Squish

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46u

New Member
Feb 14, 2016
16
2
0
Georgia USA
On an 80cc two stroke Chines engine what is the best but reliable squish to run? I what as little as possible but still reliable.
Thanks Jeff
 

46u

New Member
Feb 14, 2016
16
2
0
Georgia USA
if you have an angled head squish won't matter. Just an FYI.

Fist I will not be using a head with a angled spark plug. I have built many a motor for my 1/5 scale buggy. I all so for over 40 years built motorcycle engines including but not limited to machine work for a living. Setting the Squish is VERY important for a good running 2 cycle motor. Now all I need is to find someone that knows what is best for reliability.
Thanks
Jeff
I 1/5 scale buggy.
 

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46u

New Member
Feb 14, 2016
16
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Georgia USA
On another forum I was told .030 is a good place to start. On my 26 to 30cc 1/5 scale engines I run between .021 and .025. These motors put out between 4 to 5HP by doing the numbers.



 
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sbest

Member
Nov 3, 2015
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Nova Scotia
Jeff,
30-40 will work great, but you have to bring that squish out to the edge of the cylinder or it won't matter. I didn't try 20. I am running 0.038" at the last measure. I did have it down to 0.030" on one running, no advantage.

Steve
 

exokinetic

New Member
Mar 18, 2016
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Lake Forest, CA
Depends on the cylinder bore.

The smaller the cylinder bore the smaller you can safely go.

Up until the point the piston actually hits the head its free power everywhere in the powerband.

The connecting rod and other related parts in the engine stretch due to heat, and the centrifugal force of high engine RPMs's.

NASCAR and other huge dollar racing series run the squish so tight that at max engine temps and at redline RPM the piston is actually JUST kissing the head. When you disassemble the engine you will be able to see the "witness" marks on the head and piston.


On incredibly small bore engines (i.e. RC Engines), the parts are so small they just are not going to stretch that much. In witch case you can go as low as 0.008". In my 66/80cc China Girl, with 47mm bore diameter, the recommended range is 0.015" - 0.025".

Anything bigger than 0.060" is useless.

There is a formula that uses the cylinder bore diameter to calculate the "safe" range... But I forgot it, you can probably google it.


Don't hesitate to PM me with any more questions!




Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/
 
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sbest

Member
Nov 3, 2015
343
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18
Nova Scotia
Over the years I did a lot of work investigating squish on a bunch of 2&4 strokes from over 4" bore to under 1". What I found is there is typically a sweet spot where the most power is made. I also found on all engines that 0.010" on either side of that ideal dimension has very little effect on power.

If you do not modify (increase) the squish area and shape of the stock chamber, all you need to worry about is mechanical clearance. 0.020" or more will keep you safe. If it is performance you are after, you need to increase the squish AREA out to the edge of the gasket at least.

The book "Design and Simulation of Two-Stroke Engines. Gordon P. Blair" explains and quantifies the effect in chapter 4.5.1 page 325: "A simple theoretical analysis of squish velocity". It isn't that simple, but the gist of it is THE SMALLER THE SQUISH GAP AND THE MORE AREA INVOLVED, HIGHER MIXTURE VELOCITY AND MORE ENERGY WILL BE TAKEN FROM THE CRANK. The last 2 paragraphs of page 333 are the most important, pointing out that inducing mixture velocities greater than 20m/s could actually cause detonation.

As I decreased the squish to under 0.010" in a 50% area Yamaha Blaster head, I heard a distinct knocking sound and power was noticeably reduced. Disassembly indicated no contact. Detectable power reduction occurred at under 0.020" and over 0.040" on the Yamaha Blaster so the natural conclusion is that 0.030" was the idea gap on that motor.

Ideally I would have reduced the squish on my China Girl engine until I felt a loss of power (my "dyno" is a hill and a GPS speedo) and then I would have bracketed the sweet spot for this engine. For the area of squish that I have, there is no perceptible difference in power between 0.030" and 0.038". If you have larger squish area, you will need more gap, if you have less squish area you can run a tighter squish gap.

Here is a stock combustion chamber on the left, modified on right:


So, I would guess (from past experience) that this engine could run as little as 0.010" gap with no detonation. Why not? As you tighten the squish gap you are increasing the reversing loads on the wrist pin and rod bearing. Up to a certain point less gap acts as a cushion, but tiny amounts of gap put huge load on the wrist bearing. This is a weak point on these engines, so when I determined I was no longer gaining power, I did not go smaller.

This is an easy thing to experiment with. I stayed on the very conservative side with 0.038". I fear (from experience) suggesting anything less than 0.020" could be damaging over time with the weak wrist pin bearings.

Steve
 
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exokinetic

New Member
Mar 18, 2016
108
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Lake Forest, CA
So, I would guess (from past experience) that this engine could run as little as 0.010" gap with no detonation. Why not? As you tighten the squish gap you are increasing the reversing loads on the wrist pin and rod bearing. Up to a certain point less gap acts as a cushion, but tiny amounts of gap put huge load on the wrist bearing. This is a weak point on these engines, so when I determined I was no longer gaining power, I did not go smaller.

This is an easy thing to experiment with. I stayed on the very conservative side with 0.038". I fear (from experience) suggesting anything less than 0.020" could be damaging over time with the weak wrist pin bearings.

Steve

Thank you very much for this valuable knowledge!


To add to the wealth of knowledge...


I have only destroyed one wrist pin bearing in the race engines I build (mind you I only ever run them 10-20 hrs max before a rebuild). It was when I was experimenting with extremely tight squish gap. So I would have to agree completely with this.

And I kinda have to adjust my thinking when answering questions to the whole community, as I have a rather racing-centric view of these engines.

If one is looking for where they should set the squish gap for the most power, AND longest life, 0.030" - 0.040" seems like the best range for the 66/80cc China Girl engine.


And I also agree that if one is ONLY concerned with maximum power output, there are probably no gains to be had going tighter than 0.020" (that was my experience as well).


Thank you again sbest! Great information!




Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/