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Old 08-15-2007, 01:08 PM   #16
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Originally Posted by Frost99999 View Post
I live in central FL, one of the most humid places on the planet :\

according to my iGoogle, the humidity here in Bristol, England is 100%! now that's humid! except to have 100% humidity you need to be underwater. so tell me people, as Igoogle thinks i live underwater, how do i tune my engine to run underwater?
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Old 08-15-2007, 01:37 PM   #17
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Its the timing that is thrown off by going to a higher nitro %. A lot of people try to compensate by using the carb needles but that only goes so far. Head shims, Nitro % and plug heat range determine the timing. Change one and you have to change the others as well to keep the motor happy. The reason a motor will get hot with more nitro % leaving everything else alone is that more nitro advances the timing. Turning the needles to richen it up just puts more oil into the motor and cools if off a bit. The reason for this is that the nitro and the platinum in the glow plug wire cause a chemical reaction when put together under compression. Using the same plug the actual point of ignition will advance because you have more nitro to make the chemical reaction go. Get yourself a good loupe and learn to read the plug. That is the best way to make sure you aren't going to detonate a motor. A couple of things to look for in particular is if the coil is offset to one side after the plug has been run and the general condition of the wire. If the coil is offset you have too much compression, too low a deck height or its right on the verge of detonating. If the wire looks severely pitted then you are closer to detonating one than you want to be. The pits come from the platinum wire getting blown off by the higher than normal pressures developed in the cylinder. A couple of rules of thumb are

1. more shim retards the timing with the same plug and nitro%
2. less shim advances the timing with the same plug and nitro%
3. More nitro% advances the timing if the shims and plug stay the same
4. Less nitro% retards the timing if the shims and plug stay the same
5. Hotter plug advances the timing if the other two variables stay the same
6. Colder plug retards the timing if the other two variables stay the same

Start working with the plug shims and nitro% and you won't be messing with those needles near as much. A deck height gauge is a handy tool to have but you can measure the deck height with a piece of solder and a caliper as well. A motor will overheat if its too advanced or too retarded in timing. When its timed properly the exhaust will have a sharp cracking sound instead of a dull thud when it fires. And if you get out to the track one day and its super humid drop that nitro % and your motor will run better.

Griz
Great post Griz. Lots of good info in there. I'm going to post it in my local clubs thread, be a good read for them, hopefully you don't mind.
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Old 08-15-2007, 07:53 PM   #18
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in air humidity can be well over 100%. if your submerssed in water your drowning, not sweating. lol
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Old 08-15-2007, 08:49 PM   #19
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I learned all that stuff running top fuel dragsters. The nitro variety of course I quit when it went to brushless motors for the most part. Now I just video races. I always come home with all my stuff intact and I win every weekend Sure go ahead and share that with anyone you want too. Might help someone keep from blowing up a motor sometime.

If you have super high humidity like we have here in Tx from time to time the motor just isn't going to run that well. Mostly because you don't actually burn that much nitro only the methanol and that gets diluted by grabbing up water when it goes through the carb. Since you are primarily running on methanol go to a lower percentage of nitro to get more methanol into the motor and go to a hotter plug to advance the timing just a bit. Pretty much factory settings on the carb to get as much fuel in as possible since some of it isn't going to burn since its contaminated by water. I thought this was crazy the first time I heard it. But since it came from a 2 time World Finals top fuel guy I tried it out. After seeing him win pretty much everytime it was real humid I figured he had it right. Sure enough I ran great that day and took home a 2nd place right behind the guy that showed me the way. I know about that English humidity. One summer when I was living there there it rained every day. I was born right outside of Cambridge and lived there until I was 7 years old.

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