351w heads on 5.0?
Reply #3 –
on the smog head.
the exhaust port is smaller(bump) around the exhaust port roof, next valve guide. so to let air come in from the smog pump.
inturn cause restriction in the exhaust flow.which makes your engine work harder to pump the exhaust out.
none smog heads .
do not have the bump ,so it flows easier.
some people have ported the bump out but that only works on some heads.
like the 1974 to 1980 351cleveland/351M/400- 2v headS .you can not port the smog bump out because ford raise the top part of the exhaust port of the head to be even with the bottom of the valve guide.