During the late 70s and throught the 80s the majority of the SBC heads produced had as much material that was un essential removed from the castings. This was an attempt to lighten the motors to get better gas mileage (less weight to pull around less fuel burned to do it). They are crack proned and not good candidates for porting, because the walls are thinner around the ports leaving less material to remove to shape a port.

 

"Swirl port" was an attempt to "guide" the air/fuel into the chamber in such a fashion that it "swirled" and mixed more thoroughly before being ignited. It actually works very well on 170 HP engines, producing good low-end torque and fuel economy.

But, it limits flow through the intake port severely, so that other performance improvements, including porting, are not very effective.

They didn't come out until the 1988 model year, most 3rd gen 305's with them had 191 castings (and they all had centerbolt valve covers and the different angle intake manifold bolts in the middle), so as long as your '85 hasn't had a head swap sometime in its history, you don't need to worry about them.

But, you need to stay away from them in the junkyard.

 

What kind of material was removed from the castings? Like.. the actual metal used to make the heads, and keep them reinforced? What were the torque/mileage gains using "swirl port" technololgy? Just curious..
But I do have the original heads. I am going to basically rebuild them this winter though.

 

It's not so much that they "removed" material. They simply designed the castings so that there was less cast iron in each head. Some places ended up being so thin that they were prone to crack. Other areas were so thin that you can't remove much or any material to improve the port flow.

There wasn't a significant peak torque increase with the swirl port heads vs. non-swirl port carburated engines. But, the lower RPM torque was significantly higher (I don't have charts in front of me to give you actual values).

 

Ahhh.. now I understand the lighweight casting thingy Thanx for a little bit of a more clear explanation!