[b5m]

I have heard alot of mention about porting the exhaust headers. Basically what I want to do is remove the manifold. Clean the carbon off the manifold. Then match the manifold ports to the ports on the head using the gasket as a template. I will also polish up the manifold ports as much as possible until they are nice and shiny I wont be taking the head off and apart to gasflow this since I dont want to go this far.

A lot of Subaru owners swear by doing this and say it makes a noticable difference. The big question is will this make any difference or is it just a case of feel good factor knowing everything is aligned and shiny What is the alignment like between the head and the exhaust, is there a great big step there or is it ok?

 

[tomstickland]

I've got one or two books that tell you to mismatch the exhaust manifold! Basically they say that the manifold runners should be larger than the ports so that a step is created. They argue that this reduces backflow.

Have a look in A Graham Bell "modern engine tuning" or "4 stroke performance tuning".

 

[b5m]

Hmmm thats very interesting but I suppose it makes sense. What I am worried about it where the step is the other way and the manifold is smaller in places and is "blocking" the port!! This case would definately impede the flow of exhaust gases. I suppose ideally the manifold ports should be as large as possible but this depends on your manifold. Do you know what it looks like, are they a good match?

I unfortunately dont have those books but I am sure someone can tell me how good the current match is and how much room for improvement there is.

Originally posted by tomstickland:
<STRONG>I've got one or two books that tell you to mismatch the exhaust manifold! Basically they say that the manifold runners should be larger than the ports so that a step is created. They argue that this reduces backflow.

Have a look in A Graham Bell "modern engine tuning" or "4 stroke performance tuning".</STRONG>

 

[tomstickland]

I'm not certain, but I'd guess that you can improve the matching, because some of the tubular manifolds have nasty weld etc protruding into the flow path.

On polishing, I do think that would just give you a nice feeling. After a few miles it'd all be sooty again.

Btw, is that a 2L 16v Kadett "super boss" or something?

 

[Elmo Putney]

Just to confirm the theory that the primaries should be bigger than the exhaust ports, my SBD manifold has about a 3mm step. I phoned SBD to ask if I should open the ports out to match, the response was absolutley do not toutch the ports.

 

[b5m]

Definately worth looking at the manifold.

And yes it is a "Opel Kadett 16V super boss"

Originally posted by tomstickland:
<STRONG>I'm not certain, but I'd guess that you can improve the matching, because some of the tubular manifolds have nasty weld etc protruding into the flow path.

On polishing, I do think that would just give you a nice feeling. After a few miles it'd all be sooty again.

Btw, is that a 2L 16v Kadett "super boss" or something?</STRONG>

 

[b5m]

Thanks for confirming that.

I found an excellent links that explains porting/gasflowing well: http://www.sa-motorsports.com/diyport.htm
Here is a quote:

"In most cases, you never port match the exhausts. Many stock exhaust manifolds and virtually all tube headers will have larger port sizes than do the heads. You want that "step" from the port to the larger header tube or exhaust manifold because, as pressure pulses flow back and forth in the exhaust system, it acts as a "reversion dam" by resisting back flow of exhaust gases into the port. If you end up with the rare situation where the stock exhaust manifold has ports smaller than those in the head, you will need to port match. Use the same technique used to match the intakes. "

Originally posted by Elmo Putney:
<STRONG>Just to confirm the theory that the primaries should be bigger than the exhaust ports, my SBD manifold has about a 3mm step. I phoned SBD to ask if I should open the ports out to match, the response was absolutley do not toutch the ports.</STRONG>

[ 18-07-2001: Message edited by: b5m ]

[ 18-07-2001: Message edited by: b5m ]

 

[DarrenH]

Got any more pictures of your car, it looks well tasty.

 

[Mart]

Realy carn`t see why your bothering with the exhaust side to that extent because in about 10,000 miles it`s only going to get carboned up again.

Best thing to do is the inlet side. I removed my head and inlet manifold and went silly with the porting and polishing and while I was at it I decoked all the valves and re-ground them in, it made a difference!.