Weight!
hehe, I have Will Handzel's book "How to build high performance ls1/ls6 v-8's" and it says the aluminum LS1 is 103 but also says 107 lbs in another spot, and the iron blocks are about 88lbs more. the iron block is more sought after for performance, especially the 6.0L since you can bore it out and it's soooo much stronger, and bigger anyways. Think, it's designed for aluminum, but made out of Iron, it's ridiculously strong. the aluminum block is already strong as it is 
Trending Topics
LS1 Tech Stories
The Best V8 Stories One Small Block at Time
6 Gifts Neither Your Dad Nor Grad Will Shove Into the 'Trinket Drawer'
Brett Foote
Topdon ONE vs. Artidiag 800 BT2: Which is the Diagnostic Tablet For You?
Pouria Savadkouei
Gas Monkey Built a 6-Wheel Ferrari Testarossa With a Corvette LT4 Engine
Verdad Gallardo
7 Most Reliable High-Performance Engines GM Has Ever Built
Verdad Gallardo
Amazing '71 Camaro Restomod Is Modern Muscle Car Under the Skin
Verdad Gallardo
6 Common C5 Corvette Failures and What's Involved In Repairing Them
Pouria Savadkouei
Retro Modern Bandit Pontiac Trans AM Comes With Burt Reynolds' Autograph
Verdad Gallardo
Top 10 Greatest Cadillac V Series Performance Models Ever, Ranked
Pouria Savadkouei
Top 10 Most Powerful Chevy Trucks Ever Made!
i think the alum block BARE w/ out sleeves is 107 pounds. that came out of a similar book to will hanzels'. i forget the name of it. hold on i'll google it. it was EXTREMELY helpful to me when i put my new motor in.