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Old 04-19-2010, 04:24 PM   #1
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Default engine modding TECH question.

when you do a drag mod on a motor, it lowers the compression because the ports are higher right?.....less area to commpress

my question is that do you take out a head shim to make up for that? to help raise the commpression back up?
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Old 04-19-2010, 09:03 PM   #2
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There is what we call trapped compression ratio and static compression ratio. Static compression is when you figure in the entire volume of the cylinder and chamber with the piston at BDC. Trapped compression ratio is measured from when the piston seals the exhaust port and starts to build pressure and that is also refered to as cranking compression and in the small engine or automotive world, we use compression guages to measure that. I have seen a small guage specifically made for nitro engines so in theory you could simulate the same process. what I have found is depending on the engine, how far you raise the timing and what the original static ratio was will depend how the engine responds to it. Most engine I have done usually responded well to running somewhere between .015-.018" of squish band when most run between .020-.022" stock so taking out the .1mm or .004" shimming will get it very close to stock trapped compression ratio. I have also had engines that have had the exhaust port raised 10-15 degrees over stock and like the stock shimming the best so running a lower trapped ratio was actually better. This is something that needs to be tested by yourself but you need to be able to determine the difference between the safe zone and pre-ignition. Sometimes just trying to listen to what people refer to as "pinging" is not good enough. By then, the damage is already done.
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Old 04-20-2010, 05:58 PM   #3
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im running .015 with a p3 plug, 20% o'donnel in a 28xz...would it be safe to go up to 30% nitro? btw thanks for the help.
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Old 04-20-2010, 06:43 PM   #4
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Has the timing been altered? Is the .015 you are reffering to the current squish band or do you have .15mm worth of shimming. There is a difference. The .28XZ tends to favor stock shimming even with the exhaust raised 7-10 degrees so if you are running stock shimming, you are safe to run 30%.
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Old 04-20-2010, 07:36 PM   #5
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yes its moded, and im running .015" squish band and i have one shim...(stock is 2)

thanks for the help...so when i go to 30% ill put the stock shims back in...
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