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Well, things are slow here in the engine tech forum, so it's a good time for a little show & tell. This will be aimed at those who have never had a 351C engine apart, and have wondered what the terms open chamber & closed chamber actually referred to in regards to the cylinder heads.

2 aspects that differentiate 351C cylinder heads are the design of the combustion chambers and the size of the ports. At the bottom of this post is a picture of the two combustion chamber designs.

On the left is an open chamber cylinder head, on the right is a closed chamber cylinder head. The open chamber head has a round, bowl shaped combustion chamber that is approximately the same diameter as the cylinder in the engine block. The closed chamber head has a smaller combustion chamber that has been shrunk to tightly enclose the two valves and the spark plug, creating flat surfaces above & below the combustion chamber. These flat surfaces are known as the "squish" areas.

On a 351 cubic inch motor, replacing an open chamber 4V head with a closed chamber 4V head will raise the compression ratio about 0.8:1, eight tenths of a point. If you have a '73 to '74 cobra jet motor with a dismal 8.0:1 compression ratio, replacing the open chamber heads with closed chamber heads will yield a compression ratio of approximately 8.8:1. Increasing the compression ratio is one of the advantages of the closed chamber heads.

It has been long ago determined that reducing the clearance between piston dome and the cylinder head “squish” surfaces at top dead center to 0.045” +/- 0.010” on a street motor improves the thermal efficiency of the motor (it produces more torque) and increases the motors resistance to pre-ignition (aka knock or pinging). It is common for racing engines to set this clearance even smaller. As delivered by Ford, this clearance is approximately 0.070 which is too large to realize the benefits of “squish”. Squish aids the engine during the compression and exhaust cycles. As the piston stroke reaches top dead center during the compression cycle the squish area squeezes the air and fuel into the more compact combustion chamber so as to be more easily ignited by the spark plug while also increasing the turbulent motion of the air and fuel so that upon ignition the air and fuel burn more completely and energy extracted from the fuel air mixture is maximized. During the exhaust cycle, the squish area and smaller combustion chamber improves exhaust gas scavenging by forcing the exhaust gases nearer the open exhaust valve as the piston stroke reaches top dead center.

With a 0.070” gap between the piston and cylinder head at top dead center, the fuel/air/exhaust gases are not completely squeezed into the combustion chamber, the fuel and air left behind during the compression cycle are actually shielded from the flame front during ignition and remain partially unburned, which reduces the energy extracted from the fuel air mixture. The unburned fuel increases hydrocarbon emissions, which is why Ford switched the 4V Cleveland motor from closed chamber heads to open chamber heads in 1972. In a similar manner, the exhaust gases left behind in the squish area during the exhaust cycle are shielded from the exhaust valve and exhaust scavenging is reduced. Exhaust gases left behind in the cylinder reduce the cylinder area available for fresh fuel & air that are drawn into the cylinder during the intake cycle and dilute the fresh air that is drawn in. So unless a short block is “decked” to reduce the clearance between the cylinder head and piston dome, a closed chamber head’s only value is that of increasing the motors compression ratio. The motor shall not profit from the extra torque and pre-ignition prevention of squish.

An open chamber head can extract equal power from a non decked short block, as long as the motor’s compression ratio is adequate for the fuel and camshaft being used.

Your friend on the DTBB, George


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Derek, your chassis number tells me you have a late 1972 model Pantera. All '72 through '74 351C 4V engines were equipped with open chamber heads. Unless the heads on your motor have been swapped, they will be the open chamber variety.

All cast iron 4V cylinder heads are externally identical. There is only one externally visible number on the head casting, that's the 4 in the upper coners which signifies it is a large port head. It doesn't indicate what the combustion chamber design is.

The casting date is under the valve covers, amongst the valve gear. But I have never read anywhere what the date is when the casting of the open chamber head began or the cut off date for the casting of closed chamber heads.

Finally there is a casting number on the bottom of one of the intake runners, the closed chamber heads are D0AE, the open chamber heads are D1ZE, but to see the casting numbers, you have to remove the cylinder heads from the block, in which case you can "see" which heads you have!

