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Discussion Starter #1
I’ve searched and couldn’t find anything directly addressing this question.

And I definitely don’t want to start an argument or anything, but I have a question about aluminum vs. iron that's been bugging me for a while.

I read all the time that the aluminum 6 bolt aluminum block is stronger than the 4 bolt iron block. (We're only talking blocks, no rods, pistons, etc.)

But which is actually stronger? (Maybe better due to lower weight, resistance to impact/vibration and better heat dissipation, but stronger?)

Personally, I think the iron block is equal to or stronger than the aluminum. Now before everyone takes my head off, let me explain my reasoning. :D

I’m assuming that block casting are very similar in regard to wall thickness and boss sizes. (I’ve never seen a side-by-side comparison of the aluminum block and the iron block, so please correct me if I’m way off). I feel that aluminum is weaker than iron in this application. As an example, if you have two identical blocks, one 4 bolt made of aluminum, and one 4 bolt made of iron, the iron block will be stronger. Assuming the same casting dimensions.

So I think the extra set of bolts in the aluminum block gets you up to the strength of the iron block, not stronger than the iron block. The second set of bolts may prevent shifting of the cap, but with the cross bolting they way they are, I think that should be take care of. I just think the second sets of bolts are to provide lateral strength to the aluminum caps to keep them from twisting in the saddles.

Another reason I say this is that the new supercharged Cobra engines went back to an iron block. (Is it 6 bolt?). Since the aluminum 6 bolt block couldn’t handle the power.

Any opinions? Anyone agree/disagree? Or am I just out in left field? :D
 

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94 Daily Driven 4.6L said:
Another reason I say this is that the new supercharged Cobra engines went back to an iron block. (Is it 6 bolt?). Since the aluminum 6 bolt block couldn’t handle the power.
I'm not going to venture any guesses, as this is actually going to bring some solid answers/proof from the more knowledgeable members....

I do want to make a comment on the '03 Cobra engine. I think the only reason they went to iron would have been to keep costs down. An all aluminum engine would run the price up. In an effort to blow the doors of the C5, yet still be under $40k, an iron block would help keep the costs down. At least, this is what I am assuming.

Physically, iron is stronger than aluminum...but....that's as far as I'll go. I'll wait to see what the 'big dogs' throw out.
 

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Good question.... I want to hear the answer too...

Sam
 

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Here is some information for you

aluminum and iron are as equally strong depending on the application.

This is dependant on the processes used in nealing and also rockwell hardening.

I am not a molecular person but it has been explained to me that the properties for aluminum and iron.

Regarding the srength of aluminum; there are many in the modular world pumping outragious power and torque numbers from aluminum blocks w/o blowing up.

The advantages of aluminum is the weight characteristics compared to iron.

just .02
 

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The '03 Cobra uses an iron block from a Crown Vic/Mustang GT. It has a 4-bolt main. The 32V 4.6L '96-'99/'01 Cobra is an aluminum alloy block with steel cylinder sleeves and a 6-bolt main.

The '03 Cobra went to an iron block due to durability issues with the main caps stretching/walking. The '03 Cobra also uses a newer forged steel crankshaft. Aluminum loves to absorb heat and moves fairly easily, Ford didn't want to take a chance for longevity/durability with the Eaton Blown 4.6L.

Aluminum is lighter than cast iron and weight savings help Ford meet fuel economy requirements.

A-Train
 

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Troy...

I take issue what you have said about aluminum being as strong as iron...depending on the application. That is completely false. The alloy of iron (meaning it's not completely just iron...it has other elements in it) used for engine blocks is physically much stronger than the alloy of aluminum used in engine blocks. Aluminum is just about 1/3 the density of iron. It's physical properties are much different. In the same application, it dissipates heat faster due to its much better thermal conductivity.

Tensile strength...that is the ability to resist deformation in tension is nowhere near as good as iron. Threads will pull out of aluminum much easier than in iron. This may be the reason 6 bolts are required in the aluminum blocks.

