Aluminum vs. iron heads-This test shows no difference in power

[375instroke]

Can you really run a point higher compression with aluminum heads vs. iron, all other factors being equal?  If this motor they tested ran fine at 10.88:1 with iron heads, could it really run 11.88:1 with the aluminum heads without a problem on 91 octane gas?  10.88:1 compression, .046" quench, 260/260@.050 cam 110 lsa, identical heads, one aluminum, one iron.  They cc'd them and flowed them, very close to identical.  Tried different octanes to see if one was detonating or not, different coolant temps, different timing, both heads made the same power and torque at the same RPMs.

http://www.hotrod.com/how-to/engine/ccrp-0602-iron-versus-aluminum-cylinder-heads-test/

Some say you have to run higher compression with aluminum heads because they suck heat out of the combustion process, hurting power.  Many say you can run a point higher with aluminum because their thermal characteristics reduce hot spots in the chamber, allowing higher compression before detonation.  It could be all bunk?  Aluminum does have higher thermal conductivity, but how much power is that really sucking out of the combustion process of an event that lasts something like .003 seconds?  Apparently none.  What I speculate is that compression ratios started increasing again at the same time aluminum became more and more popular as a cylinder head material, and people were assuming the two were connected, when several other factors were being implemented at the same time, like better combustion chambers, computers with knock sensors, along with better air/fuel ratio management and ignition timing.  My Charger has the origin 440 in it, never rebuilt, and it runs fine on 92 octane with stock timing, no pinging.  Is there a reason anyone would build their motor with less than 10:1 compression?

[BSB67]

Nice discussion opener.

General comments:
1) The article does not actually have the iron head hp data.  I'm from Missouri, show me.  Why would they do all of this and not publish the data?
2) They did not actually measure the cylinder head volumes, or chose to not print them, right?  Apparently even the magazines are silly enough to print CR numbers to the 4th significant figure.  Pure nonsense.  Again, speaks to credibility.
3) The aluminum head actually flows more.
4) Did they use the same hot lash??

This is why I hate articles, they regularly skip the scientific process, don't think critically, and their conclusions cannot really be substantiated.  It does not mean they are wrong, but you simply don't really know anything for certain.

That said, there are many people smarter than me that say the same thing that is implied in the article, i.e., the real hp loss due to the heat loss from aluminum is either not truly measurable (due to minute differences in all of the variables), or is small enough that it simply does not matter.  

I really don't have a strong opinion on the CR verses head material discussions, but will offer some observations:

1) if a guy has a old set of iron cylinder heads and put on a new and similar set of aluminum heads without other changes, he is usually very pleased in the performance bump
2) if a guy has a good fresh set of iron heads and replaces them with similar aluminum heads, he is usually disappointed with the results ( something like, "...boy, I thought I was going to get more than that...")
3) CR aside, on pump gas, I have only been able to run about 165 psi on an iron head, and about 190 psi on an aluminum head.  Others report different results.  It just makes good sense to me to not leave any cylinder pressure on the table, irrespective of the CR.

I think 10:1 on an iron head is possible, depending on a lot of factors.  So it is not a yes or no question.    What is your cranking pressure?

Finally, and not part of your post, but notice the huge power difference with different coolant temps.  Go ahead, some of you, and love your 195° thermostat, just understand that you are giving up something

[c00nhunterjoe]

That article was pretty useless other then showing that higher engine temps rob power. They compared aftermarket heads against each other. Well of course there isnt going to be a difference, they are the same port designs. And they stuck a fairly large duration cam in with a wide lsa. Im not surprised it didnt ping. Im only 248 at half cranking in the upper 190s on iron heads and 93 and 42 degrees of timing- no pinging here.  Im with russ on most of his responses. When most guys see a respectable power increase they usually take off old wore out stock heads and bolt on a set of nice modern heads with bigger valves. The fact that your new heads are made out of aluminum is not why they make more power, its because they flow more then your old heads did. We can argue dynamics of iron vs aluminum, and given extreme and precise circumstances such as all out racing, then yeah the aluminum will win. Aluminum is also a preference for weight savings, yet another reason 90% of the aftermarket heads are not iron.

[Challenger340]

I surely do get a kick out of these sales articles.
Not so much for what they DO tell you..... but for what they don't, and this one is no different ?

So my question would be....
Just what in the fawk were these guys "testing" ?
Other than 91 Octane fuel will indeed run just fine on an "efficiently" chambered Cast Iron Cylinder Head at 151 psi ? and it will LIKE IT !
That's right.....
I said 150'ish psi.... NOT the articles claimed 185 ? or whatever ? because with a 79* ATDC close on the 260* pattern there is NO WAY they even saw the highside of 155 psi on the iron head with a 5.7 Rod ?

So what's the point ?
Write an entire article to test what many already know ?
and,
then compound the stooopid looks ? by running the Aluminum Head at it's lowest incipient Heat range for best power ?
to then declare.....
lookee here Martha... WE couldn't find ANY DIFFERENCE ? (because we're really 'toopid)

Best damn article I've ever seen at comparing "Apples" to "Pomegranates" by blind men.... but I bet DART HEADS is really happy !

