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Author Topic: M94 hit n miss Piston Top Shape Theory?  (Read 204 times)

ShadetreeMotorcycle

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M94 hit n miss Piston Top Shape Theory?
« on: November 17, 2020, 07:48:41 pm »
I have not had time to tune on my engine as my new job has long hrs and sizeable commute...but... my Mind is always tinkering!....SO....our very own Gil posted a YouTube video of a very nice running M92...I was pleased because he shared in detail just what he had done which besides a softer spring was remove .105 from piston face plus dish piston approx .075
I was thinking that lowering compression makes the flywheels able to carry us thru the compression stroke for a longer time...BUT also lowers the power stroke...I am not really concerned about putting This engine to work but more a nice slow display speed...However....I Do want to make the most of the power I Do make....got me to thinking about dished pistons!
   I did some math and the photo will show my Very Crude sketches of 3 pistons....
Top A = stock
Center B = full half round dished piston
Bottom C = closer to my proposed mod

The arrows on the left point to the surface top of piston...B has Double the surface area of stock but is not practical as the Volume of material removed would equal shaving .288 off the top of piston if it where kept flat....that is approx double the total amount Gil removed....also the edges of top the piston would be sharp and not ideal or recommended.

My point of all this is: If I just dish the piston rather than follow Gil's recipe...but try to match or get close to his total material removed...Am I correct in thinking if you increase the total surface area of the top of the piston...does it not push harder using same amount of theoretical combustion pressure of a flat top piston?....now I am kind of splitting hairs because Gil also dished his piston a small amount but if piston B could be made to match Piston A's compression reduction....would piston B push a bit harder on the crank?... Not sure if I have explained it well or if my crude sketch helps...But would love to hear what the experts have to say!

Adirondack Jack

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #1 on: November 17, 2020, 08:55:06 pm »
My thinking is that within a closed cylinder with immovable walls and top, the expanding gasses will move the piston to lower the pressure without significant regard to crown shape. Old time flatheads often had pistons with a cylindrical bowl cut out of the middle, (think ash tray) but they were flat heads with side valves. At low compression, as was used in pre-ethyl lead gas engines, such as the model T at 4:1, heavy flywheels meant they could idle very slowly.
I think the larger hit and miss engines benefit from scale, and from well oiled plain bearings. Ball or roller bearings are more durable, but not as silky smooth running as a bronze or even Babbitt bearing that is well worn in on a smooth shaft.
A big honking iron flywheel thumped vigorously by a large volume, low velocity push from a mildly compressed charge is like pushing a bus with a car.  Our little engines with comparatively low mass, relatively rattly bearings, larger tolerances per unit of size (.005” isn’t a lot on six inches. It’s a ton on six mm), and relatively small, perhaps more energetic firing as occurs when the resistance to initial movement is less in the small chamber of a light engine, ends up more like hitting the car with a sledgehammer than pushing it. The initial movement of the piston and rod and crank journal doesn’t translate into the same kind of low velocity, high momentum push as the big motor gives its flywheel. 
And once in motion, our parasitic drag is probably higher per unit of momentum, which degrades coasting more quickly. 

I understand what you’re trying to do. I think it won’t amount to much if any repeatable improvement. I like Gil’s method. Frankly I think the slight dishing and significant shortening is a fair compromise in not reducing wall length or material thickness too much. I don’t think the gasses pushing against it care, so long as it’s not a needle, shoving the gasses past the rings instead of moving the piston.

I say this as someone with decades of experimentation with expanding gasses within relatively cylindrical chambers with varying piston shapes at varying rates. It’s accumulated knowledge from designing bullets for handguns with varying burn rates from lightning fast to black powder slow.
 
I think another area to explore might be the Governor components. Slick and repeatable, rather than rough and clattering mechanisms eat far less.

ShadetreeMotorcycle

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #2 on: November 18, 2020, 07:43:09 pm »
I do plan to polish the governor mechanism and brass gears as well as change out the bearings for bronze...as well as add functional oilers, but I kind of like to make one mod at a time to try to document what changes are the " biggest bang for buck" or effort...I now have 3 engines to tune...the M92 sure is pretty!

Adirondack Jack

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #3 on: November 19, 2020, 05:25:15 pm »
One change at a time, that’s the only way to “science” it. Have fun. 

ShadetreeMotorcycle

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #4 on: November 21, 2020, 10:28:25 am »
Well...I got to mod my Piston today!...first pic is fresh off the lathe... second pic I have polished the dish and top half to about 1000 grit...there was a distinctive zipper or record player groove texture to the piston...for now I have left some of the texture on the bottom half for oil to get into...but may fully polish later.
   She runs slower!...I need a tach now to tell how much, but need to work on the Governer springs first to get more fine control.

ShadetreeMotorcycle

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #5 on: November 21, 2020, 12:21:32 pm »
 Only one pic attached...this is the piston dished .225 deep with a .875 ball nose cutter....but before polish.

Adirondack Jack

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #6 on: November 21, 2020, 01:58:54 pm »
That is some nice work. I don’t know that polishing was necessary, but it’s sure pretty. I like the dome dish idea. It preserved full piston length, which I think makes them run smoother, while reducing compression.

70Rcode

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #7 on: November 21, 2020, 05:47:34 pm »
Now that's  a "Lathmaster"  & a well up-graded piston !!   You've  probably reduced the factory's hi C/R  down to 'bout 5:1 & combined with those friction reducing, oil retention "record player" skirt grooves should really help getting the revs down to a more realistic full scale "hit"/coast speed....Fantastic machining job... Can't wait for you to post some video action ! ........tom.

Jim

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #8 on: November 21, 2020, 05:55:51 pm »
Really need to see a vid of this engine running now.
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ShadetreeMotorcycle

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #9 on: November 21, 2020, 07:56:54 pm »
My engine came with (2) o-rings on the piston....it seems the general consensus is one is better...I removed the one farther from the spark plug.
   It still has Plenty of compression even with the deeply dished piston....I did not really want to remove more from the piston, so I dished the Head!...I noticed the intake and exhaust are deep in the spark plug hole....I dished the head to lower the compression a bit more And try to get the path for the intake mix a bit shorter....to my surprise the dished head seems to have made Very little difference in performance!
   The photo is the head machined...with the grind on my ball nose cutter combined with the spark plug pilot hole caused quite a bit of tooling chatter. I can polish it out but it was kinda cool looking so I left it for now...lol

Nick

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #10 on: November 21, 2020, 11:01:47 pm »
Really need to see a vid of this engine running now.

Agreed  ;)
Nick

ShadetreeMotorcycle

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #11 on: November 22, 2020, 03:50:49 pm »
I did try to post a video straight from my phone...its about 130MB and am sure that is over the limit for file size, I just got the little spinny "Working" circle....of death, and it never loaded....sorry Guys!

Jim

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Re: M94 hit n miss Piston Top Shape Theory?
« Reply #12 on: November 22, 2020, 04:34:54 pm »
I did try to post a video straight from my phone...its about 130MB and am sure that is over the limit for file size, I just got the little spinny "Working" circle....of death, and it never loaded....sorry Guys!

G'day Ben, its pretty easy to set up a Youtube account to upload a vid to.

Vimeo is super easy also -

https://vimeo.com/
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Jim

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