Camel hump heads

[Anthony C.]

Hi , does anyone know did the 71 base 350/270 hp come with camel hump heads from the factory. Thank u

[Bill B.]

Re: Camel hump heads

Anthony,

According to the 6th edition of the TIM&JG, the 1971 small block heads were not double hump like the previous year. They had a particular "digital" looking waveform pattern in lieu of the double hump pattern.

According to the TIM&JG, the casting number for the '71 small block heads was 3973487. The 1970 double hump were 3927186.

The other significant difference, IIRC, was the '70 heads had flat gasketed spark plug seats, whereas the '71 heads were tapered seats. The other technical difference, again IIRC, was the '71 and subsequent heads had hardened valve seats to be compatible with the lower leaded gasoline. The leaded gas provided lubrication for the valve seats to minimize valve seat recession - the hardening of the seats mitigated such in the presence of low or no-lead gas.

One of the exceptions to the '71 non-double hump pattern was the "over-the-Chevy parts department counter "492" heads (straight or angle plug versions) which retained the double hump pattern and had the hardened valve seats with tapered spark plug seats. (I have several NOS of these) To the best of my knowledge, these "492" heads were never installed on any factory production Corvette application.

Hope this helps. Joe L. and Duke W. and others may have more definitive info on this topic.

[Duke W.]

Re: Camel hump heads

The BIG change was the increase in combustion chamber volume to about 76 cc in order to reduce compression ratios sufficiently to operate detonation-free on 91 RON unleaded fuel (equivalent to today's 87 PON) per edict from the fourteenth floor. Such fuel was not available in 1971, but the industry knew that catalytic converters would be required to met then proposed 1975 emission standards and leaded fuel would rapidly degrade catalysts. At the time the oil industry said the highest RON unleaded fuel they could produce was 91. Ford followed GM's lead for the 1972 model year, and I think Chrysler too. The '71 base Corvette engine had an advertised CR of 8.5:1 and 9.0:1 for L-82, and I believe the 454 option was 8.25:1.

As with previous engines, as built engines could have CRs up to half a point lower due to deck height increases as the broach tools wore. Also, the spark advance maps were set up to comply with emission standards, not performance or fuel economy.

I don't think induction hardened valve seats were implemented in production until 1973 at the earliest. Joe probably knows the right answer.

I recall the ...492 heads were only sold OTC for "off-road" applications and, as you stated, were never used on a production engine. Since they were intended for non-catalyst application, they wouldn't really need induction hardened valve seats, but I don't know if that ever occurred. Maybe Joe knows.

Duke

[Joe L.]

Re: Camel hump heads

The BIG change was the increase in combustion chamber volume to about 76 cc in order to reduce compression ratios sufficiently to operate detonation-free on 91 RON unleaded fuel (equivalent to today's 87 PON) per edict from the fourteenth floor. Such fuel was not available in 1971, but the industry knew that catalytic converters would be required to met then proposed 1975 emission standards and leaded fuel would rapidly degrade catalysts. At the time the oil industry said the highest RON unleaded fuel they could produce was 91. Ford followed GM's lead for the 1972 model year, and I think Chrysler too. The '71 base Corvette engine had an advertised CR of 8.5:1 and 9.0:1 for L-82, and I believe the 454 option was 8.25:1.

As with previous engines, as built engines could have CRs up to half a point lower due to deck height increases as the broach tools wore. Also, the spark advance maps were set up to comply with emission standards, not performance or fuel economy.

I don't think induction hardened valve seats were implemented in production until 1973 at the earliest. Joe probably knows the right answer.

I recall the ...492 heads were only sold OTC for "off-road" applications and, as you stated, were never used on a production engine. Since they were intended for non-catalyst application, they wouldn't really need induction hardened valve seats, but I don't know if that ever occurred. Maybe Joe knows.

Duke

Duke and Bill------


Yes, the GM casting #3991492 heads DID NOT have induction hardened valve seats for either the straight plug (GM part #3987376) or angled plug (GM part #3965742, GM #336746, etc.). I actually have a set of the GM #336746 I purchased new and used them for years---wonderful heads! In my mind, the best cast iron heads ever for a small block. All of the aforementioned heads have 65 cc, nominal, combustion chambers and are machined for screw-in studs with guide plates.

Small block heads which have induction-hardened exhaust valve seats are very easy to identify. They have a slightly raised seat area.

[Dan L.]

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[Anthony C.]

Re: Camel hump heads

Anthony,I thought so as well ..thank u bill looking at a 71 base for my son and owner claimed it had the double hump heads from factory . Appreciate ur time 👍<br />
<br />
According to the 6th edition of the TIM&amp;JG, the 1971 small block heads were not double hump like the previous year. They had a particular &quot;digital&quot; looking waveform pattern in lieu of the double hump pattern.<br />
<br />
According to the TIM&amp;JG, the casting number for the '71 small block heads was 3973487. The 1970 double hump were 3927186.<br />
<br />
The other significant difference, IIRC, was the '70 heads had flat gasketed spark plug seats, whereas the '71 heads were tapered seats. The other technical difference, again IIRC, was the '71 and subsequent heads had hardened valve seats to be compatible with the lower leaded gasoline. The leaded gas provided lubrication for the valve seats to minimize valve seat recession - the hardening of the seats mitigated such in the presence of low or no-lead gas.<br />
<br />
One of the exceptions to the '71 non-double hump pattern was the &quot;over-the-Chevy parts department counter &quot;492&quot; heads (straight or angle plug versions) which retained the double hump pattern and had the hardened valve seats with tapered spark plug seats. (I have several NOS of these) To the best of my knowledge, these &quot;492&quot; heads were never installed on any factory production Corvette application.<br />
<br />
Hope this helps. Joe L. and Duke W. and others may have more definitive info on this topic.

thank u bill looking at a base 71 for my son and owner claimed it had the double hump heads from factory . I appreciate your time

[Anthony C.]

Re: Camel hump heads

Thank u guys as always I appreciate your Expertise