I'm building a JrFuel engine - 4.125" X 3.825", 409 cid early hemi, N/A methanol, filled block, 900 hp.
We put aluminum caps (five, center three splayed 4 bolt) on a previous engine, and the block cracked after just a few passes. The #2 main web is the weak spot on early hemi's, and that is where it cracked.
We had bored out the cam tunnel (55mm as I recall), but that's a long way from the main bearing, and we figured the cracked block was probably due to the aluminum main caps. We had also bored this engine out to 4.125" which is very large for a 354 hemi.
We then bored out the cam tunnel on our original 4.035" bore/steel main caps JrFuel engine, which had been running for four (maybe five) seasons. It ran well for three passes, then started slowing down. After the tenth pass, we pulled the pan, and observed that the #2 main web had cracked in exactly the same place as the aluminum main cap engine.
Bottom line: The aluminum main caps save 10 pounds, and I have one set of aluminum and one set of steel main caps. The block is headed for the machine shop tomorrow, and I need to decide on steel or aluminum caps. If steel, and no issues, we'll never know what would have happened had we utilized aluminum. If aluminum, we'll probably learn, but it will be an expensive lesson.
I don't see any SBC JrFuelers (the only other engine currently running in JrFuel) running aluminum.
At 3.5 pounds/cid, 10 pounds equals about 3 cubic inches, which at 2 hp/cid equals about 6 horsepower.
I'm sure wanting to do it, but scared. lol