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byots,
allthough I love all of this theoretical BS, generally it doesn't mean much as the parts variability are limited, and you have to build the engine with the parts available. For the most part, the head combustion chamber size and compression you're going to run will generally dictate what style piston you will use, whether a dome, flat top, or a dish, so you don't have much input on the style of piston. I would think the piston manufacturers know this stuff inside and out, and will make you a piston that will generally give the the best performance compatable with the parts (heads) you are using.
However, if you are going to use edelbrock heads, the 428 heads have a smaller combustion chamber than their 427 version, allowing for a smaller combustion chamber, I mean cross sectional area on a transverse plane, parallel to the block deck. I realze that the combustion chambers comparing engines of equal displacement and compression ratio have the same total volume , and that it is the shape of the chamber that is different, which impacts how fast the combustion process process is. Ford 427 heads have the intake and exhaust valves spaced father away from the centerline of the bore by about .050" compared to 390-428 heads. Therefore, larger valves can be used. The edelbrock heads are different, with different spacing, but the spacing is the same whether you get their 427 head with the larger combustion chamber, or the 428 head with the smaller combustion chamber. With each head, the intake valve is spaced like a 427 head, but the exhaust valve is spaced like a 428 head, so there is no real difference in the max size valves you can use, although the 428 head may require more chamber work. When I built my engine, I got the 428 head thinking I could try to keep the combustion chamber as compact as possible, and I could unshroud the valves and still have a smaller combustion chamber in the head than if would have used edelbrock's 427 heads. As my pistons were custom manufactured, they ended up having a D-shaped dish for 10.5 compression, with a 4.2" stroke, 477 ci.
If you use factory 427 MR/TP heads or Shelby heads, I think they are 88 cc chambers or somethinki lke that, and you don't have much choice in piston design, with more of the chamber in the head and less in the piston contour.
I am no where as knowledgeable as Mr. Vizard, or anyone else you rebiulds engines, or racing engines for a living. It is my understanding that car engines work by heat, the heat of combustion heating the gases (nitrogen, CO2, H20) causing elevated pressures which then push down on the piston. You make the maximum horsepower when you can generate the highest mean effective pressure on the pistons, converting heat energy into mechanical energy. The point is that you want to get the highest average pressure on the piston. Since combustion takes time, it is not spontaneous, this is why you must run advance timing, to get the combustion process started early, before the piston reaches TDC, so you get most of the combustion completed as the piston starts to travel down, applying maximum pressure on trhe pistons. Ideally, combustion would be instantaneous, and would always occur at TDC, but thats not the case. The next best thing is to make the combustion process as fast as possible, and the best way to do that is to make the combustion chamber the most compact (I say smallest) as possible, like a egg (sphere) as opposed to a pancake. Like you said above in your post, the ideal combustion chamber would be a sphere. Yes, that is I believe assuming that the spark plug would be in the center of the sphere, as the flame of combustion would have the shortest distance to travel to any part of the combustion chamber. But in reality, the plug is kind of on the edge of the chamber, and the valves require the chamber to be oblong, but you still would like the chamber to be as compact as possible, with the spark plug as much as possible in the center of the combustion chamber.
I know from physics, that for a piston operating in a cylinder, that it doesn't make any difference the contour of the piston top, as the pressure generated by the piston for any given pressure pushing on the piston is directly related to the cross sectional area of the piston, no matter the piston's contour. However, there are many other factors that come into play in an internal combustion engine, to maximize the pressure seen by the piston, even fuel-air mixing, turbulence, efficiency of the combustion process, etc., that I'm sure the contour of the piston comes into play. I would think a dished piston allows the combustion chamber to be as compact as possible, allowing for a better combustion. I'm not sure if the dish aids in the hot gases pushing down on the piston, making more power.
This is why you want a good quench area, to keepo the combustion chamber as compact as possible. This brings up another question, how much valve unshrouding in necessary? How much does valve shrouding come into play with closed chamber heads? I'm sure the answer is one of those "it depends", with engine displacement, bore size, flow capabilities of heads/induction system, and more and more playing a role.
All in all, I enjoy understanding how an engine works, and all of this theoretical stuff. I did choose the smaller edelbrock head because of this, but I never measured the diffrerence between the edelbrock 427 and 428 heads to really determine the difference in the combustion chamber. I just assumed the 428 head woulod give me a more compact chamber, and went with it. I don't race professionally, so no matter which head I used, the edelbrock 427 or the 428 head, the outcome would be about the same. Maybe the 427 head would have been the better choice, with the bigger chamber. I don't know, but I'm satisfied with the 428 heads, my engine makes alot of power.
Maybe someone could comment about the difference between edelbrock 427 and 428 combustion chambers. Interesting topic, but unless your in the design stages of an engine, I'm not sure how relevant this info helps you with a FE427.
In addition, I saw that you're coating your pistons and chambers. I decided just on the pistons, n ot so much for an increased power aspect, but to help prevent against engine (piston) damage in transient lean conditions, especially since the coating was basically free. I figured doing the chambers would help prevent heat transfer through the heads, but you can compensate to some degree with the ability to run higher compression with uncoated heads. OIbviously it costs more. Maybe in the future, I'll do engine, and coat both.
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Last edited by Anthony; 10-17-2004 at 11:12 AM..
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