Aluminum vs. Iron Block...

[Michael Yount]

I think some of us need to get out more and look around - globally. The majority of engines being produced today (of the 17 or so million new vehicles that will be sold on the planet this year) are all aluminum - block and head. Over the last 30 years, the VAST majority of heads on engines have been aluminum - it's taken the domestic manufacturers a while to catch up, which has been their plight for years. Manufacturers use it because it's inexpensive, easy to manufacture/machine, lightweight, and designed properly is more than capable of standing up to years of use and abuse. As someone mentioned earlier - everything from severely boosted 6000HP top fuel big blocks to 19,000 rpm F1 engines use blocks made of aluminum. It's all about what it was designed for. So, left stock and designed properly, there's no reason to fear an aluminum block anymore than you would fear a properly designed cast iron block. Start doing things to either of them that reach beyond their design envelope, and you can run into problems. And because of the crush of market competition and ever tightening profit margins, manufacturers have consistently over the years built their engines with less 'room' for standing up to power modifications -- from the manufacturer's perspective, it's inefficient and economically disadvantageous.

And I just have to ask - this quote from Sean "Al block should hold out fine because of the cubical design of a V8 block which brings in a natural geometrical stifness". Can you tell us more? It's snowing here in east TN, I'm stuck at home, and I've got a feeling it will be entertaining.

 

[shatner saves]

Generally speaking advantages of using Iron block is more. Aluminum (rather use of Al as an alloy)is there for bulk savings. Metal to metal pure Al's boiling point is almost 5 times less than Iron, therefore it can only be used as an alloy to form engine blocks.

Iron's strength (rather weight) is 7 times greater than Al. This means that a castiron block is more elastic, noiseless and resistant to wear than aluminum , thus from longevity and stength standpoint over time CI is a winner. They are also quieter as they absorb shocks/vibrations more. Al alloy blocks need special pressure casting, rib formation and when it has a stroke equal to or greater than the bore then usually a CI plate is bolted to the bottom end for added strength. All this brings down the weight savings to about half over CI.

But manus make Al alloy blocks because they can be pressure die casted, the process (assembly line process)is cheaper, the final finish is always more refined and smoother, therefore less friction within the moving parts. But almost all al alloy blocks are CI lined for each cylinder. Usually CI blocks have siamesed walls but al alloy blocks can be casted s.t water jackets are formed between the cylinder walls for better heat dissipitation.

For oversquare high revving engines i would take an Al block. It holds out fine. For a stroked or blown motor CI is better , longevity over time. For a square config. like the 05 4.6l motor Al block should hold out fine because of the cubical design of a V8 block which brings in a natural geometrical stifness.

I'm sorry to say that you have been mis-informed because the majority of that post is

The boiling point of aluminum is 2467.0 °C. Iron is 2750 º C. This is not "5 times less", it is almost identical. Boiling point has nothing to do with the application of these materials since you would never see temperatures this high outside of ground zero of a thermo nuclear explosion. Melting point is 660.37 °C for aluminum and 1535 º C for cast iron. This is the reason why you don't generally see exhaust manifolds made from aluminum, but it is still well beyond what you are likely to see in a properly cooled block and heads. Engine oils would boil away long before you ever hit 600 °C and throttle bodies and carburetors also would melt before that since they are generally zinc die castings which melt at around 420 º C.

Aluminum structures have strength to weight and stiffenss to weight ratios that are much higher than comparable steel or iron structures. Since aluminum has a density of about 1/3rd of steel, (.097 lbs/cu. in. compared to .298 lbs/cu. in.), you can use more of it , still get a weight savings and achieve much higher stiffenss/strength. So what if they have to cast ribs in the block.

The "aluminum" they refer to in engine block is not there as a low cost filler or an alloying element. It is the primary element, chosen for it's many positive properties. A356 aluminum (or variants of this), which is very common in engine blocks, transmission casings, diff housings, etc. has the following alloying elements:

Si 7.0%, Cu 0.2%, Mg 0.35%, Mn 0.10%, Fe 0.20%max, Zn 0.1%max

Essentially you have over 92% pure aluminum, 7% silicon and negligible amounts of copper, magnesium, manganese and the rest are impurities.

There are aluminum/zinc alloys that are almost a 50/50 mix, chosen for a good compromise of cost, castability, strength, cosmetic appearance, but you never see them in something like an engine block.

There is nothing remarkable about low or high pressure casting, and I have seen aluminum big block v8's that were sand cast, not die cast. Die casting has it's advantages, but it is not a requirement.

