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Old 06-30-2006, 04:08 AM   #1
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Default What causes Detonation ?

I have recently had a couple of motors in my shop that have had damage from pre ignition . They have pitting on the top of the piston and button. Also both the owners experienced glowplug failures prior to this.

I would normally think this is from over compression, or using too hot of a plug ?? Is this correct ? Could there be other reasons for this ?

Any help would be appreciated..
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Old 06-30-2006, 04:54 AM   #2
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Yes, it depends on 3 things
- Glowplug
- nitro percentage
- head clearance (overcompression)

A hotter glowplug as well as a higher nitro percentage and a smaller head clearance gives an earlier ignition......

The plug used, together with the nitropercentage and the head clearance should be in balance.......
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Old 06-30-2006, 05:04 AM   #3
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You forgot one Item. To short of a tuned pipe will also cause detonation. If the head shimming is correct for the percentage nitro used and the engine continues to detonate suspect the pipe is to short.
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Old 07-02-2006, 06:08 PM   #4
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Sorry guys two more items.

A bad rear bearing will also cause det as well a streched rod.
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Old 07-03-2006, 05:57 AM   #5
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Quote:
Originally Posted by SpeedBump57
Sorry guys two more items.

A bad rear bearing will also cause det as well a streched rod.
Your right, also this is the most common problem when detonation occurs, to much play on the conrod / main bearing.......

You know why I didn't mention that, because I was under the impression that we were talking NEW engines here, because the TS said he had seen engines back in his shop with this kind of damage.

He never mentioned they were new, just my conclusion..... stupid me.....
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Old 07-03-2006, 08:45 AM   #6
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No need to feel stupid!! You gave good answers.
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Old 07-03-2006, 07:37 PM   #7
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The other thing is given the fact that he said that they were returned with glow plug failures it is possible the engines were run too lean.
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Old 07-03-2006, 11:04 PM   #8
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Correct! Running to lean can cause a plug to fail and collapse. What I didn't know was that running too lean could cause detonation. Can you explain why?
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Old 07-04-2006, 12:09 AM   #9
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Well the way i understand it is basicly the same as in a 4 stroke car engine. If you have too lean a mixture it causes the cylinder temp to become to great and causes pre-ignition, however in a car engine what usually happens is the extra heat from the lean mixture causes hot spots on pistons or valves and that is what cause the fuel miture to ignite prematurly, the other side of the coin is that the pitting could have also been caused from the element when it expired bouncing around inside the liner (not likely but a possability).

Cheers Daniel
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Old 07-04-2006, 05:28 PM   #10
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Seem to have fxed the detonation by adding extra .3mm of shims. Head clearance with .1mm shim(standard) was only @.4mm . Now with a total .7mm they ran fine.

These are buggy motors running 25% nitro and turbo plugs (C7TF) The motors in non turbo form don't seem to have the same problem.

I think the manufacturers have now increased the clearance in the latest batch.

Thanks for the help.
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Old 07-14-2006, 02:34 AM   #11
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and can be from running too much castor oil in the fuel blend...i got marks like smoke marks on the top of my piston and on the sides and stuff from running way too much fuel..like 7 gallons before i cleaned her again...i recommend that the engine be pulled apart and cleaned out (or polished) so that the carbon buildup is removed asmuch as possible...
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Old 07-14-2006, 08:47 AM   #12
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Quote:
Originally Posted by Qlder
Seem to have fxed the detonation by adding extra .3mm of shims. Head clearance with .1mm shim(standard) was only @.4mm . Now with a total .7mm they ran fine.

These are buggy motors running 25% nitro and turbo plugs (C7TF) The motors in non turbo form don't seem to have the same problem.

I think the manufacturers have now increased the clearance in the latest batch.

Thanks for the help.
If its a .21 engine 0.4mm head clearance is way to small. 0.7 is normal.

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Old 07-14-2006, 01:23 PM   #13
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Quote:
Originally Posted by ziggy12345
If its a .21 engine 0.4mm head clearance is way to small. 0.7 is normal.

Cheers
I disagree...
Everything between 0.5 and 0.8 is common.....
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Old 07-16-2006, 01:18 PM   #14
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I am having trouble with detonation on my .12 engine. What should the shim be on a .12? I think it may be shimmed wrong. I only get about 4 tanks on one glow plug if i am lucky. My engine also doesnt hold a tune. One week i ran it and it ran fine, had a good tune. The next week i started it up and about 5 laps later it over heated and when i checked the temp it was 275.

The engine is a novarossi tsr12s3
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Old 07-16-2006, 04:32 PM   #15
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Quote:
Originally Posted by ziggy12345
If its a .21 engine 0.4mm head clearance is way to small. 0.7 is normal.

Cheers
For a very few times. I agree with M7H... All depends on many factors:

- Nitro content
- Relative humidity
- Altitude over the sea level.

One of the best gadgets one can bring to a track is a small weather station (Oregon Scientific have some very nice ones) and start to get some notes about the temp, height and rel. humidity during the use of our engines and start messing with the combustion chamber height.
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