What does this mean???

I have noticed several times in the past someone would respond to a question with this statement, "Put another shim under the head." What is that suppose to mean??? Some explaination explaining what this does would be very helpful to understand what this is for. I assume another shim under the head will lower the compression of the engine. This can do several things to a gasoline engine. It increases the area inside the cylinder and allows a larger fuel air mixture inside the cylinder and produces more power at lower RPM's. It can make the engine run smoother at low RPM's. It can make the engine start easier but not in extremely cold weather.

Why is this done to a model airplane engine?

Gary,
I would think that it's for the reasons you state.
Bill <><

Pretty much exactly the same reasons. You can also add blowing plugs. Too tight a squish band clearance and/or too high compression will blow glow plugs. That doesn't apply much to stunt though.

Basically, if the engine power jumps "too much" when it switches from a 4 stroke run to a two stroke, adding an extra head gasket or two will usually tame the break down.

Pretty much the same things you already mentioned.

However, since these are generally 2 strokes, and the firing timing can be affected by the area called the squishband (The part of the head at the top of the chamber that comes the closest to the piston at TDC) Generally a ring area around the periphery of a hemisherical chamber.

THe squish clearance is critical to the timing and is affected by the clearance. Adding a shim under the head adds volume to the chamber and the other things you mention as well as retarding the ignition timing.

THis is often recommended by engine fiddlers as a way to tame an engine that is running away or generating too much power.

If the engine is otherwise designed for the application, and proper fuel and prop is being used, shimming will neuter a good engine.

For example, you can cure the FP 40 runaway problem by adding a couple (or more) shims but what you will end up with is an engine that develops the power of a good 25. ie it isnt a simple panacea for every problem engine--it is just simple to do.

JMHO

>Pretty much the same things you already mentioned.
>
>However, since these are generally 2 strokes, and the firing
>timing can be affected by the area called the squishband
>(The part of the head at the top of the chamber that comes
>the closest to the piston at TDC) Generally a ring area
>around the periphery of a hemisherical chamber.
>
>THe squish clearance is critical to the timing and is
>affected by the clearance. Adding a shim under the head
>adds volume to the chamber and the other things you mention
>as well as retarding the ignition timing.
>
>THis is often recommended by engine fiddlers as a way to
>tame an engine that is running away or generating too much
>power.
>
>If the engine is otherwise designed for the application, and
>proper fuel and prop is being used, shimming will neuter a
>good engine.
>

Up to this point you are doing pretty good. You want (or need) to lower compression if you are adding nitro. If the engine "comes on" too strong in the corner, then adding a head shim can solve the problem as well. The problem is, this can often be accomplished with the addition of 2 to 4 thousandths of an inch and people will sometimes add 2 to 4 head shims of 16 thousandths each. This is terrible, and not the way to go at all. The OS FP, for instance, comes with a 16 thousandths head shim in the RC configuration and two 16 thousandths head shims in the CL configuration. That is way too much of a difference. I have some shims of 10 thousandths thickness that in mix or match configuration can give you 16, 20, 26, 30, 32, etc thickness. This is a much better way to go. Make slight changes and you will see the difference. Make big changes and all you will see is loss of power.

>For example, you can cure the FP 40 runaway problem by
>adding a couple (or more) shims but what you will end up
>with is an engine that develops the power of a good 25. ie
>it isnt a simple panacea for every problem engine--it is
>just simple to do.
>

Yup, that is just what I was talking about. I once got an engine to rework that had 5 head gaskets under the head. The owner had to replace the head bolts with longer ones just to keep the head on. the Magnum engines come with 4 thousandths head shims. Nice to work on. The PA series (and others) have various thickness of head shims so you can mix and match to remove or add just 1 or 2 thousandths at a time if you want. And, yes, that 2 thousandths difference can make a very noticeable difference in the run.

LAST EDITED ON Nov-29-04 AT 09:29 AM (CST)
 
It seems to me that a more prefered method for taming the break would be a smaller venturi vs reduction of compression. Here's my thinking.