Cast Iron block do not necessarily have siamesed bores, wheras every aluminum block I've ever seen does, to have enough material for the cylinder liners without having ridiculous bore spacing.

GM cast iron small block V8's and big block 454's had non siamesed bores. However, the GM 502 had siamesed bores, to accomodate it's greater displacement. Since there is no coolant flowing between the cylinders in a siamesed block, this causes some amount of warpage in the cylinder, resulting in much higher oil consumtion. I used to work in the engineering deprartement of a well known marine engine manufacturer (who will remain nameless) and it was common knowledge that 502's couldn't go through an 8 or 12 hour endurance run without having to add a quart of oil at least every 2 hours. 454's didn't have this problem and GM's new 8.1L big block does not have siamesed bores.

Since aluminum conducts heat so well, 3.5 times better than Iron, siamesed bores are not an issue.

Also the aluminum big blocks we built did not have "a CI plate is bolted to the bottom end for added strength", they had individual 4 bolt steel mains. The new mustang block is a deep skirt design with cross bolted, 6 bolt mains on at least the middle 3 main bearings. definitely a lot beefier than the 2 bolt mains in my 5.0 block.

Thank you for encouraging me to pull out my old textbooks.

 

[red94fiveo]

I'm sorry to say that you have been mis-informed because the majority of that post is

The boiling point of aluminum is 2467.0 °C. Iron is 2750 º C. This is not "5 times less", it is almost identical. Boiling point has nothing to do with the application of these materials since you would never see temperatures this high outside of ground zero of a thermo nuclear explosion. Melting point is 660.37 °C for aluminum and 1535 º C for cast iron. This is the reason why you don't generally see exhaust manifolds made from aluminum, but it is still well beyond what you are likely to see in a properly cooled block and heads. Engine oils would boil away long before you ever hit 600 °C and throttle bodies and carburetors also would melt before that since they are generally zinc die castings which melt at around 420 º C.

Aluminum structures have strength to weight and stiffenss to weight ratios that are much higher than comparable steel or iron structures. Since aluminum has a density of about 1/3rd of steel, (.097 lbs/cu. in. compared to .298 lbs/cu. in.), you can use more of it , still get a weight savings and achieve much higher stiffenss/strength. So what if they have to cast ribs in the block.

The "aluminum" they refer to in engine block is not there as a low cost filler or an alloying element. It is the primary element, chosen for it's many positive properties. A356 aluminum (or variants of this), which is very common in engine blocks, transmission casings, diff housings, etc. has the following alloying elements:

Si 7.0%, Cu 0.2%, Mg 0.35%, Mn 0.10%, Fe 0.20%max, Zn 0.1%max

Essentially you have over 92% pure aluminum, 7% silicon and negligible amounts of copper, magnesium, manganese and the rest are impurities.

There are aluminum/zinc alloys that are almost a 50/50 mix, chosen for a good compromise of cost, castability, strength, cosmetic appearance, but you never see them in something like an engine block.

There is nothing remarkable about low or high pressure casting, and I have seen aluminum big block v8's that were sand cast, not die cast. Die casting has it's advantages, but it is not a requirement.

Cast Iron block do not necessarily have siamesed bores, wheras every aluminum block I've ever seen does, to have enough material for the cylinder liners without having ridiculous bore spacing.

GM cast iron small block V8's and big block 454's had non siamesed bores. However, the GM 502 had siamesed bores, to accomodate it's greater displacement. Since there is no coolant flowing between the cylinders in a siamesed block, this causes some amount of warpage in the cylinder, resulting in much higher oil consumtion. I used to work in the engineering deprartement of a well known marine engine manufacturer (who will remain nameless) and it was common knowledge that 502's couldn't go through an 8 or 12 hour endurance run without having to add a quart of oil at least every 2 hours. 454's didn't have this problem and GM's new 8.1L big block does not have siamesed bores.

Since aluminum conducts heat so well, 3.5 times better than Iron, siamesed bores are not an issue.

Also the aluminum big blocks we built did not have "a CI plate is bolted to the bottom end for added strength", they had individual 4 bolt steel mains. The new mustang block is a deep skirt design with cross bolted, 6 bolt mains on at least the middle 3 main bearings. definitely a lot beefier than the 2 bolt mains in my 5.0 block.

Thank you for encouraging me to pull out my old textbooks.

What do you do for a living? Just curious...