As A-train said, problems with aluminum main caps deforming was one reason Ford went back to the cast iron block. This is a perfect example of the lesser tensile/ductile strength of aluminum.

Now please don't get me wrong....because of the lighter weight of aluminum, a designer can afford to use thicker sections in high stress areas....sometimes doubling the thickness when compared to cast iron. When designed with the proper porportions, an aluminum main cap can be as strong as cast iron, with about 1/3 of the weight. Of couse that main cap would be much more massive that a similar one made from cast iron. And would require more screws to keep it stationary because of the lesser amount of torque the threads could withstand before damage occurs. One way to lessen the amount of screws required would be to make the threaded sections longer, but this usually isn't all that practical in engine applications.

Feel free to dispute any of the points I have made....I also don't want to start any kind of word war, but I do have a background in the metalworking industry and manufacturing engineering and do have a good idea of what I am speaking. I am in no way a professor of structural design....but I've done a good deal of it. Engine design is way beyond my engineering scope, but strength of material is more inline with my background....

Just my 2 Cents!
 

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OK well let me give a quickie answer....

Ultimate strength is not the question..... Engines are not designed to be stressed to there max.... They are engineered to have tolerences. What Were are really looking at is the Modulus of Elasticity. For steel its roughly 29,000 KSI and for Aluminum its about 10,000 KSI..... So we can see that Steel has a modulus of about 3 times Aluminum...

To put this into perspective... If you take two equal slender beams Make one of steel and the other of Aluminum and load them with the same weight.... the Aluminum bar will deflect about 3 times as much as the Steel bar will.... THIS IS NOT ULTIMATE STRENGTH its just a ratio of "Stress vs Strain" the aluminum bar may break at 1000 lbs and the steel bar may break at 1200 lbs but this does not matter since we are designing the engine NOT TO BREAK......

SO there will be MORE FLEX in an aluminum engine vs a Steel engine if the casting sizes are the same.... Leading to more wear and tear of parts with high tolerances ( bearing caps etc.... ) I do not know the differences in casting thcknesses bewteen the "Al" and "Fe" blocks.... but we can see that they did put more cap bolts on the Aluminum block to keep them from flexing as much......

So as far as ultimate strength....????? But ultimate durability will equte to having less flex in the rotating assembly for sure.....
 

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Discussion Starter #8
LOL!! I guess this was a good question, huh!

Everything everyone has said has already gone through my head! Thus my confusion. :D

A-Train: Thanks for the info on the '03 Cobra block. :thumbsup:

It would be interesting to see what physical property (elasticity, ductile strength, coefficient of thermal expansion, etc.) differences are between the two type engines.

Anyone have any side by side photos of the aluminum vs. iron blocks? I’m interested in if the webs and caps are any bigger in the aluminum blocks. (I can provide photo’s and measurements of the bottom end of a 95 SOHC block if someone has access to a DOHC block)

This is getting deep real quickly, but these type things are what I think about on my 1 hour drive to and from work each day. LOL. (Yeah, I know, I’m weird :D )

So for now, the it sounds like the general consensus is that from a strength standpoint they are probably the same with the iron maybe slightly ahead. But for all the other things the aluminum is better .

Keep it coming everyone. :thumbsup:
 

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DD94...

I don't subscribe to the opinion that aluminum is better....period. It is lighter....period! Just about 3 times lighter....which is consistant with it's density versus iron AND it's modulus of elasticity versus iron. I wasn't going to get into the whole stress over strain thing because it does get deep.

Personally, from a tool AND machine design standpoint....Aluminum is not used for high-precision, high tolerance applications...USUALLY. Of course, you could point out the bearing journals for the cams in our OHC engines...and really, this is also a compromise in function.

Steel (and iron...both ferrite based) and Aluminum expand at different rates because, as I said earlier...aluminum has greater thermal conductivity. The bearing journals in our heads get to thier expanded sizes at operating temp. BEFORE the steel jounals of the cams. Theoretically, the oil pressure would be different at temp than cold (for reasons other than oil viscosity) and possibly could effect bearing life. In this applications it's really not very critical.

