Mooning cylinders

[Walt Fricke]

The cylinders I have seen which have been mooned have had this done on the lower "sides" - so the air can flow axially, so to speak. Through the passages between the webs and the bottoms of the spigots.

Could one not also machine away the area of the cylinder skirt which lines up with the rods, where the case casting doesn't go to the bottom of the skirt?

That would let the air move through an even larger area.

As best I can tell, the cylinder skirts are not in compression - that happens up at the surface of the case/top of spigot. So they are there to guide the pistons insofar as is needed. And mooning couldn't extend above the BDC ring line, one supposes.

Is the transverse/radial part of the skirt (unmooned) needed for letting slipper skirt pistons slip?

There must be some flaw in my thinking here, or everyone would do this.

Walt

[Eagledriver]

Walt,

Not sure about what you are describing but here are my thoughts. I've seen cylinders that were shortened instead of "mooning". This seems like a very bad idea, as the piston skirts will stick out the bottom of the cylinders when they are at BDC. I would say that as long as you leave the cylinders at full length at the top and bottom where the pistons skirts go they would be fine.

-Andy

[Walt Fricke]

Here is an example of standard mooning.

What I don't know is how far down in the cylinder the base of the slipper skirt comes. If there is any part of the cylinder it does not reach, and that corresponds with an opening in the case casting, as it does in the rod travel direction, why not remove that part of the cylinder?

[boxermania]

The slots should be in line with the piston pins or at 90 degrees to the piston skirt. That being the case there isn't a solid part of the piston sliding anywhere near the slots.

That is unless the slots have been machined too far into the barrel and can interfere with the piston ring lands, but I doubt it.

BTW, nice looking P&C's

[Walt Fricke]

They are nice, aren't they. Too bad they aren't mine. A "stock" photo I had saved, first one I came up with from my stash of (mostly from Pelican) stored photos.

[Eagledriver]

Walt,

I think you are correct that removing more material would be fine as long as the piston skirt doesn't protrude. However, I think the skirt pretty much goes all the way down on most engines. Maybe not on a short stroke but the rest seem to be. Look at the wear patterns on the inside of the cylinder and you should be able to see how far the skirt goes. I suppose you could also measure it easily by putting the piston in at the appropriate deck height and then measuring the distance from the bottom of the skirt to the bottom of the cylinder. Compare that to the stroke of the crankshaft.

-Andy

[prschmn]

Eagledriver ir quite right--never ever allow the skirts to go beyond the bottom of the jug. More to be gained anyway by knifedging and or oilshed coatings.
Mark--or Judy Hs "partner in crime" as you called me at Rennsport

[bobolsn]

I am not sure what engine you have, but the early engines did not have the crankcase aerodynamic characteristics of the later engines. This created more work for the engines as they pushed air through the crankcase on the piston's way down. The cylinder mooning and the crankcase web removal (boat tailing?) is a modification that Bruce Anderson recommends in his 911 Performance Handbook. He stated that this modification would add as much as 10 hp to my 2.2 engine. He has a fair number of pictures in the book to illustrate the mods.

I hope this helps.

Bob

[Flieger]

I think for an engine that does not spend its life at redline like a race engine, windage mods will make virtually zero horsepower for all the money invested. Also, if not done properly, they can compromise the durability of the crankshaft and case.

Money is far better spent elsewhere. Removing weight, etc.

[Walt Fricke]

I agree with Max.

But the motor I am slowly assembling will spend 98% of its life between 4,000 and 8,000 RPM.

66mm crank, 95mm J&Es in Nikasil Cs, early Carrera 3.0 case, 3.2 heads, other stuff.

Case boattailed (by hand, with a die grinder - not as aesthetic as previous cases done by machinists with machines). So every little reduction in parasitic losses will count.

The 1968 912 Targa this goes into, by the way, is down to around 2,000 lbs without driver. Only large weight savings on the horizon is to remove full roll cage (1.75" thick wall) and replace with 1.5" thinner wall - should save maybe 40 lbs. Weight reduction, after a point, gets expensive too.

[fbarrett]

Walt:

Until tonight I had never heard of mooning cylinders, so thanks for helping me to understand it. I've mooned a few things in my life (maybe even you), but I'd never heard of it. Then again, I am still stuck in about 1973...

Frank

[PeteKz]

Likewise, I'm familiar with term "notching" the bottoms of the cylinders and other descriptions, but not "mooning," heretofore.

[winders]

Do this Google search:

moon cut cylinders porsche

You get over 8 million results.....

I have been hearing this term since the late 70's when I was exposed to 911 engine builds.

[PeteKz]

Then moon on!

[porschedude996]

What is the process on the pistons called.

Originally Posted by Walt Fricke

Here is an example of standard mooning.

What I don't know is how far down in the cylinder the base of the slipper skirt comes. If there is any part of the cylinder it does not reach, and that corresponds with an opening in the case casting, as it does in the rod travel direction, why not remove that part of the cylinder?

[PeteKz]

That copper color is a thermal barrier coating (TBC). Walt would have to tell you what brand it is. TBCs on the piston tops reduce heat transfer into the piston, allowing the engine to run a little cooler, and keeping more heat in the combustion chamber where it can produce more power. TBCs can also be applied to the inside of the head, the faces of the valves, and the inside of the exhaust port (I had TBC applied in the exhaust ports of my 3.2 build).