Hemi Head?

[oakman]

hey all you sawheads, here is the head off my Canadian Power Machinery 275. it's a 123cc saw, similar in design to a stihl. i was just wondering if this is whats called a hemi head. i've never seen one like this. i am curious as to the theory behind this design. if anyone is familiar with it...................................

 

[bwalker]

Looks like the head on any two stroke bike motor except for the fact that the squish band is huge. Most two cycle heads are hemispherical in nature.

 

[oakman]

here's a pic of the saw. ups lost all the rest of the parts. just kidding. i'm cleaning it up, gonna run it and see if it needs a major tear down or not. bottom end is really tight and smooth, although i haven't pressure checked it yet. this particular saw was sold by the Skil company. can anyone see the similarities to a modern Stihl? tank location, angled back cylinder, piston ported, shroud design, etc.. so its a PM, that looks like a Stihl, but sold by Skihl, er i mean Skil..............................................................................

 

[oakman]

here's the other side

 

[dbabcock]

Beginner is also Ken Dunn.

 

[John in MA]

Neat looking saw. I've never seen a PM that large. They were originally made and sold under the PM name, mostly in Canada, and then Skil bought the rights to several of the saws. At least that's what I think. Mike Acres can give a much better rundown.

I suppose that's a hemi-head, although the saws I've seen with that description usually have the entire head surface as the "hemi" part.

 

[oakman]

John in MA
there is actually a bigger one that Skil sold. i believe a model 1690. it is 8.5 c.i. i got this one off ebay, there was only one poor pic, and the seller couldn't find the model#. however i knew it was one of those 2. was hoping for the big boy, but dang, it's only 123cc's. next time.

 

[NevadaWalrus]

A true hemi would have the entire head and piston top in a matching dome.

 

[Tzed250]

Hemi

The most famous hemi heads are of course from a Chrysler Hemi. They do not have the squish areas common to a two-stroke head. Hemi=hemispherical combustion chamber.

 

[bwalker]

A true hemi would have the entire head and piston top in a matching dome.

Which would be a nightmare for a two stroke. Talk about detonation prone.

 

[NevadaWalrus]

Not sure why detonation would be a problem but it would be interesting to see the affect a true hemi would have in the exchange of exhaust for a fresh charge. There are strato charged ports and other alterations to improve the exchange, the hemi design might provide a natural swooping action that would cause an efficient exchange.

 

[Tony Snyder]

I thought the intention on the original Chrysler 4 cycle hemi head was to allow, in combination with the double rocker shaft, larger diameter valve heads, thus greater volumetric effeciency.

But I held that belief before reading Beginner's lesson on engine theory.

If the combustion area in a 2 stroke were not mostly hemispherical, wonder what else it might be?

 

[oakman]

beginner
i'd still like to know the purpose of recessing the plug like whats on my saws head. is it supposed to like super concentrate the fuel/air mixture, or protect the plug from the explosion? your statements would suggest a much more efficient combustion process if the head were a uniform hemisphere, with a tiny recessed area so the plug tip would be flush. incidently i recall working on some older snowmobile engines that used a hemi shaped piston dome. don't recall which motors, though. possibly a JLO.

 

[Tony Snyder]

I have a restored D 17 Allis Chalmers. It has power crater pistons. Those are upside down hemis.

 

[Tzed250]

Dangerous

There is nothing more dangerous than a person with a little bit of knowledge

 

[bwalker]

I don't see how a hemi design would cause detonation. Detonation is pretty much the same as precombustion, the burning of the fuel-air mixture before the correct ignition time which could be caused by a hot piece of carbon stuck in the cylinder due to rich burning or incomplete burning, which is what the hemi design does, improve fuel burning. But I am no pro, so I could be wrong.

Beginner, You seem to be confused as to what detonation and pre ignition are. Pre igntiion is the fireing of the charge before the spark plug fires. This can be caused by carbon as you mentioned, ash build up, too hot a plug, etc. detonation however takes place after the plug has fired. Imagine that a regular combustion events pressure curve where illisrtated by a line graph. The normal event would be in the shape of a bell curve while the event where detonation is present would resemble a bell curve with a rapid spike towards the end of the event. In a motor normal combustion is a even burn of the charge. detonation starts out as a even burn but then degenerates into a explosion. The resulting heat and pressure spikes destroy motors in short order. The reason pistons with severly domed crowns cause detonation is because the piston runs significantly hooter due to the surface area exposed to the heat of combustion. This increased heat can lead to deto.

Now getting back to hemi heads. Most of the two stroke heads i have seen are hemi type. The picture Oakman posted is indicative of this. The spark plug Idealy should be at the center of the combustion chamber mounted in a vertical position. Most saws dont do this because of packaging concerns.

 

[bwalker]

Thanks for the info. Obveously I need to reread the books I've been neglecting. At vo-tech school,

If you want to read some good books read anyhting by Gordon Jennings, Smokey Yunick, or A.G. Bell.

 

[NevadaWalrus]

Not to mention, Smokey is a gas.

Thanks Walker explaining why the detonation with the domed piston. Makes sense, Guess thats why they're ok in car engines or for that matter liquid cooled engines that tend to run cooler.

 

[bwalker]

The will work fine in low stress four cycle apps ie cars. Start to push them and the will make a stroke detonate also.