Aluminum heads
As said, 100% apples to apples is not going to happen due to the casting.
However, IMHO the above is missing a crucial and vital point about the heat conduction. It is correct that once the combustion process starts, the head material does not do anything, since the heat generation is so fast that the head material does not matter. But what is being missed is that the peak temperature and pressure (which is what we are interested in for managing detonation) is NOT ONLY due to the heating during the combustion process. It is a result of the temperature/pressure of the gases immediately prior to the start of combustion plus the heat added during combustion.
There is no arguing with this matter... The research that established this was done 80-90 years ago, in the 1930's, that cooler engines and mixtures are less prone to detonation. You can find the papers out there if you look. (And I don't believe that the laws of physics have changed any since then LOL.)
It is the gas temps just prior to combustion where the cylinder/chamber materials can make a difference. The thermal resistance of AL is 3/10ths that of cast iron. So, as long as the other thermal resistances are similar or the same (the chamber gasses into chamber wall, chamber wall to coolant, and then the whole effective thermal resistance of coolant to air) then aluminum lowers the overall thermal resistance from chamber surface to air. The aluminum chamber will be cooler when you need it to be: during the intake and compression time prior to the start of combustion. If that can be used to keep the gases cooler prior to the start of combustion, you have succeeded.
How much of this is a factor? IDK.... Hard to separate it all out. I will not say the YR's results are in any way wrong.... but my opinion all along it is due to other things he is doing, and other factors in the combustion process and operation, that makes the materials appear equal in his tests. (And we don't know how extensive those tests really are to be able to judge if they indicate a broad result.)
As for the 'going through the detonation conditions at tip in by mashing the throttle', that mashing is changing other combustion conditions to get away from the detonation. But that says zip, zero, nada about the chamber material's contribution to the detonation when it does occur. So that is not a proof of anything about the materials. I can only say what's indicated above.... that other factors are more important overall in YR's testing. But a different engine, setup, operating, may benefit more clearly from the materials difference.
Noting new or bad about any of this..... Research work goes to great lengths to try to contrive tests to separate out all the different effects, but sometimes, the papers confess that it just cannot be done in their particular testing setup, and all they can draw is probable conclusions. So no surprise that it cannot be done in the field.
BTW.... If you had an accurate steady state dyno, and the AL was actually pulling out more heat to keep the chamber temps lower, then the coolant temps wold rise.
The only place I know of other than marine stuff where en engine is in steady state is Talladaga or Daytona. Everything else is transient RPM so a steady state measure is kinda worthless for what we are considering.
Again, an aluminum head is at the same temperature as a cast iron head when running. It's not any cooler because it's aluminum.
Want to prove that? It's easy. You can measure a cast iron intake and record the temps. The swap on an aluminum intake and the temps will be the same. Or so close it's not worth considering.
I do nothing special to my engines other than not buying some cookie cutter junky assed off the shelf cam. But I do that regardless of what the head casting is made of.
Been doing it for decades and all brands. Had a BBC street strip deal come into the shop from another builder local to us. It came in at 555 CID on pump gas and the calculated CR was a paultry 9.25:1. Rediculous.
Then we got the cam card (engine came in because it was down on power and stuck a piston) and it was some off the shelf hydraulic roller. First things first, the cam went in the resale pile and the lifters followed the cam.
Then we changed Pistons to get the CR to 11:1 with yes, you guessed it, iron heads. Of course the owner was wetting his panties because he was a forum surfer and all the key board builders assured him it was grenade with the pin pulled.
Ordered a CUSTOM cam from CamMotion and it looked nothing like all the keyboard heroes said it should. A HR lobe with solid lifters. Again the interweb went wild claiming disaster and explosion on the first dyno pull.
That was the summer of 2006. My brother ran into the guy at a car show this summer. 13 years later, about 55k on the engine, all on pump gas and still going, detonation free.
One example of doing it again.
The facts are simple. Detonation almost always occurs at high load, low speed throttle positions. There is no way on Gods green earth that an aluminum head can reject heat so much faster, and the coolant can accept the heat so fast as to correct this.
I say the biggest issue people have is they don't understand cam timing enough to pick the correct cam so they end up with off the shelf junk cam with the Comp de facto LSA which is almost always wrong. So they give up CR (which is HP) and they give up HP because the cam timing is wrong.
Then, they believe the lie that a wide LSA makes the idle so much better that they have to have it. I used to say unhook the tach. The fact is this. If the cam timing is correct for you combo, then the LSA is what it is, and is almost always much more narrow that the keyboard builders think it should be.
Again, I'm 340 inches, 11.08:1 measured and my cam is 255 at .050 on a 105 in at 105 and my junk will idle right down to 750. I don't let it idle that slow, ever but it will. With full timing (36 total) and zero detonation on pump gas. Premium pump gas but that low grade junk is just that...low grade junk.
Does it lope? Yes. Is it miserable to drive and sit at a stop light? Nope. It's doesn't idle any rougher that a cam much, much smaller. Why is that?? Because the lobes were selected for my combo. It isn't magic. It's called preparation.
One last thing. If all the proponents of the silly idea that head materiel makes a difference believe what they say, I should be able to go to 12:1 or even 12.5:1 and change nothing but the heads, just because the aluminum rejects the heat so much faster and better.
Only a fool would do that. I'd bet everything I have and a bunch of stuff I don't have that not a single proponent of the aluminum advantage so called would ever be willing to attempt that change on their own engine. I wouldn't do that. That much CR would require a totally different cam. Regardless of head materiel.
In fact, I've never ever had a single cam grinder tell me I needed aluminum heads for detonation resistance.
The reason why the OE's love aluminum is because it's light (think CAFE standard stupidity), cheap (mo profits), and much easier to machine and easier on tooling (again...show me the money).
Never once was there a consideration of detonation in that decision.
Here's a largely unknown fact. When Honda came out with their aluminum framed dirt bikes the entire off road motorcycling world was in awe of this magical, engineering wonder.
The fact of it was, 4130 chromoly was getting scarce and expensive over on Dai Nippon. Way too expensive. They have no natural resources there and have to import everything. So they opted for the much cheaper aluminum.
It was all profit and availability motivated. I have zero issues with that. I have issue with the myths perpetuated by Honda and the media about why the change was made. It wasn't for any special, secret go fast anything. It was $$$$$.
That's why KTM, GasGas, TM and probably more still make them chassis from 4130. They can get it. And they get it cheap enough to not compromise with aluminum.
