Realistic flow differences on BBC heads

[Steve Antonucci]

Hi everyone,

I was recently looking at all the different oval port heads that GM made from
1969 through 1975 and I have some questions. What were the basic changes in
these individual heads? I know that the combustion chambers were tweaked to
find better open chamber designs. What I don't know is if the intake and/or
exhaust port designs were modified to increase flow. Does anyone here have
experience with this subject? I know some of you are ex-racers and I have
always enjoyed the level of detail you bring to these topics.

Here are some example oval port castings that would support this topic:
All share 2.06 intake and 1.72 exhaust valve sizes respectively.

3933148 - 1969 117cc open chamber ( odd one-time application )

3993820 - 1971 113cc open chamber

336781 - 1974 113cc open chamber

353049 - 1973 113cc open chamber

If one wanted to modify these heads, will all of them support the use of 2.19
intake and 1.88 exhaust valves? I'm working on a project and I have a set of
the 3933148 heads. I like these because they use the larger spark plugs style
( pre-1970 ) and would be stealthy to the eye. Others have recommended either
the '781 or '049 heads. Again, are the flow rates of these heads very
different?

I appreciate you help.

Steve

[Clem Z.]

Re: Realistic flow differences on BBC heads

there have been articles on BBC oval port heads and if i remember the best flowing ones are the 049 because they are open chamber and the flow rate can be increased with the instalation of larger valves.

[Mark #28455]

049 or 781 are the way to go

You can add the 2.19 and 1.88 valves (some are using the 2.25 intake). Then you open up the port throat to match the valve. When that is done, you intentionally leave in a venturi to help the flow. In addition, I found a little benefit from using a .100 longer than stock exhaust valve and advancing the valve into the chamber, thereby lengthening the short side radius a little, but I'm not sure the gain was worth the hassle.
Mark

[Steve Antonucci]

Re: 049 or 781 are the way to go

Clem & Mark,

This supports what I have learned. However, were there any casting differences
to the design of the intake or exhaust runners ( length, shape, volume ) of
these two heads? Could there be any differences due to water jacket layout,
conbustion chamber, etc.?

Steve

[Clem Z.]

Re: 049 or 781 are the way to go

i have no idea as i never used either type

[Joe L.]

Re: 049 or 781 are the way to go

Steve-----

The REAL "way to go" for oval ports is the '391" casting (i.e GM #12363391) and for rectangular ports the '401' casting (i.e GM #12363401). These are the current-production big block STREET heads. They're "the best ones yet".

[Mark #28455]

the alum heads Joe mentions are nice

but are obviously not iron. It's all about function vs originality. Externally, the 781 vs 049 are pretty much identical. I'm sure there has to be some difference in the casting somewhere or they wouldn't have assigned a new casting number - I just haven't figured it out yet. The 049 and 781 heads use the small 5/8" spark plugs, have a temp sender boss, and have a small rectangular casting mark on the ends about 1/4" wide and 3/4" high. Also some of the accessory mounting bolt holes on the ends may be different size than your original ones. You could probably have them pass judging on a 1970 or later (although I don't think the casting mark on the ends of the heads appeared until about 1971?). Hopefully I will get all of my parts out of storage in the next month or two and could give you more info at that time.
Mark

[Steve Antonucci]

Re: 049 or 781 are the way to go

Joe,

Just for argument sake, what would a set of these '391 heads cost approx.?

Steve

[Joe L.]

Re: the alum heads Joe mentions are nice

Mark------

Yes, the heads I mentioned are not going to pass for original heads, either the cast iron variety or the alumunum variety. If someone is most concerned about exact original external configuration, these are not the heads to use. However, if someone is concerned about performance and doesn't mind some difference in configuration, then the ones I mentioned are the ones to get. It just depends on what one wants.

[Joe L.]

Re: 049 or 781 are the way to go

Steve-----

BARE and from a discounting dealer about $1450/set.