I had been skeptical of that slot because I know when you do stuff like that, you make a dual plane manifold less of a dual plane manifold.  Then several years ago, Ted did some testing suggesting a pretty decent gain between 2500 and 3500 rpm, right where a street engine will be able to make use of it, without affecting any other aspect of the output significantly.  http://www.eatonbalancing.com/2014/11/23/intake-manifold-plenum-slots/

So I had it filled.  I also took some time to smooth the transition from the carb to the ports and plenum and make the ports a closer match to the heads and smooth some bumps that I could feel in a couple ports.  I didn't get too aggressive with this because the intake flows better than I currently need it to out of the box.  So just smoothing and port matching, trying to err on having the intake ports a little smaller than the heads - don't know that that is the best choice, it just seems that tumbling over a ledge will be better than hitting a wall.  I'm sure a lot can still be improved there, if need be and hopefully I didn't remove too much to ruin anything.

I also cut a 4-hole gasket into a 2-oval gasket to properly isolate each plane as much as possible.  There is still some crossover going on with the carburetor, but you can't do much about that.

After all this, there is a subtle yet noticeable improvement starting at about 2000 rpm and I gained 1" hg, for 17" which is great considering that the cam is the repop of a blower cam that Mummert produced a decade ago.

It cost me $50 to have that slot welded.  If you have one of these and it is in a street vehicle, it's a good $50 to spend.










Lawrenceville, GA

Charlie.  THANKS for sharing the information on modifying your BT intake.  Appreciated your pictures, too.  Well done!

Regards,


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

Charlie.  Good job on getting that manifold slot filled back in.  The increase in manifold vacuum is typical in getting that manifold back to a true dual plane design.  You did mention awhile back about how the pictures on some of my website articles would not enlarge when clicked up; that was a result of my website being updated awhile back.  That ‘plenum slot’ article was one of the ones I haven’t had a chance to fix yet so I went ahead and fixed the pictures and graphs so they would blow up to a larger size once they are clicked upon.

Lorena, Texas (South of Waco)

Or you can file a little piece of Aluminum to fit in the slot.  Inverse 'V' each end of the slot and  'V' the vertical ends of the piece. JB Weld  the Aluminum piece in place.  Be sure the spacer and gasket you use will hold the aluminum piece securely when the carb is in place. A pure two plane does give nicer running in the normal street conditions.  

  

Doug T

The Highlands, Louisville, Ky.

This has really been a great improvement. The bad thing about a dyno is that it can't measure anything below a certain rpm, in an rpm range which street vehicles actually use regularly.  The increase in responsiveness is fantastic.  Of all of the changes I've made in the past couple years to fine tune the engine, this is probably the best, and about the cheapest.  If you have one of these manifolds, do this modification asap.


Lawrenceville, GA

good report , good to know.



"Came too close to dying to stop living now!"