Head Porting

[Blackcoog]

I'm going to swap another 3L in my Cougar. My 3L is burning a bit of oil (ring issues). I plan on doing a little extra porting on the new motor than I normally do. I was researching porting bits and I ran across this link:

http://www.vaglinks.com/Docs/Misc/SA-Motorsports.com_Basic_Head_Porting.pdf

I was going to gasket match the exhaust ports again but then I found this in the article above:

"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."

Is there any truth to that? Should I avoid opening up the exhaust ports? It sounds like I should stick to polishing them up and skip the exhaust port gasket match.

 

[svttour]

i was almost worried about clicking that link when I saw vaglinks and "Porting" lol good read.. not sure about the exhaust ports however.

 

[blu_fuz]

I honestly couldn't say either, but every time I have seen someone port the exhaust side they seem to have good results (IE, no back pressure problems :shrug: ).

What would happen if back pressure is changed? Less HP/TQ?

 

[Blackcoog]

They are trying to state that the air could flow back into the heads easier than if the lip wasn't there. Air flowing back could create more turbulence and a loss of hp I suppose.

 

[Heywood]

It makes sense logically - the step promotes flow out of the heads which will increase flow through the cylinder thus increasing HP. More pressure behind the exhaust valve will reduce flow. [source: 1st year fluid dynamics]

I had a friend that said he lost power when he put a bigger exhaust because of lack of back pressure - back before all this stuff made any sense to me - so this may be a point against, bull•••• on his part or maybe it depends where in the exhaust system this occurred (PS not a turboed car).

 

[B3NN3TT]

I had a friend that said he lost power when he put a bigger exhaust because of lack of back pressure

He lost power due to lack of exhaust velocity and the natural scavenging effect it creates. It's got nothing to do with "back pressure". People keep using that word... I do not think it means what they think it means.

 

[mmc757]

For what its worth, (not much) I spent a LOT of time researching exhaust a while back and I read the same thing from several other sources. That seems to be a pretty widely accepted theory.

 

[jimbbski]

I have ported a few heads and built a few headers and yes that "step" at the exhaust port can help performance. It makes the most difference at low to medium engine speeds as that step keeps spent exhaust gases from flowing back into the cylinder during the period that both the exhaust and intake valves are open. Also by keeping the spend gases inside the header pipe and not the heads exhaust port it helps cylinder filling of fresh air/fuel. On an engine that will see high speeds I don't thing you will see much of a difference.

Bottom line it helps a street engine but very little for a "racing" engine. On a racing engine a "stepped" header works better. A stepped header is one that changes pipe ID size at some point before all of the pipes join together at the header collector. The change in pipe size and the point that change is made can only be determined on a dyno.

 

[crewchiefpro]

The only engines that need reversion bounce are two strokes. Street engines need this little vortex tornado in the exhaust to stop gases from returning into the cylinder during the overlap cycle of the camshaft. We don't even port match the exhaust on my 1100 HP drag race motor.