Too much compression?

Old 12-20-2014, 09:23 PM
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Default Too much compression?

Hello all! I recently bought a new engine since my last one was completely worn out. I have a VX .16 and I just finished the break-in and had to go buy a new glow plug since my last one fouled out. I bought a McCoy MC8 medium glow plug since I am using Werks 30% race blend. As soon as I installed the new glow plug, not even half a tank later the engine randomly died. I took out the glow plug and the whole middle section of the coil completely broke off and was hanging on the last little piece of top coil. I'm glad it didn't fall into the combustion chamber! The coil was still very shiny and had no discoloration on it. I have a spare O.S. 8 plug but I don't know if I should use it since it's quite a bit shorter than the stock plug. Would the extra combustion space from the shorter plug help the extra compression? Would it damage the engine?
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Old 12-20-2014, 10:34 PM
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Are you sure the engine is shimmed for 30% nitro?
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Old 12-20-2014, 10:46 PM
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Originally Posted by Jaz240 View Post
Are you sure the engine is shimmed for 30% nitro?
It never came with a booklet and I can't find any information on it but I know people who ran 30% on it and it worked perfectly fine. The stock plug never broke when I was breaking it in like the McCoy did. Should I try my O.S. 8 plug and see if the extra space with the shorter plug solves the compression issue as I think it is?
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Old 12-20-2014, 10:51 PM
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It is still running extremely rich from the break in so I haven't had time to tune it before the glow plug blew. Could the rich setting have done that and made the compression too high?
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Old 12-20-2014, 10:54 PM
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Originally Posted by guitarguy19 View Post
Hello all! I recently bought a new engine since my last one was completely worn out. I have a VX .16 and I just finished the break-in and had to go buy a new glow plug since my last one fouled out. I bought a McCoy MC8 medium glow plug since I am using Werks 30% race blend. As soon as I installed the new glow plug, not even half a tank later the engine randomly died. I took out the glow plug and the whole middle section of the coil completely broke off and was hanging on the last little piece of top coil. I'm glad it didn't fall into the combustion chamber! The coil was still very shiny and had no discoloration on it. I have a spare O.S. 8 plug but I don't know if I should use it since it's quite a bit shorter than the stock plug. Would the extra combustion space from the shorter plug help the extra compression? Would it damage the engine?
The length of the plug will affect tuning. I don't believe it can destroy the engine directly.

I was running short plugs in an engine since break-in and I couldn't figure out why it would never warm up correctly. I had to lean it to get it to warm up and then catch it in time and adj it so it doesn't over heat. Didn't catch it in time one time and it over heated.
That engine never acted right. I didn't know the plug was too short. I recently tried a longer plug and it ran right. Warms up like it should without adjustment.

So in a way, ya you could damage the engine if you, because of the plug, can't tune it correctly.

So it depends on if, when using the shorter plug, the engine is tunable or not.
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Old 12-21-2014, 11:20 AM
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Running short plugs in a head button designed for long plugs is not really a good idea. I guess you can consider it a means to reduce the compression ratio but the way plugs are designed to work the element should be flush with the top of the combustion chamber. Keeping things simple the shape of the combustion chamber is specifically designed to direct the flame path basically dictating how the air/fuel mixture burns and where the energy from that is directed. By recessing the plug element up in the glow plug hole (like when you run a shorter plug) you are basically throwing all of that design work out of the window and it causes excessive turbulence and the mixture will not be burned as efficiently all of which can cause tuning issues. Additionally the threads at the bottom of the glow plug hole are now exposed to the combustion process which could cause them to heat up and glow, which can cause pre-detonation and result in additional tuning issues. So in a nutshell using a short plug will more than likely result in more problems than it's worth.

Now on to the plug issue that you mentioned. First if the compression ratio is too high, change the deck height by increasing the shim stack. If the compression ratio is too high you will see it in the coils of the plug being compressed, the will look smushed together. If the coil is just broken that is not necessarily an indication of the compression being too high, that can also happen when running the engine abnormally rich i.e. during break in. So my suggestion would be to finish break in, get your engine tuned and then check for signs of pre-detonation etc. If you see those then go with a colder standard length plug not a short length plug.
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Old 12-21-2014, 03:10 PM
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Originally Posted by Werks View Post
Running short plugs in a head button designed for long plugs is not really a good idea. I guess you can consider it a means to reduce the compression ratio but the way plugs are designed to work the element should be flush with the top of the combustion chamber. Keeping things simple the shape of the combustion chamber is specifically designed to direct the flame path basically dictating how the air/fuel mixture burns and where the energy from that is directed. By recessing the plug element up in the glow plug hole (like when you run a shorter plug) you are basically throwing all of that design work out of the window and it causes excessive turbulence and the mixture will not be burned as efficiently all of which can cause tuning issues. Additionally the threads at the bottom of the glow plug hole are now exposed to the combustion process which could cause them to heat up and glow, which can cause pre-detonation and result in additional tuning issues. So in a nutshell using a short plug will more than likely result in more problems than it's worth.

Now on to the plug issue that you mentioned. First if the compression ratio is too high, change the deck height by increasing the shim stack. If the compression ratio is too high you will see it in the coils of the plug being compressed, the will look smushed together. If the coil is just broken that is not necessarily an indication of the compression being too high, that can also happen when running the engine abnormally rich i.e. during break in. So my suggestion would be to finish break in, get your engine tuned and then check for signs of pre-detonation etc. If you see those then go with a colder standard length plug not a short length plug.
Not saying it should be done but back in 10th scale nitro offroad (GAS TRUCK) we used either or , a little tuning difference but short plug in standard length never seemed to adversely effect the engines running characteristics in a detrimental fashion or long plug in short OS engine really for that matter

Just dont lean it to 350 degrees and im sure u will be ok ,lol
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