Compression? How much?

[Steve Best]

Jaguar is talking good stuff. He is right. More compression drives heat into the head and piston and small chambers do kill top rpm which equal speed. There is minimal gain to the last bit of compression and it does get destructive. His 0.85mm measurement is what I found as well, by experiment on my Grubee. Less than that is not helpful.

Steve

 

[Nanonevol]

I will try this today! It's not too hot yet. 80 no humidity but getting warmer in the next month and then Summer.
What number plug? Any specific brand or go with ngk?Is a higher plug number cooler? I have a 7 in now and seems good.
Gapped at .30
Mike

No relation between outside air temperature and spark plug temperature rating!

 

[Steve Best]

<SOME SNIPPED>
Years ago HS proved you can't have a squish band head on these small engines and not have too high an engine compression.
<SOME SNIPPED>
Squish bands were invented to reduce piston heat. Research has already proved that there is no power benefit other than not having power loss due to excessive piston heat.
The research paper is "Squish Velocity in the Combustion Chamber of a 2-Stroke Cycle Engine" by Avraham Ziv of Engineering Div., McCulloch Corp.
Here is my infor page about squish bands: http://www.dragonfly75.com/moto/SquishVelocity.html

I might have been reading a bit too fast.
A squish band head is still advantageous on these engines. (can you lead me to "HS" information?)
You are right but when used to cool the piston squish zones are commonly referred to a "quench" area.
Both effects are real so the terms are interchangeable.

Avraham Ziv's paper was from 1973 I believe and conflicted some of the results of other people who were doing quite well in the racing world. I don't remember the exact reasons why. I read your webpage on squish and think it is a wonderful resource. Great work.

Modern automotive engines are not using quench or squish area because of the effect on hydrocarbon emissions hiding in that cooled area.

Steve

 

[Deleted member 12676]

HS is "headsmess". message him and ask for the info of his investigation as to whether such a small bore engine can have a squish band and still have low cranking pressure (below 140).
A squish band is advantageous especially on an air cooled engine by lowering the piston temperature. But they always raise compression too much on a small bore. I shouldn't say always. It is possible to have a minimal area squish band if the combustion dome is made large enough. It's just that no aftermarket head manufacturer has done it so far for this engine. So you have to do a lot of work with a dremel to enlarge that area.
I modified a stock head to have a squish band and the cranking pressure was 150psi. I guess I got tired of messin' with it and so never got it down to the desired 135psi. It didn't make any difference in power though. (.85mm clearance)