Stock SB manifold question.

[petes74ttop]

What is the purpose of the recess that connects the secondaries and why is it such a odd shape?

 

[lars]

By connecting the upper and lower planes of a dual plane manifold with a slot like this, the engine effectively "sees" a larger manifold inlet plenum. This produces a slight increase in midrange and top end power due to opposing cylinder banks being able to share some plenum volume - the effect is more pronounced with more material removed between the two plenums - the factory notch shown in the photo above produces only a very slight effect. There is little or no change in part throttle performance since the mod is not done under the carb's primaries, just under the secondaries. A common "street trick" to get better power out of a stock dual plane has been to mill down the wall between the secondary bores about 1/4-1/2" to produce more of the same effect. If you look at a Performer RPM manifold, you'll see that Edelbrock does this little "tuner trick" on their out-of-the-box manifolds now. Many of the GM big block low-rise manifolds were cut down up to 2" between the secondaries in order to get enough plenum volume on the short manifolds. NHRA allows cutting the plenum divider on post-1970 GM manifolds now.

Lars

 

[petes74ttop]

Hi Lars,
Thanks for the explanation. It's amazing that you could produce a little more midrange and high end power by adding such a small space. You and the other GM engineers sure know your stuff.
Regards,
Pete.

 

[lars]

You can increase this effect significantly by installing the thick (1/4") open-plenum carb base gasket. By playing with the thick open-plenum gasket versus a closed 4-hole gasket, you can "tune" your intake setup and tailor the torque curve a little.
Lars

 

[petes74ttop]

Someone posted that by using the 4 hole gasket mileage would improve so I tried it for a few months. That didn't work for me so I went back to the big hole gasket.
With 190 HP I'm not to concerned with changing my torque curve. LOL