6.2 head flow vs 6.5

[scottm]

Is there any truth to the rumor that 6.2 heads have better ports for more air flow, since they were designed for NA operation? If so, wouldn't they be a better choice for a turbo'd application too?

[john8662]

The heads both flow the same CFM of air through the intake and exhaust.

The key differences between the two heads performance wise is:

A: The pre-combustion port diameter (pre cup)

B: The injector angle in the head.

My theory has been that the injector angle change in the head way before the 6.5L Turbo manifold was not just for clearance but because the angle change in the head also increased efficiency in the engine. Engines equipped with the later heads (1990 and on 6.2's) started better and ran cleaner both at idle and under load.

[scottm]

Ok thanks. I take it the 6.5 na engines also use the exact same head?

[scottm]

I knew I saw somewhere that the 6.2 had bigger valves. In 1982 they had 1.96" intake and 1.63" exhaust. It might be 1982 only that had them, idk.. Check the pictures in this article:

http://www.dieselpowermag.com/features/chevy/1006dp_500hp_65l_diesel_race_engine/

[john8662]

Yeah, some Light Duty 6.2's got the larger exhaust valves. How much this makes a difference I don't have any data for (yet).

Early 82 heads have these valves, as well as some scattered light duty engines through 86,

J

[More Power]

GM's experimenting with large valve heads in the early 1980s wasn't all that successful. Larger valves reduced the amount of metal between the valves and in the valve seat area. Less metal contributed to more and larger cracks. GM fiinally standardized valve diameters in the mid-1980s, which was then carried through into 6.5 production.

But, you're right, the larger valve diameter trial was supposed to allow the engine to be more efficient. Truth though, is that it didn't make all that much difference in these low RPM engines.

That said, matching exhaust manifold ports to the heads allows for theoretically lower exhaust backpressures and fewer hotspots to attract heat. That's easy to do if you have a die grinder and a little time.

Jim