I might have missed something on this but know the reason for porting headers and there is lots of thumbs up for doing this job on an Impreza.
My question is, having removed the entire front exhaust system I find that the head exit port is practically the same as the one in the header. The original gasket is bigger that the head port and the manifold port and the rim of gas mark appears to be the same on both. The walls of the manifolds appear quite thin. Are early manifolds different or STI heads different.
I have Pics but can't post.
Any comments greatly received.

 

pretty sure that all the manifolds are the same

 

The Headers are normally taken out to the fire ring on the gasket.
Do the same on the link pipe from header to header,right hand header to turbo
up-pipe and do the same on the up-pipe to turbo flange.
Some people remove the exhaust housing of the Turbo and port that to match the up-pipe gasket.
The quicker you get the exhaust gas out the heads to the turbo - the better.
Its a very messy,time consuming job to do though.

 

Exactly what this guy said!

One other point is that you will create an anti reversion lip, which in short reduces the pressure pulses from the other cylinders traveling the wrong or rather the undisired direction.

Check the outlet of the cross pipe too. Many have the weld protruding into the pipe. Dont make the outlet any larger, just remove the obstruction by grinding the weld flat with the tube. The uppipe outlet can also be worked in the same way.

Some early classic's have a more restrictive NS collector than later. Dont have any pics though. Its not dependant on the type of car, ie STI or UK.

Last thing, wrap them with some decent heat wrap.

Wayne.


Have a look here. Please bear in mind that I only do these for the Sheffield guys.

Sheffield Scoobys :: View topic - Ported OE Headers

 

Thanks a million, that reverse pulse wave bit makes sense, shame I'm not closer to Sheffield, you appear to be spot on with a Die Grinder.

 

You get pretty good after doing a few sets. I also used to do head porting in my days with pinto/x-flow engines. I made a mess of quite a few cylinder heads before I figured out it was all about flow rather than size of the ports.

The only problem is the old vibration white finger is setting in.

I would love to see Harvey's fingers to see what colour they are!!