Since reading the recent post here about filling the Plenum slot in a Blue Thunder intake & a similar article in issue #140 of Y-Block Magazine, which also included a 2nd article on modifications to an ECZ-B intake, both writen by Doug Thrasher, I noticed a variation in the intakes.. The old "B" intake had what I'll call siping or ridges running in the Plenum floor lengthwise towards the runners. The Blue Thunders have a smooth Plenum floor & checking photos of a Mummert intake, it has ridges running crossways in the Plenum. The question being, is there specific reasons for the differences? 


Paul

The reason for the Blue Thunder intake having a smooth plenum floor is due to those ridges being difficult to cast.  I've talked about this with Art Francis in the past and I'll bring it back up again the next time I talk to him.  The ridges are beneficial if for no other reason than giving a place for excess fuel to puddle and evaporate rather than allow that same fuel to run directly to the cylinders.

Lorena, Texas (South of Waco)

Ted: I never thought of that.. My guess on the ridges, particularly in the Mummert intake, was that maybe it was to create mixture turbulence?. 


Paul

I did get to talk with Art this week again about putting some kind of ridges in the plenum floor of the BT intake.  He said that this next run of castings is already scheduled but would try to do some kind of plenum floor modifications after that.  I suggested some kind of “X” pattern.  Here’s a picture of the 2010 EMC manifold that had the plenum floor modified to create some additional turbulence in that area.
 

Lorena, Texas (South of Waco)

I wonder if there is enough material in the floor of the BT to ad several rows of dimples with a drill. 


Lawrenceville, GA

With regards to ridges in the intake plenum, I have heard that they were there to aid in vaporization of the fuel especially in cold start situations.  The added surface area would evaporate droplets that drop out of the carb.  Having the ridges crosswise would hold the fuel in the valleys,  but the Mummert manifold has no heat there to aid vaporization while the BT has the heat but no ridges....Hmmm!

If the ridges do anything would make an interesting test. I would not be too surprised to find out the ridges don't do too much at any speed above a brisk idle.

  When I do an Iron B manifold I start the radius down to the lower runner so that it takes out some of the ridges which in the B manifold run fore and aft as shown.   BTW   I worry about falling into the exhaust plenum at that point.




Doug T

The Highlands, Louisville, Ky.

I asked A.T. about the floor thickness and while he didn’t have the exact number on the top of his head, he replied that it’s thin there.  As a side note, I will be doing a dyno test later this month confirming the effect of the dimples on the plenum floor versus a smooth floor.

Lorena, Texas (South of Waco)

Just got the latest Y-Block Magazine and it has the article about plenum floor dimples.  For the future, May - June 2018, Vol 25, No. 3, Issue 146.

Question 1 is since the plenum floor is thin, is it a good idea for street use where reliability is a concern and the crossover passage may be in use?

Question 2 is how deep are the dimples?  Since a 1/4" ball mill was used, are they 1/4" deep?

Next is more of an observation, Ted says it is subtle and you won't feel it strongly in a before and after situation.  The plenum slot modification for V1 and 2 manifolds suggest similarly subtle gains and after having done that myself, it is one of the 2 best modifications I have made since actually building the engine.  The dyno doesn't show what happens at typical street rpm and it is fantastic, even below the 2500 rpm test range.  If I can get that again, without sacrificing the reliability of the manifold, I'm in.

How do the averages and test range scores work out?


Lawrenceville, GA

The dimple depth for the test is a minimum of 1/8” with nothing over 3/16”.  Widths of the dimples would be limited to 1/4" or the diameter of the ball end mill that was being used.  Likely deeper than it needed to be but I was shooting for overkill or a worst case scenario when doing that particular test.  In the real world, a 1/8” depth and potentially less will net similar results.  Just enough depth to create some turbulence or fuel mixture bounce back in that area.
 
I did use that modified BT intake on a customers’ pump gas engine and was rewarded with 300HP on 298 cubic inches with the tried and true 228/238 Isky camshaft ground on 111° lobe centers.  And that was with the shop temperature at 100°F.  The torque numbers were stout and the engine idled fair at 800 rpms with a 750 cfm carb.  That engine was still pulling 1.2”Hg at 5500 rpms so it was far from being over-carbureted.   There was some light port work performed in the heads but this is an engine using cast pistons with old school ring technology so it did well.  I’d like to think that the dimpled plenum floor was likely just enough to get that magic 300HP number and not having to settle for a number just shy of 1HP to the cubic inch.
 
On a scoring standpoint, the numbers for the plenum test look like this.
No Spacer before plenum mods: 1717 pts.
No Spacer after plenum mods:    1756 pts
 
1” Prototype Spacer before plenum mods: 1769 pts.
1” Proto type Spacer after plenum mods:   1812 pts.
 
2” Prototype Spacer before plenum mods: 1789 pts.
2” Prototype Spacer after plenum mods:    1799pts.

Lorena, Texas (South of Waco)

In lieu of dimpling the plenum which is thin, could something be added/attached to the BT plenum to achieve desired result?



.150 Stroked Y-Block:327.25 ci @ >1hp per ci