1. Addition of shims and the resulting reduction of compression will reduce power all over not just the break. As it was stated .. a wild 40FP can quickly become a sweet 25.

2. Venturi reduction has less degrading effects on on overall power but reduces the sharpness of the break.

My opinion of what causes the break is a change in head from the fuel pickup to the opening in the spraybar. Add to that the "suction" which is the difference in the pressure from the fuel pickup to the spraybar hole and is constant at some speed. Call that pressure difference - DP (delta P) When the nose is rotated up, the head increases. The DP doesn't so engine tends toward lean.

If I decrease the size of the venturi then the velocity thru the venturi increases and the DP increase. When the nose is rotated up, the same head increase exists but now with a greater DP there is less leaning of the engine.

The venturi will have an effect on power but reasonable changes should be overcome by the pumping efficiency of the engine. There will be a small drop in HP but should be considerable less sensitve to the head change.

(Learned after attempting to get an 1100 cfm carb to work on a 283 chevy)

I'm inclined toward smaller venturi to stop windup, Just to start an argument for others:9 The break is the engine's response to changing loads on the prop. The break due to changes in fuel head is too slow to accomplish what we want. I think this was documented by the Wright brothers. Have fun!

LAST EDITED ON Nov-29-04 AT 12:17 PM (CST)
 
Gary,

...to add to Jim T's point, WHY the prop loads affect 2/4 break is how our engines ignite the fuel/air charge. (IMHO, though I have run some number studies that seem to agree:) We have semi-"diesel" engines. The combination of compression heating and a heated catalyst coil (containing platinum and in the presence of alcohol) gives good ignition, and the combustion helps keep the plug coil hot.

Quote marks around "diesel", above, because ours are not quite what the rest of technology considers diesels. Those others inject fuel at the critical time into essentially dry piston-compressed air in the cylinder. Wanna try that in 2.5cc (.15 cu in) size? Me neither.

The 'compression ignition' engines, then, adjust ignition timing by varying compression ratio. That windup knob on top? Higher setting is a smaller combustion chamber, thus higher compression ratio, thus quicker and higher compression heating. Earlier ignition.

For big, slower turning props, a lower setting retards ignition to match.

Now, change fuels back to glow -- the catalyst coil and the alcohol are much less fussy about mixture ratio, so we usually run a pretty wet fuel/air charge. It will ignite. Alcohol is an excellent coolant -- check the temperature at the shaft housing just under the venturii while it runs. It stays cool, even cold, there. That, and the amount of oil, absorbing heat but not burning, modify the catalyzed compression effects. The chillers dampen the tendency to ignite. Just as lowering compression on a 'diesel' engine does... So adding a gasket or two pre-sets the compression ratio a bit lower than it had been. That adds to the other ignition-suppressing effects and broadens the range where the regular misfire can occur.

That makes it a bit easier for us to find and set it.

Jim T related the break to the prop loads. I'm with him on that. A sharp corner, per Bill Netzeband, could generate 35g wing lift. Induced drag coefficient increases as the square of the lift coefficient change. Actual numbers? Roughly - induced drag on a 4 lb stunter flying level may be 1 to 4 ounces. In a 35 g turn, lift must be 140 lbs, and induced drag can reach 20 to 30 lbs.

The prop had been loafing along, adding thrust only to hold speed against wing induced drag, line drag and skin surface drag -- perhaps 8 ounces? Corners don't last long: those flash load values don't either. Still, the effect is to yank the plane slower with the induced drag at the high lift. Tell me pulling aftward with 25 lbs or so isn't going to slow your model? Unless something flexes or gives, the whole model slows under that drag force.

The only flexible thing involved is the air the prop is clawing its way through. The prop load rises dramatically. The prop slows.

Where we had mixture (needle setting), compression ratio, and prop RPM balanced to a nice chugging 4-cycle, we now slow the prop, leaving the piston more time to pass through TDC. ...more time to start and generate combustion... Instead of a misfire every other, we can get ignition every revolution when the prop is loaded.

At least, this is how it seems to me...