Inconel actually is not that light, it does weight more than titanium, but because it has incredibly high temp characteristics, including holding heat in, a good thing in an exhaust, it can be used in a header using material much much thinner than titanium, and that is why it makes the lightest header available. Less material equals less weight.
To find the weight of the metals displayed, multiply the weight of plain steel by the factor provided in the table.
METAL WEIGHT CONVERSION TABLE
Aluminum
0.3357
ARMCO 18-2 MN
0.9929
Brass (70-30)
1.0883
Brass (80-20)
1.106
Bronze, Commercial (90-10)
1.1237
Bronze, Phosphor (95-5)
1.1307
CARPENTER No. 20 Cb-3
1.0318
CHROMAX
1.0148
Copper, Pure
1.1413
EVERDUR 655
1.0883
Gold (24 Kt.)
2.4664
HASTELLOY Alloy B
1.1802
HASTELLOY Alloy C-276
1.1343
INCOLOY Ailoy 800
1.0141
INCOLOY Alloy 825
1.0389
iNCONEL Ahoy 600
1.0742
INCONEL Alloy 601
1.0283
Iron, Wrought
0.9929
Molybdenum
1.3074
MONEL Alloy 400
1.1343
Nickel 200
1.1343
NICHROME
1.053
NICHROME V
1.0707
Nickel, Pure
1.1343
Platinum
2.7385
Silver, Fine
1.3392
Stainless Steel (18-8)
1.0247
Tantalum
2.1201
Titanium
0.5795
TOPHETM
1.0353
TOPHET 30
1.0353
Tungsten
2.4734