High-quench, reverse-dome pistons look cool

I was running a flat top piston in my 493" RB, and with the 76cc chambers in the Indy EZ heads it was near 12.2 to 1 compression. I enjoyed the nearly 7 years I had it on the street. I always burned 101 octane unleaded fuel through it, which was available locally 24/7 at the pump.

I pulled the valve covers for routine inspection, and found bits of the rocker arm bearing cages laying in the heads. I posted that story here a few months back. Turns out valve train harmonics from my hydraulic roller cam and some weak valve springs beat up my exhaust rockers, cracked 3 valve seats and compromised all my valve train parts.

I tore into the motor. In consultation with Dwayne Porter, I upgraded to a mechanical roller, replaced the entire valve train and had him freshen up and repair the heads. Great man to work with by the way!

While it was out, I bit the bullet and dropped the compression to a milder 10.25 to 1 compression, so I could run common 91 octane pump premium. Dwayne was instrumental in helping me spec out the pistons to get the compression where I wanted it.

No one made an off the shelf piston for my combo, so I contacted Ross. They made my previous set, and had the Job Card on file. I gave them the info Dwayne calculated and paid my money.

These 514 gram beauties showed up the other day, and I dropped them off at my local builder. Only recently had I seen a few pictures of what big block Mopar high-quench, reverse-dome pistons look like, so I've attached a few here for your viewing pleasure. I had the skirts coated to reduce friction.

Nice!

Is that actually a reverse dome, or a flat top with a dish?

Looks nice.

Hmmm ... look like 440Source "off the shelf" items !!

Ross used to be the standard 440Source piston.

Mine were custom 34cc reverse dome or dish (same thing?) By the time Ross got around to making them 3 months after promised date, Brandon had made them a shelf stock item so he refunded my custom up charge.

Kevin

It looks like the only difference between the two is that the deck is thicker under the reverse dome on this style of piston. I was told they could not carve a 22cc reverse dome on a standard flat top, as it would leave too little deck thickness.

My original flat tops are from 440source. They don't use Ross anymore. In this case, the piston is lighter, has a slightly different compression height, and is larger in diameter to allow for a fresh hone job on the block. The shape of the 22cc reverse dome is also matched to the Indy EZ combustion chamber.

Yes, of course it is forged to allow adequate thickness. I would still call it a dish.

There are pistons out there that actually step-up opposite the spark plug that also have a dish. Those I would call a reverse dome.

Yes these are off the shelf 440Source D-dished pistons as I am also running a 493 with the standard EZ heads. I was up around 12.0 myself when I had flattop pistons in it at first. Mine is a street car that I use 92 pump in and it is now at 10.6 and works great since it has good quench. And I use a flat tappet cam from Dwayne Porter. They are nice looking pistons you have. Ron

The difference between a reverse dome and a dish has to do with the shape of the hollowed out area.

A dished piston top has a round depression, usually centered in the center of the piston. This leaves some of the SQUISH area of the cylinder head uncovered.

A reverse dome has the hollowed out area under the open part of the head's combustion chamber. The squish area of piston and head match, almost exactly. It's called a reverse dome or inverted dome because if the piston had a dome it would go up into the open area. The inverted dome has the piston surface go away from the cylinder head over the open area, just like if you took the dome and turned it upside down.

You can argue that an inverted dome is a dished piston and that is technically correct, but the terminology was developed so you have a better understanding of what the shape of the hole in the top of the piston is.

It's easier to say "inverted dome" than to say "it's a dished piston with the dish mirroring the open part of the combustion chamber."

Semantics.

R.

D dish is the term. The Diamond catalog simply calls them dish.

I use the term "inverted dome" for that style piston, which is also how JE catagorizes that design.

Here are some diamonds that are like that:

And they are called dish.

D Dish pistons were available way before anyone even used the words inverted dome, which was developed for building quench in open chamber heads.

There are pistons, I believe Icons, that actually have three different levels, Why, I don't know, but could be called an inverted dome with a dish.

I'm not a wordsmith, or a piston manufacturer....... But JE, Wiseco, Mahle, SRP, DSS, and CP all refer to pistons that are of the same configuration as what the OP has a picture of as "inverted dome".

The Icon pistons that have the two levels along with the inverted dome, they refer to as a "step dish".

A standard "dish" piston is exactly that, the WHOLE top is just a dish shape and round. Inverted dome, reverse dome, stepped dish whatever you want to call it refers to pistons pictured here. Why? because if you had a regular DOME piston to make big compression, it would be contoured to fit inside the chamber. These are inverted or reversed in order to lower compression and still have a quench pad.

I can show you plenty of "dished" pistons.........the ones above are NOT them

Side by side view. 15:1 dome piston on left, 11:1 reverse dome on right.

Then I guess it is semantics. You could buy D-dish pistons in the 1970s, and I don't recall anyone calling them inverted domes. I guess back in the day we were overwhelmed by logic and the simple definition of the words dome and dish.

I stand corrected. Today, this an inverted dome.

Now, think of all those beautifully machined sharp edges surrounding the dish and valve reliefs.
They're like little waterfalls, in which the overlap leaving the valve seats near TDC flows.
Yes, the flow trips over them, and reduces the effective area of your overlap triangle.
If you cam is too small, you're losing power that could be recovered by adding a 1/8" radius to all those edges.

A finished piston may have sharp edges to control flow, but not as shown.

Great explanation!