73 Heads and distributor

[greg meiers]

I found a set of 1970 454 heads-open chamber,marked PASS- they are not the correct date of course,but in a quest to build a car that at least appears to be original and a daily driver never to be sold but billed to be otherwise correct, would these heads fit the bill??? my car was an orginal 454 with air,smog,ps and i want it to at least appear that way again. currently, it has a built 350. Besides the engine the car is all there in excellent shape. Also, in the course of rebuilding my distributor the replacement parts call for a shaft with high performance or exc.high porformance. since that year had only one 454 to the best of my knowledge(which is limited)it should only have one part number as a replacement. the prices appear to be the same sooo i just want the correct one any help appreciated!!!!!!

[Joe L.]

Re: 73 Heads and distributor

Greg-----

You didn't mention what the casting number was for the 1970 heads which you are looking at. However, I'm going to assume that they are the 3964290 castings and that is what I am going to base my reply on. Your 1973 Corvette LS-4 originally was equipped with casting number 353049.

Both of the above cylinder heads are oval intake port, so the 1973 intake manifold will work without problems. Also, both heads have a very similar configuration and appearance. Both have the same valve size---2.06" intake/1.72" exhaust. The 70 heads may be configured for the tapered seat spark plug or the gasketted type plug. Most likely, though, they will be tapered seat. All 1973 heads were tapered seat. In any event, it doesn't matter since you can use the 1970 heads with the appropriate spark plugs.

The major difference between these two heads is the combustion chamber size. The 70 heads will have a combustion chamber volume of, nominally, 101 cc. The original 73 heads have a combustion chamber volume of, nominally, 113 cc. So, the use of the 1970 heads will raise your compression ratio significantly. However, since the 1970 engines which used the 3964290 cylinder heads and the 1973 engines which used the 353049 castings used pistons with a similar dome configuration, the heads should work without piston interference problems. Without actually calculating it, I expect that your compression ratio would be raised from the stock ratio of 8.5:1 to the 1970 compression ratio of 10.25:1.

As far as the distributor main shaft goes, I don't recommend using either of the two available mainshafts. As I have said many times before, the only difference in the point type tach drive mainshafts is the "auto cam" which is brazed to the top of the shaft. This "auto cam" is one of the factors in determining the centrifugal advance characteristics of the distributor(the others are the weights and springs). Originally, there was a large number of mainshafts available due to the large number of different "auto cams" which GM used. GM did not use all of these different "auto cams" because two different ones, as are currently available, would have worked just as well to service every engine they made.

I recommend that you take your stock mainshaft and send it to Hi-Tech Innovations. They will remove your "auto cam" from your shaft and re-braze it to a new shaft. Then you will be able to keep your stock advance characteristics. Hi-Tech can be reached at 1-727-942-4003.