 

[Michael Yount]

Well said Cap'n Kirk!! All this alloy talk has me pulling my data out.....

 

[Sean`03///Zed4]

Good post Shatner...I was pulling the data out of memory, hence apologize for the boiling melting point mix up.
But metal to metal Iron is more elastic than Al. Elasticity being defined as the ability to regain the original shape and form after removel of deforming forces; Iron is more elastic thus for a given mass compared to Al, it's stronger. You can mix a variety of other elements in varying ratios but it will not reach the strength of Iron.

Thus comparing an Al alloy block of the same design to a CI block, a CI is stronger. Pressure die casting is not necessary to create an Al block. GM uses the lost foam tachnique but the former is a good way to make an Al block stronger to reduce the strength advantage of CI. Another alternative is the Nicasil process. Heck if one is going for Al which has a natural strength disadvantage over Iron might as well do it the right way.

Siamesed walls. Yes most manus have siamesed walls, it's not required to pull off water jackets between the walls, but it's an advantage. Here is another fact almost all high performance CI blocks can get away with the use of non-synthetic oils , but you use a high performnce Al block, most manus advise the use of synthetics.

Skirting the block is not a bad thing but what i am getting at is, a CI block can be designed simpler, Al blocks require more design structuring to achieve a similiar strength to weight ratio. But in today's assembly line design world this cost of producing complex blocks is a one time cost. Another reason many are switching to Aluminum.

Al blocks have to be designed right because corrosion and porosity issues with Al blocks is more over Iron.

This is the chief reason you see blown motors with CI blocks. A NA V8 oversquare can be done good with an Al block because of the natural cubical design achieving the geometrical stiffness. But if you want a 5/5.7 litre V8 to run strong for 16-20 years, giving you the same ground pounding torque, you want a CI block. That's why older high performance engines were CI. With time increase in production costs, emission issues, meeting the supply-demand curve, reduction of weight, better front-weight ratio, mileage issues and all that has prompted switching to Al alloy block. Rarely does one hold on to the same car for a period of 15 years or so,more so with sports cars. Nowadays the average buyer prefers to lease a sports car. If they like it they may buy it, if they don't they move on, perhaps to try something else.

Variety is an essence now; it's the spice of life and longevity is no longer as big a concern as during the times of our forefathers.

In the end it boils down to design/technique, engine config.. An Al inline 6 because of it's natural balance will possibly outlast a cheap designed CI block inline 4.

 

[shatner saves]

What do you do for a living? Just curious...

At this moment I am a fresh air inspector (damn economy). I fill the hours mostly by posting on stangnet, and in my spare time I'm trying to get my Bill shatner cult off the ground.

Always know that shatner saves.

 

[351CJ]

Always know that shatner saves.

Except if you happen to be his last wife.

 

[holler]

btw, the new 2001+ BMW M3's (considered the technological "wünderkind" of german sports sedans) uses an "archaic" CI block...

 

[rhumbline]

btw, the new 2001+ BMW M3's (considered the technological "wünderkind" of german sports sedans) uses an "archaic" CI block...

I know some of the earlier M3s had an iron block, but thought they all (3 Series) went AL now -- could be wrong though.

In any case, even Ferrari used an FE block a couple years back in their Formula One cars, though they've returned to AL (and/or Magnesium?) now.

So while FE has been used time to time in fresh sheet hi-po designs, that's very much the rare exception.

 

[shatner saves]

BMW 3 series cars use aluminum blocks except for the M3, which still uses iron. Even though I6's are the best balanced configurations, the long, skinny blocks tend to have poor torsional rigidity/resonance characteristics when compared to a V8. Since the M engine makes about 100 hp and 1500 rpm (!) more, they decided cast iron was the best solution.

Ferrari wasn't the only ones to use iron blocks, but I think the were the last ones to abandon it for aluminum. An F1 or Champ car is much different from a passenger car because the engine and geabox do double duty be serving as the chassis from the fuel tank back. The engines are rigidly bolted to the rear bulkhead, gearboxes to the engines and all the rear suspension pick up points are on the gear box and bell housing. All the underbody and rear wings are also bolted to these parts and there are no other structural chassis components back there. The exception to this was back in the '70's when Ferrari was using a flat 12 and added a rear subfame to help take the loads.

 

[ryanrule]

just make it outta titanium allow

 

[Michael Yount]

Still remember an article back in the early 80's Car&Driver did on a then-exotic, twin-turbo Nissan (maybe Datsun then) IMSA GTP car. They said that the hubs were made of an exotic alloy - unobtanium.

