[Jay S]

I have been intrigued by this topic too. Looked for a pattern over the years and it is hard to follow. It seems largely to do with the combustion chamber shape of the heads, there may be multiple reasons. Not sure if the inclination angle was a driver, but it certainly effected the shape of the chamber.

I recall Steve25 flow testing the chamfer on a head maybe last year and it pickup up cfm at .100” lift and below, but IRC didn’t show much up at higher lifts. The later heads had narrower chambers and did away with the chamfers. My 2 bbl and 4 bbl 71 455 blocks I have do not have chamfers. Being that I haven’t seen every block made, I don’t know which blocks had bore chamfers and which didn’t. But the 68-70 blocks have the bore chamfer the same as the photo below with the chamfers higher. It seems largely do to the width of the chamber shape in the head. After 71 the chamfers became less and less common. Sometime in the mid 70s the 350s received chamfer on the exhaust only, which is unique, all the other engines had them in the intake or both. I thought I had a handle on what they did until I noticed exhaust only chamfers. It seems more like the chamfers are to unshroud certain combustion chamber shapes.

Here are some photos. If you notice these are both 1967 blocks xxxx133 castings and one block has the chamfered much lower. Both on the intakes.

The oily photo is from a 75 350, exhaust chamfers only with no valve reliefs. The other 2 are 67 400s.

[MarkS57]

Originally Posted by Jay S

<snip>

The oily photo is from a 75 350, exhaust chamfers only with no valve reliefs <snip>

Interesting, didn't all 350 / 400 / 455 have 2.11 intake valves 75 up?