Numbers Question

[Bill E.]

Hello:

The base engine in my '74 was modified when the car was restored about 14 years ago. Although I can not verify the internal work which is said to include oversized domed pistons, double roller timing chain and porting and polishing, I am trying to document what I can see.

Here's what I found along with an Edelbrock intake manifold and a great sounding exhaust: the cylinder head casting number (seen thru the oil fill cap) is 3991492. Above that is "P133" I think, its a little hard to read. Is this an LT1 head with 2.02" intake and 1.60" exhaust valves or something else? Also, the exhaust manifolds numbers are 3797901/02. I think these are for a 62-63 high performance 327 ci engine with a 2 1/2" outlet. Does this make sense?

Being a recent tranfer from British Sports cars, I have not modified a SB Chevy engine. Are the above items normal for pumping up a base engine? Any comments would be appreciated. Thanks, Bill

[Joe L.]

Re: Numbers Question

Bill-----

GM #3991492 casting cylinder heads are SERVICE-only small block cylinder heads with 2.02/1.60 valve size. They have 64 cc, nominal, combustion chamber volume and are similar to the 1970 LT-1 cylinder heads.

Your exhaust manifolds are, indeed, the 62-65 Corvette SHP manifolds. They were never originally used on any Corvette after 1965.

All 1974 Corvettes, except L-48 with THM-400 and federal emissions, were originally equipped with AIR. The change of the manifolds may have been partly done to "rid" the engine of the AIR-fitting manifolds as well as gain "extra performance" from the larger outlets (little, if any, extra performance is actually gained, though).

The above items are often used as part of "performance-enhancing" engine mods.

[Bill E.]

Re: Numbers Question

Thanks Joe, I should have mentioned that the car is a non-california with turbo hydra-matic without the air pump so the manifold change must have been just for performance, I assume the larger exhaust. What do you think of the dome piston claim, is that just BS? Bill

[Joe L.]

Re: Numbers Question

Bill----

With domed pistons and the heads you have, you're going to be talking about comprssion ratio in the 11+:1 range. If the engine runs well on pump premium, I sort of doubt the domed piston claim. The installed camshaft can have an effect on this, though. Generally, you don't want to have domed pistons. Flat tops with these heads will produce about 10:1 and that's probably as high as you want to go. So, in this case, I'd say it's better if the domed piston claim is BS.

[Bill E.]

Re: Numbers Question

Joe, for what's it worth, it runs fine on premium but doen't like regular. I guess the only way I'll ever know for sure what's inside the engine is to tear it apart. That's one alternative I can avoid, Thanks for your comments. Bill

[Bill E.]

Re: Numbers Question

Joe, one last question on this issue, is the SERVICE-only head #3991492 the correct replacement head for an LT-1? Thanks, Bill

[Joe L.]

Re: Numbers Question

Bill----

First of all, GM #3991492 is a casting number and not a finished part number. The current finished part number for the cylinder head using the 3991492 casting is GM #12480092. That cylinder head is the current SERVICE replacement cylinder head for 1970 LT-1 engines (Corvette and Camaro). It is NOT the SERVICE replacement head for 1971-72 LT-1s. The 71-72 LT-1 SERVICE cylinder head is GM PART #464045 and CASTING #462624.