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iron vs aluminum

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Old 04-30-2006, 06:22 PM
  #11  
luckythirteen13
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Default RE: iron vs aluminum

the weight savings alone make it worth buying but aluminim gives a little which helps fight ping
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Old 04-30-2006, 06:23 PM
  #12  
ikeybmg
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Default RE: iron vs aluminum

another great article on this..

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Choosing Heads: Aluminum vs. Iron (Which Ones Do You Need?)

Some basic guidelines on how to choose heads


With the current availability and popularity of aluminum heads for both Big and Small Blocks, the question of which to use is more pertinent now than in the recent past. Since we port, test, sell about an equal number of each and love both of them we can offer some unbiased opinions and information. With the number and types of aluminum heads now available, the Mopar racers need to understand all of the quirks about them and terms used with them.

There are several types or levels of heads available and there is a lot of confusion over what is what. This confusion is in part created by the terms "bolt-on" and "stock replacement". They are used indiscriminately and interchangeably, and no one has stepped forward to either clarify or classify them. We will try!

A "stock replacement" head is one that has the stock location of the ports and rocker arms and these original components will along with the rocker covers and other odd brackets will attach like they did on the original iron heads. No spacers, oil lines or other adapters are required.

"Bolt-on" can generally be interpreted, with cylinder heads, to mean that they will have head bolt holes located in the same "location" the stock ones. So they will "bolt-on" with the correct bolts, which may or may not be the same as stock, usually not. It does not mean that they are a "stock replacement" head and you should not expect any OEM pieces to fit. This "bolt-on" type head will be a race type head and have the ports raised from the original location and require special intake manifolds or spacers and Special exhaust components, and many times special rocker arms and maybe even external oil lines.

The difference in cost between the two is very high too, as many who have called our shop can testify. They have been sold as so-called stock replacement (S/R) heads (by other sources) that "bolt-on" and need many expensive parts to complete the job. That takes them out of the "stock replacement" group no matter what you call them, you can put lipstick on a pig, but it's still a pig. And no, we don't know of any lower cost way out of that dilemma, except to send the heads back and get what you wanted in the first place.

We are dealing with two different types of heads from a performance stand point too! A stock replacement type head must have the port locations in the same position as the stock head, so the manifolds etc. will attach. This feature has drawbacks because raising the ports is one of the best ways to improve air flow, but with that alteration comes the special manifolds, spacers, headers, rockers etc. putting the head into the raised port bolt-on group. In our shop we consider the following heads to be true "stock replacement" heads:

Small Block
Edelbrock RPM aluminum
M/P P4529269 iron
M/P '785 aluminum
M/P '310 aluminum
'574 factory iron
'768 factory iron
'459 factory iron

Big Block
Edelbrock RPM aluminum
M/P P4529992 Stage V iron
M/P '311 383/440 aluminum


We often get requests to compare these "stock replacement" heads to heads that are not on this list, in other words raised port, "bolt-on" heads. This is unfair and misleading; they are not apples to apples comparison. The stock replacement head are just that, a stock replacement head in a high performance version. The raised port, "bolt-on" head is a head designed for racing. If you need a "bolt-on" raised port head why would you even care about a "stock replacement" head? On the other hand if you need a high performance, "stock replacement" head, buying the raised port "bolt-on" heads can be a expensive mistake.

As we always tell our callers, you must build your car/engine with a plan or a goal in mind. You can use horsepower, ET or money, they will all end up at the same point, but you must have a plan. If you are starting from scratch and need 750 or more horsepower and cost is not a problem go with the fully ported raised port bolt-on heads which should have the capability to develop the most power. Here we run into another problem. Some of the more popular raised port bolt-on heads barely out flow a set of, well ported, stock replacement heads. Check carefully before buying. Heads with ports in the stock locations can be ported to flow about the same amount of air whether they are iron or aluminum. Read that sentence again! All "stock replacement" performance aluminum heads are designed to develop more power than the iron heads they are replacing. However the gap between what was designed and what leaves the factory can be worlds apart.

