Cast vs Alum....combustion characteristics

pistonhead · Mar 10, 2008

Hello everyone. I have worked with both cast heads and alum heads. Besides alum being lighter, easier to do port work, and better heat sink are there any cons to running an alum head. I amthinking along the lines of the actual internal combustion only. Like does the lower melting point of aluminum matter? Does the softer metal effect drivetrain geometry? Does the expanding characteristics affect the way valves sit?

There is alot of questions here so I am looking for answers or even better would be some good resource material.

Thanks for reading

Sicarius428 · Mar 10, 2008

Cast heads are actually better if you are trying to milk every HP compared to Aluminium... The heat dissipation is what soaks the HP too. On the flip side, Aluminium can run higher timing because they have less chance of detonation.
Kevin

dcurtis · Mar 10, 2008

Properly built you can run high compression on pump gas, just about 6 or 7 months ago I built a 12.3 to 1 chevy 350, it ran on pump gas all day long, it ran mid to low 9's in a 1000 foot. That engine is long gone already because my friend has a heavy foot and thought he could daily drive it.

NIKwoaC · Mar 10, 2008

Sicarius428 said:
Cast heads are actually better if you are trying to milk every HP compared to Aluminium... The heat dissipation is what soaks the HP too. On the flip side, Aluminium can run higher timing because they have less chance of detonation.
Kevin

+1

The basic principles of thermodynamics show that because cast iron doesn't conduct heat as well as aluminum, more heat energy is available for work production during the combustion process. But also, because aluminum heads will operate at lower average cylinder temperatures (because of its ability to transfer heat to the coolant better than cast iron), higher compression ratios can be used without detonation. So really, there isn't much to be gained using a CI head over aluminum, but its up to debate. The advantages of aluminum heads far outweigh what could possibly be gained from using CI, IMHO.

On the other hand, a lot of traditional hot rodders still run CI heads, but its more of a nostalgia thing than it is a power thing.

mastrdrver · Mar 12, 2008

pistonhead said:
Hello everyone. I have worked with both cast heads and alum heads. Besides alum being lighter, easier to do port work, and better heat sink are there any cons to running an alum head. I amthinking along the lines of the actual internal combustion only. Like does the lower melting point of aluminum matter?

No, since there is a thermal barrier that keeps the high combustion temps from doing any damage to the heads/pistons/etc. That is why an aluminum head looks like it has had a torch taken to the chamber when it has gone through preignition. The detonation creates a sound wave that knocks that thermal barrier away from the cylinder head.

Does the softer metal effect drivetrain geometry? Does the expanding characteristics affect the way valves sit?

It does not effect geometry since no valvetrain is anchored by the aluminum. It does not effect the valves since the seats are not aluminum and the valve guides are brass (I think).

85_SS_302_Coupe · Mar 12, 2008

Sicarius428 said:
Cast heads are actually better if you are trying to milk every HP compared to Aluminium... The heat dissipation is what soaks the HP too. On the flip side, Aluminium can run higher timing because they have less chance of detonation.
Kevin

Exactly. If you have to absolutely identical heads and one is iron and one is aluminum the iron heads will make a tad more power because they hold the heat in the cylinders better. The trade off is obviously the weight. Also aluminum headed engines run hotter than an iron headed engine (although honestly i have no proof of this, it's just what i've always heard).

dcurtis · Mar 12, 2008

Personally I will always run iron heads, mainly because I'm not worried about the weight.

Fast63 · Mar 13, 2008

I have heard aluminum heads make the engine run cooler because of heat dissipation.

85_SS_302_Coupe · Mar 13, 2008

Fast63 said:
I have heard aluminum heads make the engine run cooler because of heat dissipation.

I've always heard the opposite because the fact that the heads are soaking up the cylinder heat is what makes the whole engine hotter, which requires more cooling. Iron heads don't draw the heat out of the cylinders. Aluminum heads transfer that heat right up into your intake also, more so than iron heads do. Think about a heat sink used on something like a computer processor...they're aluminum so they soak up the heat...the heads do the same thing with the cylinders which you actually don't want. Of course we're not talking about so much that it ruins the engine combo, all you need is a more efficient radiator and a good cooling fan and it'll keep things in check.

Again this is just what i've always heard over the years...it would be really cool to see a back to back dyno test of 2 identical cast heads and see how they make power and do temperature checks on both.

Fast63 · Mar 13, 2008

85_SS_302_Coupe said:
I've always heard the opposite because the fact that the heads are soaking up the cylinder heat is what makes the whole engine hotter, which requires more cooling. Iron heads don't draw the heat out of the cylinders. Aluminum heads transfer that heat right up into your intake also, more so than iron heads do. Think about a heat sink used on something like a computer processor...they're aluminum so they soak up the heat...the heads do the same thing with the cylinders which you actually don't want. Of course we're not talking about so much that it ruins the engine combo, all you need is a more efficient radiator and a good cooling fan and it'll keep things in check.

Again this is just what i've always heard over the years...it would be really cool to see a back to back dyno test of 2 identical cast heads and see how they make power and do temperature checks on both.

Yeah, that does make sense. It would be nice to have that temp comparison... it's too bad no one ever has any articles on that sort of thing. Everything is about HP and torque and that's it. Not much science is explained

NIKwoaC · Mar 14, 2008

Fast63 said:
Yeah, that does make sense. It would be nice to have that temp comparison... it's too bad no one ever has any articles on that sort of thing. Everything is about HP and torque and that's it. Not much science is explained

If you really want to know, then go to college to be a mechanical engineer. That's what I'm doing.

There is a reason the best quality cylinder head designs on the market are aluminum, not cast iron…

Fast63 · Mar 14, 2008

NikwoaC said:
If you really want to know, then go to college to be a mechanical engineer. That's what I'm doing.

There is a reason the best quality cylinder head designs on the market are aluminum, not cast iron…

lol, already went to college to study criminal justice. have several mechanical engineers in the family though. I don't have time to take the classes but a simple article in a mustang mag would be nice.

squeak_cg · Mar 14, 2008

Minor point, heat sinks don't exactly 'soak' up heat from electronics. The heat has to go somewhere and heat sinks enhance the heat transfer (or heat dissipation) to the ambient air by increasing the surface area. Sorry for the silly comment.

haha, this is coming from an ME

85_SS_302_Coupe · Mar 14, 2008

squeak_cg said:
Minor point, heat sinks don't exactly 'soak' up heat from electronics. The heat has to go somewhere and heat sinks enhance the heat transfer (or heat dissipation) to the ambient air by increasing the surface area. Sorry for the silly comment.

haha, this is coming from an ME

Yeah, that's still the same point i was trying to make. The heat source is the cylinder and the head just increases surface area and the heat dissipates through the heads...same thing.

NikwoaC said:
If you really want to know, then go to college to be a mechanical engineer. That's what I'm doing.

There is a reason the best quality cylinder head designs on the market are aluminum, not cast iron…

The reason is that people want the light weight of aluminum and because aluminum is much easier and cheaper to cast and much easier to port...it has nothing to do with what i'm talking about. Like i said this is just minor details, it's not enough to sway people away from aluminum heads.

pistonhead · Mar 23, 2008

Thanks for the info guys. That really shows the combustion process from a different perspective than I have been looking at it previously.

Cast vs Alum....combustion characteristics

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