Hi; I was reading on the web somewhere that if you add a shim to the head of an engine it that it will decrease the compression
ratio. What advantage would increasing or decreasing the comp. ratio of a glow engine have? I know this sounds like a dumb question but I really would like to know!!! Thanks....Veco

On a 2 stroke glow, the only way to set the "timing" (when the detonation occurs) is by compression ratio, nitro content, glow plug heat, engine load, pipe length, atmospheric conditions, or a combination of the above.

Being able to adjust the compression ratio will help you get the optimum power from your engine.

Lowering the compression ratio will retard when detonation occurs, the same as lowering nitro, reducing load, colder plug, or longer pipe.

The main advantage to lowering the compression is to eleminate pre-detonation..where the charge fires prior to the optimum for the nitro/load/plug...all that. If the engine is pre-detonating, it is working against itself and causes a loss of power, added wear on internal components, extra heat, burned glow plugs, etc.

A far easier method than adding a head shim is to reduce nitro or run a prop that doesn't load the engine as much.

This is just a broad overview of compression..I'm positive you will get a much more detailed explaination shortly.

The problem with our engines is the wide variation in fuels we can buy, meaning nitro content. Methanol by itself can withstand very high compression before it detonates and high compression makes for efficient combustion. Nitro will only tolerate a much lower compression before it detonates. The problem then comes when you mix various percentages of nitro with the methanol. The more nitro you have the lower the compression before it detonates.

Most engines have a compression ratio that's optimised for a fuel with something like 15-20% nitro so if you want to use more nitro then the compression has to be reduced by adding shims. Conversely, if you want to get the best out of the engine by using less or zero nitro you have to remove shims or even machine the head to raise the compression enough.

But there's a complication here. Raising compression also advances the ignition timing which in itself can lead to pre-ignition (and of course, lowering compression retards ignition timing). Fortunately for us, fine adjustment to ignition timing can be made by using different heat range plugs. A hot plug advances timing and a cold plug retards it.

The only way to know what's best for any engine is trial and error (and a good tach ). Decide what fuel you want to use and check max revs before doing any mods then add a shim and check again. If the revs go down then start removing shims. If you want to leave the shims as is then try different amounts of nitro until you find the mix giving the highest revs and then play with plugs. Going to a cooler plug may then allow you to use slightly more nitro.

What you're then doing is optimising the fuel, compression and plug for that engine and no other but by then it'll be so critical that even changing the brand of prop (let alone size) will need at least a change in plug used.

As a guide though, in another thread you mentioned the Enya 60X. I run mine on zero nitro but have raised the compression to 13.5:1 which gave a 19% increase in HP although I haven't bothered experimenting with plugs.

Wow, That was very, very helpful. Thanks guys. Looks like I'm going to have to brake my test stand out and buy a tech.and start doing what I like best, experimenting with engines. I am really glad I found the RCU website and having knowable folks like you on it.
I would be lost without you guys. Thanks.........Veco

It all sounds very scientific and complex.

In actual use, I don't mess with it. If I'm blowing plugs (too much compression) I run a colder plug, or a smaller prop. I usually always run the same fuel. If I have a prop I really like, and am still blowing plugs, I might put a shim in. Not really rocket science, or something to dwell over. After a while, you'll become accoustomed to what sounds right and works good.

I just bought a VMAR engine test stand yesterday and I'm getting a tech. off of e-bay next week and I already have a compression tester.I wish I had a horsepower tester. I am going to rpm,% nitro, plug and shim experiments on all of my engines just to learn what works and what don't, its a good way to get to know each of your engines, rainy day stuff Thanks........Veco

Ask and you shall receive... Since you know rpm's, getting horsepower is easy.

http://www.bmaps.net/software/goodies.html

I've thought of add a shim to the head of my MVVS engine to decrease the compression.
The big advantage of decreasing the comp. for me would be to run 10% nitro instead of special ordering FAI, or 5% fuel.

You shouldn't have to special order 5% fuel, Fox makes and sells lots of it and it is excellent fuel, I use the half syn. and half castor blend in all my two strokes.

10% and up is what the local hobby shop stocks.
I order it by the case to get them to get it for me.