Research into Colder Spark Plugs - Z27AG & CT9A & WRX STi - 14mm dia × 26.5mm LongReach - 16mm

I previously posted some info here http://ayu452.blogspot.com.au/2011/09/z27ag-spark-plugs-ilzfr6c-k-ikh22.html about plugs.
But I have recently found out a good rule of thumb is to run a colder plug ever 100hp increase.
At the moment i am guessing i run about 260HP and want to go one step colder.
From my research the stock plug is a #6
I want to go a #7 so this should include the HKS M35iL or a NGK MAX Iridium Spark Plugs LFR7AIX-P or Denso Iridium Power IKH22 (is one heat range colder than IKH20 whichi  believe is a 6).

Gapping - http://www.ngksparkplugs.com/
Another consideration that should be taken into account is the extent of any modifications that you may have made to the engine. As an example, when you raise compression or add forced induction (a turbo system, nitrous or supercharger kit) you must reduce the gap (about .004" for every 50 hp you add). However, when you add a high power ignition system (such as those offered by MSD, Crane, Nology) you can open the gap from .002-.005".

Colder Plugs
Denso description -

When the air temperature is high (summer):
The inlet air temperature also becomes higher, increasing the load to the engine. Choose a plug with a higher heat range.
If horsepower has been increased through tuning:
The increase in explosive power leads to an increase in combustion chamber temperature, making pre-ignition more likely. Choose a plug with a higher heat range and a higher level of heat resistance.

Different Tips>>>



Heat Range Conversion Chart.
HKS  DENSO  NGK
25  16    5
30  20    6
35    22    7
40  24    8
45    27    9
50    31    10
52.5   32    10.5






HKS SUPER FIRE RACING M35iL (Heat Range 7) - Used in Evo 9/350Z
I also found these cars: 2010 Hyundai Genesis Coupe 2006 Mitsubishi Lancer 2006-2007 Subaru Impreza
http://www.hks-power.co.jp/product/plug/m_series/index.html
M-iL = Type for Longreach

http://www.hks-power.co.jp/product_search/product/326
φ14×26.5mm - 16mm - NGK7 - SUPER FIRE RACING M35iL - 50003-M35iL - ¥2,310 (¥2,200)

These should fit but at a colder plug in the HKS Range (based on the dimensions):
φ14×26.5mm - 16mm - NGK8 - SUPER FIRE RACING M40iL - 50003-M40iL - ¥2,310 (¥2,200)
φ14×26.5mm - 16mm - NGK9 - SUPER FIRE RACING M45iL - 50003-M45iL - ¥2,310 (¥2,200)

Found some handy info from the http://www.speedtechnz.com page when looking at EVO 9 service packages for spark plugs:

Stage Power Level (ATW) Plug Type Plug
Stage 0 Stock Iridium NGK - ILFR7H
Stage 0 Stock Iridium Platinum HKS - M35IL
Stage 1 up to 250kw Iridium Platinum HKS - M40IL Suitable Models >>>
Stage 3 400kw plus Iridium Platinum HKS - M45IL

---JDM Part Numbers and assorted---
Looks like its harder to find apart from Japan CT9A. I believe its because of the so called MAX

NGK Spark Plug IRIDIUM MAX - LFR7AIX-P - Stock #2900 - This was because it was higher HKS.
Was in the NGK iridium MAX LFR7AIX-P this time → 2 → DENSO Iridium Power IKH22 → HKS Fire Racing M35iL × genuine.
Screw diameter 14mm - Thread length 26.5mm - Hexagon width across flats 16mm -

Mitsubishi genuine spark plug [P#1822A005]- ILZFR6C-K - Stock #5617 [for a Z27AG and a Z27A - 4G15 Mivec Turbo]
Denso\IRIDIUM TOUGH- VKH20 can be cross referenced with ILZFR6C-11K
IRIDIUM POWER - IKH20

Equivalences (according to http://japansparkplugs.com):
- NGK LFR7B / ILFR7B / LFR7AIX / LFR6AIX (Colder) / LFR6AIX-P (Colder) / LFR6B (Colder) / ILFR6B (Colder)
- DENSO IKH22 / VKH22 / K22HR-U11 / IKH20 (Colder) / VKH20 (Colder) / VKH20Y (Colder)

Information From - http://ngk-sparkplugs.com
NOTE: The heat rating of each NGK spark plug is indicated by a number; lower numbers indicate a hotter type, higher numbers indicate a colder type.

Parts No. : LFR7AIX-P
Stock No. : 2900
NGK MAX Iridium Spark Plugs LFR7AIX-P
Design Symbols used in NGK Spark Plugs
LF - Plug Type: Long thread reach plug Gasket type (26.55mm reach)
R - Resistor
Heat Rating Numbers : 7
A - Firing End Construction: Special Design
IX - Firing End Construction: High Performance Iridium
IX-P - Spark GAP: Iridium MAX Spark Plug


Parts No. : NGK Iridium Plugs ILZFR6C-K
Stock No. : 5617 / 6645
NGK Iridium tipped types Spark Plugs ILZFR6C-K
Design Symbols used in NGK Spark Plugs
I - Plug Type: Iridium Platinum
L - Metal Shell Size: ?10.0, 12.7mm, (1/2¡¨) Gasket 16.0mm, (5/8¡¨)
Z - Plug Type: Extended Gap
F - Metal Shell Size: ? 14, 19 mm (3/4") Reach, 16 mm (5/8") Hex.
R - Resistor
Heat Rating Numbers : 6
C - Firing End Construction: Low Angle Ground Electrode
K - Spark GAP: Vibration-resistant Ground Electrode


Parts No. : ILZFR6C-11K
Stock No. : 4673
NGK Iridium tipped types Spark Plugs ILZFR6C-11K
Design Symbols used in NGK Spark Plugs
I - Plug Type: Iridium Platinum
L - Metal Shell Size: ?10.0, 12.7mm, (1/2¡¨) Gasket 16.0mm, (5/8¡¨)
Z - Plug Type: Extended Gap
F - Metal Shell Size: ? 14, 19 mm (3/4") Reach, 16 mm (5/8") Hex.
R - Resistor
Heat Rating Numbers : 6
C - Firing End Construction: Low Angle Ground Electrode
11 - Spark GAP: 1.1 mm (.044")
K - Spark GAP: Vibration-resistant Ground Electrode

Cross Reference Denso Parts No. : VKH20
Denso IRIDIUM POWER : IKH20
Denso IRIDIUM TOUGH : VKH20

---USA Part Numbers and assorted---

NGK Rating #7
NGK LFR7AIX Iridium = also fits 06-07 WRX, 04-07 STi, 05-08 LGT & 10+ Genesis 2.0L T
NGK ILFR7H Iridium\Laser Iridium Stock #5245

NGK Racing Plugs GAP .024:
NGK R7437-8 Stock #4901 = NGK #8
NGK R7437-9 Stock #4654 = NGK #9
NGK R7437-10 Stock #4655 = NGK #10


Denso Iridium Power
IKH series
IKH20 = Heat Rating NGK #6
IKH22 = 1 Step Colder (therefore Heat Rating 22 or NGK #7)
IKH24 = 2 Step Colder = Heat Rating NGK #8
- 14mm dia. X 26.5mm length long reach type.
- For Fits Subaru 2.5 Turbo Models. 04-12 STI & 06-12 WRX, Nissan, Peugeot, Citroen, and Yamaha Marine.
- IRIDIUM TOUGH VKH20, and VKH22 are also on sale.