Chamfering the edge of a combustion chamber

[Rick Pirrone]

Hi all

I have seen this done to some head buttons and was wondering what the pros and cons in doing this

thanks cheers rick

 

[anthony_marquart]

I have read that people have done this to lower compression just a touch,.. maybe when running very well but using too many plugs,... That's just what I have read though.. I have smoothed mine a bit when cleaning up a head button with some detonation or damage from eating plugs,. never really saw any difference.

 

[camaroboy383]

the benefit would be easier on plugs like anthony said....

 

[steve wood]

supposed to reduce detonation.i don't do on brass buttons,aluminon only

 

[Terry Keeley]

Here's my stock 45 VAC button, looks like they tried to open it up a bit, still measures only 0.29cc!

 

[anthony_marquart]

Is it just me,.. or does that volume seem really low?

 

[Don Ferrette]

I started doing it years ago when a 45 motor I had in my sport 40 was breaking the element on every run. The boat ran great, the plug would look perfect but wouldn't light after the run. I can't remember who suggested it (maybe Finch?) but all I did was screw an old glow plug into the head, chucked it up in my 3/8" drill (didn't have my mill and lathe yet) and just ever so lightly knocked the sharp corner off where the squishband transitioned into to bowl. Put the head button back in and with no other changes whatsoever the motor stopped breaking the plug element. Been doing it on aluminum head buttons ever since. B)

 

[Jim Allen]

What does a chamfer or radius at the intersection of the bowl & the squish band effect, besides adding a little volume to the chamber?

 

[anthony_marquart]

Jim,.. is that a rhetorical question that you, of all of us, would know the answer to? I'm sure it has an effect upon the way fuel and exhaust move around the head button.. But I have know idea what that would do..

 

[Terry Keeley]

What does a chamfer or radius at the intersection of the bowl & the squish band effect, besides adding a little volume to the chamber?

Slows the squish velocity a bit?

 

[Jim Allen]

Jim,.. is that a rhetorical question that you, of all of us, would know the answer to? I'm sure it has an effect upon the way fuel and exhaust move around the head button.. But I have know idea what that would do..

There is nothing rhetorical or ostentatious about this very simple question.

 

[Jim Allen]

What does a chamfer or radius at the intersection of the bowl & the squish band effect, besides adding a little volume to the chamber?

Slows the squish velocity a bit?

Bingo! Only it slows it down a large amount. It also will de-attach the incoming charge from the piston crown. Not a desirable feature.

 

[anthony_marquart]

A wealth of knowledge as usual!! So,.. can you tell us more about reduced squish velocity? Effects? advantages? Disadvantages?

oh,. and I didn't mean anything disrespectful with the previous comment. I just remember that you often asked me questions as a way to get me to think about the answers I was already looking for..

 

[Don Ferrette]

What does a chamfer or radius at the intersection of the bowl & the squish band effect, besides adding a little volume to the chamber?

Slows the squish velocity a bit?

Bingo! Only it slows it down a large amount. It also will de-attach the incoming charge from the piston crown. Not a desirable feature.

Jim can you help me understand why by just knocking off the very end edge stopped the motor from breaking the element and no change in how the boat ran?

 

[Terry Keeley]

Is it just me,.. or does that volume seem really low?

I get 13.7:1 trapped CR, that's with 0.012" head clnc and a 0.02cc dish in the piston.

I used to think 10:1 was high! :lol:

 

[Jim Allen]

The following information comes from testing different squish band types, different head shapes & different piston crown shapes with 80% nitro fuel. There are other factors to be considered, but this short discussion will be about what I have found. Excessive squish band width, squish band chamfers or radii in combination with excessive squish height inclose a large volume of fresh charge in the squish band area. This combustable mixture is blocked from the initial combustion process. If this volume of fresh charge begins to burn from the high pressure & temperature already in the chamber, detonation occurs. The results of that detonation can be seen in areas under the squish band as seen in photos previously posted.

When the squish band width & height are small enough, there cannot be any detonation in this area. The very high maxium squish velocity developed promotes a much faster flame speed, faster burning, a steeper pressure rise & a higher end pressure. Since detonation temperatures need time to start, the faster burning that results from a high MSV counteracts detonation.

Jim Allen

 

[dwilfong]

Jim what do you consider a max MSV?

David

 

[Jim Allen]

Jim what do you consider a max MSV?

David

What I do is make the verticle squish height close to the point where the piston will just kiss the head when the engine is over- reved. I can easily do this because of the very tight clearances used on my steel connecting rod. I want the squish area ratio between 40 & 50%. The squish velocity will be in the high 30m/s (98ft/s) with the shape of the squish area following the piston crown, & a sharp transition edge from the squish to the bowl.

If the squish is working properly it will cause much greater turbulence in the end gases, increasing flame speed, burning more combustion gases faster, killing detonation. When operating the engine in this manner tuning can be critical. The use of a toroidal head makes the tuning much easier because the point of ignition can be moved to the center of the chamber. The close proximity of the piston crown to the toroid shape forces the flame front outward horizontally very early in the combustion process. This horizontal movement encounters the high squish turbulance approximately 10* BTDC. All things working together, ignition point in the middle, very high MSV & a horizontal flame front, create a much faster burn.

This is the magic of a high performance two stroke, "you never know when something really exciting is about to happen!"

 

[Jack ODonnell]

Jim what do you consider a max MSV?

David

What I do is make the verticle squish height close to the point where the piston will just kiss the head when the engine is over- reved. I can easily do this because of the very tight clearances used on my steel connecting rod. I want the squish area ratio between 40 & 50%. The squish velocity will be in the high 30m/s (98ft/s) with the shape of the squish area following the piston crown, & a sharp transition edge from the squish to the bowl.

If the squish is working properly it will cause much greater turbulence in the end gases, increasing flame speed, burning more combustion gases faster, killing detonation. When operating the engine in this manner tuning can be critical. The use of a toroidal head makes the tuning much easier because the point of ignition can be moved to the center of the chamber. The close proximity of the piston crown to the toroid shape forces the flame front outward horizontally very early in the combustion process. This horizontal movement encounters the high squish turbulance approximately 10* BTDC. All things working together, ignition point in the middle, very high MSV & a horizontal flame front, create a much faster burn.

This is the magic of a high performance two stroke, "you never know when something really exciting is about to happen!"

Question; How do you know the point of ignition with a glow plug ?

 

[RonShaw]

The following information comes from testing different squish band types, different head shapes & different piston crown shapes with 80% nitro fuel. There are other factors to be considered, but this short discussion will be about what I have found. Excessive squish band width, squish band chamfers or radii in combination with excessive squish height inclose a large volume of fresh charge in the squish band area. This combustable mixture is blocked from the initial combustion process. If this volume of fresh charge begins to burn from the high pressure & temperature already in the chamber, detonation occurs. The results of that detonation can be seen in areas under the squish band as seen in photos previously posted.

When the squish band width & height are small enough, there cannot be any detonation in this area. The very high maxium squish velocity developed promotes a much faster flame speed, faster burning, a steeper pressure rise & a higher end pressure. Since detonation temperatures need time to start, the faster burning that results from a high MSV counteracts detonation.

Jim Allen

Explains how my old friend Mike Rushing was able to develop so much power from his "zero squish clearance" theory. Sometimes he even went a little negative!!!!!.... Great for power, not so great for piston top, head, rod, wrist, and crank pins!!!!.....

Finding that perfect compression rate that works for your setup, and figuring out how to get as much of the total fuel mixture on top of the piston as possible, into that small bowl, but not cause stress and premature wear on the internal components of the engine, is a major challange.........