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tmhutch · 05-13-2010, 06:14 AM

Unique Ford DOHC Characteristics

Every engine combination is more or less unique in its cam requirements but I’ll focus on the Ford DOHC engine and some of the major design elements that make specifying their camshafts unique from the brand X alternatives.

The most obvious departure from traditional domestic architectures is the DOHC 4 valve layout. When Ford designed the new modular engine family they knew the small cubic inch blocks were at a disadvantage and that the tiny bore size would never be big enough to support enough valve for reasonable performance applications. As a result they employed a 4 valve per cylinder, dual overhead camshaft combination. Compared to typical two valve wedge heads it was very expensive to engineer, produce, assemble and package but it is far superior to conventional pushrod and 2 valve variants in terms of horsepower per cubic inch. I was always surprised that the 4.6 didn’t get more press for producing almost identical power numbers to the LS-1’s of the same era and they did it with 50 fewer cubic inches, smaller camshafts and better fuel economy. Here’s why:

The biggest restriction to getting air into and out of an engine is the valve. This is how David Vizard puts it:

“The most important factor in making hp is the circumference of the intake valve in relation to the cylinder's displacement. All two-valve V-8s are under-valved for the cubes those valves have to feed.” David Vizard

In other words, almost all domestic V-8’s from the inception of the internal combustion engine has been under valved. It’s not hard to imagine when you look at this nice straight pathway the port provides, only to plug the end up with a 2” hole and a valve that only lifts a half inch off the seat to allow air to pass. Valve limitation is the nature of the beast because a cylinder can only be so big before the dimension of the block will no longer fit within a reasonably sized engine bay. Four valve architecture, expensive as it is, does away with this problem by allowing a lot more valve area to be applied to a given combustion chamber/cylinder bore size. To put this into perspective we can compare the valve size of the 281 cubic inch 4.6L (1.456”) with those of a traditional 350 cubic inch Chevrolet small blocks (1.94” – 2.02”), the LS1 346 cubic inch small blocks (2.00” valve size), the current model LSX 380 cubic inch 6.2L small block (2.165”) and finally, the venerable Z06 427 cubic inch animal at 2.2”.

To accurately compare valve sizes we have to convert the diameter of both the DOHC valves to their area, add them and then convert back to the diameter for a single valve. When we do this we get 2.06” equivalency. That’s a 2.06” valve to feed a paltry 281 cubic inches, or 4.6L. With larger and larger cubic inch engines coming on line, Detroit is producing heads with massive ports and large valves to go with them. But even with modern day technology it’s impossible to match the area of a four valve head. To bring more perspective to the table let’s look at what size valves the Chevrolet’s would need to equal the valve area per cubic inch of the 4.6.

The 347ci LS1 (2.00”) would need a 2.28” valve.
The 380ci LSX (2.165”) would need a 2.4” valve.
The 427ci LS7 (2.2”) would need a 2.54” valve!

It is safe to say the DOHC 4.6 is not “under valved”, which accounts for the superior horsepower per cubic inch figures they deliver, especially with mild cam timing. In addition to superior valving over their domestic 2 valve brethren, the 4 valve layout utilizes a semi-hemispherical combustion chamber that places the intake and exhaust valves directly across from each other instead of side by side. As a result, the incoming air doesn’t have to make a 90* left hand turn followed by a 90* right hand turn in order to exit the cylinder. Airflow direction remains unaltered traveling straight in and straight out during the overlap period. That is a huge advantage.

With the 4 valve layout, maximum airflow potential is impressive but more important is the low lift flow figures which are nothing short of excellent. Remember that what happens in the beginning of the valve event determines the success of the entire induction cycle.

Here are a few things you may or may not have known about the 4 valve layout:

More valve area allows the use of smaller ports for increased velocity while achieving the same peak airflow numbers as a larger port. The increased velocity also provides even better low lift flow and improved fuel atomization and inertial ramming.

The smaller valves way less and don’t need to be lifted as far off the seat to move an equivalent amount of air so the valvetrain utilizes lower spring rates and lighter components for improved high RPM operation.

Superior low lift flow allows a smaller cam to perform as well as a larger cam. More specifically, a 260 degree cam in the 4 valve engine has the equivalent cylinder fill capacity of a 280 degree cam in a two valve engine.

The same is true for overlap and lobe centers. Superior low lift flow and opposing valves instead of side by side allows wider lobe centers and a minimum of 20 fewer degrees of overlap.

Compare 4v cam specs to 2v cam specs and you will see dramatic differences between the two. Camshaft engineers didn’t suddenly forget how to design cams when they went to work on the Ford DOHC. They have wider lobe separation angle (less overlap), later intake centerlines, less duration and less lift for very specific reasons.

The characteristics that improve the function of the intake side carry over to the exhaust as well. The exhaust of the four valve design is so efficient that the valves can be held closed deep into the blowdown period and still efficiently rid the chamber of spent gasses. This results in more low end torque even with very aggressive cams. And coincidentally, better fuel mileage.

Without the 4 valve layout, a streetable 450+ flywheel horsepower, naturally aspirated, pump gas, 281 cubic inch engine wouldn’t be possible. That’s 1.6 horsepower per cubic inch. Comparable horsepower per cubic inch figures for a 6.2L LSX Chevrolet would be 608 horsepower. How streetable do you think that would be? There is no comparison. Of course, cubic inches rule so the little DOHC leaves a lot to be desired in that department. If Ford pulls out all the stops and gives us some DOHC cylinder heads for the new 440 cubic inch capable Boss block, the blue oval will be untouchable. Did someone say Boss 429?

