Squish

[45u]

What is a smallest squish that is not going to affect the reliability.
Thanks
Jeff

 

[butre]

about 30 thou squish clearance is the tightest you'll ever want. start there and use a thicker head gasket if it's too tight. too tight can actually reduce performance long before you actually start experiencing reliability problems.

 

[45u]

Thanks! On my RC gas car engines I have gone as low as .020 but I rebuild them often. I will start at around .030 trying different thickness copper head gaskets. I am going to run the engine in well before I do much. Motor kit is on its way and hoping to buy a bicycle in a few weeks or less.
Thanks
Jeff

 

[sbest]

I have run a Yamaha Blaster 66mm piston as low as 0.010" (10 thou) to see the effect.
There was no sign of contact, but a distinct noise difference and reduction in power.

This book talks about the reasons and gives calculation for best squish based on mixture velocity:
http://dragonfly75.com/motorbike/2StrokeDesign.pdf

Basically the theory is that you are trying to create a turbulence in the combustion chamber that will rapidly spread the flame front throughout the entire mixture charge. More is not better, there is an optimum speed so there is an optimum squish gap and area for whatever you want to do. A closer gap favours a lower rpm motor.

What I suspect is the tight gap acts as a cushion, turning piston speed into mixture velocity, with a loss of net energy through heat. I suspect the noise is a sonic crack. For the 66mm piston and approximately 50% squish area I found 0.030" to 0.040" gave the best power, and no difference to either side of the tolerance. Another 0.010" in either direction and you could feel a reduction in power.

Steve

 

[Purple Haze]

Once again, sbest, you've done your homework! When I raced sbc engines, I found the perfect squish gap to be .035 Can you give me some pointers on tumble? I believe proper mixture tumble will improve performance and economy at all rpms.

 

[HeadSmess]

the turbulence factor is only a concern with diesels. it is irrelevant on carburetted petrol engines that dont require 16:1 or more compression ratios to run. it can actually do more harm than good by "blowing" the flame kernel out. you only get one spark to light up the charge. velocity gets too high and it wont even light.

diesels work on completely different principles.

 

[sbest]

Once again, sbest, you've done your homework! When I raced sbc engines, I found the perfect squish gap to be .035 Can you give me some pointers on tumble? I believe proper mixture tumble will improve performance and economy at all rpms.

Yeah, I worked on sbf engines. Had to deck the block or buy pistons to get that spec. Tumble was important in the wedge and 4 valve engines, but not so much 2 strokes I feel. Once that piston closes with the head all tumble is overwhelmed by the turbulence on the central chamber. It becomes a torroidal (donut shaped) tornado. Having the right plug heat range and placement helps to keep the flame from being snuffed out. If the plug is too hot it will run with no spark:


I find the more pocketed and smaller the chamber, the more the engine favours mid rpm power.
The more shallow and open favours high rpm.

 

[45u]

Thanks all for the input. I am far from being a beginner when it comes to engines. I have made my living for the past 45 plus years not limited to but including doing 99% of my own machine work on motorcycles. I have my own little machine shop. Now I do work on more 4 cycles then 2 cycles and have built many a crank for both. I have rebuilt many a connecting rod IF over size bearings are available. I have disassembled many a crank replace or rebuild rods assemble and true. I all so on the big Harley s balance the cranks and so much more.

Not trying to toot my horn but at 61 I am quite familiar with the combustible engines. Just wanting to let all know I am FAR from being new to engines.

When it comes to 2 cycle engines I have been building them for my 1/5 scale buggy for many years. Have 4 engines I built I have not even broke in yet. Yes TOO little squish is VERY departmental to the motors heath. On my 26 to 32 CC motors a good squish on the safe side is around .021 to .025. On another forum I was recommend no less then .030 on these bicycle engines. What do y'all think?
Thanks
Jeff

 

[45u]

Thanks all for the input. I am far from being a beginner when it comes to engines. I have made my living for the past 45 plus years not limited to but including doing 99% of my own machine work on motorcycles. I have my own little machine shop. Now I do work on more 4 cycles then 2 cycles and have built many a crank for both. I have rebuilt many a connecting rod IF over size bearings are available. I have disassembled many a crank replace or rebuild rods assemble and true. I all so on the big Harley s balance the cranks and so much more.

Not trying to toot my horn but at 61 I am quite familiar with the combustible engines. Just wanting to let all know I am FAR from being new to engines.

When it comes to 2 cycle engines I have been building them for my 1/5 scale buggy for many years. Have 4 engines I built I have not even broke in yet. Yes TOO little squish is VERY departmental to the motors heath. On my 26 to 32 CC motors a good squish on the safe side is around .021 to .025. On another forum I was recommend no less then .030 on these bicycle engines. What do y'all think?
Thanks
Jeff

Here is my little 27.5 to 30cc engines for my 1/5 scale buggy.

 

[45u]

My little 1/5 scale buggy.


One of my babies