C-3 3919840 heads

[Michael Ketterer]

Hello forum,
I have a set of 3919840 heads all books say that they have 1.88 exhaust while the intake is 2.19
But this set of heads 3919840 with 1.72 exhaust and 2.19 intake
has anyone heard of this before?
And the archives had no info on this

Thanks Michael

[Wayne M.]

Re: C-3 3919840 heads

From "how-to-Hotrod BB Chevs", Intake valves in the closed chamber BB square port heads are 2.19". Exhausts are 1.72" in all of the cast iron closed-chamber square port heads except for the 1970-71 450hp 454 engines, which have 1.88".

Exhausts in the closed chamber aluminum heads used on 1st design L88 engines (ie. 392's and 842's) are 1.84" in diameter. I have a set of early 842's and have confirmed that they are 1.84". The 6074 2nd design L88 aluminum heads are also 1.88" exhaust.

A. Colvin says 1.72" on 842's and 392's.

[Michael Ketterer]

Re: C-3 3919840 heads

Thanks,
Will the 1.88 vs 1.72 flow better in the 3919840 head?

[Wayne M.]

Duke Williams may respond to flow; I can't *NM*

[Jack H.]

Re: C-3 3919840 heads

Typically, a cylinder head's raw casting number tells you very little about how it was finally 'dressed' in production (or after going through a rebuild). That's one of the reasons there's a difference between casting numbers and part numbers...

The final part number (doesn't appear anywhere) comprehends the 'secondary' operations done after the base part was cast. These include, drilling, tapping, milling and 'stuffing' the casting with internal/external components.

Quite a few of the cylinder heads used on Corvette were also used in either lower HP Corvette engines and/or in Chevy passenger car engines where the most common carb was a 2-Bbl and max flow rate wasn't needed/wanted resulting in smaller intake/exhaust valves being installed.

[Duke W.]

Re: C-3 3919840 heads

Briefly, the 1.72" valve is very restrictive on the rectangular port heads and the E/I ratio is only about 0.60, so a "single pattern cam" like the SHP cam is not a good match. These heads either need more exhaust flow or more exhaust lobe duration.

Pocket porting/port matching these heads and opening up the exhaust valve seat to accept the 1.88" valve is definitely the way to go, but it's a good idea to sonic test the valve seat area to make sure there is enough material, and in most cases there should be unless the cores were out of alignment.

With the above done the E/I ratio improves to about 0.75 and the single pattern SHP cam is spot on.

Duke

[Michael Ketterer]

Re: C-3 3919840 heads

Thanks
Jack and Duke,
The heads were sent down to Barns and Reese for the seat cuttings.
Wanted to make sure thats not a mistake, on this set of heads before morning,
thanks to the forum and its great members
Michael

[Joe L.]

Re: C-3 3919840 heads

Michael---

The GM #3919840 cylinder heads were originally fitted with intake valves GM #3864808. This valve is 2.19" OD. They were also fitted with exhaust valve GM #3860002. This valve is 1.72" OD. No exceptions that I am aware of as far as a stock GM head is concerned. As far as later field modifications are concerned, who knows?

[Joe L.]

Re: C-3 3919840 heads

Wayne-----

Yes, Colvin's books are incorrect on the exhaust valve size for the GM #3909392 and 3919842 cylinder heads. Both used 1.84" exhaust valves. These valves were known as GM #3879619. As far as I know, the only use of these valves was for these 2 cylinder heads.