This image shows the three problems that can arise when using aluminum for the casting material for the cylinder head. These challenges can be addressed. They can be dealt with. They must not be ignored.

The three problems:

...the already mentioned headbolt area distortion

...the brinelling of the head where the fire-ring of the headgasket contacts the head

...the distortion of the sides of the combustion chamber

All three problems are related to the nature of the aluminum metal. Aluminum is softer than iron. It has an thermal expansion rate that is about 1.5 times that of iron...ie the aluminum will get larger than the iron as it heats up. It has a specific heat capacity that is much higher than iron. The specific heat capacity and the heat transferance capacity of the aluminum are where most of the problems stem from.

Rather than get into a long discourse on heat, heat acceptance, heat absorption and conductance, I'll summarize it as it applies to these heads.

*****
I realize that my abbreviated summary is less than completely explanatory. I do advise further research by the reader to clarify his understanding. It is a rather complex subject.
*****

Aluminum does not readily accept heat; and it takes a lot of heat to get aluminum hot [specific heat capacity]...ie aluminum does not want to accept heat easily; and if you want to heat aluminum up, you are going to have to turn up the flame. Once the aluminum does get hot, it will conduct the heat fairly well [heat conductivity, or transferance]. Copper accepts heat much more readily than aluminum. Silver, copper, and gold all conduct [transfer] heat much better than aluminum.

An example to illustrate heat acceptance and transferance:
...why copper is a better material for a heatsink on a CPU in a computer: it accepts and transfers heat faster than aluminum. Copper is also heavier, denser, and more expensive. An often used compromise that saves weight and expense is to use a copper wafer or small plate as the contact material at the CPU, but have the copper wafer mated to aluminum fins to transfer the heat away. Such a heatsink design quickly accepts and moves the heat away from the CPU, and then transfers and dissipates the heat reasonably well. I have found some Volvo a/c condensers and evaporators from 1970s models that were constructed with copper tubing and aluminum fins....excellent heat absorption from or to the freon; reasonable heat transferance through the aluminum; much lighter and less costly than all copper, but more efficient and more expensive than all aluminum.

Iron accepts heat easier than aluminum; expands less as it is heated; and conducts heat through itself more slowly than does aluminum.

One might ask why use a material that is so dissimilar to the iron block. Aluminum is much lighter than iron; and can be cast into more intricate shapes with more consistency. A single overhead camshaft cylinder head definitely qualifies as an intricate casting. ['Complex' might be a better adjective than 'intricate'.] And even though aluminum is very lightweight, it is a very rigid material. It will try very hard to maintain its shape. The castability, lightness, and rigidity all help to make aluminum a good material for use in a cylinder head.

The early heads were what is known as BCP heads [big coolant passages]. They were fairly durable and reliable on NA [normally aspirated] engines. When volvo started to turbocharge these engines, the strength of the castings was challenged. The castings were revised to improve port flow; and to add material to improve rigidity and strength in order to withstand the pressures encountered in a boosted motor. The newer castings are known as SCP heads [small coolant passages].

As stated, aluminum does not readily accept heat; will absorb a lot of heat before it gets hot; and will transfer [conduct] heat fairly well. When the BCP heads were revised into SCP heads for more strength and improved air flow through the ports, three things happened:

...rigidity was enhanced: the thicker deck and internal structure of the head was more stable; boost pressures in the cylinders did not overpower the aluminum and cause flexing and head gasket failure as easily as before. SCP heads are about 2 pounds heavier than BCP heads.

...emissions due to quench cooling was reduced. The combustion chamber surfaces in the head were operating at a higher metal temperature; thereby reducing the amounts of unburned fuel in the exhaust.

...the entire head was operating at a higher metal temperature.

How so? If you increase the thickness of a conductor, it takes longer to move the heat through. The rate of heat transfer is the same, but there is more material for the heat to pass through; so it takes longer to complete the heat transfer. The result is a higher metal operating temperature.
To restate that another way: the aluminum still conducts the same amount of heat; but because there is more aluminum for the heat to go through, it takes longer for the amount of heat to get moved. And the longer the heat stays there, during the process of the transfer, the temperature of the metal will be higher.

*****
It is the differences between temperature and heat that is often the source of confusion and misunderstanding. And that is why I did and do suggest further reading on the subjects to clarify the matter.
*****

There is another item that contributed to the higher operating temp of the SCP aluminum head: higher temp thermostats. By raising the operating temp of the coolant, the rate of heat transfer was slowed down even more. How so? Heat transfer is dependent on temperature differentials: the greater the difference in temperature of two materials in contact with each other; the faster the transfer of heat can occur from the hotter one to the cooler one. [and unscientific terms like 'hotter' and 'cooler' may not be the best terms to use in this explanation; but for many reading this, those terms are familiar enough to help explain what is going on] The higher temp thermostats was also part of the emissions reduction program: a hotter cylinder head reduces unburned fuel emissions.

To reiterate: the SCP head is stronger and more rigid; and it operates at a higher metal temperature. The higher metal temp is good for reducing emissions.

The higher metal operating temp is bad for durability.

Aluminum, either pure or in alloy, is strongest at room temperature. At 200F, it has lost some of its tensile strength. At 400F, it has lost about half of its tensile strength. At 600F, it has lost about two thirds of its tensile strength.

A casting made from aluminum is the strongest it will ever be when it is first made and heat treated. Over time, as it goes through heat cycles, it will lose some of its strength. Which does have implications for an aluminum head: the greater the heat load during those heat cycles, the faster the loss of strength will occur. There is a cumulative effect at work: the distortion is cumulative; the brinelling is cumulative; the softening is cumulative. And, if welded, even if re-heat treated after the welding, it will never be as strong as it was originally.

Looking at our SCP cylinder head: it is thicker dimensionally; it runs hotter. The higher metal temp causes more thermal expansion. The increased expansion fights the headbolts, putting more static pressure on the fire rings of the head gasket: this causes increased indenting [brinelling] of the deck surface of the head. The same increased static pressures also contribute to the distorting of the aluminum at the headbolts.

**** UPDATE 8-22-2004 ****

the following paragragh will be re-edited to update the information soon to reflect the fact that I have concluded that the head bolts are NOT TTY bolts; but instead are TAT bolts.

**** **** ****

One of the reasons for using TTY headbolts is because the TTY bolt provides a more uniform clamping throughout the operating temp range; ie they can accomodate the thermal expansion better than the old style headbolts. To also put that in regular language: the TTY headbolts can give and take better with the aluminum, as it expands and contracts, than can the old style headbolts. But even with the TTY bolts, we find the distortion, or pulling, of the aluminum at the headbolt areas.

The distortion of the aluminum around the combustion chambers is indicative of the higher metal temps of the SCP heads; and of those higher temps for a number of operating cycles/period of time. Remember, the head surface was originally machined flat all the way across; the distortion of the aluminum edges making those high ridges around the combustion chamber is a result of heat...and evidence of a lot of heat.

Over a period of time and after a number of operating cycles, combined with the distortion of the aluminum at the headbolts, the brinelling of the deck surface will cause sufficient indentation to result in insufficient clamping force being exerted on the fire ring areas; the head gasket will no longer be able to contain the combustion pressures: the gasket will fail.

This head is almost there. The brinelling between number 1 and number 2 cylinders is almost 0.005in deep. It would have failed within another 10,000 miles; possibly a lot sooner...


...to be continued