Don Terrill’s Speed-Talk

As i read through this discusion the thought occured to me that a cam with a lot higher lift will also most often have more duration at each step of valve lift. That more agressive profile i assume would offset lost flow from a head ported to gain flow up high at the expense of some mid range? I also assume this may most likely be true ONLY in certain circumstances, like restrictive heads that need help, like most 60s stock performance heads?

I asked to have my heads flow tested again, this time with a 37 degree back cut. We will see what happens!

gregsdart wrote: ↑Wed Aug 03, 2022 5:02 pm As i read through this discusion the thought occured to me that a cam with a lot higher lift will also most often have more duration at each step of valve lift. That more agressive profile i assume would offset lost flow from a head ported to gain flow up high at the expense of some mid range? I also assume this may most likely be true ONLY in certain circumstances, like restrictive heads that need help, like most 60s stock performance heads?

I am not an expert, but I have a flow bench, and seem to get some good out of it from time to time. I have fixed a couple of indy heads, both were CNC ported and they backed up just as you described. I attended a Morgan head seminar and his ideas on port velocity seem to ring true. I widened the short side slightly and blended a small ridge left from the CNC porting and it fixed the heads. The velocity around the short turn was very high, over 400 ft. per second. The engine they came off of picked up a tenth, so something got better, no other changes. If they are flowing your heads again and they have a velocity probe have them check it. Also the old string on a metal rod will show the issue as well. At higher lifts a string should stick to the bottom of the port on the short turn, if it does not that is an issue.