In my opinion, when weight isn't a factor at all, Steel and ferrous based alloys are much better overall in all catagories relative to wear, strength, precision, hardness and all that.

A perfect example are land-based turbine engines, such as those used to generate electricity....and flying turbine engines, such as those that power aircraft. They are basically the same types of engines with the same applications, but they use different materials for thier internal components because of the weight factor.

Weight = Cost in aircraft. It doesn't make a difference in land-based turbines. Compressor and turbine blades in aircraft engines are made from titanium where temperture permits. It is over half the weight of steel and is 5 times stronger (OK, for those who insist on technical precision...it's modulus is over 5 times that of steel!). Compressor and turbine blades in land-based tubines are made from steel and nickel....much heavier....but much cheaper also!

Well...I'm sorry to ramble....but I hope you catch my drift...er...opinion...er...***hole!
I prefer ferrous alloys to aluminum alloys for most everything.
Try to tell that to Ford, who is trying to get thier HO Mustang to get 30 MPG on the highway.....

I'm done for now!
 

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rustyul said:
DD94...

. I wasn't going to get into the whole stress over strain thing because it does get deep.

Nah it aint getting to deep, you know that its the basis for all Civil/Structural/ Mechanical.... Engineering. it had to be mentioned... LOL... :)



But I would agree with you the only thing Aluminum is for sure less dense (weight) tolerences issues are big factors when you start looking at rotating body's of mass...



Oh and one other side note..... I enjoy having a REAL conversation about intellignet things here.... It beats a lot of other threads...
 

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I was reading this thread and can't figure something out. He asked if aluminum or iron is stronger. We came to the conclusion that iron is stronger but aluminum is lighter. The thing I can't figure out is, is the block really put under that much stress that it matters? I understand if we were talking about rods or a crank, but the block? The part of the block that I understand is under stress are the cylinder walls. If thats so then why would it matter? Since both engine use iron sleeves. I personaly have never heard of somebody ripping or cracking an aluminum or iron block. So does it really matter in automotive engines other than weight?
 

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Discussion Starter #12
A perfect example are land-based turbine engines, such as those used to generate electricity....and flying turbine engines, such as those that power aircraft. They are basically the same types of engines with the same applications, but they use different materials for thier internal components because of the weight factor.

Weight = Cost in aircraft. It doesn't make a difference in land-based turbines. Compressor and turbine blades in aircraft engines are made from titanium where temperture permits. It is over half the weight of steel and is 5 times stronger (OK, for those who insist on technical precision...it's modulus is over 5 times that of steel!). Compressor and turbine blades in land-based tubines are made from steel and nickel....much heavier....but much cheaper also!

Rustyul: That is an excellent example!

Pooperscooper: That is a good question. I think the crank caps and rod caps probably see the most stress. Yes, the cylinders have to contain an explosion, but the rod caps and the main caps have to handle the huge stresses of everything stopping and doing a 180 within milliseconds. But that is only my opinion. All the blown engines I've see were from first rod failures, then piston pin failures (pulling the pin out of the piston), then main cap failures. That is excluding a broke piston or a valve train failure.

But my original question was about the blocks only. What my main concern was that everyone I hear keeps saying the 6 bolt DOHC block is better that a 4 bolt SOHC block. And I just wasn’t buying it.

Now to throw more fuel on the fire: Isn’t an aluminum block less susceptible to shock failure than an iron block. As an example, take a hammer and whack the side of a iron block. Good chance you’ll crack it if you hit it in the right place. Won’t an aluminum block just “dimple” at the impact point and not crack? I don’t know, that’s why I’m asking. (Well, I know for a fact about the iron block cracking…. Don’t ask. :leftright )

But thanks to everyone that has posted. This is some good info. :thumbsup:
 

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I think everyone is ignoring the fact of resonate frequency. AL being less dense has a lower freq before it starts to resonate. Because of this resonace it induces stress which eventually lead to failure.
 

