1976 Engine with 1965 Heads - Problem?

[76Stingray]

I bought my '76 Vette 3 years ago at Bloomington Gold and have loved the car ever since. It won't win a lot of races, but I was always surprised as how much zip she does have for a '76. This weekend I found the reason. Somewhere in this car's life, someone installed 1965 Camelback (double hump) heads on it. That has answered a lot of questions for me, and left me with a couple more. Should I be concerned that the heads on this car are from the Leaded gas era, but ever since I've had it I've been using 87 Octain Regular Unleaded? I'm pretty sure the previous owner did also. I replaced the spark plugs two years ago with the ones that are supposed to be on a '76. Should I be using something else? Again, no problems of any kind in three years. I want to keep that streak going. Any suggestions?

 

[JohnZ]

What are the casting numbers and dates on the heads? Heads up through '69 use gasketed 13/16" hex spark plugs, and from '70-up they use tapered-seat (no gasket) 5/8" hex "peanut" spark plugs.

 

[76Stingray]

3782461 - K204 (November 20th of 1964)

 

[Vettehead Mikey]

If your car is running well on 87 octane (ie no 'pinging or preignition) then leave it alone. Most probably the heads are not being used to their best advantage though. No telling without stripping the engine down to measure compression ratio or learning the cam characteristics.

Listen to what John said about the plug type.

 

[Hib Halverson]

Cylinder heads of that vintage typically have smaller combustion chambers than do heads from 1976. In fact, the difference in chamber displacement may be significant. I don't have access to my data base right now, but I'm going to guess it's 64cc for the early heads and 76cc for the mid-70s head.

The smaller chambers make for an increase in compression ratio.

It's interesting to me than the car has been run on 87 octane gas. To be able to do that without detonation, means either the pistons were changed, the heads were modified to increase chamber displacement, multiple or thick head gaskets were used or the spark curve had been significantly modified to reduce or eliminate detonation.

Without your doing some testing or investagative disassembly work, it would be hard to know for sure.

 

[tracksnblades1]

The 461 head was used as far back as 1962 on 283s and 327s. There were two 461s. The 461 and the 461X. The 461 was manufactured with 1.94 and 2.02 intake valves and the 461x with 2.02 intake valves. The 461x were the desired ones in the day. The 461s were suppose to have 160cc intake runners and the 461Xs, 170cc. Actual measuremnts may vary. The 461s' used the above mentioned spark plug type. The spark plug's location was higher in the chamber than the 327 only 462 head that came later. This worked well with domed pistons. The 461s' both had two quench pads the second one located right below the spark plug. The unleaded fuel will eventually claim the factory exhaust seats on your 461s especially if she's a daily driver. If your 76 has the L48 factory dished pistons, I don't think you'll see a whole point gain in compression over the original head. It would be interesting to run it through the many calculators though. If you are a 9:1 L82 you maybe marginal on 87 octane. With the factory .021 steel shim gasket and the excellent quench characteristics of the old 461's it may. This head swap was done often in the past on 8.5:1 350sb with good results. With hard exhaust seats even better. I would never use multiple head gaskets to alter the compression on the sbc. The factory engineered the sbc with the proper quench in mind to suppress detonation and promote the proper flame propagation let alone rocker arm geometry. In the past mix and match sbc heads had some interesting results. Ever wonder the compression ratio of a stock 400sb wearing a set of 283 powerpack heads, of course with the steam holes drilled.

 

[JohnZ]

3782461 - K204 (November 20th of 1964)

You need to change the plugs you're using - the '76-type tapered-seat "peanut" plugs won't seal reliably against the counterbore in those heads that crushes the gaskets on the gasketed plugs. You need AC R45 or R45S plugs, or their heat-range equivalent in a different brand.