Some Thoughts on Spark Plugs

[jluetjen]

OK; A quite day today so I figured that I'd screw some plugs into the heads for my one-day race engine and get some baseline cc measurements for the heads.

1) I screwed in a WR5DP plug that I had into one of the heads. Hmmmm it's ENTIRELY shrouded by the plug hole. I was expecting that the tip would have protruded into the cylinder a little bit, but nope, it is entirely buried down in the spark plug hole. Even after torquing it down. Any thoughts????

2) So I did a few searches in my library and on this BBS and double checked the plug type. Yup, WR5DP is the commonly recommended plug for 911's.

3) I went to the Bosch page and found some interested FAQ's including this interesting chart which I figured others might find valuable. They also have this handy decoder table.

It's interesting that the only difference between the WR5DP and the WR5DPO which is recommended for the early engines is that the "O" appears to signify nickle plating on the tip.

In the interest of fair disclosure, NGK also has some useful information.

Any thoughts on how shrouded the plug was in the head???

[Steve@Rennsport]

Hi John:

What you saw with your correct 3/4" reach plug is normal. They DO sit down into the recesses of the plug holes.

In a perfect world, this is not the best way to position the plug gaps since flame propagation is relatively slow in these hemispherical chambers and this exacerbates it. This is why projected nose plugs are so important in most 911 engines. Porsche does this for piston dome clearance (in high compression engines) and to help prevent pre-ignition from hot spots at any exposed threads that collect carbon. Years ago, we experimented with some different flame trenches in the heads on single-plug motors looking for additional torque. Worked pretty well,.....

The Bosch "DPO" plugs are just the good old (horribly expensive) platinums used from '69 on to help prevent plug fouling in the MFI's engines. They used to be called W250P21 and W265P21; same plug.

Using a projected nose plug is a good thing on all of these engines as long as one has piston dome clearance for them. Not until one goes over 10.5:1 CR does this become an issue. The further into the chamber that you can position the plug gap, the less total advance you will need for best power and that's a good thing. Like anything else, everything is a tradeoff and this issue has all its own,....

[jluetjen]

Now that my wife came home I went and checked all 7 of my heads (I have a spare) and the other 6 were not as recessed as the first head that I checked. Looking at it closer the plug on the first head was not even bottoming in threads because they were all caked up with carbon. I cleaned them out with a tap and now the plug sits comparably with with the others.

I also checked the heads in "as removed" condition, but using the same 2 valves in each head and they were all 70 cc's. So it looks like they're in pretty good shape in that regards. Of course I'll check them again after I've had the valves done just to make sure.

Thanks for the response Steve.