL lamp if your ECU detects a fail safe condition.

I have two switches on my dash that can individually turn off the coils to each set of six plugs (3 top on one side and 3bottom on other side). A quick flip of each switch (black ones) will tell you if one or more plugs is/are missing on that circuit.

http://forums.pelicanparts.com/uploa...1528888522.jpg

Cheers,
Johan

I’ve read quite a bit about this on other threads when using twinned boxes

I haven’t looked but I think Henry smidt is one of the contributors if you want to search in his name or msd twin plug

If you add a dual AFR display on your dash you can catch the problem before it gets too hot.

http://forums.pelicanparts.com/uploa...1528898447.jpg

You’d lose power as now your timing is retarded

I've got twin plugs and twin MSD's on my 81 3.0L in my 72 911. I put a switch, in the engine compartment, that turns one MSD off to check that they are both working. It works like doing the Mag Check on an airplane before takeoff.

Here is the factory RSR solution for twin plug switches (looks like carried over from the 550 spyder of 1955)

http://forums.pelicanparts.com/uploa...1476205390.jpg

Aha!

I found the thread...

http://forums.pelicanparts.com/911-engine-rebuilding-forum/669447-msd-twin-plug.html

Well there ya go, the "problem" has already been solved (of course). Yes, I like that idea of switches to cut out coil sets as a check. Thanks for everyone's inputs.

Interesting side fact. When a 964 comes in for service a guy is supposed to pull each coil quick disconnect off one by one to check each circuit’s ability to run the car...

The other option discussed in the other thread is to run two coils off just one box

( much discussion ensued)