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Engine basics => Engine set up tips => Topic started by: Steve Fitton on October 01, 2009, 06:54:10 PM
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I should know this, but, I'm curious for some opinions. In engine tuning, if you want to increase/decrease power, can changing the number of head shims in a given motor act the same as going up or down in nitro? In other words, if you wanted to use a certain fuel, could you just adjust the head clearence for the ambient weather conditions and run that way-kind of the opposite of the guys who tweak nitro for weather conditions?
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Along the same line of thought, are venturi filters or venturi size changes going to do the same thing as nitro changes?
Popping in a head shim isn't that tough a job, nor is adding an air filter or another layer of panty hose discs. Not only is carrying around two or three jugs of fuel a PITA, just getting a decent fuel in more than one percentage of nitro content is about impossible around these parts in the last year. The LHS dropped Powermaster and gets us CL peons SIG fuel...in quarts...for about $10/each. Not going there! :P Steve
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I did some experimenting with compressions on an Enya 60X and gained a tad over 20% more HP (an extra 800 revs) using zero nitro fuel. I also experimented with a CL G51 by lowering compression to try to tame a 2 stroke run away. It certainly tamed the extra 2 stroke power but it also cut back the 4 stroking power by a similar amount, not what I was looking for. So I guess the answer to your question is that yes, adding/removing shims seems to do the same as altering nitro percentages although I never use nitro so can't be certain.
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I think there are at least two things going on. n~
First Nitro brings some oxygen to the combustion process, so it allows you to richen up the mixture (more fuel) and still burn it efficiently. So that is like having a larger venturi that lets the engine breathe more air (with oxygen) and bring in more fuel. In both cases you are burning more fuel, so that gives more power.
Head gaskets help change where the ignition point occurs. That can help or hurt power "depending" . With 1/2a at least, when I go up in Nitro (serious increases up to 25-30%), I also add head gaskets so preignition doesn't occur.
I am sure changing gaskets will affect power, but I am not sure how easy it is to pre-figure it out. I am sure some guys have.
I think the easiest may be the air filter trick- which is equivalent to changing venturi size (small changes I think). If you tune up the engine so that cool dry days use the most filter, then when you hit the hotter/more humid days you strip off the filters.