Your friend on the DTBB, George
Thanks George,
I definately have the large port heads - I've removed the intake manifold in the past.

Are 4V open chamber heads the same as Cobra Jet heads?
Does a late '72 motor mean I have a 4V or a Cobra Jet and what is the difference?

I've no history for my engine. I think it's stock, but it had a 700cfm holley 4778-2 and an Edelbrock Torker when I bought it so who knows... Confused

p.s. I got a couple of heads in the garage that look very similar to the closed chamber head in your picture. They are from a Sunbeam Rapier - though these were used with dished pistons.
Derek, focusing on the 351C 4V motors with self adjusting (hydraulic lifter) valve train manufactured in the US from 1970 through 1974 there are 3 distinct motors.

1) the 1970 and 1971 351C 4V, had closed chamber heads with 2.19" & 1.71" size valves, an engine block with 2 bolt main bearing caps, a very mild cam shaft, a 600 cfm Autolite 4300A carburetor, a very light harmonic balancer, and cast flat top pistons. Ford rated the compression ratio at 10.7:1, but it was actually about 9.5:1. The ignition consisted of a single breaker point distributor. The motor was rated at 300 bhp in 1970. That year, AK Miller, one of Ford's own contractors, tested a production motor and found it produced only 288 bhp. The results were published in Hot Rod magazine, so the following year the motor was rated at 285 bhp.

2) the very late 1971 through 1972 351 Cobra Jet, had open chamber heads with the same 2.19" & 1.71" size valves as the previous motor, an engine block with 4 bolt main bearing caps, a Cobra Jet camshaft having specifications identical to the FE 390GT & FE 428 Cobra Jet motors, a 750 cfm Autolite spread bore 4300D carburetor, a decently heavy harmonic balancer, and the same cast flat top pistons as the earlier motor. The compresion ratio was accurately rated by Ford at 8.8:1. The ignition consisted of a dual breaker point distributor. This motor was rated at 280 bhp. This motor ran stronger and rev'ed higher than the earlier motor.

3) the 1973 through 1974 351C 4V motor was actually a Cobra Jet motor, Ford simply reverted back to calling it a 4V. This motor differs from the 1972 Cobra Jet in 4 ways. The 2.19" & 1.71" size valves in the open chamber 4V head were replaced with the 2.04" & 1.65" size valves of the 2V motor; the camshaft timing was retarded 4 degrees; dished pistons replaced the flat top pistons, resulting in a drop of the compression ratio to a dismal 8.0:1; and exhaust gas recirculation was added to the motor, evidenced by an "EGR plate" sandwiched between the intake manifold and carburetor. This motor was rated at 264 bhp.

Your friend on the DTBB, George
Thanks George,
So I probably have a 351CJ.

I get 308bhp (262 at wheels), which is more than the Ford rating, but about the same as I've seen quoted for european model Panteras.
Would the tubular euro exhaust header an Edelbrock Torker and a holley 600 be enough to make the extra 20+bhp or would this need a cam change as well?

Do european spec Panteras use a different camshaft to the US spec cars?

Hey George, if we paid you for all this advice you'd be rich Big Grin.
Originally posted by Rapier:
Thanks George,
So I probably have a 351CJ.

I get 308bhp (262 at wheels), which is more than the Ford rating, but about the same as I've seen quoted for european model Panteras.
Would the tubular euro exhaust header an Edelbrock Torker and a holley 600 be enough to make the extra 20+bhp or would this need a cam change as well?Derek, the Torker intake will make 20 more bhp at peak horsepower at the expense of less bhp at 4000 rpm and below. No cam change necessary. The DeTomaso exhaust may even provide 10 additional bhp compared to the oem exhaust of a Torino or Mustang.

Do european spec Panteras use a different camshaft to the US spec cars? To the best of my knowledge, no. Export motors may have received a different intake & carb, but I have never read of export motors ever having a cam change, although Ford did manufacture hotter hydraulic cams for the 351C.

