3991492 Cylinder Heads

[Kevin Whiteley]

Ok, another question. The 327 NOM in my 68 coupe has the 3991492 heads dated September 1971. I searched the archives and learned that these heads were never a production item. However, I found one post which indicated that there were two? variations - one with straight plugs and one with angled plugs.

Looking at a head from the side (either pass or driv), the front two plugs angle towards each other maybe about 15 or 20 degrees and the back two plugs do the same. Using the keyboard, they resemble " / / ". Are these considered the "angled" heads?

And, although I know value is subjective, are these worth anything? In other words, are they so common, that there really isn't a "market". Thanks again for all the help.

[Joe L.]

Re: 3991492 Cylinder Heads

Kevin-----

Yes, the configuration that you describe is the "angled" plug configuration. These cylinder heads were a SERVICE-only item and were never used in PRODUCTION. Given the date on yours, these must have been some of the very first ones released. The finished heads were known as GM PART #3965742.

Since they were not used in PRODUCTION for any engine, certainly no restorers would be looking for them. However, these are an excellent performing cylinder head and must have some value from that perspective. I have a set of the later (circe 1975) angled plug heads based upon this same casting and known as GM PART #336746. I wouldn't part with them for love nor money.

[Michael F.]

Joe why wouldn't you part with them

Joe, why wouldn't you part with your heads??? Are they that better over the stock LT-1 heads??

[Joe L.]

Re: Joe why wouldn't you part with them

Michael-----

Personally, I think they're a lot better than the stock LT-1 (straight plug) heads. I added a set of these to a 350 many years ago. The combustion chamber volume of these heads was the same as the heads I removed (confirmed on both sets at 64 cc, nominal). No other changes were made to the short block and the same thickness head gaskets were used with either set of heads. With the original straight plug heads, the engine "pinged" mercilessly, regardless of what I tried. Even Chevron Custom Supreme gasoline (this was in the old days, of course) and timing retardation would not quiet it. With the angled plug heads, I could run the engine on regular gas with up to 4 degrees more initial advance than stock and the engine wouldn't ping, at all. Also, there was a noticeable increase in power over the original heads.

[Dick W.]

Re: Joe why wouldn't you part with them

When these head first came out, I bought a set and sent them to Joe Mondello to have them ported and cc'd. Installed them on a 350 12.5 cr, Crane roller cam, 2 660 cfm holleys on a tunnel ram. We later bought a 69 Z-28 that had a set of angle plug heads on it. Seemed to run a little better that std heads.

[Michael H.]

Re: Joe why wouldn't you part with them

Dick,

I also had a set of the 1st issue angle plug heads in the early 70's. There was, as you said, a slight power increase across the board. We also saw a power increase when the timing was set back from the typical 40 degrees total to 36-38 degrees. Never was anything incredible but was still an increase.

[Mark #28455]

why not run cast iron bowtie or vortec heads? *NM*

[Dick W.]

Re: Joe why wouldn't you part with them

Michael, on the Z's anything to help until you got your RPM's up. The were lacking on the lower end. But with a little gentle massaging..140 cam, headers, tweak the carb and distributor, massage the ports, look out when you got to 5,000 rpm's and above. But then again, in those days there was no substitute for cubic inches, unless it was a pile of cubic money

[Michael H.]

Re: Joe why wouldn't you part with them

Dick,

Yes, they were flat until 5000 when setup that way and even worse with the factory crossram. With the entire Trans-Am package installed, a 6 cylinder probably made as much torque as the 302 did at 2000 RPM but then, suddenly, the fire within came alive at 5000. The complete 302 Trans-Am motor with crossram made over 465 HP at 7400 RPM. Amazing package. I still have the dyno sheets. If you send me your email address, I'll send the dyno sheet. Very interesting.

Michael