Read carefully. Those aren't my heads. They are pictures of DrJay'* heads from his website.

I have just started reading about reversion as per your suggestion. Immediately I noticed that the Edelbrock web site lists the D-shaped exhaust port as a "feature":

3. "D" Shaped Exhaust Port
The "D" shaped exhaust port is designed to provide a larger area for increased flow and velocity as well as to create a mismatch between the exhaust pipe and the exhaust exit. This purpose-built mismatch reduces "power-robbing" reversion by keeping the majority of the exhaust gasses from going back into the engine.

Very interesting. Thanks for the info, Will. I guess for anybody who wants to port S1 heads, opening up the sides and the top a bit is OK but leave the flat bottom of the ports alone? If I understand correctly, this is what you did, right?

DS, please go back and read MY last reply. Particularly the last line. This topic is about Porting Exhaust Manifolds.

I'll also grant you (making us both equally guilty of being off-topic) that I'm not surprised those are Jason'* heads. Not at all. That'* a long story.

Right, sorry about that... back on topic with a couple more questions (you guys must hate me... it'* okay to tell me to shut up, I mean it...)

Before some of you mentioned in passing a relationship between KR and cracked manifolds. Does a cracked manifold cause false KR or something?

And Will, what I'm taking from your first posts is that unless I port my heads, I would probably be hurting performance with PEMs from loss of velocity? If this is true, you saved me a lot of wasted time...

Unless you port your heads beyond the Inside Diameter of the factory weld in the runners, there'* no reason to port.

Cracks don't cause false KR. The knock sensor isn't sensitive enough (generally) to hear it. Again, it'* exhaust gas velocity that is hurt. Slightly by escaping gasses, and partly by 'cold spots' where the crack is. Inefficient flow, decreased velocity.

You want the manifolds to not leak, and to hold heat. Hotter EM'* flow faster.

Wouldn't cracks also cause your O2 to read false lean, causing fuel mixture problems also?

Yup. I kinda left that one out. Cracks can suck in outside air, and send it downstream to the O2 sensor. This will cause the O2 to richen you up a bit. From what I understand (and I may be mistaken here), too rich can cause KR as well to a point, but it'* not as much of an issue as lean.

Rich can hurt performance as well as lean though.