Hey George, if we paid you for all this advice you'd be rich Big Grin.

The opportunity to assist others does make me rich, in the way that is important to ME!
Yes indeed.
We all must join in thanking you George & of course PI for making it all possible and not to forget THE PROFESSOR; Jack della Rich....
I have to tell you George... I recently went to the big National Cobra Day event here in Johannesburg to shoot some DVD footage armed with your excellent piece / posting re. the fine design details of the orginal 351C 4V closed - chambered headed NASCAR equipped cars results plus your: - i am sure, little knowen facts, of the german dude trouncing the Factory Porsches with the 351C Pantera...and then also Chevy copying the 351C head design to try and catch up after all the various restrictions imposed on the Fords you mentioned.... ( Plse wont u post that agin somewhere where it will be read by more of US. Its such a extraordinary bit of info.....who out there knew this ??) - Anyhow, there I was with well over a 100 Cobra's awash with Chevy engines reading them all your interesting info....well we certainly caused quite a stir to say the least. Lots of fun, banter / joshing around all in good humour....what never ceases to amaze me is the fact that virtually always when you get a crowd of PetrolHeads/ Car Freaks chatting together you go away with a great feeling of;.......what should one call it ?? - bon amee, cameradery, goodwill, companionship ???? Its the "payment" you talked about of course hey George...
Many thanks agin to all concerned.....
Yes, the Cobra Jet motors (late '71 through '74) were specified with 4 bolt mains. From time to time one will show up with only 2 bolt mains. I have seen a 2V motor with 4 bolt mains too! The '72 through '74 blocks all had the same casting number, D2AE-CA, whether they were 2 or 4 bolt. Somehow at the factory the wrong blocks were sent down the wrong assembly line, or blocks were borrowed from one assembly line to make up for shortages in the other.

In general, if the motor was equipped with the spread bore Autolite 4300D 4 barrel carburetor, it "should" have 4 bolt mains, but the only way to be sure is to drop the oil pan, because there were exceptions.

Your friend on the DTBB, George
In your initial post, I understand you to say that the open chamber head results in a less complete burn due to the lack of "squich" as compared to the closed-chamber heads. This unburned fuel then results in increased hydrocarbon emmisions, "which is why Ford switched the 4V Cleveland motor from closed chamber heads to open chamber heads in 1972."
I must be missing something as why would they change to a design that increases emmisions?

Thanks - Mike
'74 L 6328

You've got it backwards, the open chamber heads produce less hydrocarbons (and less NOx too).

Closed chamber heads on a "non-decked" block create small pockets at TDC where fuel molecules are hidden from the flame front.

Decking the block, closing the gap between piston dome & head at TDC, reduces the amount of molecules that are hidden, it actually produces a "squeeze" on the gases in those "squish areas" that evacuates the gases from them very well.

So now you're wondering, since the oc heads combust all the fuel, why don't they make equivalent or more power than cc heads with a decked block? Because there's more to it than simply burning all the fuel. The cc heads focus more of the burn on the piston dome, the turbulence spreads the pressure of combustion more evenly across the piston dome, and the cc heads aid in controlling the timing of the occurence of peak combustion pressure at the right time during the piston's stroke.

Also, I am told by those who's profession is porting heads, that the cc heads actually flow better than oc heads, because as the fuel air mixture is flowing past the open intake valve, it hugs the wall of the cc heads and is therefore directed away from the valve head; the fuel air mixture separates from the wall of the oc heads and crashes into the valve head. This is why, on a professionally ported set of Cleveland heads, the combustion chamber around the intake valve is left pretty much as is, the intake valve is not "unshrowded".

The modern aftermarket heads for all brands, all race heads for the last 30 years, even the heads in modern production engines with over head cams, incorporate features of the Cleveland combustion chamber. It's a good one. It was the last cylinder head developed by Ford engineers during the total performance era, and combined everything they had learned (and borrowed from others) up to that point.

Your friend on the DTBB, George
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