351C cylinder heads - squish and other issues

I didn't want to steal Dennis K's thread, so to show respect I started a new one.

Yep ... sorry guys ... another long-winded George post.

I'm not an engineer, this is the info about the 351C combustion chambers the best I understand it and can remember it. I don't want to get into an argument with anyone. I understand the whole small block Chevy/squish thing as taught by Smokey Yunick and repeated over an over in magazines, by SEMA, etc is hard to get out of your head once you've been indoctrinated. What Smokey taught in regards the small block Chevy may very well be true, but it just doesn't apply to the 351C, the cylinder heads aren't even close to being the same ... apples and oranges.

I respectfully submit the following to the open minded members of the 351 Cleveland forum.

Both varieties of the 351C cylinder head, quench combustion chamber and open combustion chamber, are poly-angle cylinder heads. The quench combustion chamber head is simply a poly-angle head with some small quench areas added on either side of the valves ... it is not a wedge style combustion chamber. Eventually you'll learn to think of the two styles of 351C heads in the same terms, instead of thinking of one as different or superior to the other.

The combustion chambers of Fords poly-angle heads are shallower than the combustion chambers of a true hemi-head, and this gives them two important advantages: (1) the first advantage compared to a hemi combustion chamber the 351C combustion chambers have less volume and can achieve adequate compression with pistons having flat tops, or smaller domes. Since the pistons have flat tops or smaller domes they are lighter, and they interfere with the propagation of the flame front less. (2) The second advantage compared to a hemi combustion chamber the shallow depth and poly-angle shape of the 351C combustion chambers promote better mixing of the fuel air mixture making them much more detonation resistant, this is true for both the open combustion chamber head and the quench combustion chamber head. This is also one reason for the 351C's higher volumetric efficiency.

Like a hemi head, the flame front in the 351C combustion chamber propagates at a rate, in a direction and above the piston in such a way that makes more horsepower than the flame front propagation of a wedge head.

Detonation occurs when a stagnate pocket of fuel air mixture self-ignites within the combustion chamber before it is ignited by the flame front; the source of ignition is normally pressure or temperature. The shape of a wedge combustion chamber, like the combustion chamber of a small block Chevy or a small block Ford, promotes the formation of such stagnate pockets in the area furthest away from the spark plug, which is shielded from the flame front by the large flat surface of the cylinder head, known as the squish area. The squish area of these heads is almost ½ the width of the cylinder itself. To prevent detonation with this combustion chamber design the clearance between the piston and the squish area at top dead center must be kept relatively small, thereby forcing any pocket of fuel air mixture out of hiding and towards the flame front. In comparison, the fuel air mixture in a 351C combustion chamber is dynamic or active over a significantly greater area of the combustion chamber; this is due to the relatively shallow depth and the poly-angle shape of the combustion chamber.

Squish promotes turbulence or swirling/tumbling of the fuel air mixture, it also squeezes fuel air mixture away from the edges of the cylinder and directs it towards the center of the combustion chamber where it will be ignited by the flame front. As a motor increases in speed (rpm) the piston reciprocates faster and faster within the cylinder, there is a point in which the turbulence created by the pistons motion makes the turbulence created by squish unnecessary. As a generalization then, squish becomes less important in the prevention of detonation as the speed of a motor increases. Whether or not squish is needed to improve horsepower at higher motor speeds shall be dependent upon how efficient the shape of the combustion chamber is at promoting swirling of the fuel air mixture within the combustion chamber. Wedge shaped combustion chambers are notoriously poor in this aspect whereas the shape and depth of the 351C combustion chamber (both quench and open chambers) promotes better mixing and swirling over a larger area.

Squish is not the only means by which detonation is avoided in the design of a combustion chamber, as in the case of the 351C for example, the poly-angle shape and shallow depth of the combustion chamber combats detonation. The design of the water jacket is also capable of combating detonation. The coolant path of the 351C cylinder head was specifically designed to increase coolant flow and reduce the temperature around the exhaust valve.





Torino Pat:

The Ford Off Highway Parts manual, 351C section, pages 8-9 states the quench chamber head promotes better low rpm torque and makes it easier to raise the motors compression ratio (see above). These are the only advantages ascribed to the quench combustion chamber head by Ford. The intake ports and valves of the M code and Q code heads are the same size and shape. There is nothing in the design of the Q code heads that would make their ports flow less than the ports of the M code heads. If a person is going to assert that the open combustion chamber heads flow worse than the quench combustion chamber heads the burden of proof should be upon them to prove their point, not me, because there is no physical reason why this should be the case.

To my knowledge no piston manufacturer offers an off-the-shelf piston with a dome designed specifically for the open combustion chamber heads. If there are problems encountered with the open combustion chambers in drag racing I would tend to believe the piston domes are the most likely culprits, not the combustion chambers themselves. Obviously there are domed pistons designed specifically for the quench combustion chamber heads, and the quench heads will make it easier to achieve a certain compression ratio. So from that outlook, the quench heads are a better choice and a good recommendation.

One last note about this ... my interest is street cars, not drag racing. I wasnt there and I dont claim to be experienced in the realm of building full boogie drag racing motors.

In the 1970s I helped a gentleman modify his 1972 Q code motor. He wanted to raise the compression without resorting to using quench combustion chamber heads. We had the decks leveled, and then the block was milled until the decks were the same height as the piston domes, i.e. it was zero decked. We had the heads milled until the combustion chamber volumes measured 70cc (milled about 0.025 inch). This machine work gave his motor a true 10.0:1 compression ratio at all 8 cylinders. We installed the D1ZZ-BX camshaft and headers. On the dyno his motor made about 355 peak horsepower. Over the years I assembled several motors with almost identical specs excepting for the cylinder heads ... I always used M code heads instead ... they all made 350 to 360 peak horsepower on the same dyno. I must admit, I was leery about building a 10.0:1 compression ratio motor with open combustion chamber heads, I was worried about detonation because detonation was a big issue in that era. I was relieved when we found the motor was no more prone to detonation than the motors I assembled with quench combustion chamber heads.



For those of you who have never seen this before, I thought you might enjoy seeing the small block Chevy cylinder heads that "almost were". In the same era that Ford dropped its racing program and abandoned further development of the 351C as a race motor, Chevy also dropped the development of these cylinder heads ... and for the same reasons ... to focus their budget and time on tailpipe emissions and passenger safety.


-G
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