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"Thank-you for your recent inquiry about the Suzuki RM125 and the recommended spark plugs for this application.
First I would like to explain why the R6918B-8 is so expensive. This particular spark plug was designed and engineered for this specific application. Due to the fact the production volume is lower and the number of spark plugs sold is much lower then that of the BR9ES, the cost of the spark plug is increased. The production costs and the
fact this spark plug utilizes gold palladium, a precious metal the cost is much more.
The Suzuki RM125 is a high revving, high compression engine and has vibration issues throughout the power band. We've tested with Suzuki and found the ES, EG, EV and EGV and determined these spark plugs fail under certain loads and conditions. If you compare the two spark plugs you'll notice a major difference in ground electrode design.
The R6918B has an electrode designed to withstand severe vibration and possible detonation associated with these bikes. A standard spark plugs ground electrode is
welded on the outside of the metal shell and can vibrate like a tuning fork throughout certain rpm ranges. This severe vibration causes fatigue and the ground electrode fails. This is why the R6918B was chosen for this application. The R6918B is very reliable and durable in these conditions. Ignitability with this spark plug is better then the
ES series due to the diameter of the center electrode. Any time you have a racing engine you need to choose a spark plug designed to withstand this type of environment.
I would also like to inform you I am the person who takes care of damage
claims at NGK Spark Plugs. I have had more BR8ES and BR9ES spark plugs
cross my desk every month with broken ground electrodes and receipts for
repairs. When the ground electrode fails it bounces around leaving indentations in the cylinder head and the top of the piston. If your lucky the ground electrode will sneak out of the cylinder, however if your not the
ground electrode wedges itself between the piston and cylinder wall and intern destroys the engine.
I hope I have helped you in better understanding why this spark plug is used in this application. If you have any questions or concerns please feel free
to contact me at (248) 926-6900 ext 248.
Sincerely,
Jon MacQuarrie
Jon MacQuarrie
Technical Customer Service Representative
jmacquarrie@ngksparkplugs.com"
The Shop
Compare it to an BR8EIX
R6918B-7
Racing Plug,
14mm Thread,
19mm (3/4") Reach,
13/16" (20.6mm) Hex Size,
Gasket Seat, Resistor,
Solid Terminal,
Platinum Tipped Fine
Wire Center Electrode,
Angled Ground Electrode,
.024" (0.61") Gap,
Heat Range 7
BR8EIX
Iridium IX Plug,
14mm Thread,
19mm (3/4") Reach,
13/16" (20.6mm) Hex Size,
Gasket Seat, Resistor,
Removable Terminal,
Fine Wire (0.6mm) Iridium Center Electrode,
Tapered Cut Ground Electrode,
.032" (0.8mm) Gap,
Heat Range 8
Thats service!
Thats service!"..........
DITTO.
Dave O.
Usually fine to dial it in with cheap plug. But limited use!
I can actually modify the heads so you don't need to run the expensive plug.
I've ran standard plugs, iridium, and NGK's "super plugs" through several RM trials using an exhaust sniffer hooked to PC graphing software to see burnt fuel mixture ratios. The data didn't lie. The standard plugs burnt the least efficient, while the "super plug" burned the most efficient. This was a controlled test where jetting was spot-on from dyno runs and a consistent race fuel was used. The only difference was plug swapping. I actually saw a big of a difference as 3.5% parts per thousand of unburnt fuel through the exhaust sniffer between the cheapest and most expensive plug.
I also found the mid-line iridium series plugs burn't virtually to the same specs as the super plugs. The iridium seems to be a great compromise between the two, since it's only $10-15, but mind you it also is like the standard plug because it does not have the fail-safe design to prevent breakage during failure, potentially to damage the engine. I run the iridium series in every engine i build, besides any RM's.
In my mind, i've always thought of the $30 plug as cheap insurance as one more thing I can eliminate that could potentially damage my engines.
Maybe you should try selling him off on your blueprinting and balancing instead of a squish-band reshape to compromise the compression ratio.
One could possibly argue that changing the compression ratio could reduce some internal stress on the engine, but thats merely putting a band-aid on a wound that needs stitches. the real problem of the RM engines is the extreme vibration caused by some not so perfect engineering.
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