Dished pistons for more power?

[SuperMag]

OK... in previous discussions we learned that in engines of equal displacement and all else being the same, the engine with the larger bore size/shorter stroke will make more total horsepower than the one with the smaller bore/longer stroke. Force x area and all that...

So my question is this-- Forgetting about the effects of quench for a moment (let's assume open chamber heads) --and assuming that we keep the compression ratio the same by some means-- will dished (or domed) pistons make more HP than flat tops? They have more surface area after all...

 

[Fred]

No. The thing that counts is the push down the bore. If you have pressure on an angled surface only the component of the force that is directed down will contribute to the power of the engine. Basically you can make the piston any shape and all other things being equal, you will make the same power.

 

[80broncoman]

Yes, and ...Maybe but for a different reason. Yes I agree the area of the bore is the same then IF the pressure is the same then you have the same net output.

Here is where it CAN make a improvment, What if the shape of the top of the piston helps the head flow more air? More air + fuel = more pressure and thusly more net output.

 

[SuperMag]

Area of the bore... only the downward vector matters... got it. I suspected as much. Thanks!

Although, I suspect that flat tops are still the best ticket: less exposed area = less heat lost through the piston crown, correct? And ditto for a shallow, closed chamber head, no?

 

[Thad]

Ever hear of the Herron design head used by Ford UK. No combustion chamber in head, perfectly flat except for the two valves. The combustion chamber was in the piston top.

Oversquare , bore larger than stroke, engines tend to make more HP.
Undersquare engines tend to make more torque. Torque does the work.

Read up on the"Engine Challenge"series in Pop Hot Rod, engines are evaluated on sum of power "under the curve", both torque and HP. Almost all the engines entered and IIRC all the winners SB or BB have been undersquare engines.

 

[Geezer 300]

Sounds like the 348 or 409 combustion chamber

 

[rbohm]

8) with equal compression ratios, flattop pistons and dished pistons will make more power than domed pistons will. the reason is that there is nothing in the way of the flame front to prevent smooth travel across the combustion chamber. that said, it has been found that in some engines, dished pistons make more power than flattop pistons. it has to do with the shape of the combustion chamber matching the shape of the dish in the piston.

 

[6bangerbill]

I have no data to back it up, but I think that just maybe the high velocity quench gasses being introduce from the sides of center of the chamber volume, rather than at the bottom of the chamber volume( as with a flat top pistons), may produce a better mix of air-fuel for a faster and more complete burn (fewer polutants too).

This chamber fuel mixing idea has cought on in detroit, with the trend now of zero fuel vaporization in the intake manifold (dry manifold). This thorough mixing is a must. The results of vaporization of fuel occuring in the combustion chamber "only" allows more space for air in the intake manifold (no space used up by liquid or vaporized fuel), more heat absorbed in the chamber from fuel vaporization(like air conditioning), and allows all gas and fuel expasion to occur in the combustion chamber where it can develop a bigger bang.

This dry manifold idea is sometimes accomplished by injecting fuel in a narrow pattern against the back of the intake valve.The vaporization of this narrow spray chills the valve, causing heat transfer to cool the combustion chamber.The idea is to reduce temperatures(less detonation), reduce polutants, and produce more power. An even better method, used by gm on its new ecotec engine, is direct chamber fuel injection (using extreemly high fuel pressures). With direct chamber injection, a stock turboed ecotec engine produces 265 hp(325 hp with a stage II kit) with only a 2 liter engine. Think about it, thats more than the stock 400 ci carburated engines of the past with less than a third of the weight and cubic inches! Maybe this chamber stuff works.

 

[Anlushac11]

8)

In the 2.3L turbo forums I lurk on dished pistons are prefered because as the air/fuel mixture enters the chamber the dished pistons are supposed to promote better tumbling which provides more even air fuel mixture and cylinder filling allowing for more power.

 

[6bangerbill]

The best thing to appear on this forum is Freds '40? ford coupe picture. Flatheads forever! Anybody want to critique the flathead ford chambers? I dont think the flathead designers ever intended it to run, just a bad practical joke on Henry. Seriously, my newly built drag racing 286 flathead in 1968 ran 13.50s while missing and popping. When I cut the block reliefs(about 1/16 deep in the chamber between the valves and cylinder bore), the engine stopped missing and ran 12.50s. The plugs were bone white before and grey after the change. Maybe it was detination or pre-ignition, I ran 40 degrees fixed advance with a mag, and .030 piston to head clearance. It was in fiberglass a T- roadster with a t-10 2.20 low gear fourspeed and 10.5 wide M&H slicks. Not as cool as a '40 coupe.

