The hemi refers, IMO, to a bowl shaped chamber with no squish band at the edges.  Anything with a squish band should be referred to as a squish band head.  This nomenclature gets really confusing, because merely cutting the inside corner off the squish band and rounding it can be called a hemi conversion.  Other common head types are a wedge, like a stunt fox, seen with baffle pistons, a trumpet, like a Cox tee dee, and a double bubble, a squish band with a first bowl inside the band, and a second deeper bowl inside the first.  For our purposes, squish band and hemi are what is most common.  The hemi shape seems to give a slower burn, and a smoother break.  As originally  configured, the squish band head was used for racing.  The idea was to get the deck height small enough to make the piston come within 10 thou or so of the head, and force all of the mixture to rush to the center bowl at the instant of ignition.  This generated extreme turbulence and rapid flame travel, which made the combustion rapid and the break somewhat harsh.  Stunt engines typically have more deck height, say 15 to 20 thou, but the  harsh quality of the break seems to persist as log as the squish band is left square.  Tom H.

Not a thing... In order to get the break right for stunt purposes, the deck height gets so high that there is no practical difference between a single or double bubble. The double bubble does have some effect for racing applications.  Tom H.

That mod should lower the effective Compression Ratio quite a bit, too. Right, Bob???

Thanks Bob, that picture really tells the story. I'll put that one away for my FP40 when it comes time to build a classic model for it.

The compression question links up with my other enquiry... What kind of compression ratio are you typically ending up with after such a mod? It seems like a much bigger change than a few head gaskets will get you. Can someone give me some before/after figures for compression ratio changes?

I took to my GMS76 head with the lathe and did some test flying today. The geometric compression ratio is at about 8.4:1 now. I have no idea if that is in the right ballpark, but is certainly running more like I want it to.

It could use a little more power or perhaps it just needs pitch. Its fine in level flight and is doing a subtle but undefined 2-4 kind of thing, but seems to lose power or slow down too much in loops and overhead and then as you pull out, it picks up for a lap before settling back down to an easy 4-stroke.

What does that sound like? Undercompressed? Tank issues?

I have controlled runaway in a K&B 4011 by running a smaller venturi.  Prop choice also has something to do with runaway, or at least excessive windup.  If you let your fox 35 get enough castor crud on the outside, it will heat up and runaway on you.  Clean it up and problem solved. 

Have any of you compared a head which is a tight slip fit into the liner with one of the usual with several thou clearance?