another plug thread: what temp plugs for 9.5:1 CR?

[mikesarge]

I just got back from a track day, and in my 3rd session, my car started misfiring badly. I got home, pulled my plugs and we're SUPER fouled. Cylinder 3 was the worst, with what looked like a little oil fouling on there too (sure hope not! this top end was done really recently!)



but the rest looked like the other pic attached. These plugs have only been in here for ~1k miles max, so it's a bit concerning.



The engine is a 3.1L with 9.5:1 CR and 964 cams, RHD ITBs and MS2 controlling fuel only, MSD street fire w/MSD blaster coil, stock dizzy, non-resistor rotor, clewett wires

The plugs are NGK 7s of some kind (BP7ES?).

Is this right? What temp plugs should I be running? I'm thinking of warming up to a NGK 6. Part of the reason, is also that my "coolant" temp sensor (mounted in the breather cover) only ever hits like 170F even after 20 minutes on track, and the oil temp stays in the "warm-up" area, or maybe gets just barely above it.

I swapped the plugs for a new set of the same, but I'm curious what others are doing

[al lkosmal]

Mike,
Were you logging during your track day?
I'd move to the hotter plug, but.......It is possible that you are running too rich.....



regards,
al

[mikesarge]

Hey Al-

Thanks for the reply. I'll check my data logger- it was in the car, but I'm not sure I hit "save" which sometimes I have to do. I know I pulled it out once it started breaking up and it was false-lean (from the misfires) at that point.

For what it's worth, I did have the car tuned on a dyno, but he left it somewhat rich (like in the 11s to 12s near the top at WOT). this was a sopping wet track day, so not a huge amount of WOT, and my AFR table calls for pretty average high 13s and or 14s through the low load range, and I have EGO control on with pretty substantial authority (15%) and operating up to 97%baro

I'll check for logs when I get a chance and I may email you if I have some.

[al lkosmal]

Mike,
greetings....yes, if you have a log, feel free to send it to me. Dyno runs can definitely get you a good tune, but it is a pretty controlled accel ramp/load and may not fully represent the conditions you see at the track. It will be interesting to review the logfile(s)

regards,
al

[mike sampsel]

I use the plugs recommended for the 9.5 CR Porsche. These are one number cooler than the stock SC recommendations. They are bosch 7’s instead of the 8’s for the lower 8.5 CR SC. My plugs look great and I burn premium. 20/21 cams stock CIS.

[mikesarge]

Originally Posted by mike sampsel

I use the plugs recommended for the 9.5 CR Porsche. These are one number cooler than the stock SC recommendations. They are bosch 7’s instead of the 8’s for the lower 8.5 CR SC. My plugs look great and I burn premium. 20/21 cams stock CIS.

That's a helpful data point- the 7s are about the same as an NGK 6, according to this chart I found on the internet (and therefore it must be true)




I've got a set of NGK 6s coming in the mail, I'll give em' a shot. For what it's worth, I swapped a new set of 7s in and drove maybe 20-30 miles at temperature with them, they look kinda busted already: each of them has one half really dark, and one half lighter (see first photo below for gradient), and they look like they're burning okay but I think they're a bit too cool for this setup.