Shimming: How do you know compression is right?

The compression is ideal when the engine produces maximum power. In theory it means the ignition occurs just right before the piston reaches its top dead center, and heat input to the cycle occurs thus at the highest average temperature. (This comes from the Carnot cycle theory and laws of thermodynamics)

To early explosion caused by excessive compression means part of the combustion energy will work against the piston moving upwards. Too late ignition i.e. on the top DC or slightly afterwards will not result in the highets possible mean pressure in the cylinder, because part of the fuel will burn with the piston already near to opening the exhaust opening and the burning mix will leave the engine before spending its useful energy to drive it.

I guess setting the ideal compression is more or less an empyrical stuff and exact, one-fit-for all answer cannot be given. You must exercise by starting up the engine, peak it by setting the carb, take rev readings on a tachometer, stop the engine and change the number of shims. Obviously the compression is ideal when the peak rpm of the engine is the highest with a given prop and fuel blend.

The more shims under the head, the lower the compression.

As a rule of thumb, this is my experience:
Normal glow heads require one or two shims under the head. This is true up to 20% nitro (I do not use higher than 20% nitro)

High compression heads need 3-5 shims. Too high compression typically becomes manifest in very hot engine temperature and premature firing of the engine. (When you flip the propeller by hand the engine will backfire much before you turn the prop over the top dead center of the piston.)

Higher nitro content (say, above 20%) causes higher engine temperature that triggers premature ignition, the prevention of which will certainly call for more shims.

Sorry if it was not quite an exact methodology.

This is somewhat confusing to me too. Until recently (since joining CEF) I wasn't aware that high-compression Cox glowheads existed. From what I have come to understand.. they would only be useful when using a fuel with a lower nitro content. Am I correct here? If you need to add copper gaskets when increasing to a high-nitro fuel.. you're effectively reducing compression.. so; wouldn't "switching" to a standard head accomplish the same thing?

Here's a question..   ... does the difference in compression between the two heads work out to be less than.. or possibly "half of" a single copper gasket's difference? If so; then I could understand how using the high-compression head allows for more fine-tuning possibilities.

The high compression head can be as effective on a Babe Bee as a TD. The outcome won't be the same because the designs are different. However a bump in compression is always a bump in power no matter what fuel you use. To a point...

Anyway to answer your question there is a significant difference in compression between a standard head and a TD head. And way more than a gasket.

Ron

You can lap a head (grind or sand against a plate) too. This can increase compression to a middle value.

Think about compression as a timing adjustment.  Higher compression advances timing (earlier than top dead center or tdc) for a given fuel and glow plug.  Similarly a hotter glow plug will advance timing.

Don't forget the plethora of drop in heads available.

Phil

I'll probably get myself slammed here, but the way I see it, Cox R&D & QC people surely would have ensured that all Cox engines came from the factory in a state that would be as efficient as possible. As far as I know, all Cox engines came from the factory with one head gasket and with the expectation that they would be run on (at least) the standard 25% nitro Cox fuel. Otherwise, the instructions would state that the engine should be run on 10% maximum. Tee Dees had much higher compression than babe Bees but came out with the ssme head gasket, and only one. Was there any official documentation from Cox that stated that more gaskets should be used? Why would any Cox engine using a standard head ever need more than one gasket, when it's quite feasible to install a Tee Dee head and get more power? The QZ was an example. It was basically a non-SPI Babe Bee with a Tee Dee head installed to up the power to compensate for the muffler. I'd be interested to see if the QZ instructions state the need to run more than one gasket, or any Cox engine instructions for that matter.
Clearly, many of you have disproved my thoughts by experimenting and getting more power from your engines, but I feel that any shift from original design in anything mechanical can comprimise efficiency. In my job I refuse to perform modifications to people's engines for that very reason. Can of worms!!

Rod.

(What have I done?? )

Rod, that was something I was thinking too. Or, if not set up for peak power, maybe for ease of operation.

I don't think as kids we ever used more than one shim, and with a chemist father who whipped us up everything from 0-50% nitro, it sounds like we should have.

Yes, I suppose it was set up to run out of the box from the factory. Maybe I mistakenly misunderstood the intent of the post and I answered as pertaining to performance.

Anyway, if you just want to have a hands free engine out of the box then run it as is. If you want a little more then play around.

A little story, I was having a very hard time starting an engine a couple weeks ago. I was told the engine was a consistent runner. I also noticed that it was doing the prop wobble which is a sign of over compression. I added a gasket and we were in business next flip.

What changed? I don't know for sure, have a few theories, but I say all that because it helps to understand what's going on and how things work. It could have made for a crummy day if I hadn't taken the time to learn a few things beforehand.

Just a thought.