05-13-2010, 06:14 AM	#8
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond Unique Ford DOHC Characteristics Every engine combination is more or less unique in its cam requirements but I’ll focus on the Ford DOHC engine and some of the major design elements that make specifying their camshafts unique from the brand X alternatives. The most obvious departure from traditional domestic architectures is the DOHC 4 valve layout. When Ford designed the new modular engine family they knew the small cubic inch blocks were at a disadvantage and that the tiny bore size would never be big enough to support enough valve for reasonable performance applications. As a result they employed a 4 valve per cylinder, dual overhead camshaft combination. Compared to typical two valve wedge heads it was very expensive to engineer, produce, assemble and package but it is far superior to conventional pushrod and 2 valve variants in terms of horsepower per cubic inch. I was always surprised that the 4.6 didn’t get more press for producing almost identical power numbers to the LS-1’s of the same era and they did it with 50 fewer cubic inches, smaller camshafts and better fuel economy. Here’s why: The biggest restriction to getting air into and out of an engine is the valve. This is how David Vizard puts it: “The most important factor in making hp is the circumference of the intake valve in relation to the cylinder's displacement. All two-valve V-8s are under-valved for the cubes those valves have to feed.” David Vizard In other words, almost all domestic V-8’s from the inception of the internal combustion engine has been under valved. It’s not hard to imagine when you look at this nice straight pathway the port provides, only to plug the end up with a 2” hole and a valve that only lifts a half inch off the seat to allow air to pass. Valve limitation is the nature of the beast because a cylinder can only be so big before the dimension of the block will no longer fit within a reasonably sized engine bay. Four valve architecture, expensive as it is, does away with this problem by allowing a lot more valve area to be applied to a given combustion chamber/cylinder bore size. To put this into perspective we can compare the valve size of the 281 cubic inch 4.6L (1.456”) with those of a traditional 350 cubic inch Chevrolet small blocks (1.94” – 2.02”), the LS1 346 cubic inch small blocks (2.00” valve size), the current model LSX 380 cubic inch 6.2L small block (2.165”) and finally, the venerable Z06 427 cubic inch animal at 2.2”. To accurately compare valve sizes we have to convert the diameter of both the DOHC valves to their area, add them and then convert back to the diameter for a single valve. When we do this we get 2.06” equivalency. That’s a 2.06” valve to feed a paltry 281 cubic inches, or 4.6L. With larger and larger cubic inch engines coming on line, Detroit is producing heads with massive ports and large valves to go with them. But even with modern day technology it’s impossible to match the area of a four valve head. To bring more perspective to the table let’s look at what size valves the Chevrolet’s would need to equal the valve area per cubic inch of the 4.6. The 347ci LS1 (2.00”) would need a 2.28” valve. The 380ci LSX (2.165”) would need a 2.4” valve. The 427ci LS7 (2.2”) would need a 2.54” valve! It is safe to say the DOHC 4.6 is not “under valved”, which accounts for the superior horsepower per cubic inch figures they deliver, especially with mild cam timing. In addition to superior valving over their domestic 2 valve brethren, the 4 valve layout utilizes a semi-hemispherical combustion chamber that places the intake and exhaust valves directly across from each other instead of side by side. As a result, the incoming air doesn’t have to make a 90* left hand turn followed by a 90* right hand turn in order to exit the cylinder. Airflow direction remains unaltered traveling straight in and straight out during the overlap period. That is a huge advantage. With the 4 valve layout, maximum airflow potential is impressive but more important is the low lift flow figures which are nothing short of excellent. Remember that what happens in the beginning of the valve event determines the success of the entire induction cycle. Here are a few things you may or may not have known about the 4 valve layout: More valve area allows the use of smaller ports for increased velocity while achieving the same peak airflow numbers as a larger port. The increased velocity also provides even better low lift flow and improved fuel atomization and inertial ramming. The smaller valves way less and don’t need to be lifted as far off the seat to move an equivalent amount of air so the valvetrain utilizes lower spring rates and lighter components for improved high RPM operation. Superior low lift flow allows a smaller cam to perform as well as a larger cam. More specifically, a 260 degree cam in the 4 valve engine has the equivalent cylinder fill capacity of a 280 degree cam in a two valve engine. The same is true for overlap and lobe centers. Superior low lift flow and opposing valves instead of side by side allows wider lobe centers and a minimum of 20 fewer degrees of overlap. Compare 4v cam specs to 2v cam specs and you will see dramatic differences between the two. Camshaft engineers didn’t suddenly forget how to design cams when they went to work on the Ford DOHC. They have wider lobe separation angle (less overlap), later intake centerlines, less duration and less lift for very specific reasons. The characteristics that improve the function of the intake side carry over to the exhaust as well. The exhaust of the four valve design is so efficient that the valves can be held closed deep into the blowdown period and still efficiently rid the chamber of spent gasses. This results in more low end torque even with very aggressive cams. And coincidentally, better fuel mileage. Without the 4 valve layout, a streetable 450+ flywheel horsepower, naturally aspirated, pump gas, 281 cubic inch engine wouldn’t be possible. That’s 1.6 horsepower per cubic inch. Comparable horsepower per cubic inch figures for a 6.2L LSX Chevrolet would be 608 horsepower. How streetable do you think that would be? There is no comparison. Of course, cubic inches rule so the little DOHC leaves a lot to be desired in that department. If Ford pulls out all the stops and gives us some DOHC cylinder heads for the new 440 cubic inch capable Boss block, the blue oval will be untouchable. Did someone say Boss 429?