flat squish bands on a .21?

[eric ekern]

who has experience with this? results? problems?

 

[ggillman]

Hi Eric,

I believe that Tim D. is who you might talk to about this,

hang on he should be checkin in.

Gene ;D

 

[eric ekern]

the reason i ask is that i read about removing some material off of the bottom of the head button (.040") of my o.s. engine in order to reduce head volume and increase CR. ive notice that the button has a slightly angled squishband. removing the material would make for a flat squishband. before i do it i just wanted to see if anybody had first hand experience. thanks, eric

 

[ggillman]

I have messed around with bowl shape and enlarged it a bit, some of the aftermarket head buttons have the steped bowls.

( Flat steps in a reducing radius toward the glow plug )

Gene ;D

 

[eric ekern]

did you have any problems with detonation?

 

[C.Dickson]

Eric,

If you are trying to reduce head volume, you must reduce head clearence. To reduce head clearence you must either remove material from the top of the sleeve or remove material from the area of the head button that sits on top of the sleeve. Both of these methods will allow the head button to desend into the sleeve the desired ammount, thus reducing total head volume. If you remove material from the squishband area it will actually increase head volume. The way I understand it, a flat squishband will promote detination and overheat and rob your engine of power(something I dont think any of us want going on in our engines). When you reduce the head clearence, you might find that you start mangling glow plugs. You will then want to start opening up the bowl in the head button, but I always like to take material off the inside corner of the squishband to promote fuel flow to the glow plug(more direct route to the heat source).

Hope my ramblings have done more good than harm.

Good Luck>>>>>> C.Dickdon

 

[ggillman]

Eric,

I have had no problems at all, I was increasing the size of the bowl, with a smoother radius.

Gene ;D

 

[GTR]

Hi Guy's,

Have you checked out the combustion chamber in a racing 2 stroke - a lot like a xmas bell. radius curved squish to a small round curved combustion bowl. Does anybody make a head button like this?

cheers

GT 8)

 

[eric ekern]

i suppose i should have mentioned this is an o.s. engine. the head button does not sit down into the sleeve like most engines, it actually just sits on top of the sleeve. because of this shaving material off the head button doos not change the squish band width but it does reduce the head volume........it will also give me a flat squish band which leads me to my original question.

 

[C.Dickson]

Just remove material from the top of the sleeve and leave the squishband angled. Why take a chance of detonating when you dont have to?

 

[eric ekern]

cause then in also changes the timing of the engine

 

[eric ekern]

and makes the squish band width narrower

 

[craig]

OK, I quit.

Good luck

 

[Chris Attebery]

Eric,

I run flat squish bands on my 21s and 45s all the time. No problems here. If you start popping plug you can raise the head .004" or add 1 degree of angle over the inner half of the squish band.

Good luck,

Chris Attebery

NAMBA 786

 

[eric ekern]

thats's what i was looking for, thanks chris

 

[TimD]

Hi guy's,

Sorry for not jumping in sooner.

A completely flat squish will make the motor feel like it is a little out of balance. Try making the outer half of the squish flat and the inside half at 1 1/2 degrees - good compromise.

Reducing the head clearance does not nescessarily reduce detonation. You will get more power from running a very tight squish clearance and making the bowl volume larger to avert detonation. (especially with 21's)

Taking metal off the top of the liner will not change the timings. Raising or lowering the liner in the case does that.

My 2 cents.

Tim.

 

[Chris Attebery]

Tim,

I agree with you that a flat squish band will make the motor vibrate more than a angled band, but it will make more power too. The motor will not clean up as much as with the angled band because you need to run a bit richer.

I also agree that increasing the squish clearance may not reduce detonation, actually it can make it worse.

The best thing to do is run a minimum squish clearance .006" to .008" and reduce the head volume until the motor just starts to detonate and then open it back up slightly. I run very small bowls, flat squish bands, and tight clearances. You HAVE to run rich, but it makes a ton of power.

Good luck,

Chris Attebery

NAMBA 786

 

[TimD]

Thanks Chris,

I forgot to mention about the good side of the flat squish! I just mentioned the bad bit and the compromise!

Tim.