Do aluminum heads really reduce the octane requirement...

[F-BIRD'88]

Aluminum conducts heat (a change in heat) faster than cast iron.
But hot aluminum is just as hot as hot cast iron. So when a motor goes WOT it starts making heat. The alumnium sucks the heat away at first (to the cooling system) but once heat soaked the cast iron catches up. Once you let off the gas the cast iron head retains the heat longer. So the aluminum head has a short period where it will have a small knock threshold advantage and small power diference. But the two materials even out eventually. Hot is hot.

So for drag racing it has a small brief advantage (or disadvantage depending on how you look at it) and can tend to tolerate a bit more cr.

Once running WOT steady state there is no difference.

Many people make way too much of any practical difference.

You will see a significant difference at WOT steady state, the heat transfer of the aluminum will cause a lot more heat dissapation into the cooling system, lowering the temp inside the chamber.

I contend that this is absolutely true, at first. But as time progresses and the two materials heat soak and the cooling system heat soaks, the difference in heat transfer speed difference to the cooling systems decreases and the two different power outputs of otherwise identical engines becomes less of a difference.

In a steady state WOT dyno test comparison, to prove one way or the other, all the other factors would have to be controled.
I contend that at first there is a big difference but the difference gets a good bit less over time.

[ap72]

IF your cooling system is incapable of keeping up then yes there will be little difference, BUT if your cooling system can keep up it will transfer more heat through the radiator with aluminum heads and the chambers will be a little cooler. It becomes a cooling question rather than a cylinder head question at that point though.

[Patriyacht]

On a boat there is no build up of heat in the cooling system. We have an unlimited supply of cooling water. Supercharged engines typically run no thermostat and the n/a guys run between 120 deg. and 160 deg thermostat. The rule of thumb that we use is that the greater heat dissapation will allow 1/2 to 1 point greater compression.

[rfoll]

One of the magazines ran the iron/aluminum test a while back, I believe using Dart heads, and found very little HP difference. But the test was done by magazine guys....

[franklstoner]

So why would anybody want to run aluminum heads. I know their better to weld up and their lighter. But if your running a 13 to 1 comp 406 and not wanting to change any other parts. Does afr have a cast iron duplicate to the 220 heads. What about thermo coating your already existing aluminum heads. Would you just want to coat the combustion chambers?

[plovett]

Here's the article:

http://www.carcraft.com/techarticles/cc ... index.html

It does seem to contradict all that I've heard over the years. I had read this article before and it made me wonder if anybody else has actually tested iron vs. aluminum.

paulie

[bigjoe1]

If you increase the compression ratio with the alu heads like I already stated, the alu heads will be the same as the iron. Again, this was on a mild 350 test engine. The alu heads are so much nicer to work with, in ever way. I would never go back to iron unless the rules required it


JOE SHERMAN RACING ENGINES

[plovett]

Anybody have any ideas why the test by Freiburger doesn't show more power with iron heads? The article doesn't show all the data, but it states the dyno curves were almost identical between the iron and aluminum heads.

paulie

[bigjoe1]

If the alu heads were 4 cc smaller in the chamber, that would make up the difference


JOE SHERMAN RACING

[plovett]

The article says:

"Externally they appear to be the same casting, and to verify that they are functionally identical, we cc'd the chambers (around 63.8 cc each) and intake runners (almost 217 cc on both)."

I'm not endorsing the article and I definitely wasn't there. I'm just trying to figure it out.

paulie

[franklstoner]

maybe the lower ve% the motor achieves the less the difference between the two heads. Just a guess why the article didn't see a difference.

[F-BIRD'88]

While Joe's power and cr difference statements are true, in testing its hard to control all the other critical variables that effect engine power output and A B test variation. Just raising the water temp up from 100F to 180F will typically drop the power by +/-20hp.

And does not reliably predict any specific octane requirement difference. There are way too many other more critical operating variables/conditions that cannot be so tightly controlled in the real world.