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www.300below.com

works on the molecular level. Saves money and is used by many in the racing, industral, commercial, and aero industries.

Food for thought.

Regarding the information I stand corrected and always learning something new.

I have only the basics but know where to go to get the job done.

Now for deeper subject the block is actually a MarK VIII block and the cams are Navigator cams.

Any questions on this email Mark M (TCCoA - VP) he is the one who provided thsi information to me.

Bottom line work with what you feel best in your application. Both will do the job depending your application.

The knowledge has been given by many reputable people here on this post.

.:uppoint: :ztoohot: :uppoint:
 

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Well, I prefer the aluminum head-iron block combo... strength in the bottom end and lite weight and heat dissapation up above. I've done some custom machinework on aluminum blocks used for racing and the material is a bit stronger(I can tell this by the way it machines) than the crap aluminum used for bell housings and transmissions, however, especially with a supercharger I would feel safer with the iron block.
 

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I have a DOHC aluminum block sitting right here in my bed room. I just don't have a digital camera so I can't take pics from various angles.
 

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aduty: I think everyone is ignoring the fact of resonate frequency. AL being less dense has a lower freq before it starts to resonate. Because of this resonace it induces stress which eventually lead to failure.


Resonant frequency doesn't really work like that.. i dont think anyway

i know that resonant frequencies are made when vibrations are made through a metal, and when a 'natural resonance' is reached the metal becomes stressed, otherwise not really anything happens, or to a point where it would matter. And since the motor mounts are rubber or liquid filled this dampens the vibrations to a point that the natural frequency can not be reached. I also know that to find the natural frequency it takes calc 4 BLAH!!!!

ok well thats what i think
 

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Ton6y...

Just about everything has a resonant frequency. In some objects it is either too high or too low to affect anything.

An engine block does indeed have a resonant frequency, and it does have a point at which, if left at that frequency, will fatigue the structure of the metal and cause it to deform. I'm not sure I could comprehend the dynamics of the forces that could do that to an engine block, but I assure you, it could happen!

I would theorize that the resonant frequency of just about any engine block would be so high, that the vibration caused by the recipricating mass of the crank and pistons would never cause resonance....even if it were aluminum. Of course, this is only an opinion. As I've said, I have never designed an engine....
 

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Funny this question came up...

I am just finished a course in metalurgy and this was one of the questions we covered. Well this exact question but the whole steel and iron vs. aluminum. The only problem is that all my books are back at school so i cant bring up any numbers.

Anyway here are my general thoughts. Only the absolute highest strength steels can come close to rivaling aluminum for the strentgh to mass ratio. With cast iron it isnt nearly as good. The blocks of the 4.6 are made out of gray cast iron which isnt very strong but has pretty good thermal conductivity.

So for the str. vs. mass aluminum has a big advantage....

Until you figure for fatigue. At which point aluminum really starts to suck. This is why you dont see street engines with aluminum rods. Even the best aluminums fatigue down to crap after 50 million cycles. However with a block this doesnt really matter that much, however if you want to make a massive amount of power for incredibly long periods of time your aluminum block will fail.
 

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Okay almost forgot natural frequency.

Natural frequency refers to how the incoming force hits at exactly the precise moment to coincide with the force of the vibration thus increasing it. So you are all thinking, "What the hell does that mean?"

Think of jumping on a diving board or trampalene. If you want to get really high you dont just jump as often as you can, as that really doesnt accomplish a whole lot. If you want to get really high in the air, you time your jumps to coincide with the diving board or trampalene so you are jumping at the same time it is throwing you up into the air. Now in an engine this depends not on both the timing of the firing event, (your jump) and the thickness and geomentries of the cylinder block (think of this as the spring rate or your diving board or trampalene.). Now ever thing in this world has a natural frequency, the whole challenge to engineers is to make sure that this natural frequency isnt at a range that your engine could be running at. Now i dont know what the natural frequency for the 4.6 blocks are, (rpm) however i would guess it to be at least 10,000 rpms, just to try and avoid any of these problems.

Anyway just some randome thoughts of mine.
 
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