Purpose of raised center in a dished piston

[user-23911]

It's because if you made a normal dish, you'd end up with a hole in it.

Don't know how this one is still going?

[Mark O'Neal]

People eat with their eyes

[DCal]

I can't say why they did it but "the industry" does that when the piston has a short compression height and a dish is needed . Often the rod ( as pantorman mentioned) gets too close to the under side of the piston and the "bump" allows rod clearance to be machined and the surrounding "donut" area lowered. Diesels use it for fuel spray/distribution IIRC.

This is the reason we do it now. But way back Nick Arias believed that it pulled heat away from the ring area and drew it into that smaller raised dome in the center of the piston , he called it a "Thermal Dome" and did a lot of Nitro pistons that way. No CNC machines in those days. It took a real machinist.

[ptuomov]

It's because if you made a normal dish, you'd end up with a hole in it.

Don't know how this one is still going?

Subaru has turbo pistons with same compression height out with square dish, round normal dish, and round donut dish with a raised center. None of them have a hole in the dish.

[2seater]

With a compression height of 1.449", I wouldn't think the rod clearance would be an issue? I know this is isn't about a high performance engine but is one of the most common that powered many of the GM large fwd cars for a decade and a half and made Ward's ten-best list multiple times. It was also used in rwd form in some F'bodies which is probably closer to the use in the Holden. Millions of these engines were used here in the U.S. with conventional dished pistons so when a different design is in use in a different market I simply wondered why.

[user-23911]

It's because if you made a normal dish, you'd end up with a hole in it.

Don't know how this one is still going?

Subaru has turbo pistons with same compression height out with square dish, round normal dish, and round donut dish with a raised center. None of them have a hole in the dish.

Subaru actually have quite tall pistons (33 to 34 mm comp height depending on version)
There's more space to the pin.
I've measured them with thought on re using for something else.......too tall.

When you get down to a 29 mm comp ht you need the middle higher.

[Cutlassefi]

clearance for the top of the rod ???

That would be my vote. Very common.

[ptuomov]

It's because if you made a normal dish, you'd end up with a hole in it.

Don't know how this one is still going?

Subaru has turbo pistons with same compression height out with square dish, round normal dish, and round donut dish with a raised center. None of them have a hole in the dish.

Subaru actually have quite tall pistons (33 to 34 mm comp height depending on version)
There's more space to the pin.
I've measured them with thought on re using for something else.......too tall.

When you get down to a 29 mm comp ht you need the middle higher.

So why do some factory Subaru pistons with tall compression height have that raised center?

[user-23911]

Not sure that I've seen subaru pistons with raised centre but .......
For more compression, as in non turbo.

[ptuomov]

Not sure that I've seen subaru pistons with raised centre but .......
For more compression, as in non turbo.

They’ve got dished turbo pistons with that raised circular island on top. And there are also square dishes and round normal dishes. All in the same ball park of compression.

The normally aspirated pistons have the usual 4-valve crown so I’m not talking about those.