Rod has a good point. Maybe we've been telling each other this stuff for so long that we believe it to be gospel. I never was one to read documentation so if the instructions ever mentioned extra head gaskets I wouldn't have known. But the blue Cox can I always used was 15% nitro and the rule of thumb I now refer to is one gasket up to 20% nitro and add one for every 10% after that. However the red Cox racing fuel can was 30% nitro but I didn't use it until the last days of Cox. I do know that I never blew a plug with the one gasket I ran until I was back into it and using 25% post Cox internet fuel. Then started popping them in the air and learned of this new-to-me rule of thumb and quit blowing them again.
Rusty

Oldenginerod wrote:I'll probably get myself slammed here, but the way I see it, Cox R&D & QC people surely would have ensured that all Cox engines came from the factory in a state that would be as efficient as possible.  As far as I know, all Cox engines came from the factory with one head gasket and with the expectation that they would be run on (at least) the standard 25% nitro Cox fuel.  Otherwise, the instructions would state that the engine should be run on 10% maximum.  Tee Dees had much higher compression than babe Bees but came out with the ssme head gasket, and only one.  Was there any official documentation from Cox that stated that more gaskets should be used?  Why would any Cox engine using a standard head ever need more than one gasket, when it's quite feasible to install a Tee Dee head and get more power?  The QZ was an example.  It was basically a non-SPI Babe Bee with a Tee Dee head installed to up the power to compensate for the muffler.  I'd be interested to see if the QZ instructions state the need to run more than one gasket, or any Cox engine instructions for that matter.
Clearly, many of you have disproved my thoughts by experimenting and getting more power from your engines, but I feel that any shift from original design in anything mechanical can comprimise efficiency.  In my job I refuse to perform modifications to people's engines for that very reason.  Can of worms!!

Rod.


(What have I done?? )

Rod,  

TeeDee049 and 051 engines came with the below instruction, stating 3 factory installed shims were  under the head.



I believe the number of shims must be set by the engine user empyrically because quite a number of factors may cause ignition timing to be different between 2, otherwise identical stock engines:

1. The dome geometry inside the head (I saw many different high-compression head domes)
2. The balljoint "play" and wear of the ball top that may result in a lowered piston crown thus lowered compression
3. Thickness of the castor varnish burnt on the top of the cylinder
4. The piston/cylinder fit...even though my experience is this is very precise and lasting compression is maintained in COX engines even after many tens of hours of running (except one has leaned out the engine many times or run it with dirt in the cylinder)
5. Elevation above see level (the higher you live the thinner the air..)
5. Last but not at least, fuel blend/nitro content

In such small engines the variance of the above conditions/geometry will significantly impact the ignition timing thus a one-fit-for all solution i.e. number of shims cannot be expected..

Cribbs74 wrote:


A little story, I was having a very hard time starting an engine a couple weeks ago. I was told the engine was a consistent runner. I also noticed that it was doing the prop wobble which is a sign of over compression. I added a gasket and we were in business next flip.

What changed? I don't know for sure, have a few theories, but I say all that because it helps to understand what's going on and how things work. It could have made for a crummy day if I hadn't taken the time to learn a few things beforehand.

Just a thought.

Prop wobble? Is that when the engine fires but cannot get past TDC, then cranks backwards to fire again but again cannot get past TDC? If so, I think you may have just saved one of my hanger queens

Yep, that's exactly what it looks like.

flyjsh wrote:

Cribbs74 wrote:


A little story, I was having a very hard time starting an engine a couple weeks ago. I was told the engine was a consistent runner. I also noticed that it was doing the prop wobble which is a sign of over compression. I added a gasket and we were in business next flip.

What changed? I don't know for sure, have a few theories, but I say all that because it helps to understand what's going on and how things work. It could have made for a crummy day if I hadn't taken the time to learn a few things beforehand.

Just a thought.

Prop wobble?  Is that when the engine fires but cannot get past TDC, then cranks backwards to fire again but again cannot get past TDC?  If so, I think you may have just saved one of my hanger queens  

A year ago I thought that was one of the most infuriating things on the planet! Half my engines did this!

Yes, a gasket or two will save your hanger queen and possibly make it a favorite runner.

Phil

What do you call a hanger queen?

I think I have found out...planes with defunct engines hanging on the wall of your shop?

Yep, you got it.

HAZAAA! Okay, I finally put down the balsa on my current build and took the PT-19 out for a spin. Fiddled with shims and was able to go from mid 13s to high 14s/low 15s (according to my crappy phone app tach)! Quite pleased with the first attempt. Now I guess I need to buy a good tach.

flyjsh wrote: Now I guess I need to buy a good tach.

Nah, not really. Just set it for 2 blades and point it at a fluorescent lamp. Count any amount over or under 3600 RPM as your offset to add or subtract from your actual engine readings. It should be consistent.
Rusty

The phone tach is acoustically driven. Set for 1 cylinder 2cycle it bounces from 15k to 30 k, so I think it sometimes doubles the revs. I bounces around a bunch. I figure it is+/- 500 at any time.

flyjsh wrote:The phone tach is acoustically driven.  Set for 1 cylinder 2cycle it bounces from 15k to 30 k, so I think it sometimes doubles the revs.  I bounces around a bunch.  I figure it is+/- 500 at any time.

Yeah, I see what you mean.