Originally Posted by magnethead

I don't think there is room for bigger valves. It's a very tiny combustion chamber.

edit- I just looked it up. The biggest valves that will fit the 3.9/5.2/318 head is a 1.88 intake and 1.60 exhaust, which is the stock size for a 360, and plus 1 tenth per valve over stock (1.78/1.50).

Read this. The 3.9 is a 318 minus 2 holes. They only got 330 from a 318, you'll be hard pressed to get 300 from 6 holes. http://www.buschis-place.de/mopar_31...erformance.htm

Per Wes Garret:

Originally Posted by Hahns5.2

Looks like that article is discussing LAs, not Magnums. Magnum 5.2 and 5.9 heads are the same and have 1.92" intake valves.It would stand to reason the 3.9 is no different.