Aluminum vs. Iron: Apples-to-Apples - Chevelle Tech
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post #1 of 16 (permalink) Old Jul 18th, 17, 9:38 AM Thread Starter
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Aluminum vs. Iron: Apples-to-Apples

Recently had the opportunity to R&R a pair of iron heads and replace with aluminum. The block is an aftermarket iron.

Platform=BBC, 9.8:1(flat-top pistons), mech-roller, 100% pump-gas compatible.

Absolutely not giving ANY brands here, it is NOT about the mfr's! Both head choices are compatible with the sole exception of the material.

I'll call them heads "AL" and "CI", based on the material. Both sets were very close in runner size, not enough to have any major effect in the larger picture.

Here are the flow numbers for both sets.

Heads "AL", flow: 370/280 @ .700"
Heads "CI", flow: 375/270 @ .700"


Flow numbers for both pair throughout the entire lift range were very close. There was NO add'l extensive porting on either heads, some very simple bowl work and identical on both sets!

Unit made 795 HP/750T with the "iron" and 742 HP/725T with the "aluminum". We've been well aware of this difference between the material components for years now. Aside from our own previous testing this had come up in a long discussion with a "very-high-up" (owner) at Dart back when, mid '80's or so.

This is as close to a "true" test as we can report on, really not looking to get into any "deep" debates over this issue, "it is what it is", at least for us.

Thanks, Gary in N.Y.

P.S. Again, I'm NOT giving any head brands due to the fact it would have made absolutely no difference in our results, we really don't care about brands due to our own very highly experienced in-house head porter. We simply wanted both pairs of heads as close to identical as possible.
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post #2 of 16 (permalink) Old Jul 18th, 17, 10:41 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

Gary,
is the difference the heat in the chamber is greater then the aluminum?
I "think" I've read that somewhere yrs ago.
What's the weight diff between the 2?
Thanks,

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post #3 of 16 (permalink) Old Jul 18th, 17, 10:43 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

Yep, sorta matches everything I've read or seen in the last 30 years. The "standard cure" is a bit more compression on the AI heads, but in your case that would be apples and oranges.

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post #4 of 16 (permalink) Old Jul 18th, 17, 10:58 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

Sheer curiosity, what's the bore\stroke on the engine?
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post #5 of 16 (permalink) Old Jul 18th, 17, 11:47 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

Only question I have is are the iron heads aftermarket or OEM?
Even if they're aftermarket there are so many other variables involved this is hardly conclusive based on material differences alone.

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post #6 of 16 (permalink) Old Jul 18th, 17, 9:57 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

That's a pretty big swing right there. How about headflow throughout the curve? Low lift a lot better on iron ones? Chamber design?


Interesting.....got any dyno sheets to see the curve/peaks/after peaks??

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post #7 of 16 (permalink) Old Jul 19th, 17, 5:58 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

Carl, please refrain from attacking others.

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post #8 of 16 (permalink) Old Jul 19th, 17, 6:15 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

TBH,Im not a dragracer,so I favor the iron heads above the aluminums on the street.Theyre cheaper,and they dont mess with my ride height,lol.

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post #9 of 16 (permalink) Old Jul 19th, 17, 7:18 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

They actually sank my car's front end nicely. Damn boat anchors.

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post #10 of 16 (permalink) Old Jul 19th, 17, 10:12 AM
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Re: Aluminum vs. Iron: Apples-to-Apples

For what it's worth, back in the '70's, before everybody started manufacturing aftermarket heads, there was quite a bit of testing done comparing the open chamber Big Block Chevy Aluminum Square Port Heads with the Cast Iron Square Port Heads. Both were over the counter items.

Out of the box, the Iron Heads always made a little more power. Everybody attributed it to the better heat retention of the Iron Head. On a flow bench, the Iron Head's square Exaust Ports flowed better than the Aluminum Head's Round Ports. That could have had something to do with it as well.

I ran both on my APBA Super Stock Race Boats. In that class, you could do absolutely nothing to the ports, just a "competition valve job" as described in the rule book. Honestly, I could not tell much difference in the RPM at top end.

Anecdotal at best. But all all things being equal, I preferred the Iron Heads. There were simply too many problems with the Aluminum Winter Casting Heads to justify the weight saving.

Of course, things are different now. Modern aftermarket heads are light years ahead of those old Chevy Aluminum items.
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post #11 of 16 (permalink) Old Jul 19th, 17, 12:26 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

Yes, i would look at entire flow curve of the head.

It be nice if there was a test where identical cnc programs were run thru two identical castings, one being alum and one being iron, to therefore have identical copies just different material

Also, is there a difference in port surface finish comparing iron to aluminum? Could alter boundary layer flow, much like discussions on burr finishes.

Minor changes in valvetrain alignment/geometry due to thermal growth differences in alum and iron also is something i ponder. Enough to change cam lift and duration a deg or two? I dont know.

But it is interesting. Always heard heat loss in chamber from alum required alum headed deals to have more compression to make same power

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post #12 of 16 (permalink) Old Jul 19th, 17, 12:36 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

Unfortunately without the heads being identical with the exception of material, it isn't quite apples to apples.

I have seen other tests, I believe it was with Dart heads that showed a power increase for the aluminum heads.

I never understood the whole heat retention theory. Does it give a better burn across the chamber? We know cold air/fuel makes more power, and we know lower water temps make more power too.

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post #13 of 16 (permalink) Old Jul 19th, 17, 12:57 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

990 casting vs. 074 casting would be a fair test in my mind...


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post #14 of 16 (permalink) Old Jul 19th, 17, 1:02 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

Quote:
Originally Posted by mr 4 speed View Post
990 casting vs. 074 casting would be a fair test in my mind...
Those are the heads we tested back in the '70's. I doubt anything has changed.

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post #15 of 16 (permalink) Old Jul 19th, 17, 2:01 PM
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Re: Aluminum vs. Iron: Apples-to-Apples

Quote:
Originally Posted by -SS454- View Post
Unfortunately without the heads being identical with the exception of material, it isn't quite apples to apples.

I have seen other tests, I believe it was with Dart heads that showed a power increase for the aluminum heads.

I never understood the whole heat retention theory. Does it give a better burn across the chamber? We know cold air/fuel makes more power, and we know lower water temps make more power too.
As long as its not fuel limited, i would think the more heat in the chamber leads to more pressure upon combustion, and thus more torque. Want cold dense air going in, combust it and turn that heat energy to pressure on the pistons. Heat lost to coolant or surroundings is energy lost.
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