When aluminum heads are cast there is a lot of core shift, more than with iron heads. That core shift ALWAYS reduces the air flow in the port. The question is how much flow is lost and is it significant in reaching your goal. We constantly check the "out-of-the-box" flow of heads that we are porting and find them to be down 15 to 30 CFM. The CFM from port-to-port will also vary 15 to 30 CFM in the same head (please refer the our Flow Bench section for further information).

In an effort to reduce manufacturing costs on ready-to-bolt-on heads, "finished" seat rings are installed in the heads by pressing or driving. This procedure causes the seats to go out-of-round. We have seen them out-of-round 0.014" to 0.016". The limit should be 0.002" or less. Seats this far out of tolerance will cause hard starting, rough idle, and reduced power output. Originally, performance aluminum heads were designed for porting shops to finish and correct the manufacturing flaws before the head was installed. That way the problems of core shift and out-of-round seats never reached the end user.

Aluminum heads dissipate (get rid of) heat much faster than iron heads. The rapid heat loss from the combustion chamber results in reduced combustion chamber pressures (heat equals pressure); similar to the effect that lowering the compression ratio lowers cylinder pressure. When switching to aluminum heads you MUST raise the compression ratio to prevent a power loss. This is a good opportunity for our customers to take advantage of our computer calculated compression ratio service. This software assures them of getting the maximum power out of their purchase; it's a no-charge service for our customers.

Aluminum heads will not automatically create more power than iron heads unless they flow more air and are installed on a higher compression ratio engine. In cases where you have a decent port job on a set of iron heads it is possible to bolt on a set of out-of-the box aluminum heads and lose power. Aluminum is automatically lighter but not faster. Yeah, yeah I've read all of those magazine articles that told you how they installed brand X widget and picked up 500 HP and 4 seconds in the 1/4 mile (we have even been the victim of these articles). What you must keep in mind is that those magazines and the writers (???) are paid by the advertisers, not by that $19.99 you send in once a year. So you can expect that advertised products always look good even when they screw the test up and the "results" are not what they should be, (been there done that!). They are under a deadline and the rent has to be paid so the facts sometimes take a back seat. In some cases the engine would have produced more power with a set of well ported, iron heads, and at a lower price.


Quick Facts: Aluminum vs Iron


Weight: Aluminum heads weigh about 45 to 50 lbs., a pair, less than iron heads
Durability: "Ported iron heads can crack if the engine is badly over-heated". Yes this
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Old 04-30-2006, 07:42 PM
  #13  
infantrycav
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Default RE: iron vs aluminum

there was an article in car craft a about a half year ago about this. With identical casting and flow numbers and chambers the two castings showed no difference in power. They also mentioned the alum. having more timing potential but they tried it and found no gain and they had detonation at the same degree as the iron.
i think the engine was a BB chevy but not sure which one.

the only problem I've heard of is having an iron block with alum. heads and the different expansion rates sometimes causing head gasket problems but i really dont hear of this ever just something we were taught in school.

I plan on spending the extra $$$ on alum. purely for weight, that and most mid-higher end 5.0 heads are alum. anyways. That and if I decide to go with the alum. A4 block instead of a dart block I'll fix that so called expansion problem and shave a sh#t load of weight off the front end
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Old 05-01-2006, 12:55 AM
  #14  
gspfunk
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Default RE: iron vs aluminum


ORIGINAL: 94BlackCobra

besides weights savings...is there really any other difference in perforamce from a set of good iron heads compared to aluminum
It depends on the heads you're comparing. The GT40 P's flow almost as much as the GT40 X's, but the X's are alum. and lighter. Now if you're talking about the AFR 165's or TFS twisted wedge's, they flow a helluva lot more than any of the GT40 heads and give you a lot more power.
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