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Info guides and mods

451 Spark Plug Specs

The new 451 uses Denso spark plugs as standard. Here is all the information about the OEM plugs.

Modification Details

What Are The Standard Spark Plugs?

The factory fitted spark plugs are Denso XU22HDR9.
From the end on 2012 smart started also using FXU22HR8

What Do All The Numbers Mean?


XU = 12mm thread and a 16mm hexagon top.
22 = Heat range of 22.
H = 26.5mm reach.
D = Insulator is 1.5mm projected, spark position is 3.5mm.
R = With resistor.
9 = 0.9mm electrode gap.



F = Iridium
XU = 12mm thread and a 16mm hexagon top.
22 = Heat range of 22.
H = 26.5mm reach.
R = With resistor.
8 = 0.8mm electrode gap.

Thread And Hex

12mm is the thread size that screws into the engine.
16mm is the size of the spark plug socket required for removal.

Heat Range

Companies use different numbers to represent the same heat range.

Denso = 22
NGK = 7
Champion = 7 and 8
Bosch = 5


Obviously the electrode has to sit in the correct position in the combustion chamber.
The reach is the length of the 12mm thread mentioned previously.

External Construction

The ceramic insulator is projected by 1.5mm and the spark position is 3.5mm.

Internal Construction

Between where the power goes in and the spark comes out is a resistor (on the smart
OEM version), this controls the strength of the current to ensure optimal spark strength.


This is the gap between the centre electrodes and the surrounding earth electrodes.
Denso plugs come pre-gapped so you should not need to alter the spacing.

Performance Spark Plugs

They have taken their time but Denso have finally started shipping the Denso Iridium performance
sparkplugs to the UK. The part number is IXUH22. These are available from SmartMods.

If you have a preferred make of spark plug, simply match the information to that above.

Please remember that a lot of plugs claiming to be performance plug are anything but.
As is the case with the area of engines, there is a lot scientific rubbish out there.

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