How rich do you have to run to start fouling plugs?

Steampunk · Postby **Steampunk** » Mon Jul 21, 2008 10:26 am

Generally speaking, how rich does an engine run, on a constant basis, in order to foul the plugs?

Rotary · Postby **Rotary** » Sun Jul 27, 2008 8:53 pm

Depends on the plug

Colder plugs will foul much easier than warmer plugs,

to give you an idea, when i played with tuning the Ancient injection computers (microtech D4),
the richness of the computer would foul the very cold B9ES sparks sometimes in 1 hour, each warmer plug would take a bit longer to foul,
B8ES about a few hours, B7ES a few days and B6ES would rarely foul.

This was a Very rich mixture though, so rich the plugs were always wet when pulled out, but even so the B6ES would not foul, so i used warmer plugs for tuning.
Also these would foul on the old Coils with less spark, but with the later Transistor Coil packs even the colder plugs would take a long time to foul

So I guess with most setups fouling would not be easy unless running very cold plugs.

Steampunk · Postby **Steampunk** » Mon Jul 28, 2008 1:43 pm

Cool.
I'm just running standard heat-range plugs (whatever that may be), those Bosch Super 4 things.

I'm running my MegaSquirt at slightly richer values across the entire rev-range just to be on the safe side, and I was just worried that it may be too rich.
At high revs and Kpa (above 4500RPM and 80Kpa) I have it around 12.8:1 AFR, and cruising around 13.5:1 AFR.

Cruising at 13.5:1 AFR shouldn't foul plugs should it?

Juffa · Postby **Juffa** » Mon Jul 28, 2008 1:59 pm

Unless you are running boost rather than vacum then I would not see the need to run rich. You will use more fuel and reduce the potential power output. Of course you need to consider the timing and fuel octane rating you are using. Did you MS come with recommendations for MX-5 AFRs? Are you running a knock sensor with your MS?

J

Steampunk · Postby **Steampunk** » Mon Jul 28, 2008 2:06 pm

the base map on the MS is even richer through most of the rev range! As you know, they have pretty shite fuel over the US, and thus overcompensate for it.

I don't have a knock sensor, yet, so I am erring on the safe side.
I realise that I don't have optimum AFR for power, but I would rather produce less power and save the engine.

Timing is only a few degrees more advanced than the base MS map, which, again, is fairly conservative.
I always fill up with Caltex Vortex 98.

I think you get an understanding of how paranoid I can be regarding this sort of stuff Jeff

Rotary · Postby **Rotary** » Mon Jul 28, 2008 5:34 pm

I think it should be OK, Not sure of exact specs of the Bosch plugs, but the normal heat range would be pretty hard to foul, and definately not with anything above 12:1 ratio

Your A/F of 12.8 for Power is ok, every engine varies but it wouldn't be that far off the best A/F ratio for power.