What can Ford teach us re: piston design

[engineermike]

Some time back I disassembled a gen3 coyote and was mesmerized by the fact that built-in stock features resembled what we did on performance engines 20 years ago. Domes, floating pins, cap screws, coatings, etc all from the factory. The only thing missing was being forged material.

So now I’m building a predator and I’m finding stuff on the stock pistons and rods that we *didn’t* do 20 years ago. For instance, there’s no anti-rotation tangs on the big end bearings. Furthermore, the pistons ring and land geometry is really interesting. I’ve attached pics to show that the 2nd ring is a complex geometry and the bottom of it isn’t fully supported by the piston. There’s a big groove between the top and second ring, though a smaller one does exist on the gen3.

Accepted logic is that tight ring gaps will butt together and break pistons, so I’ve always run .020-.026 on the top ring. I was surprised to see ford now running .010 or less in all their engines, Including the predator and ecoboost. The spec for the gt500 is .008-.012, so I checked it and sure enough it’s .010. Is anyone aware of any predators breaking ring lands (or the stock powdered metal opg for that matter)? The second ring is much wider than the top ring as well.

I don’t know if any of this geometry is what allows them to run such tight ring gaps in boost, but I’d be interested to hear any theories anyone has.

Edit: Ford calls the unique 2nd ring geometry a "hook feature". A quick google search revealed this design is called a "Napier" feature that scrapes oil from the cylinder wall. I do not recall this being included in the Gen3 ring design.

Also, I've attached a pic of the top ring to show it has a bevel on the ID, which is also unique.

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[Grimreaper]

Ive got more to learn than add but what's the difference in ring material and its impact on ability to take heat and retain its size before closing the gap?

What are you measuring for the ring width and thickness differences on the slugs you have? 1st/2nd ring on gen3 and predator?

 

[engineermike]

Unfortunately, I let the Gen3 go so I can't measure the rings. But on the Predator, the stock ring dimensions are:

Top
.121" (3 mm) wide
.047" (1.2 mm) thick

2nd
.152" (3.9 mm) wide
.047" (1.2 mm) thick

I don't know the material, but they are all most certainly majority iron, so the coefficient of thermal expansion is going to be roughly the same.

Another interesting fact is that the factory Predator piston-to-wall clearance spec is only .001-.002. I verified on OEM pistons and a new 5.2 block, the clearance is right at .001.

 

[GregO]

A quick google search revealed this design is called a "Napier" feature that scrapes oil from the cylinder wall.

There are several great threads on Don Terrill’s Speed-Talk regarding the Napier ring.

 

[wish i was fast]

Some time back I disassembled a gen3 coyote and was mesmerized by the fact that built-in stock features resembled what we did on performance engines 20 years ago. Domes, floating pins, cap screws, coatings, etc all from the factory. The only thing missing was being forged material.

So now I’m building a predator and I’m finding stuff on the stock pistons and rods that we *didn’t* do 20 years ago. For instance, there’s no anti-rotation tangs on the big end bearings. Furthermore, the pistons ring and land geometry is really interesting. I’ve attached pics to show that the 2nd ring is a complex geometry and the bottom of it isn’t fully supported by the piston. There’s a big groove between the top and second ring, though a smaller one does exist on the gen3.

Accepted logic is that tight ring gaps will butt together and break pistons, so I’ve always run .020-.026 on the top ring. I was surprised to see ford now running .010 or less in all their engines, Including the predator and ecoboost. The spec for the gt500 is .008-.012, so I checked it and sure enough it’s .010. Is anyone aware of any predators breaking ring lands (or the stock powdered metal opg for that matter)? The second ring is much wider than the top ring as well.

I don’t know if any of this geometry is what allows them to run such tight ring gaps in boost, but I’d be interested to hear any theories anyone has.

Edit: Ford calls the unique 2nd ring geometry a "hook feature". A quick google search revealed this design is called a "Napier" feature that scrapes oil from the cylinder wall. I do not recall this being included in the Gen3 ring design.

Also, I've attached a pic of the top ring to show it has a bevel on the ID, which is also unique.

the beveled edge your talking about is to help the cylinder pressure to get behind the rings and push them against the cylinder wall to help with sealing

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