Pontiac valve back cut angle

I have been working on some pontiac 670 cylinder heads and was just wondering if there was a magic angle to back cut the valves.
I seem to remember that Grumpy said 22.5 for a small block chevy.
Any one done some flow bench work with these heads I understand that the back cut if done correct will make big improvement on the flow of the heads.
Thanks. James

 

The most common backcut is 15 less than seat angle, 45 would be 30. Typical mid flow improvements are 4 -5 cfm on the intake.

 

I think Pontiac intake valves are cut at 30 degrees rather than 45.

 

Yes intake are 30 and exhaust are 45.

 

use the factory angles and make sure the machinist does not try to use Chevy angles.

http://www.highperformancepontiac.com/tech/hppp_1106_performance_valve_job/viewall.html

 

some places cut the ridge / sharp part off (blend) and at no exact deg*...
some cut the valve (the 45's) at 44*... 29 on the 30's.
i ain't saying who....

 

treb11 thanks for the article but I guess the angle is super secret.

 

the secret is to run them the way the factory cut them , you can back cut the valve behind the seat area to improve exhaust flow out of the cylinder , but do not do a 3 way chevy cut on the seats were the vlave sits it will cut flow as the pontiac chanmber allows the flow to stay close to the chamber roof also blending in the short side radius and the throats will help . and if any material is to be removed its on the top side of the port never the bottom unless you have a flow bench and know what to do . also Ponchos respond to flat valves better than tulips on certain heads so if you have flat valves use them , if the heads have tulips stay with them . for the best info get the H-o specialties book , the superstock copy has port charts to show the differences in the heads and also were to port them .

 

back in the day I had customer that ran Pontiacs. I messed with the heads a lot. we ended up with 45 degree seats on both valves. The problem with 30 degree seats is they build up with carbon real easy. 15 degrees less on the back cut is good. (not on 30 degree seats. I ended up building a set of ramair 5 heads that I cut the top edge of exhaust port back about 1/4 inch to nothing on bottom . Dont remember exactly how much. This really helped the exhaust. Had to use larger size header bolts with helicoils,but it worked. Custom made headers.

 

I was not thinking of changing the valve angle but just a back cut on the valve.
So it sounds like I would not back cut a 30 deg valve ?
Guess thats another tool to add to the shop a flow bench.