 

[powerband]

I think this might be on topic:

One of the mysteries of my "legacy" 250 engine, built in the Clifford shop years ago, was the Pistons. The buildup correspondence with the original owner and the Clifford's shop indicated it had Jahn's 10:1 Pistons but I could see through the plug hole the top of the piston read "STD - 470NP". The numbers off the pistons turn out the application is for an AMC 258CID six. These pistons have a deep @21CC's worth of dish and a slightly higher "Compression Distance" or distance from the piston pin to the top. The STD size works out to @ .070 overbore on a 200 or 250.

Here's the comments on my finding from David Schjeldahl co-author of "The Falcon Six Performance Handbook" from a previous forum thread:

Thanks for sharing that discovery. I did a quick search on the Silvolite website and found a similiar piston. It is part #2227. They list the cc volume of the recess at 21ccs- which makes it ideal for this application. It lists the depth of the recess at .185".
With the .070" overbore the bore is 3.75", stroke of 3.91, a Felpro head gasket at .050", a zero deck height, chamber volume of 60ccs and a piston recess of 21 ccs, gives an ideal street compression ratio of 8.8:1!!!
And, the recess is not "D" shaped, but appears to more closely mirror the shape of the combustion chamber of a small ford six. The dish is more of a bathtub shape.

The complete thread is at:
http://fordsix.com/forum/viewtopic.php?t=34610

I'm still working on the best drivetrain match but so far this engine performs awesome!. It has low end grunt as well as top end and I've not had any detonation or knock with any spark advance variations I've been experimenting with. I'm anxious to have it dyno'd...

Jack Clifford, legendary six cylinder racer, record holder and engine builder's shop, performance built Ford 250 six engine- 10:1 Pistons, Clifford rods, Clifford 272H Cam, line bored & balanced bottom end, Cloyes timing set, Fisher balancer, 1.88 int. valves, Ported and relieved, Offy 3X1 w/3 Holleys, Re-curved distrib., Hooker dual out headers, the works, a few mysteries...

With 7448 centr carb as of 9/06:

Powerband

 

[Anonymous]

a dished piston should come closer to the ideal spherical shape, hopefully lowering the surface area/volume ratio of the chamber.

 

[351wF-250]

I actually have a friend that put Blower pistons(dished pistons) in his truck for the express reason that it helps get the most energy from the combustion process. I asked him to clarify. He said that if you were to look at how it works. If you have flat tops it pushes straight down right. If you have domed pistons it pushes around the top of the piston and most of the force is exerted toward the cylinder walls, and that mostly domed pistons are good for raising compression when other rules would stipulate things that couldn't. He said he chose the dishes for the reason that like flat tops it exerts most of the force in the downward motion but also as the fuel/air mixture hits the surface it swirls and this is beneficial for many reasons. If it wasn't do you think that so many companies are after swirl designs. I personally take his word, he's an engineer, and drives a open wheel modified on the weekends.

 

[Lazy JW]

... I asked him to clarify. He said that if you were to look at how it works. If you have flat tops it pushes straight down right. If you have domed pistons it pushes around the top of the piston and most of the force is exerted toward the cylinder walls...

There are many good reasons to use dished or domed pistons but this isn't one of them. A study of basic hydrodynamics will show why; Pascal's law states that pressure in a contained chamber is equal in all directions, therefore any side thrust from a projection or dish is cancelled out by the equal and opposite thrust. The net result is a push straight down. The dome and dish both increase surface area though.
Joe

 

[Stubby]

I have often wondered about high pressure cylinders. Why have concave or convex ends? Is this just to locate the welds in a less critical area?

I believe the dish might help flow in some engines. The dish or dome is an extension of the combustion chamber. Domes and valve reliefs have always caused some flow restriction. This affects flow in the low lift area and that can be a critical area. I wouldn't think there would be as much difference between flat tops and dish as there would be between flat tops and domes.

Most manufacturers kill the quench in the early seventies and then brought it back in the late eighties and early nineties. When they killed the quench, MPG suffered greatly. I will point out the FE engines and the 351m and 400m. Good engines, but very thirsty engines.