The 351 Cleveland Cylinder Head a Deliberate Design part 3 by George Pence

Question for ya George…

March 3 2012, 4:36 PM 
I’ve often thought that the open chamber 4V head looks very much like a Hemi head and really don’t understand why its not called a Hemi head, other than back then MOPAR was using that name in their marketing, and that chamber design was used on the more pedestrian 2V head, so I’m sure Fords marketing folks passed over the whole idea.

Great history, and I think the whole topic helps remind us what a special slice of automotive history the 351 Cleveland was.

Thanks for taking the time George, I know you spend a lot of time on these “mini dissertation” which may seem unappreciated, but I think we all learn, and do indeed appreciate the time you spend on your “classroom presentations”

Poor man’s hemi

Blizz … Cleveland heads are not hemi heads, they are too shallow to be hemi heads.

Chrysler may have had the rights to the name Hemi, I don’t know. But Chrysler also invented the canted valve head, and referred to their canted valve motors as “Poly-Spherical”. Motors with canted valve heads are supposed to approach the performance of motors with hemi heads without the expense of hemi heads. A poor man’s hemi you see.
So somebody coined the term poly-angle head, very similar to Chrysler’s poly-spherical.

The 351C has shallow poly-angle combustion chambers. Be proud of that fact. Its one of the things that gives the 351C the superior volumetric efficiency it has. Understand your motor, there are fundamental differences between a poly-angle head and a wedge head … they are very very different. Don’t apply wedge head thinking to a poly-angle head.

The 351C open combustion chamber head is superior to any wedge combustion chamber design. It makes as much power as the quench chamber version … its not prone to detonation any more than the quench chamber version. Poly-heads don’t depend upon squish for their power and resistance to detonation, those characteristics are achieved by the shallowness and poly-angle shape of the combustion chamber. The two versions of the 351C head were developed at the same time, the quench version was designed mainly to decrease chamber volume, that’s all.

The canted valve head that Chevy was developing in 1969 also had an open chamber design. They didn’t feel the need for squish, and it was a racing head!

Detonate?–Bill pewterboss351

i know of some strong running open chambered engines
I am not doubting the power output of an open chambered head
But i have personally had 2 barrel clevelandengines. One in perticuar in a 1972 gran torino
that Detonated. you might think it was out of tue, maybe a little bit.
but this car would smoke alot of basic 350 4 barrel chevys , that were in lighter cars.
and must we not for get all the 77 to 82 ford pickup out there with 351m 400 engines
that seemed to pink all the time.
so yes a cleveland head will detonate
sorry george i disagree there

Re: Detonate?  George Pence

I don’t mean to imply the 351C can’t detonate … the point I would like to make is the shallow poly-angle combustion chamber doesn’t rely upon “squish” to prevent detonation, this is why the open chamber version of the 351C head is no more prone to detonation than the quench chamber version. There are some members who are under the assumption that the open combustion chamber will detonate easier than the quench combustion chamber, or is even prone to detonation, and that the open combustion chamber cannot possibly make as much horsepower. But those assumptions are largely unfounded and influenced by the fact they apply wedge combustion chamber characteristics to the 351C combustion chamber. To those who think that way I write the open combustion chamber heads can make as much power and they don’t detonate (any more than the quench heads).

It is of course possible to get any engine combination so far outta whack that it will rattle, ping and detonate.

In the case of the 351M & 400 I think Miller hit the nail on the head, the extreme amount of deck clearance in the later versions of those motors destroyed the shallow combustion chamber’s ability to swirl the mixture around. Stagnate pockets of air & fuel are what cause detonation.

Sometimes what we refer to as detonation isn’t really detonation. Having worked on old Fords … I can point a finger at one aspect that causes problems … worn out or gummed-up centrifugal advance mechanisms that get stuck at full advance. Of course that’s not really detonation, that’s just too much advance. Pre-ignition (pinging) is another issue that’s not related to combustion chambers. Ignitions and carburetors that are out of “whack” will of course create detonation sounding problems. Lean air-fuel mixtures, weak ignition modules, worn out spark plugs or cross-firing ignitions will do this.

All these problems occur when things are outta whack. But wedge heads are another situation. By their very design the portion of the cylinder farthest from the spark plug, directly below the squish area of the head, is completely shielded from the flame front. Pockets of stagnate fuel and air collect there easily at low rpm. Wedge head motors can go into detonation even when everything is well maintained and seemingly working properly. This situation will not happen with a 351C unless … as Miller pointed out … the deck clearance is extremely large.

Picture – The First Poly-Angle Combustion Chamber

Does the combustion chamber in the picture below look vaguely familiar? Its the combustion chamber of a mid-1950′s Chrysler Poly Spherical canted valve head.

The Chrysler Poly Spherical head was the first canted valve head, the one Chevy and Ford copied. The combustion chambers of the big block Chevy and the big block Ford don’t resemble this too much, but the Cleveland is a dead ringer.

[linked image]


Wedge verses Poly – understanding combustion chambers a little bit

A Wedge Combustion Chamber has the cross sectional shape of a wedge, due to the angle of the valves This combustion chamber does not control how the flame front impacts the piston dome very well It creates an area of “dead space” shielded from the flame front, below the large squish surface This area is a source of detonation and reduces volumetric efficiency. This is why the small squish clearance is so important in these motors

[linked image]

The Chrysler poly spherical head didn’t have a “dead space” and had no squish area. The valve angles are smaller, the chamber is relatively shallow. The chamber cross-section is not wedge shaped. The raised areas surrounding the valves adjust chamber volume, allowing the motor to utilize flat top pistons rather than domed pistons which were required by their hemi motor. Like a hemi the shape of the combustion chamber is designed to focus the flame front on the center of the piston dome

[linked image]

The 351C chamber is very shallow, the valve angles are merely 9-1/2 degrees. There is no dead space. The
chamber cross-section is not wedge shaped. This shape evenly distributes the flame front over the entire surface of the piston dome.

Engine designers realize the combustion chamber is the heart of an internal combustion engine. The best designed intake port in the world cannot produce superior torque and horsepower if it is supplying fuel and air to a cylinder with a poor or average combustion chamber. The 351C had the highest volumetric efficiency of any mass produced push-rod (OHV) V8 not solely because it had well designed ports and large valves, but also because it had a well designed combustion chamber. Although it may not be intuitive, what you are looking at in the picture below is one of the best performing combustion chambers of any mass produced push-rod V8.

[linked image]

The 351C quench chamber is nothing more than the open chamber with raised areas on either side of the valves primarily there to adjust chamber volume, although the turbulence this creates also improves low rpm torque. Since the smaller chamber is very well centered in the cylinder, the flame front is focused upon the center of the piston dome.

[linked image]

Chevy LS7 combustion chamber, even though the head still uses in-line valves, this is not a wedge combustion
chamber, valve angles are only 12-1/2 degrees, the chamber is very shallow and it does not have a wedge shaped
cross-section. There’s no dead space. The small amount of squish is for adjusting chamber volume, possibly for
creating turbulence, but not for preventing detonation.

[linked image]


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