1-3/4" headers on 964 ?

[billjam]

What is the usual way of handling 1-3/4" headers on 964 ?

Looking at the exhaust ports, it seems that there is very little room for port enlargement due to the gasket ring groove. Is this groove normally welded up when bigger headers are fitted, or is it OK to live with a narrow (about 1mm) land once the port has been taken out to 1-3/4" ?

[Tyson Schmidt]

You also have to remove the ceramic port liner material.

You should be fine with the thin ring. You don't need to remove much to port match to the 1 3/4 headers. You're going from roughly a 1 5/8ths port to a 1 3/4 port. That's an 1/8th inch diameter change, so you only would be removing 1/16th of an inch from the edge.

[billjam]

Originally Posted by Tyson Schmidt

You also have to remove the ceramic port liner material.

Thanks Tyson,
I wasn't aware that the exhaust ports were ceramic lined. They were all coated in carbon when I sent them off to the head shop for new guides.
I assume it's a fairly thin coating (like paint?) so I can munch through it with a HSS burr.

EDIT: Just been reading up on this. Found the following info in Bruce Anderson's book ...
"In addition to adding the second spark plug, the heads had been changed by increasing the cooling surfaces by 17 percent and casting in a ceramic port liner in the exhaust port to reduce the heat transfer and improve the heat flow. ... The ceramic liners reduce the temperature in the cylinder heads by approximately 104 deg F (40 deg C) in that portion of the cylinder head."

I guess that puts paid to any serious porting of the exhaust port!

[175K911]

You can still do some cleanup around the valve pockets, and use a 993 valve which has an 8mm stem vs 9mm for the 964 which allows a bit better air flow. And you can gain a bit by cleaning up the intake ports some too. But doing any of this is best left to the experts who know where to remove material and where to leave it. My heads will be going to Steve Weiner for his magic touch.