Discussion in 'Performance Mods' started by 45u, Feb 18, 2016.
What is a smallest squish that is not going to affect the reliability.
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.
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.
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:
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.
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.
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.
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.
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?
Here is my little 27.5 to 30cc engines for my 1/5 scale buggy.
My little 1/5 scale buggy.
One of my babies
I started as a machinist, rebuilt engines as a hobby, then switched over as an industrial mechanic, electrician, then electronics and programming, troubleshooting in a factory. Not as specific experience as you so nice to have you here working on this stuff and sharing.
The beauty of these engines is the head comes off and on in 5 min so experimenting is easy. Previous experience show just about any engine can run at at 0.010" with no immediate damage but typically lose power at that small a gap. I did not progressively try different gaps on the China Girl engine, but why not you give it a try? According to Gordon (and others) the sweet spot should be somewhere between 0.010" and 0.040". Easier to take metal off than to add it so maybe try shaving the head 0.010"or less at a time from 0.040"? When you feel it loses power throw a paper gasket under the base to bring the squish gap back up to where the power was. The ports will hardly notice a 0.006" rise. Like the RC engines Jeff, these are wonderful to play with.
As for damage, I was amazed that even with the noise made at 0.010" there was no metal contact on the 200cc engine. The engine heated up noticeably and produced ( I'd guess 10%-20%) less hp. I suspect there was sudden piston deceleration at tdc that would eventually result in the bearing or piston failure. I'd also guess that the rings are taking a pounding too. So after that I always erred on the top end of the tolerance. In other words if I felt no power difference between 30-40 thou, I went to 40.
I do the same with compression too. All these things give diminished returns. For racing you want everything you can get so you may run it to max, but for longevity I run it to the safe side where I cannot feel any difference. As the cc's decrease closer to detonation, less and less power gain is felt. So if I have detonation at 16cc, I'd probably be safe at 17cc, but I really didn't feel any power gain from 18.5cc so that is where I left it. The problem with heat is that it causes a rapidly cascading failure. Run forever while everything is right but when something fails it rapidly cascades into meltdown. I love these engines, I'm only out $30 for another top end!
So Jeff, if you don't experiment with squish gap, when the snow melts, I probably will.
Steve I do not even have my motor yet but should be here tomorrow. Then I have to find a bike so it will be a few more weeks at least. We get very little snow down in middle GA and is good for me as this is my only transportation as I do not like cages and do not own one. LOL Been this way for most of my 45 plus years of riding.
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