I have an 06 Cummins and am debating on getting a hellman intake, while I have the head machined I was wondering if it is possible to have larger valves installed to flow more air and if it will still run fine on the street?
You can install bigger valves, but in all honesty with the plenum off and porting done for a street truck I wouldn't worry about it. It should flow more than enough air. And valve work is when the machining starts to get really expensive.
