I'd have to find it again, but I seem to recall that they are canted at 14*. I agree, the Pontiac bath tub chamber is terrible and have no idea what the engineers were thinking. I've roughed in all the chamber mod's (as I watch even more compression wash away..) but still need to cc each chamber to balance them out. I'm going to tell the machinist to shave as much as he is comfortable doing. Worse case, if the chambers end up too small I'll grind some more out. I've read numbers from 68cc to 80cc's. Using a 68cc with a stock shim head gasket would put the engine at 9.0:1 compression, but that is not what I've measured.

Here are some flow measurements that the guy that helped develop the port and chamber shape of the good head. Reading, up on the work he did, he started with a '69 1bbl like I am using (it's what I have), and testing 4bbl heads that have about 0.2" longer short side fattened up flow up to about 0.2" lift, balances out around .3" and matches up from .4" on. Which of course the loss of low end flow does hurt, but at least it's a fast ramp cam design.

The longer short side radius helps as long as it raises the port enough, but on this port, it makes for a noticeable cutoff right after mid-lift. By allowing the air a more direct path to the valve, the high lift flow is enhanced. I think of it as using more of the perimeter of the valve, but it may also reduce turbulence like you said.
One trick that I use is to get a spray can of machinist's dye and give it a quick squirt into the intake while on the flow bench. This simulates the wet flow of the fuel and tends to deposit the dye on the back side of the chamber, but it will tell you if you have made any gains on the short side by widening the spray pattern. Using the entire perimeter of the valve is the key, and is why a "long" approach to the valve head works so well IMHO.

Here's what I got on the stock 69 250 1bbl head #9795456
28" plenum pressure, STP corrected
.05 ----28.92----26.02
.10 ----54.09----49.59
.15 ----76.10----69.72
.20 ----99.78----85.96
.25 ---122.14----98.76
.30 ---143.22---103.80
.35 ---153.62---109.29
.40 ---165.69---112.83
.45 ---172.95---116.19

.50 ---176.03---117.87
.55 ---179.05---119.51

'69 1bbl head. No port work, cleaned up behind the valve and chamber work.


The first time I ported a set of heads I swore I'd never do it again. There is a reason professionals are paid good money, as my limitations are quite apparent. Hopefully though there is a net positive result.

Rebuilding an OHC Pontiac 250 with EFI and a Turbo