Spark Plug Question, Colder plug..............? Options

[Lab Rat]

I have a modified 1.7 in my '73. Running 96mm Mahle pistons, mild Webcam / lifters, stock D-Jet FI and Pertronix Ignitor. I'm running in a PCA Dr. Ed. event this Sat. at Road America, and am concerned about heat generation. Currently I'm using Bosch Super PO 282 (W7CC) plugs. I bought a set of Bosch Super RO 488 (W6DC) plugs, being advised that they are a colder plug. One concern is that they "reach" almost 1/4 inch longer than what I've been using.
Will they hit the piston tops?
Are they actually a colder plug?
I've had a heat problem in the past. Timing is set at 29 degrees advance. Fuel mixture setting at "two clicks" from richest setting. Any help/advice is appreciated.

[Aaron Cox]

A bosch 6 series is colder than a Bosch 7 series.

i assume if all you wanted to change was plug heat range you would have supplemented your w7CC with W6CC....

[Cap'n Krusty]

A D series plug has an extended reach, something not in the plan for a 914, although they probably won't hit the pistons. You might be better off with a W6CC. 6 is colder than 7 in the Bosch numbering system. It does NOT mean the engine will run cooler; it simply means the transfer of heat away from the tip is more efficient. The knob you think sets the mixture only sets it for idle. Once the throttle is cracked, that circuit becomes meaningless. You can bump the fuel pressure a couple of pounds before it messes up the spray pattern, so it won't get you much. D-jet doesn't much like any cams other than stock, so your idle suffers, hydrocarbons go up and driveability suffers. Increasing the bore has raised your compression. Increased compression means more heat. Probably a lot more. I don't know if you condidered this when you built the engine, or whether you took steps to correct the compression, but it's a BIG factor, one that changing plugs doesn't even begin to address. The Cap'n