 

[shatner saves]

I believe F1 teams are using Ti gearbox housings now.

 

[SCORPION]

there has to be something for iron blocks seeing the 2005 concept cobra that was tested by 5.0&FF mag came equiped with the iron block. that same cobra ran the 1/4 in sub 12's on sticky street tires. sounds like a winner to me.

 

[Michael Yount]

This whole thread in my opinion wasn't about the pros or cons of either material for a given application; rather, it was about realizing that when material selection, intended use and design are done well and appropriately, one need not have a concern about it - cast iron or aluminum blocks. As for the Cobra running sub 12's on sticky street tires, I just came from the strip this weekend where a late model Camaro with stock LS6, bolt-on supercharger (pro-charger), Th350 automatic and sticky street tires was running 7.2's in the 1/8th; he's run 10.99 in the 1/4. Aluminum block; 3500 lb. race weight - he spent the afternoon really pissing off all the supercharged Cobras that were present. Either approach can work just fine.

 

[Ray III]

Michael what about dealing with overheating issues? No matter how strong the block is, aluminum still has a pretty damn high thermal expansion rate. It's kind of hard to design a block well enough to defeat physics. Aluminum is great to play with on the track, but I doubt that all the people who have posted on the corral about something in the cooling system breaking and pegging the temp gauge before they noticed it would have gotten away with an aluminum 5.0. I still believe that for endurance applications cast iron is the way to go, and for race applications aluminum is best.

 

[Sean`03///Zed4]

BMW 3 series cars use aluminum blocks except for the M3, which still uses iron. Even though I6's are the best balanced configurations, the long, skinny blocks tend to have poor torsional rigidity/resonance characteristics when compared to a V8. Since the M engine makes about 100 hp and 1500 rpm (!) more, they decided cast iron was the best solution.

Sorry shatner your info on BMW I6 being aluminum for all 3-ers is not true. And the reason the M3 gets a CI block since it's evolution in 95 (atleast in US) is not for the stroker. In fact the first two production M3's 94-95 were non stroker motors, they were technically oversquare CI blocks
BTW upto 99 the M3 were NOT making 100 hp more than the non-M3 3-ers. and they were not redlining 1500 rpmore (where did you read that!)

From 92-95 for all 6-cyl. 3-ers (excl. the M3) the block is CI and it's oversquare I6 (the M50 engine) commonly know as the 325 model.
Motorsport GMbH (M series) increased the stroke of the M50 slightly to make it an almost square engine at 86*85.8mm and thus the M3 retained the CI block of the parent M50 2.5l engine it was enginereed from.

In 96 the M50 motor was abolished for a 2.8l square CI block and it was the E36 328 series (M52 motor). 97 onwards all 3-series (excl the M3) were updated to the M52 Aluminum block.

Why the M3 stayed CI. Because the 97 M3 motor was still based upon the 95 M3 motor (whcih was based upon the original M50 CI motor). ok the 97-99 M3 had the S52 engine CI block which was again a increase in bore and stroke over the S50 95 M3 motor . The 97-99 M3 is an undersquare measuring @86.4*91mm.

The current M3 (S54) has an increase in bore and stroke over the S52 to 87mm and 91 mm respectively. The comp. ratio is increased to 11.3:1. Co-incidentally these figures match the 97-99 Euro spec M3, with the Euro S52 motor producing 321 horses @7400 rpm(86.4mm*91mm @11.3:1), the same time when the same engine was making 240 ponies here in US.

The reason the current US M3 has 333 horses over 240 is because the US spec 97-99 M3 had been de-tuned. The Euro S52 engine made close to 325 hp and for the current E46 M3, Motorsport GmBH released those extra ponies by increasing the redline of the S54, bringing the S54 mtor specs close to the Euro S52 motor.

That's the reason the M3 has always had a CI block and it's not because of it has more stroke over bore.
BTW: From 97- today all 3-ers including the stroker motor of 2.8l M52 and 3.0 l M54 have Al blocks.

Now this information is from my collection of BMW 3-series enthusiasts CD (right from Motorsport GmBH in Munich), so i guess i have to believe in the history of the Inline sixes. And while we are at it, quoting it from the CD
"... all Al alloy block inline sixes have steel liners to prevent excessive wear and together with aluminum suspension components could only help handling and operating economy...the net savings in weight using an Al alloy block over a CI block for the 2.5 litre engine is approximately 70 lbs...."

