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Old 02-09-2008, 12:03 PM   #1
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four stroke detonation

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i was at a four stroke instruction site, and they where talking about detonation, they explained how it happens, however the did not say how to fix it. i'm not very familiar with four stroke engine yet, so there are some few question, for the guru's out there.

1. How do you recognize detonation. it was explained that it is bad for the engine.
2. how do we fix it.

thanks in advance

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Old 02-09-2008, 01:58 PM   #2
Gary Maker
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I think everyone who has owned a 4 stroke engine has experienced detonation at some point. Detonation is usually caused by having your mixture setting too lean expecially on the low end or throttle idle side.

The fuel air mixture fires or ignites too soon before the poston has made its full upward travel which can stop the piston travel abruptly and what happens is the engine Pops and throws the spinning propellor right off the shaft. Thats one sure way to recognize detonation.

How to fix is fairily easy. What I do to set up my needle settings is to tune the settings for maximum RPM and then back off the needle dropping the RPM about 3 - 400 RPM. This will assure that your not running too lean and should take care of the detonation.

I have also been told that under rare circumstances, that detonation can also occur if the compression of the engine is too high. You will hear a sharp popping sound as your engine is running at even higher RPM settings. A friend said he had this problem but was able to fix it by using a shim under the head or the cylender reducing the amount of compression. Perhaps someone who has more knowledge about this aspect of the engine should explain this!!!

I really wouldn't worry too much about detonation unless you are experienceing your engine popping and throwing the prop. Just run your engine slightly richer and you shouldn't have a problem.
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Old 02-09-2008, 11:37 PM   #3
David McMillan
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What your talking about is pre-detonation or effectively knocking or dieseling.

before fuel injectors, badly tuned engines that got hot could experience run-on. that sputtering that can wiggle your car for 5 to 10 seconds after you shut it off.

Same effect. older automobiles have a distributor where you can set the detination point with respect to Top Dead Center (TDC) Now it''s usually controlled with electronic ignition to optimize combustion.

All 4 stroke engines have a spark advance, as the objective is to reach maximum compression for maximum power. You want the fuel to reach peak heat, and peak pressure during the burn cycle when the piston reaches TDC.

If the rpm is too low and the cyllinder is hot you can generate enough pressure before TDC to kick back the piston and drive the model engine backwards.

A Few things you can do:
Find a slower burning or cooler running fuel.
Reduce compression as mentioned (and power)
Keep your rpm a little higher at idle.

My first suggestion would be to find a heavier Prop. If you can add rotational inertia to the engine, it will be less succeptable to kickback.

Think Bigger flywheel... more rotational inertia and so the force needed for kickback or detination will be higher.

Hope this helps...

David McMillan
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Old 02-10-2008, 03:41 PM   #4
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answer,explanation and solution. exactly what i need.

thanks Gary and dave
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Old 02-11-2008, 08:31 AM   #5
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A few months ago I purchased an ASP .61 4S. When I googled it this article came up on detonation. The cure was put a .020 " aluminum shim under the head and switch to 5% nitro fuel. This engine was designed to run on FAI fuel and the compression is very high. I made a shim of .025" aluminum and installed it. The only fuel I had was 10%. at full throttle it still pinged. I have found a hobby shop with FAI fuel so I am going to try that. Pinging on this engine sounds like the cylinder is being tapped with a screwdriver unevenly.
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