[mopar0166]

is weight considered,  heat exchange?

[375instroke]

Nice discussion opener.

General comments:
1) The article does not actually have the iron head hp data.  I'm from Missouri, show me.  Why would they do all of this and not publish the data?
I don't know.  To save space?  If they're lying, they could just print the same chart and change a couple numbers, too.
2) They did not actually measure the cylinder head volumes, or chose to not print them, right?  Apparently even the magazines are silly enough to print CR numbers to the 4th significant figure.  Pure nonsense.  Again, speaks to credibility.
"we cc'd the chambers (around 63.8 cc each)"
3) The aluminum head actually flows more.
6 and 9 CFM at .2 and .3, but the same elsewhere.  That could account for "If anything, we could squint and guess and mumble that maybe aluminum heads were better by 2-3 hp."  The point was "the one thing we could never say is that the iron heads retained more heat [in the combustion chamber] and made more power than the aluminum."
4) Did they use the same hot lash??
You mean did they keep checking it, or deliberately use different lash for the two heads?

This is why I hate articles, they regularly skip the scientific process, don't think critically, and their conclusions cannot really be substantiated.  It does not mean they are wrong, but you simply don't really know anything for certain.

That said, there are many people smarter than me that say the same thing that is implied in the article, i.e., the real hp loss due to the heat loss from aluminum is either not truly measurable (due to minute differences in all of the variables), or is small enough that it simply does not matter.  

I really don't have a strong opinion on the CR verses head material discussions, but will offer some observations:

1) if a guy has a old set of iron cylinder heads and put on a new and similar set of aluminum heads without other changes, he is usually very pleased in the performance bump
2) if a guy has a good fresh set of iron heads and replaces them with similar aluminum heads, he is usually disappointed with the results ( something like, "...boy, I thought I was going to get more than that...")
3) CR aside, on pump gas, I have only been able to run about 165 psi on an iron head, and about 190 psi on an aluminum head.  Others report different results.  It just makes good sense to me to not leave any cylinder pressure on the table, irrespective of the CR.

I think 10:1 on an iron head is possible, depending on a lot of factors.  So it is not a yes or no question.    What is your cranking pressure?
I don't know.  It has 130,000 miles on it.  Doesn't burn oil or smoke, but it feels slow to me.  The last iron head motor I built (Ford 302, sorry) came in at 10.1:1 219°/227°@.050 on 112°, .043" quench, and runs fine in 24 hour endurance races on pump gas.  I haven't compression checked it, but just ran the numbers through a dynamic compression calculator.  Looks like I'm probably right on the edge, doesn't it?
Static compression ratio of 10.1:1.
Effective stroke is 2.44 inches.
Your dynamic compression ratio is 8.40:1 .
Your dynamic cranking pressure is 169.33 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.40 :1.
V/P (Volume to Pressure Index) is 126 

Finally, and not part of your post, but notice the huge power difference with different coolant temps.  Go ahead, some of you, and love your 195° thermostat, just understand that you are giving up something
Do higher thermostats give better mileage and lower cylinder wear?  What other reason?

[375instroke]

They compared aftermarket heads against each other. Well of course there isnt going to be a difference, they are the same port designs.  The fact that your new heads are made out of aluminum is not why they make more power, its because they flow more then your old heads did.

I believe the point of the article was to see if the hypothesis of aluminum makes less power than iron heads, because it extracts energy from the combustion process, therefore requiring more compression to make the same power.

"Because we question stuff. You know-bench racing stuff like, "you can get away with more compression ratio with aluminum heads because they dissipate more heat," and "iron heads make more power because they keep the heat in the chambers." We've heard those things a million times, and we've written it a bunch too. But has anyone really tested these claims, or is it just theory?"

[XH29N0G]

Can someone explain what about the lower temp cooling water led to the higher power?

• Is there an effect on how much air/fuel mixture can be sucked in?  (I would think that is air temp not coolant temp)
  
• Is it just that the charge of air fuel is not heated as much as it enters the cylinder?  (I would think this would not make a difference.)
  
• Is there something with the way the combustion proceeds?  (Do not have a clue)
  
• Is there an effect on resistance of the moving parts? (I would think this would do the opposite)
  

So I am flummoxed about this and would really appreciate understanding more about it.

[Challenger340]

IMO,
The "problem" with the article as a test of power between Aluminum Vrs Cast Iron Heads is this....
On "RACE" Engines....
once the incipient heat of combustion is satisfied for the Fuel requirement on either Head, but NOT exceeded, (and it is NOT for the Cast Iron Head at 150 psi on 79* ATDC closing @ 10.88 static)
Then.... there indeed would be very little difference in power output exhibited between identical Aluminum or Iron Heads that flow the same.

However,
if the article truely wanted to "test" the heat theory,(rather than SELL Dart Cast Iron Heads), they would have used one Head "out of it's range" so to speak ?
Like...
down around 9.0:1 with a 55* ATDC cam closing, and pull lower down in rpm(harder on rods). Wherein I believe they would have found a necessity to apply a greater timing advance curve earlier to the Aluminum Head to keep up with the Cast Iron Head powerwise,(once rpm is up they would again close the power gap at higher rpm)

As for the cooler head temps making more power.... really simple, just a denser A/F charge in the cooler head is more BOOM = more power... that's it. that's ALL !