 

[Michael Yount]

Ray - all I can tell you is that a large majority of engines being produced over say the last 5 years, and the VAST majority of the heads on engines of any type of engine during the last 25 years or so are aluminum alloy. And we're talking extreme numbers here - the industry globally has made anywhere from 13-18 million new cars/engines a year over that period. If you do the math, that's a lot of aluminum. The market will figure these things out better than anything else -- designed properly, there's not a fundamental problem with either choice, it's that simple.

It doesn't do any engine - cast iron components or aluminum - any good to overheat it. And the heads are probably more prone to damage than the blocks - they're what see the really high temps. I've seen my share of damaged CI components from overheating as well - CI is not immune. But in my mind what that speaks to is being certain that the cooling system (including gauges) is in good working order and more importantly the operator pays attention and doesn't overheat the engine. The designers and manufacturers get some credit too - cooling systems are much more reliable and efficient than they used to be. Not to mention the fact that most of the ecu's out there now will shut the engine down if coolant temps get too high. It's not foolproof; but I believe it's certainly safe enough to conclude that those aluminum components are gonna perform as they were intended to.

In my opinion Ray, the reason you see so many posts about cooling system issues on the boards is because, for the most part, you've got young (inexperienced) people modding cars that are 10-20 years old, and in a lot of cases, with original cooling system components in them (including borderline gauges/idiot lights). That's a recipe for disaster. Heck, if I followed the logic of your conclusions on this topic just a little bit further, we'd all still be driving up-draft carburated, cast iron flat heads. C'mon Ray - embrace the 21st century!!

 

[shatner saves]

Sorry shatner your info on BMW I6 being aluminum for all 3-ers is not true. And the reason the M3 gets a CI block since it's evolution in 95 (atleast in US) is not for the stroker. In fact the first two production M3's 94-95 were non stroker motors, they were technically oversquare CI blocks
BTW upto 99 the M3 were NOT making 100 hp more than the non-M3 3-ers. and they were not redlining 1500 rpmore (where did you read that!)

I didn't mention anything about a stroker motor and I didn't mention anything about "up to '99".

The current M3 has a redline of 8000rpm and makes 333 HP. A current 330ci makes 225 hp, a difference of 105 HP. I used the redline from the z4 (6500 rpm) although the 330ci has a redline of 6100 rpm. The engines in these cars have the same displacement, bore and stroke, so I don't understand where the difference comes from. It's possible BMW is just chose to wind it out higher for the sportier car. So you have a difference of 1500-1900 rev's.

The engines in all these cars are *slightly* undersquare:

M3, bore: 87mm, stroke: 91mm
330ci and Z4, bore: 84mm, stroke: 89.6mm

http://www.roadandtrack.com/assets/download/592003155629.pdf
http://www.roadandtrack.com/reviews/roadtests/pdf/2001_01_type_cast.pdf
http://www.roadandtrack.com/reviews/roadtests/pdf/2001_02_bmw_m3.pdf

 

[Sean`03///Zed4]

I didn't mention anything about a stroker motor and I didn't mention anything about "up to '99".

The current M3 has a redline of 8000rpm and makes 333 HP. A current 330ci makes 225 hp, a difference of 105 HP. I used the redline from the z4 (6500 rpm) although the 330ci has a redline of 6100 rpm. The engines in these cars have the same displacement, bore and stroke, so I don't understand where the difference comes from. It's possible BMW is just chose to wind it out higher for the sportier car. So you have a difference of 1500-1900 rev's.

The engines in all these cars are *slightly* undersquare:

M3, bore: 87mm, stroke: 91mm
330ci and Z4, bore: 84mm, stroke: 89.6mm

That may be true if you are comparing the current E46 bigger inline 6(3.0) and M3(3.2) motor. But prior to that (96-99) the bigger inline 2.8 produced 192 hp, the equivalent M3, 240 hp;
Prior to that (92-95) the bigger 2.5 inline produced 185 hp and the 95 M3 236hp.
So the difference has not always been 100 hp and 1500 rpm even after BMw switched to Al blocks for non-M3'ers 97 onwards.

But i think we are making the same points as Michael said. CI or Al, both blocks should work if designed right or blow up if not done right.

For instance till date about 63 S54 M3 2001 motors (with CI block) have been reported to have blown up (like connecting rod failure) @about 6k rpm.
And that's about 2k prior to redline, so engine block material is not the only criteria.