simpkins_44

As you have noticed by the title this post is requesting for help on choosing one of two pairs of pistons. One of which is dished with a volume that adds 14.20cc to the combustion chamber of my 58cc 302 heads and the other adds 4cc. To understand my questions fully I feel you should know the background.

At first I was leaning on the dished pistons and I know that the static compression ratio will be a low 9.149:1 but this would be perfect for if I decide to turbo my car. Although this may seem like a really low compression ratio I would have had a cam ground for my car that will opt for a high 8.5 dynamic compression ratio. This will allow for a large amount of air and gas to be compressed close to the maximum amount a air and gas(91 octane) mixture (at a ratio of 14.7) will permit. Now on the other hand will this not produce less turbulence in the cylinder and will it eliminate the effects of a proper clench distance(.038) that I have tried to obtain??? If so wouldn’t the flattop piston fix this creating more turbulence and making the air and gasses more stable???

This brings me to the flattop piston being in favor. however the flattop would change the static compression ratio to 10.469:1 an unfriendly ratio for a turbo if I later decide I want one (although I don’t really need to add one and not really sure I will be but I don’t want to base my decision on a turbo). This also means instead of getting the cam ground for an early intake valve closure now it would need to be later on to permit the 8.5 dynamic compression ratio. this will not allow for as large amount of gas and air to be compressed as the dished would have (although it would be better on gas). In other words I’m trying to maintain the pressure inside the cylinder a constant between both pistons. both will be compressing equal ratios of air and gas(14.7:1) and will be compressing them to an equal rate(8.5 times the initial volume). The only variable is the initial amount of air and gas(volume wise) in the cylinder at the instant compression begins (when the intake valve closes).

It is understandable that a larger amount of gas and air (compressed 8.5:1) will allow for more power then a smaller amount would at the same compression(thus why I wanted dished pistons). How ever this isn’t my question. my question is would it be out balanced by turbulence (or lack of) in the cylinder making the gas and air mixture more unstable and easier for pre-ignition(thus robbing me of power)???

Please if you will give me your opinions and post a reply, and thanks to those who took the time to read this.

Bubba 95SN

Lots of snow, cabin fever set in?

67mustang302

9.1:1 isn't low, a custom cam on an n/a engine in the low 9's still fills the cylinders well.

Go dished where you can, when the piston is at TDC a dished piston causes the overall chamber shape to be more circular(actually more ellipsoid) than semi circular, and it promotes better combustion. Modern cars run small chambers and dished pistons.

And just fyi, I'm at 9.25:1 static.

Venomantidote95

Flame propgation will be better with a dish but the flattop would produce more power everything else being equal! 10.5:1 isnt to high for boost u just cant go crazy with the amount! If u are worried about pre-ignition or detination keep ur car stock! But then under the right curcumstances it can still happen! Unless ur sure ur going high boost go flattop!

67mustang302

Everything else being equal the dish will make more power, but maybe not enough to notice.

simpkins_44

Hey I seem to be getting mixed results. I have now been told two completely different things that contradict each other. Venomantidote95 you stated that the flattop will produce more power yet the volume of air and gas is quite smaller as compared to the dished piston. Why is this true??? When thinking of it I would believe the dished would make more power because of the larger volume of air and gas will have a larger reaction would it not??? and in terms of energy it will have more mass(because of the larger volume) and thus wouldn’t the pressure change in the cylinder caused by the explosion be greater as well, thus having greater potentially for power??? Not to mention that the dished it will promote ignition as stated by 67mustang302.

I did the math and in terms of keeping the cylinder pressure and the total cylinder volume the same the 14.2cc pistons need a closed cylinder volume(in relation to intake valve closure) of 687.65cc (41.9629776ci) and the flattops will only need 600.95cc (36.672219ci), that’s a difference of 86.7cc (5.2907586ci) per cylinder!! 67mustang302 you stated that the power with a dished piston will be greater but wouldn’t be noticed in comparison to the flattop, Why is this??? When it seems the cylinder volume as stated here will be extremely greater then the flattops. wouldn’t the dished pistons have an extremely larger improvement(in terms of power) over the flattop because of this??? Just to relate this in terms of volume, Boring out a 302 .030 will only add 10.0805903cc (0.61515536ci) to a cylinder. so in relation to these numbers choosing a piston of this difference is a more important decision then whether or not to bore out a 302 motor. Just food for thought.

its still hasn’t really been raised on whether or not this potential power gains and losses will be out weighed(or not) by the theory of quench and whether or not it will be greatly affected but the pistons shape/volume change. When in theory it should.

Thanks for your posts and hope they keep coming in. thanks for your support as well. hit me back!

68CSTANG

http://sbftech.com/index.php/topic,3439.0.html

Venomantidote95

More compression equals more power sometimes we have to look past formulas and look at what the actual result will be! The only way a lower compression engine would make more power is adding cu in!

simpkins_44

No offence venomantidote95 but I don’t think you understand in what terms I mean compression and compression ratio. There are 2 if not more terms of talking about compression, one of these being static the other is dynamic. Static compression is the is the ratio of total available divided by the combustion volume. The problem with this is it doesn’t take into account the actual compression of a motor because it doesn’t take into a fact when the intake valve closes. It is easy to see that you cannot compress anything in the cylinder if the cylinder is open. Think of it in terms of a pop bottle being squeezed with the lid off, nothing gets compressed everything simply comes out the top. Now one of the best ways to calculate cylinder pressure is to take this into mind as well as other variables and dependents. This calculation gives up dynamic stroke length, the length left in the stroke after then intake valve closes. This then gives us the volume of actual initial compression and if we divide that by the volume it will be compressed to we get the dynamic(actual) rate of compression. Now in terms of this this means that the compression is dependent on the intake valve closure time depended on by the cam. This time can be manipulated easily to allow for a larger volume of air and gas to be compressed hence the ability to equalize the compression ratio. In other words the compression ratio is the same as I have stated previously. So in your post by saying more compression is more power is correct however there are limit’s you can only compress gas and air so far before it starts to combust or become unstable. Also the compression is equal for both parts both the 14.20cc(dished) and the 4cc(flattop) as I can alter valve timing to make up any differences. Therefore both piston options can have the same compression. So the question again remains what will be better the dished or the flat top both having equal compression ratios(as adjusted by valve timing)???.

You also stated that the only way to lower compression was to add cubic inches. This is proven 100% wrong with out a doubt. This is also a common though by people who are unaware of dynamic compression ratios(as stated earlier). Cylinder Compression is dependent on a lot of things the cam and valve timing is a big one. The compression of a cylinder can only happen when the intake valve is closed. Therefore by advancing the intake closing time it will allow for a larger volume to be trapped in other words higher compression. The retarding of the intake closing time will only allow for a smaller amount of volume to be trapped thus LOWERING compression. Maybe you should start looking at the formulas and see what power your overlooking. Food for thought!

Further more my questions have still not been answered and still look forward to hearing your guys opinions. don’t be afraid to post I put this desiccation up to communicate with you guys, to sit down and think it over and see what is the best option. Thanks you and hope to hear more. Hit me back!

67mustang302

It's about chamber shape. A dish in the piston makes that dish part of the chamber. It creates a chamber where the bottom isn't a flat surface, but concave. During combustion the flame front expands outward away from the plug, with a flat top part of the leading edge contacts the piston and redirects combustion pressure towards the side of the cylinder. With the dish the "bottom" of the chamber(which is basically the piston) is shaped more like the expanding combustion wave, and you get less interference with the direction of the combustion wave.