Shims & Pre Detonation

[Zilly]

Can someone please explain what pre detonation is and how to tell if it is happening?

I am running a YS 91 with Widcat 20% and no mods.

How do shims affect pre detonation?


z

[marked23]

I'm an OS guy, but I recently fought predetonation issues and got the problem solved rather nicely by adding a shim.

Predetonation, as I understand it, means that the air/fuel mixture is igniting before the optimal time during the compression stroke.

You can tell you have predetonation when you try to lean out your engine…and it begins to run rough as you lean it out. If you have just broken in an engine by running it rich, and you go to lean it out, and it starts kicking the tail, instead of gaining power, this is a sign of predetonation.

Engines are designed to produce maximum optimal compression because that produces the most power. However because of manufacturing tolerances, local atmospheric conditions, glow plugs, and fuel choices all vary, sometimes this maximum optimal compression is too high.

As the fuel/air mixture is compressed, its flashpoint goes down. The higher the compression, the lower the flashpoint. The piston keeps compressing the air/fuel mixture until the flashpoint of the mixture is lower than the temperature of the glow plug filament. At this moment, the air/fuel mixture starts to burn.

So predetonation is really all about timing. You want the air/fuel mixture to start burning at just the right time. If you have predetonation, your timing is advanced too far and you need to retard it.

Options for retarding timing:
Run the mix excessively rich
Use a cooler glowplug
Use a fuel with a higher flashpoint
Lower the compression of the engine

Running the engine rich smoothes out the predetonation problems, but it wastes fuel and does not produce as much power as the engine is capable of.

Using a cooler plug. I tried this, but it didn’t work for me. I think I tried using an OSA5 plug. All it did was cause the engine to die when going through the transition.

I never tried changing fuel, I have cases and cases of the fuel I use and I wasn’t about to go try something else.

To lower the compression ratio of your engine, add a shim. If you can’t find a single shim that is thicker than the stock shim, you can stack two or more shims on top of each other.

My results from adding a shim were downright dramatic. I had heard stories about a loss of power by adding a shim. If you actually have predetonation, then adding a shim will increase your power, because now you can finally lean out the engine properly.

I think that’s all I know on the subject.

-Mark

[cdrking]

Are you guys talking about preignition or detonation?

[marked23]

Let me check....
googling...

preignition:
The ignition of fuel in an internal-combustion engine before the spark passes through the fuel, resulting from a hot spot in the cylinder or from too great a compression ratio for the fuel.

detonation:
Detonation in otto-cycle engines is caused by the detonation of the unburnt portion of the fuel (knocking) (due to its overly high sensitivity to heat and pressure of the particular fuel under certain conditions) resulting in a pressure wave. This force is extremely destructive to engines, and often results in holes blown through the top of pistons or cracks in cylinder heads.

From this site: http://www.streetrodstuff.com/Articles/Engine/Detonation/
Confusion and a lot of questions exist as to detonation and pre-ignition. Sometimes you hear mistaken terms like "pre-detonation". Detonation is one phenomenon that is abnormal combustion. Pre-ignition is another phenomenon that is abnormal combustion. The two, as we will talk about, are somewhat related but are two distinctly different phenomenon and can induce distinctly different failure modes.

Final answer: I don't know.

But we're talking about early timing of an otherwise normal ignition... so I'm guessing we're talking about preignition. Detonation would imply that something is causing the fuel to burn too quickly... which in my case, was not happening.

-Mark

[Clintstone]

If it fires early as the piston is rising I would say that is pre-ignition as described above. If you are having to change the timing by running rich then you could add shims and get the mixture the way you want it and bam , the stars are all lined up and all is good in the heli world. Now if you are chasing the needles all day then there is a good possibility that the timing is off a little. If the motor is firing too far past the top stroke of he piston I would cal this detonation.......in this cae I would say take shims out. While there are alot of Good fuels out there the practice of changing brands back and forth is not a good idea, as similar as they are the motor wants to run the same fuel all the time. I run cool power fuel and it is working great for me as it is consistant and it is available everywhere. If I were running Magnum I would run 24 thou worth of shims as I am running the OMI Klien carb on a 91 Cspec with an OMI Viper Head. As I am running cool power 30% , I run 16 thou on my shims, I could get away with 12 but since I travel so much and it gets warm here in GA. I choose to run 16 thou as I get good power all the time and on the warm days it keps cool and still performs.
Think about this , if you start out in the morning and you are making great power but as the day goes on you have to richen up the motor to keep it cool what is going on? The air temp is going up so the air is changing, in a perfect world you should be able to stay the same or even lean down. If you find yourself constantly having to richen up as temps go up, I would recomend the shim or adding a shim. There are so many variables , moisture in the air ( we have a high humidity here in GA but it is dryer out west. I think it is more critical to have the motor shimmed properly if you are in an enviroment that changes rapidly.
If you feel the motor is not getting on the pipe or you just can't get the head speed up you can try taking shims out , this works both ways and if you shim too far out the motor can lay down and not perform. It sounds like a big pain and trust me it is but man when it all works out it is what it is all about.


Well forgive me for the long discussion but I just woke-up....... I guess to sum it up , the most consistant combination I have found is the Cool Power 30% with the Viper Head shimmed 16 thou with 2 enya #3 glow plugs and the Klien bigBore carb setup. I am also running a new piston from East Coast with some Italian special sauce called The Fat Mod. In short Dino is have the piston modified with extra rings cut for oil and the result is a motor that pulls without getting hot as the section that would drag in a hot condition is relieved as OS did in the viper motor which can be bought over seas. It came stock with the ViperHead from OMI and this special piston. I hope this is not too much info.