Testing "Identical" Alum vs Iron heads article

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BradH
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Testing "Identical" Alum vs Iron heads article

Post by BradH »

I'd been meaning to post something after having come across the subject Car Craft magazine article online recently. For those that haven't seen it -- and it's actually from 2009 --, the magazine attempted to compare two Dart SBC heads of almost identical configuration, other than one is iron and the other aluminum:

http://www.hotrod.com/articles/ccrp-060 ... eads-test/

One of their intents was to prove/disprove the belief that an engine with aluminum heads NEEDS additional compression ratio vs the same build using iron heads to compenstate for aluminum dissipating heat more quickly, which results in an aluminum head not creating as much power. They also did a couple of other comparisons, as well, such seeing if the different materials required different ignition timing settings.

They do state near the end that they did NOT run any test that would prove/disprove the hypothesis that you CAN run more compression w/ aluminum heads than iron, all other factors being the same. But the results of their testing certainly seems to debunk the idea that you NEED more compression to make the same power. I know engine builders who still subscribe to the "aluminum needs additional compression ratio..." belief. However, if the testing done in this article is legit, that hypothesis seems to be invalid.

Therefore, I'm interested in what others' thoughts are w/ respect to this article's findings. Do you think the testing approach generated accurate results? Do you see flaws in their methodology that makes you question their findings? Do you have any first-hand experience that either supports or contradicts their conclusions? Thanks - Brad
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Re: Testing "Identical" Alum vs Iron heads article

Post by GARY C »

I always questioned the theory due to the fact that compression heat happens instantly and best results from Dyno, Drag Racing and NASCAR qualifying is done with hot oil and cold water.

I could see the possibility of running more compression with alum in street trim for cruise and part throttle detonation resistance.
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Re: Testing "Identical" Alum vs Iron heads article

Post by JoePorting »

I think it's an old theory from the 70's. I really doubt it matters one way or the other. At most, any marginally difference will be within the margin of error.
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Re: Testing "Identical" Alum vs Iron heads article

Post by Walter R. Malik »

I think the octane requirement of the gasoline is less with aluminum heads but, when detonation is not a factor, there is not much difference at all.
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Re: Testing "Identical" Alum vs Iron heads article

Post by Newold1 »

Obviously depends on the engine and the build on the engine and the intended use of the engine as well fuel octane to be used. I think back in the early days the difference in cost of OEM iron heads most likely already on the engine and the much higher cost of aluminum heads and limited aluminum head variants more "cool and exotic" in many cases made the choice of aluminum a more critical decision. Ignition systems were much more primitive as well as cooling systems and such. as

If you are comparing something today like for a BBC let's say the EQ 320 iron heads and a decent Flo-tec 320 aluminum head is probably almost dead equal in nominal compression of let's say 10.0 -12.5 to one with good fuels. The newer head offerings tend to have optimized combustion chambers and greater port and air flow performance. The other problem today is that new iron heads are limited offerings and may not fit as many applications as the aluminum variants available do. A great performing cylinder head is so important in many builds to a point that the improved aluminum heads available are almost a must.

If the engine is being used for towing and high torque loads the compression ratios depending on fuel type need to be lowered to that 8.5 to 9.5 to 1 with an iron head to keep combustion chamber temperatures in ranges that make detonation less of an issue.

In racing applications the bigger importance of aluminum heads over iron heads is on bigger v-8 engines is WEIGHT savings and ease of modifications in my opinion. IN the BBC's, BBF and big block Mopars it's almost a 75-100 lb weight reduction. The aluminum heads are easier on porting, CNCing and welding is possible for mods and repairs, which in iron heads welding and brazing are very difficult and expensive. If the engine is going to use higher compression ratios above 12.5 to 1 then aluminum is almost a must to control detonation.

The big differential in cost is very quickly disappearing iron versus aluminum so the old affordability is not a big issue in mild to moderate head purchases or rework versus new.

Unless you have a current set of iron heads in great shape that meet your performance requirements or you are restricted by class racing rules -GO ALUMINUM, you'll usually be glad you did. JMHO
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Re: Testing "Identical" Alum vs Iron heads article

Post by PackardV8 »

Obviously depends on the engine and the build on the engine and the intended use of the engine as well fuel octane to be used.
Very obviously. The classic example is when, in 1970, Harley-Davidson built an all-iron OHV XR750. It produced plenty of power and they were ready to win. When taken to Daytona, sustained WOT on the high banking, the iron heads were getting more heat input than the material could dissipate. The tune that made the power also made them blow.

There was nothing possible to make the iron heads both competitive and durable. Two years later, the answer was a clean sheet all-aluminum engine. (Of course, with H-D, there's no real clean sheet, when the outline of a 45-degree pushrod V-twin is already there.)
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Re: Testing "Identical" Alum vs Iron heads article

Post by MadBill »

Would that be a bad case of screen burn-in? :lol:
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Re: Testing "Identical" Alum vs Iron heads article

Post by BILL-C »

If someone just has to know the answer, i can run identicle cnc porting program in a pair of iron DART 200 sbc heads and a pair of aluminum version for a very reasonable cost.
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Re: Testing "Identical" Alum vs Iron heads article

Post by MadBill »

I'd say the definitive program would be to set both heads up for 12.0:1 CR and run most of the tests on 87 octane, first with conservative common advance that would run knock-free on both, then with each at knock-limited or max power advance, whichever came first. Depending on results, a final high octane run off might be merited.
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Re: Testing "Identical" Alum vs Iron heads article

Post by PackardV8 »

Yes, we'd learn something from that, but mostly how it works for short dyno pulls. Those iron H-Ds previously mentioned could live through heat races on short tracks, but would not survive on Daytona.

I build obsolete engines for which there's no aluminum available. Dyno-tuned, cool fuel, cool air, cool water, they can work very well. Put that same tune in a 220-degree engine compartment and in a long uphill climb on a 100+degree day, it's gonna ping.

Blocked exhaust heat risers, fuel pump bypass return line, outside intake air and instructions to downshift that sucker! all help iron live, but ultimately it will have lower limits than aluminum.
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Re: Testing "Identical" Alum vs Iron heads article

Post by groberts101 »

Last 2 posts pretty much sums it up, IMHO. That video magazine test is not going to give the same final picture as a motor contained in an engine bay with 150-200° air blowing and swirling around the intake manifold. Add hot air entering the air cleaner and just adds insult to injury.

The other thing those tests never suss out is the actual loading and acceleration rates of an engine trying to get the vehicle moving along. Since WOT peak power numbers are usually the main point of such test they never test a fully heat saturated engine over and over again with pulls down into the high idle ranges 1,800-2,000 rpm. That's where most engines ping trying to get the vehicle moving along. Taller gears only exacerbate the issue.

IOW's heat saturated engine lugging is where the bulk of the differences between iron and aluminum becomes audibly obvious. So it makes the most sense to actually test the differences between the two materials in that same manner.
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Re: Testing "Identical" Alum vs Iron heads article

Post by Roundybout »

Would the BSFC or BMEP be different between aluminum and iron heads?

My thoughts were aluminum dissipates and conducts heat quicker making less power under identical conditions compared to an iron head. You can compensate for that with more compression, timing, fueling ect. with aluminum. Now that the engines are not equal does that compensation give the edge powerwise to the aluminum head? And a worse bsfc?
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