Why the 12mm Spark Plug?

YTNUKLR · YTNUKLR's Garage

I saw what seemed like factory 2.5 911S-T Race Heads on Ebay a while back, and the listing stated they were 12mm plugs (both). It seems silly for someone to have welded up the standard 14mm plug so I assume those were factory race heads.

I have also heard that Porsche used 12mm plugs as the secondary plugs in the twin-plugged race engines.

Why is this the case? What is the benefit to running a 12mm spark plug?

Cheers

Scott

juicersr · juicersr's Garage

Scott, pm sent

MBruns · 08:11 AM

It is common practice and sometimes even 10mm, there are great selections of race plugs ie. motorcycle plugs, and the main reason is the chamber vol. in the head is not increased like with the larger plugs.

Mike Bruns JBRacing.com

manu72S · 08:24 AM

Exactly the same opinion as Mike

cgarr · cgarr's Garage

I only do twinplugged heads with the 12mm second plug because you leave more material in the head and with the 12mm plug you do not have any clearance problems with the stock head stud nuts.

930gt-40r · 930gt-40r's Garage

Quote:

Originally Posted by cgarr

I only do twinplugged heads with the 12mm second plug because you leave more material in the head and with the 12mm plug you do not have any clearance problems with the stock head stud nuts.

Is it also good for combating cracking between the second plug and the exhaust valve seat area? This is what I have always thought to be the reason.