You're not going to like my suggestion...Run a functional EGR valve. You'll lower the combustion temperarture and increase fuel economy for cruise and highway.

There is ZERO truth to the oft repeated nonsense that aluminum heads can use a point more compression than iron. None. Guys running iron heads can run the same compression as guys running aluminum heads. It’s ridiculous to keep saying that falsehood. There is no possible way in hell that running along at a cruise and running into tip in rattle will stop if you switch to aluminum. Common sense says the increase in heat load happens so fast it wouldn’t matter if the heads were made of unobtainimum it wouldn’t matter.

There are no “tricks” to running more compression than the “orthodox” standards we have been lead to believe. Common sense engine building, cam selection and chassis isnt trickeration. It’s common sense.

You're not going to like my suggestion...Run a functional EGR valve. You'll lower the combustion temperature and increase fuel economy for cruise and highway.

Aluminum helps reduce detonation not just because of what happens over the course of one particular engine cycle, it helps because over the last several thousand cycles previous to that one particular cycle the heat has spread out more evenly throughout the cylinder head instead of staying more in one spot like it would have if the heads were iron. You can experience the same phenomenon when heating a head to remove a broken exhaust bolt, you put the heat on a broken bolt in an iron head and it will very quickly heat up the area the bolt is in and the rest of the head temp barely changes temps if at all and that spot will stay hotter much longer than if you do the same thing on an aluminum head, that hot spot around the broken bolt cools off waaaaay faster and the heat is much more quickly spread out through out the rest of the head. The aluminum spreads out the heat over time and prevents hot spots much more effectively than iron. You are ignoring the heat dissipation over time and focusing on one small slice of time where both heads are perfectly exact temp through out but over running time that is far from what is happening inside the engine.

There is no way that the aluminum head can dissipate heat so fast as to reduce local combustion temperature to reduce detonation.

Aluminum, thermal conductivity 136 btu/hr

Cast iron, thermal conductivity 46 btu/hr

So, aluminum triple thermal conductivity as cast iron, but no difference in operation?

That would be a no.

Don't get me wrong, you are stating the advantages of an aluminum head over an iron head is....?

And I agree in terms out output, iron comes out ahead everything else equal because of thermal efficiency, but it would also put a specific combination closer to detonation on a specific fuel.

Can't have it both ways. Thermal efficiency is keeping heat in the chamber (=power) but that can aggravate detonation sensitivity on pump fuel as well.

A general thought of a point in compression is not the definition of working miracles, it's general understanding of thermal conductivity between two different metals.





What I’m saying is you can run the same compression ratio with iron heads as you can with aluminum. It’s as simple as that. While aluminum rejects heat faster than iron, it still can’t react fast enough to allow more compression. It’s physically impossible.

I disagree. (Or would say, I'm ok with agreeing to disagree).

A few others thoughts on the matter-



But could the thermal properties of iron offer a power advantage over aluminum? That question gets complicated.

“Thermal conductivity has been debated for many years,” said Kevin Feeney of RHS, Memphis, Tennessee. “Historically, an iron head was considered more durable, and able to make more power due to the fact that it would not dissipate the combustion heat as quickly. With everything else remaining equal, there exists some merit to this argument.”

Mike Downs of Trick Flow Specialties, Tallmadge, Ohio, pointed out that the thermal conductivity of aluminum is “four or five times that of iron. This means an iron head will usually operate hotter. On the positive side, this means the fuel is pre-heated in the intake runner and easier to ignite in the cylinder.

On the negative side, it means the preheated air/fuel mixture will expand, reducing the effective flow into the engine and increasing the risk of pre-ignition. A properly designed aluminum head will transfer heat more quickly to the coolant, leaving the intake runner cooler and therefore able to flow more air-fuel mixture into the cylinder. High-energy ignition systems easily compensate for the cooler intake charge and help achieve maximum fuel burn.”

“The greater thermal conductivity of aluminum is a great advantage,” agreed Chris Frank of Frankenstein Racing Heads, Joshua, Texas, “especially in power-adder applications. That ability to dissipate heat quickly allows for more aggressive tune-ups.”

“Aluminum heads dissipate heat quicker than cast iron,” echoed Torrance, California-based Edelbrock’s Smitty Smith. “This can be an advantage in elimination-style drag racing, keeping the head temperature consistent round after round.”

With aluminum, Tony Mamo of AFR (Air Flow Research), Valencia, California, concurred, “detonation is less likely in an engine on the ragged edge, as it won’t hold as much heat. But that also firms up the argument that a cast iron head on an engine without detonation issues would make more power for the very same reason!”

“You need to build more heat with aluminum to make the same power,” agreed Bill Mitchell Jr. of Bill Mitchell Products, Ronkonkoma, New York, “or compensate by coating the chambers to keep more heat in the cylinders.”

I would love to contribute to this analysis, but I would have to dig out my thermodynamics books again.

Part of this discussion needs to separate out steady state equilibrium conditions VS transient conditions.

It's not about the temperature of the head, if it was you could just make up the difference by altering the coolant temperature.

It's about how quickly heat is conducted away from the chamber and into the cooling system. Just some numbers as an example but lets say you have peak cylinder pressure of 1200psi and peak combustion temperature of 4000 deg F, obviously that's a huge delta between say 200 degree coolant. How could a material separating those 2 one of which multiple times more thermally conductive -NOT- drop peak temperatures and therefore pressure?

Combustion heat/energy that could be pushing the piston down is lost to the cooling system which is then dissipated by the radiator.

As long as the cooling system isn't on the brink of being overloaded you could easily have more combustion heat being wicked into the system (by a head with much greater thermal conductivity) while maintaining the same coolant temp.

If that was not possible you would have a very large swing in coolant temp at say idle vs high rpm wide open throttle.

Hey, at least we agree that HP is king

I have no idea what video you are referencing.

The problem with cast iron is one spot can be significantly hotter than another spot, a ridge in the combustion chamber for example, you may have a 200 degree coolant temp but a hot spot that is 300 degrees because the heat is not transfered away very fast and the rest of the iron just slightly above the coolant temp perhaps 220 degrees. Aluminum on the other hand can also have the 200 degree coolant temp but the same thickness aluminum as the steel was but that one spot won't get as hot, the heat can move away from that area into the surrounding area quicker so you have a more consistent temp of the aluminum in the actual chamber, the temp in the same hot spot might get to 250 and the rest of the aluminum perhaps running 225 degrees so the average heat of the chamber is about equal but there are not as dramatic of hot and cool parts. That is why I brought up about the heating broken bolts, the aluminum head needs more heat applied to that spot to get it hot enough to remove the bolt and that entire aluminum head will get warm and the iron head will not but look out because that one spot you heated will burn your finger for several minutes. There is not a significant heat transfer from just one split second of heat but after thousands and thousands of those flashes of heat there are going to be hotter areas in the iron head than an identical aluminum head would have achieved.

Maybe a better analogy for you would be a well insulated and sealed up house with the thermostat set to 70, the entire house is close to that 70 degrees but then you got a drafty old house where you may achive 70 right there at the thermostat but there are much hotter and colder places in the house.

I don't understand how someone can claim the heat transfer is enough to make a difference in efficiency because less heat is absorbed by the iron, but on the other hand not understand how in one isolated part of the chambers temperature enough to make a hot spot that can start preignition when the fuel and air mix is squeezed to 250PSI against one spot that is already hot because it can't transfer it to the coolant fast enough...

You certainly CAN reduce detonation with coolant temperature. An engine at 190 degrees is much more detonation prone than one running at 160. It IS as simple as that.