iam porting a 12a, SP do i port the primarys and the secondaries or just the secondaries.. they look the same shape just the bowls and runners are bigger on the seconds.. thanks for your help it would make sense that all get ported the same.

 

I've never done porting before, so this is just my guess. If you modify just one set you'll be changing your intake timing already, so you might as well do both sets to get the most benefit (airflow) out of it. There are runners inside the intake manifold that connect the primary and secondaries.

This does bring up an interesting porting idea: blocking off those runners in the intake and doing a more aggressive port on the secondaries than the primaries. The idea would be to try and get a comprimise of both worlds: Good compression at low end, and good airflow at high end. However I doubt such a setup would work very well. I'm sure low end could be maintained with only one set of intake ports but I doubt the secondary throttle plates could hold back enough blowback for decent compression at low end. Hmmm... What kind of valves do they use on 6 ports for variable intake timing?

 

^ No.

Porting the primaries and secondaries differently works great so long as you've got separate barrels and intake runners for each. Why do you think half-bridgeports are so popular? And the channels in the manifold are primarily used for sharing vacuum signals, not for sharing intake charges. Check out Jeff20B's thread on the topic.

To the OP, generally you would port both the primaries and secondaries. How much to port each is up to you.

 

If you have to ask, you're not ready to port. Head over to nopistons and start reading up in the Engine Building and Porting extion.

 

I'm no expert on the subject obviously, but intakes aside I thought the main reason they do half-bridgeports was because if they did the bridge port on both side housings it would demand the use of one piece apex seals. Can somebody correct me?

 

^^ You're correct and Trochoid wants you to go here http://www.nopistons.com/forums/index.php?showforum=81

Nopistons is THE site for enginebuilders.

 

some of my exh work.

 

i ported the primarys and secondarys the same iall post pics tomorrow. looks real nice and wasent to hard at all.