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very hard question to answer..lets say you save 75 pounds. So if total motor weight is 950, thats about 9%
If the motor is pulling 1000 hp you may lose 1.5 % in the aluminium block ALL things being equal. (at least thats my best guess from dynoing for 25 years) so you save 75 pounds and lose 15 HP... power to weight goes from 1.05 hp/lb to 1.125 hp/lb with alum. So in this case you have a net gain with the aluminium block. Now given lets say 100 octane you can bump compression with alum a bit over cast...soooo...same power..bla bla bla ie aluminium block is a good thing....:D
 

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I never checked the weight on my 9.800 Dart aluminum block, but Dart says it is approximately 140 pounds. What does a cast iron block weigh? Then if you compare aluminum vs. cast iron heads, the total package should be alot more than 75 pounds less, don't you think???
 

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I never checked the weight on my 9.800 Dart aluminum block, but Dart says it is approximately 140 pounds. What does a cast iron block weigh? Then if you compare aluminum vs. cast iron heads, the total package should be alot more than 75 pounds less, don't you think???
Well according to Dart the Big M with a standard 9.8 deck height and 4.5" bore weighs 260lbs so there's 120lb savings right there.

Depending what heads you compare to, factory iron 67lbs (if my memory is right) vs as cast aluminum like a Dart Pro1 29lbs or say Dart 320 37lbs the heads alone almost always save 60-65lbs per pair just by themselves.
 
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