gvnick

I have a dished piston 5.3 in my old 69 C10. I am wanting to start making a few upgrades (cam, etc.. mild stuff), and first wanted to know what I am losing by keeping with the dished pistons.

Specifically my question is about quench, and how this effects timing and detonation, and ultimately how much HP you can make on pump gas. Has GM designed the head to work well with the dished piston?

Obviously flat tops up the compression, which equals more power, but thats not what I'm looking for. I want to know if a 9.5 compression engine with dished pistons, and a 9.5 compression engine with flat tops would run the same.

If the answer is that dished pistons suck, why would GM use them? As opposed to a larger combustion chamber to decrease compression.

Thanks guys.

redtan

iTrader: (5)

The shape of the piston doesn't matter, it's the final compression that does.

So you could have dished, flat and dome pistons...if they all come up to 9.5 compression they the cars would all run the same.

Dish pistons don't suck, they have their place. It's easier to design different shape pistons, from dome to dish, to get the designed compression ratio. But you can only open up the combustion chamber of a head so much...so GM decided to keep that relatively the same but rely on piston shape for final compression.

On newer engines, like the LQ9, they kept flat tops but used a 317 head with bigger chambers to lower the compression ratio.

gvnick

Thanks for the reply. I guess my point is this. When building a motor, everyone agrees you want to get the quench (piston to head) clearance as tight as possible without the piston actually touching the head. Many race motors actually do lightly touch the head on the corner because they run them so tight.

The benefit is not to raise the compression. The goal is to squeeze the a/f mixture at a high velocity into the cumbustion chamber, more complete burn, blah blah blah...

I dont see how a dished piston comes even close to accomplishing this. The dished piston only has a small area around the perimeter to give any "squeeze".

I would assume a dished piston motor would be more prone to detonation, and would have to be tuned much more conservativly than an equal compression flat top motor with a tight piston to head clearance.

Is there some benefit of a dish piston that I am unaware of? This is a question that has been on my mind for a long time. I dont know why a dished piston would ever be used in an engine.

chrisfrost

iTrader: (8)

That answer is'nt totally correct ,,,,,,,if You have say .035" to the outside part of a dished piston compared to .035" to the top of a flat top piston You will have a little more quench with the flat top thus You will make more power as the computer will detect less pre-ignition and won';t have to retard timing as much .

mark21742

With a smaller chamber and dished pistons the dish in the piston actually becomes part of the combustion chamber...on the other side if you run a domed piston you direct more pressure out toward the weaker ring lands on the power stroke, vs running a smaller chamber head and a dishes piston the majority of force stays in the stronger center of the piston.