[BSB67]

Nice discussion opener.

General comments:
1) The article does not actually have the iron head hp data.  I'm from Missouri, show me.  Why would they do all of this and not publish the data?
I don't know.  To save space?  If they're lying, they could just print the same chart and change a couple numbers, too.
2) They did not actually measure the cylinder head volumes, or chose to not print them, right?  Apparently even the magazines are silly enough to print CR numbers to the 4th significant figure.  Pure nonsense.  Again, speaks to credibility.
"we cc'd the chambers (around 63.8 cc each)"
3) The aluminum head actually flows more.
6 and 9 CFM at .2 and .3, but the same elsewhere.  That could account for "If anything, we could squint and guess and mumble that maybe aluminum heads were better by 2-3 hp."  The point was "the one thing we could never say is that the iron heads retained more heat [in the combustion chamber] and made more power than the aluminum."
4) Did they use the same hot lash??
You mean did they keep checking it, or deliberately use different lash for the two heads?

This is why I hate articles, they regularly skip the scientific process, don't think critically, and their conclusions cannot really be substantiated.  It does not mean they are wrong, but you simply don't really know anything for certain.

That said, there are many people smarter than me that say the same thing that is implied in the article, i.e., the real hp loss due to the heat loss from aluminum is either not truly measurable (due to minute differences in all of the variables), or is small enough that it simply does not matter.  

I really don't have a strong opinion on the CR verses head material discussions, but will offer some observations:

1) if a guy has a old set of iron cylinder heads and put on a new and similar set of aluminum heads without other changes, he is usually very pleased in the performance bump
2) if a guy has a good fresh set of iron heads and replaces them with similar aluminum heads, he is usually disappointed with the results ( something like, "...boy, I thought I was going to get more than that...")
3) CR aside, on pump gas, I have only been able to run about 165 psi on an iron head, and about 190 psi on an aluminum head.  Others report different results.  It just makes good sense to me to not leave any cylinder pressure on the table, irrespective of the CR.

I think 10:1 on an iron head is possible, depending on a lot of factors.  So it is not a yes or no question.    What is your cranking pressure?
I don't know.  It has 130,000 miles on it.  Doesn't burn oil or smoke, but it feels slow to me.  The last iron head motor I built (Ford 302, sorry) came in at 10.1:1 219°/227°@.050 on 112°, .043" quench, and runs fine in 24 hour endurance races on pump gas.  I haven't compression checked it, but just ran the numbers through a dynamic compression calculator.  Looks like I'm probably right on the edge, doesn't it?
Static compression ratio of 10.1:1.
Effective stroke is 2.44 inches.
Your dynamic compression ratio is 8.40:1 .
Your dynamic cranking pressure is 169.33 PSI.
Your effective boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.40 :1.
V/P (Volume to Pressure Index) is 126  

Finally, and not part of your post, but notice the huge power difference with different coolant temps.  Go ahead, some of you, and love your 195° thermostat, just understand that you are giving up something
Do higher thermostats give better mileage and lower cylinder wear?  What other reason?

It appears that you missed most of my points.  The article is just not credible.

Lie, no, but the details matter.  They HAVE the numbers and they chose to not publish the numbers.  If the data and facts are a forgone conclusion just because Steve said so, why didn't he just write a paragraph that simply says the iron = aluminum.  Apparently that would be enough to satisfy some readers.

"...around 63.8 cc"  what is the tolerance of "around"?  They took the time to measure all 16 chambers, but did not spend 1 minute putting the data in the article.  Their statement is a generalization and could mean anything.  I've measured a lot of cylinder heads, and even my CNC chambers would not fit in my definition of being  all "around" the same number. The more I read the article I become more dumbfounded that the cc, actual measured cranking pressure and hp data is not presented.

You do know that you motor is not 10.1:1, right?

You do know that the "dynamic calculators" don't match actual numbers very well, right?  Although 170 psi is probably about right for your motor, your analysis is more likely two wrongs canceling each other out.  Just measure it.

If your muscle car is about high mileage and gas mileage, the 195° thermostat is for you.

Like I said, I have no strong position one way or the other on the aluminum/iron/compression ratio/heat transfer/power issue.  I will say that there is nothing in that article that is persuasive.

[heyoldguy]

So far there aren't any better tests in print that the one first referenced to and this one. And doing this kind of test with a Chrysler head isn't going to happen unless one of us begins to cast iron and aluminum heads for a test. The aluminum heads available for Chryslers should show some detonation resistance simply because they have a better combustion chamber.

http://www.superchevy.com/how-to/83858-iron-vs-alloy-engine-heads/

[c00nhunterjoe]

The closest i think we could do is with a set of indy heads. They offer the 440-1s in iron and aluminum, identical castings other then the material. The question is, who has a set of the irons?

[heyoldguy]

We ported a set of the iron INDYs about a year ago. Near as I can tell they don't make them anymore. They weren't my heads and we didn't have a pair of the aluminum heads here at the same time to make a test on the dyno.