Don Terrill’s Speed-Talk

Any of you guys do any head porting on diesels? I just did a valve job and mild bowl porting on a 12 valve Cummins head, I came up with a about a 15% improvement on the intake side and a 3% improvement on the exhaust. Didn't spend a lot of time, and I read that you need to cut off the intake to get the two end cylinders anyway. On gas engines I shoot for 70% exhaust flow compared to the intake flow, this diesel comes in around 95 percent. Looking at the valve sizes and the fact that its a turbo engine, this makes sense. I am fairly pleased with the results, What kind of experience do you guys have that port these heads?

Now bolt up the Exh Manifold and check what the Exh flows!

Recently I did a flow test on head on a gas motors Exh side , all I did was clean up the normal carbon and rust scale that takes place and picked up a total of 16 cfm between .200" lift and .450" lift, this was with only a 80 grit sanding roll and the port is no more then 3 inches long overall.

Here's the before and after shots and as you can see it's no where near fully cleaned up and polished, in fact I did not even really even roll over the edge of the unused air injection boss!

Just think of what flow is lost in the Exh Manifold with the far greater wall area that they have!