Real life consequences re: squish gap

[weefek]

There's a lot of discussions regarding squish gap, from what I've gathered on these china dolls 0.8-0.9 is ideal.

What's missing from these discussions is the reason and theory behind this. I mean, I guess I understand the theory (piston clearance to head = whatever compression ratio , depending on what head you're using). Same goes with a lot of other very generalized discussions on squish gap regarding 2 strokes in general, not specific to these engines. Let's be real, most of our engines regardless of brand or whatever are basically the same.

What are the real world results? My stock Grubee GT5B 69cc (china units 80cc) (the engine on my bike right now) stock engine has a 2.7mm squish gap with two base jug gaskets. To me this seems slightly excessive.

On my new engine I'm working on (and have been for awhile) I decked the head now to where I'm about 1.2mm squish gap with 1 base gasket. (keep in mind, no heat cycles, head not torqued down properly, let alone torqued properly after a few heat cycles). Copper head gasket is @ 0.45mm.

From what I've read (and the information is quite sparse regarding this topic other than the fact that 0.8-0.9 is optimal) this will improve low-mid range torque but not really do anything for top end. Can anybody in the know clarify this for me? I'm pretty sure 2.7mm way too much... I mean the engine runs well, I have no complaints for what it is and what it does other than I will admit low to mid range torque is pretty bad on this engine. I need a pretty long run-up to get to full speed. I have no problems with top speed, 32-35 mph is more than enough for me especially on this cheap chino frame I'm running on. But I would prefer more low end / mid torque and generally a better running engine over all.

I come from the automotive 4 stroke world where squish gap isn't really a thing, it's all about compression ratios and whether valves will clear or not. So forgive me my ignorance.

 

[Cisco]

stock engine has a 2.7mm squish gap

Both of my engines are about like that, and they run great. It's easy to see that the pistons don't come all the way up the cylinders (even with only one thin base gasket). What is the best way to reduce the gap, milling the top of the cylinder?

 

[weefek]

I have access to a mill but I don't want to interrupt work flow @ work. I used a piece of glass and sandpaper. Also I'm not sure where the best points to pick up a flat surface on the head would be on these "mildly ok" cast jugs. I took measurements all the way around from mating surface to the first cast fin, obviously they're not consistent all the way around due to casting inconsistencies but they gave me a good idea of how much I was removing over time. I used 180 grit for main removal, 400 for clean up, 600 for final pass. Some people go all the way to 2000 but I really don't think that's necessary.

 

[Cisco]

I'll try it on my PK80 first. It shouldn't take too long to lap it down. Sounds like a three-beer job

 

[weefek]

Yeah it's pretty easy. Without having run the new engine yet (still have to balance the crank) I honestly have no idea what the real world results would be like... which is basically what I'm asking in this post.

I got the pane of glass from my father, I'm not sure where he got it from. It's about 20" by 16" and I used to use it to lap processor heatsinks back in the day. As long as you're using a flat piece of glass there shouldn't be an issue. It fits a standard piece of sandpaper plus about 3" on each side, roughly. Keep dousing with water, cleaning the paper, etc.

**EDIT: I should say I've also lapped the head to about 0.3 mm above the casting. I mainly did it to make the mating surface completely flat, as a lot of people have done before me. Nothing new there. Did the 400/ 600 for that as well. If anyone can make an argument for going farther than 600 im open ears but to me it doesn't make much sense.

 

[Chainlube]

Yeah it's pretty easy. Without having run the new engine yet (still have to balance the crank) I honestly have no idea what the real world results would be like... which is basically what I'm asking in this post.

I got the pane of glass from my father, I'm not sure where he got it from. It's about 20" by 16" and I used to use it to lap processor heatsinks back in the day. As long as you're using a flat piece of glass there shouldn't be an issue. It fits a standard piece of sandpaper plus about 3" on each side, roughly. Keep dousing with water, cleaning the paper, etc.

**EDIT: I should say I've also lapped the head to about 0.3 mm above the casting. I mainly did it to make the mating surface completely flat, as a lot of people have done before me. Nothing new there. Did the 400/ 600 for that as well. If anyone can make an argument for going farther than 600 im open ears but to me it doesn't make much sense.

A narrow squish gap forces the fresh charge to the center of the combustion chamber making the pop bigger, where as a wide squish gap lets the newly ignited charge squeeze out to the edges reducing torque.
I use 120/320 that's it.

 

[weefek]

A narrow squish gap forces the fresh charge to the center of the combustion chamber making the pop bigger, where as a wide squish gap lets the newly ignited charge squeeze out to the edges reducing torque.
I use 120/320 that's it.

Makes sense. I had a feeling my 400 / 600 was even too much. To be honest the mating surfaces @ 180 don't look terrible, they'd probably be fine. But I'm kind of OCD about it.

I'll try this in practice with my new engine and report on it in that thread eventually. I would love more opinions though. Although yours is basically what i've seeen so far. Always respect your opinion @Chainlube

 

[Chainlube]

Makes sense. I had a feeling my 400 / 600 was even too much. To be honest the mating surfaces @ 180 don't look terrible, they'd probably be fine. But I'm kind of OCD about it.

I'll try this in practice with my new engine and report on it in that thread eventually. I would love more opinions though. Although yours is basically what i've seeen so far. Always respect your opinion @Chainlube

A few light swipes across the 120 will give you a quick indication on where the high spots are, once the high spots are level, then it's a matter of sanding a bit and measuring, sand a bit, measure, sand a bit, measure, you get the picture. Probably a full six pack job.

 

[weefek]

Flat a across the board. Been doing redneck measurements from the head surface to the top fin. Extremely unreliable for consistency due to it being cast. But a consistent decrease across the board.

Took the same amount off all around roughly within a few points of mm.

 

[weefek]

cant edit* so i should say i already decked it to be flat but never cared about squish. now i am.