Howdy All:
Since MarkP opened the topic of 30 degree valve seats, it sparked a memory. After considerable searching I found the article. It is in "Hot Rod", Oct 1994. Author, Jeff Smith, did the flow bench testing using a SBC (no surprize) head. He did a series of changes and made all comparisons to the base line. I know that a test on a SBC will be an affront to die-hards, but the information is information and it is well done and can be extropolated to any similiar, wedge type head. It is worth looking up and studying.
Angles used were-
45- 30, 45, & 60.
30- 15, 30, & 60.
Test #1- 1.83" valve with a standard 3 angle 45 degree seat.
Test #2- 1.94" valve with a standard 3 angle 45 degree seat.
Test #3- 1.94" valve with a standard 3 angle 45 degree seat & 30 degree back cut.
Test #4- 1.94" valve with a standard 3 angle 30 degree seat.
Test #5- 1.94" valve with a standard 3 angle 30 degree seat & 22 degree back cut.
Significant flow increases were evident at .100", .150", .200", .250" .300" & .350" above that, flow was about even or slightly less, when compared to Test #1.
Back cutting the valve made a positive difference in both angle tests.
This test revealed significant flow increases at low and mid range lift and a slight loss of flow at higher lifts. The best increases are in the lift range of a stock Ford Six cam. A plus is that each valve sees low & mid range lift twice for every time it sees max lift- once on the way to max and again on the way closing. The stumble is that you must use a larger valve face to be able to cut it to 30 degrees in the same seat. Using the later 1.75" intake valve in an early, 1.69", head with a stock or mild performance cam with a max lift of .400" to .450" sounds like just the ticket. Some one with a C4 in a mostly stock type street cruiser would benefit most.
So, someone who is about to do a valve job on a pre '75 head with 1.69" intake valves, here's your opportunity to test out the effects on our engine.
I have never tried back-cutting on the exhaust valves, mostly because of advice from racers and machinists who should know, but I'm becoming a bit of an iconoclast in my old age. While that may be good advice for an all-out "roundy-round" or "stab-it-n-steer-it" engine, it may be too conservative for our, street uses. And we can use all the help we can get on the exhaust side. I think that it will be imparitive to maintain at least a .100" margin, to allow good heat transfer. Only way to know durability is to try it.
Anyway, it's worth a look up and a study.
Adios, David
Since MarkP opened the topic of 30 degree valve seats, it sparked a memory. After considerable searching I found the article. It is in "Hot Rod", Oct 1994. Author, Jeff Smith, did the flow bench testing using a SBC (no surprize) head. He did a series of changes and made all comparisons to the base line. I know that a test on a SBC will be an affront to die-hards, but the information is information and it is well done and can be extropolated to any similiar, wedge type head. It is worth looking up and studying.
Angles used were-
45- 30, 45, & 60.
30- 15, 30, & 60.
Test #1- 1.83" valve with a standard 3 angle 45 degree seat.
Test #2- 1.94" valve with a standard 3 angle 45 degree seat.
Test #3- 1.94" valve with a standard 3 angle 45 degree seat & 30 degree back cut.
Test #4- 1.94" valve with a standard 3 angle 30 degree seat.
Test #5- 1.94" valve with a standard 3 angle 30 degree seat & 22 degree back cut.
Significant flow increases were evident at .100", .150", .200", .250" .300" & .350" above that, flow was about even or slightly less, when compared to Test #1.
Back cutting the valve made a positive difference in both angle tests.
This test revealed significant flow increases at low and mid range lift and a slight loss of flow at higher lifts. The best increases are in the lift range of a stock Ford Six cam. A plus is that each valve sees low & mid range lift twice for every time it sees max lift- once on the way to max and again on the way closing. The stumble is that you must use a larger valve face to be able to cut it to 30 degrees in the same seat. Using the later 1.75" intake valve in an early, 1.69", head with a stock or mild performance cam with a max lift of .400" to .450" sounds like just the ticket. Some one with a C4 in a mostly stock type street cruiser would benefit most.
So, someone who is about to do a valve job on a pre '75 head with 1.69" intake valves, here's your opportunity to test out the effects on our engine.
I have never tried back-cutting on the exhaust valves, mostly because of advice from racers and machinists who should know, but I'm becoming a bit of an iconoclast in my old age. While that may be good advice for an all-out "roundy-round" or "stab-it-n-steer-it" engine, it may be too conservative for our, street uses. And we can use all the help we can get on the exhaust side. I think that it will be imparitive to maintain at least a .100" margin, to allow good heat transfer. Only way to know durability is to try it.
Anyway, it's worth a look up and a study.
Adios, David
