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Old 08-26-2008, 11:40 AM   #31
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ok, i have a question about fuel.

say i buy a nitro engine that can use 16-40% nitro. i break it in using 20%.

do i have to do anything different besides tuning to run 30%? i've heard shims being mentioned here, but dont really know how it relates to fuel %.

can someone give me a dummies breakdown of nitro content?
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Old 08-27-2008, 12:14 PM   #32
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The fuel/air mixture will ignite earlier with an increase in Nitro content. It will also ignite earlier with a reduction in head clearance. If an engine is properly shimmed for 20%, one should add a .1mm shim for 30%. This is to keep the ignition from occuring too early (pre-ignition).
Pre-ignition will cause pitting of the piston dome and the combustion chamber, also blows plugs.
One can get by with less head clearance for drag racing, due to the short duration runs.
Hope this helps.
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Old 08-27-2008, 12:25 PM   #33
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actuallly, thats exactly what i needed to know.

So i cant just switch from 20 to 30% without first shimming or else i will mess up my engine.

the other question i have is are shims manufacturer specific or is a shim a shim?

thanks!!!!
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Old 08-27-2008, 02:58 PM   #34
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You MAY be able to switch without adding a shim. It depends on how you are operating the engine and how closely it was shimmed before. In other words, if your engine is shimmed a bit on the + side for 20%, it would probably be OK on 30. You can inspect the piston and chamber after a few minutes run time. If it is nice and shiny, it's all good. If it is begging to dull, it's time to add a shim. You could just add one to be on the safe side, with little loss of power.
The shims are not brand specific. They are interchangeable between engines with the same bore.
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