View Full Version : Spark plug gap/heat range?

05-23-2008, 10:39 PM
So the Bugeye runs pretty well...but now time for some fine tuning. The car has a 948, 40 over bore, Carillo rods, Cosworth pistons, still has the 1-1/8 carbs, high compression, cam...

So I want to look into how cold of a plug to use..I have NGK BP8ES and BP9ES plugs to run and wonder what to gap them atand if I should run a particular needle in the carbs?



Michael Oritt
05-25-2008, 08:59 AM

In the Elva I use BP9ES's gapped at .30--this with a Lucas Sports Coil and Pertronix ignition. Sorry I don't know the needle offhand.

05-27-2008, 03:36 PM
"""""So I want to look into how cold of a plug to use.."""""

Depends on the compression ratio of the motor. at 10:1 you might want to start with a 7. At 11 or 12:1 you might want to start with a 8. over 12:1 start with a 9.

Get your fuel curve close. Use a suitable fuel to not have any detonation. After a hard lap cut the motor and coast back. Inspect the plugs with a Coddington light. You might read up on how and what part of the plug to examine. You want to use the hottest plug you can based on examination.

Hap Waldrop
05-28-2008, 05:48 AM
As always Jerry B gives great advice, I'll throw this in for what it's worth, on really high compression ratio motors (14+ to 1) we've used NGK B10ES or NGK B10EV, the higher the number with NGK, the colder the plug.

Doug you got any idea what compression ratio your 948 is?

I think if you run the B9ES with a gap of .030" you'll probably be ok, just keep a eye on everything. You got EGT guages?

05-28-2008, 10:31 AM
And for reference, my 10.25:1 comp ratio Spridget is running 0.032" gap with Pertronix control unit and "Standard" 12V coil and ballast resistor (intended for '65 Chevy V8). No fouling so far.

A temporary wide band 02 meter pegged my exhaust at about 13:1 A/F ratio (after some re-jetting...but I do not use SU carbs). My plugs are nice, brown/tan and I swap them about every other event.

I've tried to do the "plug chop" method, but it's hard for amateurs at most race tracks since you really need to motor into the cold pits before doing any work

I know everyone has opinions about plugs....here's a few choices that are a cross to the plugs I've used.

<span style="font-style: italic">NGK- BP8ES
N6YC Champion
Autolite 51
Accel 112</span>

I've mostly been using the Autolites just because they're cheap and easy to get. No complaints so far.

Michael Oritt
05-28-2008, 12:29 PM

What is a Coddington light? I googled the term and only got a reference to your post.

05-28-2008, 01:57 PM
A V4 hot rod :crazyeyes:


05-28-2008, 07:38 PM
Ah, the old Champion and Ac spark plug inspection light,

Gave mine away a couple of decades ago.

Hap Waldrop
05-30-2008, 03:27 PM
Doug, be careful with the "P" in a NGK spark plug part number , lower race compression ratio engines with flat top and dish piston can get away the "P" (P stand projected tip) in the part number, but a 948 with pop up pistons or any pop up piston motor could make contact with the projected tips, so lose the "P" in NGK part number, I would go for a B9ES if were me. A conservative 948 race motor would be atleast 12.0 to 1, a realy go-getter could be as high as 16.0 to 1, you need to call me someday and let me tell you about head gasket prep on a 948 as well.