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Flat top vs dome top pistons

Engine Building Fundamentals

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Referring to high compression, N/A engines here.

I've heard that flat top pistons are the most efficient design. They allow the A/F mixture to be evenly distributed, and the flame front and cylinder pressure will spread more evenly. So they produce the most efficient combustion, using up as much of the A/F as possible.

Of course there's a limit to how much compression you can get from a flat top. So dome top used to raise compression. But it comes at the cost of efficiency. I would also assume that if the pressures are unevenly distributed, it would increase the chances of knocking in those areas, over and above the knock potential from the increased static compression.

Can anyone share any knowledge or experience on this? Is it worth trying to keep a flat top piston if possible? Or getting a more subtle dome rather than a more aggressive dome? Or does the extra efficiency not really matter, and it's better to just maximize compression without regard for piston shape?

Been a while, and it's more oriented OHV engines, but as I recall...

There are several considerations, such as...

a/ combustion chamber shape. This has an affect on quench/squish which can introduce important turbulence for mixing the air-fuel, spark plug placement, and how the flame front moves through the chamber.

b/ dome shape, including height. As this is basically the bottom of the combustion chamber, it has much the same affect as the head's shape, especially as it's compressing the gas. It can also affect the overlap breathing. In context, depending on a/ and b/, it can compromise the flame front if it's too close to the head, even partially quenching it. Fire grooves, or slots, can be machined in the dome to give the flame front passage from one side to the other, if the ignition point is on one side of the chamber.

c/ the fuel characteristics. Primarily pre-ignition/detonation resistance. If you're planning on using "pump' fuel you're going to have to plan around the minimum you're likely to get - I know of several instances where people have used higher compressions but actually lost power because the fuel meant timing was rather compromised to avoid detonation. If you have ready access to high alcohol percentage, or actual race, fuels you may find you can go rather high on CR.

d/ the dynamic compression ratio and rpm range. The former because it affects the octane requirements, and the more "race" oriented, the more compression can usually be tolerated. The latter because while the burn is roughly constant at the same pressure, the combustion cycle time means a faster burn, from compressing the mixture more densely, uses the fuel more efficiently.

e/ spark plug clearance. This is more of an aside - with some high dome designs, there can be a problem with clearance for extended tip spark plugs and a requirement for standard height centre electrodes - even recessed or surface discharge designs in some cases. Some piston designs actually have a dip, or hollow, under the spark plug for this clearance.

f/ diminishing returns. If you have, for example, a 10% gain in torque going from 9 to 10:1, you may only have 1% going from 14 to 15:1.

g/ assembly. When you're pushing things to maximise compression, you're going to need to make sure ALL the clearances are optmised, especially quench/squish, as peak pressures are going to be even higher there, increasing the detonation risk. Valve to piston clearances will also be reduced to as close as is possible - heck, with OHV engines it's not un-common to have theoretical interference between these, relying on valve train deflection/flex to give the working clearances.

In practice, I'd say the first thing is to try and avoid re-designing the mousetrap - in that most engines have already had people working on this and have found the problems and benefits already, in different engine configurations, so do as much checking as you can for your engine on different forums, preferably ones featuring your engine and/or vehicle.

Next may be to see what's actually on offer from the piston manufacturers and 'tuners', especially those that specialise on your engine, and use that as a guideline. If you're really sure you want more, several custom and/or race manufacturers will machine whatever you want, but at a price.

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