[steve25]

The OP had asked if it's worth converting his #15 castings over to 2.11" and 1.77" valve sizes and here are the results from my doing that to my # 15 castings over the week end.

Here's some things to note about the stock set up.

1) the Intake valve seat OD is only 1.876" so there is some air flow left to be found even with the stock valve size by going out to a OD of 1.945".

2) The stock Exh seat OD is 1.620" and this is way small and could be taken out to 1.645" and in doing so you could even add a small 60 degree bottom cut.

3) The issue with doing either mod will be that you will sink each valve by some .020" which will drop off some compression.

Here's the stock Intake flow numbers and in the second column the numbers for the 2.11" valve with a quick 3 angle valve job ( 30/45/60 ) done to get it into the head, and the third column is the air flow gain or loss.

Flow@28" with stock 30 degree seat .

.050". 41.4. 42.6= +1.2
.100". 75.9. 87.5=+11.6
.200". 139. 149 =+10
.300". 173. 174.8=+1.8
.400". 180. 179.5=-.5
.450". 181.3. 181=-.3
.500" 183.8. 185.8=+2

Now here's the stock Exh flow numbers for the 1.66" valve and then for the 1.77" valve with a 60 degree bottom cut added.
The column are the same as above.

.050". 22.4. 26.3=+3.9
.100". 43.8 55.2=+11.4
.200". 89.9. 112.4=+22.5
.300". 120.2. 147.6=+27.4
.400". 149.5. 170=+20.5
.450". 156.4. 176=+19.6
.500". 161.3. 180=+18.6
.550". 166.6. 185=+18.4
.600". 167.2. 189=+21.8


Average flow gain with the 1.77" valve up to .550" lift is 17.8 cfm.

Exh to Intake ratio now with the 2.12" / 1.77" combo up to .550" lift is 88.9% , so this set up in no way needs a duel pattern Cam if your running even half way good Headers!

The added air flow from stepping up to a 2.11" valve should be worth 3 to 4 hp per cylinder.