I always thought Magnesium was the lighter of the two, but on Popular Hotrodding TV this morning they stated that Mag was heavier than AL. Am I wrong? What gives?

You watch to much TV

i know

They musta been breathing to many exhaust fumes - Magnesium is approximatly 2/3 the density of Aluminum - (grams per cubic centimer).

That said, "fully mag" wheels went into disuse because if you scraped a curb, they would ignite. Mag burns white hot (ouch ). It requires special machining procedures.

Mag alloy wheels are not as touchy.

We had a set of true mag wheels for the race car and I will guarantee you they were considerably lighter than the aluminum ones we had.

Specific weights:

Mg 1004-T6 0.066 lbs/cu.in
Al 6061-T6 0.098 lbs/cu.in
Steel 1013 0.287 lbs/cu.in
Ti 6A1 0.160 lbs/cu.in

Mg, in addition to being about 60% of the weight of Al, also has about 60% of the stiffness and strength of Al. Titanium is 60% heavier than Al, but is better than 3x as strong.

well - there's always that unstated condition that goes something like "for equivalent strength..." (for the record, a pound of aluminum is just as heavy as a pound of magnesium ...)

so there's a few factors. density (weight per unit volume) is one of them. Mg is weaker than Al in several important areas we care about, especially in a wheel context (it is more brittle, less ductile, and lots more corrosion-sensitive). so a dimensionally identical Mg wheel will be lighter than an Al wheel - but it will also be weaker. by the time you add enough material to make it an equivalent strength, it's not only heavier, but larger...

the line on Mg engines cases was that they are strong - "for their weight." that's not the same as 'strong.'