intake port to intake manifold. match or mismatch?h

[RW TECH]

I just ran a SBF on the dyno a few weeks ago that has Brodix SPEC heads on it per class rules. The intake manifold rules for this particular venue are wide-open and all that's required is the manifold is a casting. The intake on this engine is a extensively modified Edelbrock 2924 that was milled to fit a lower deck height block and ported to match a well flogged-out NMCA Hot Street combination.

The port mis-match is gynormous but the engine makes every bit of power the air flow would allow and it's fast on the race track.

[1989TransAm]

"The port mis-match is gynormous but the engine makes every bit of power"

So are you saying the port on the intake manifold is larger than the port in the head and there is a step at the head in the airpath? Just curious. Sounds like you are confirming what Troy said in the previous post.

[Kdub]

The big mismatch made the oval port head think it was feeding from a larger plenum manifold? The bigger runners on the square port heads would make the manifold have more volume.

kwilliams,

When you say big mismatch,,,,, did you leave the intake oval port shape oval ?? or did you possible grind a funnel shape(head side oval to square) to match the square port intake shape ??

I have seen this done before and wondered if this would help or hurt the air transition from the intake into the port.

Thanks for looking

Ron

I was commenting on Ted's post above mine. I have an old Offenhauser book that says to do funnel shape the opening of a Ford BBF head to match their big port intake for a performance increase.

To me this topic is interesting.

[MadBill]

Tapering the mismatch would seem like a no-brainer, but a previous thread dealt extensively with this on BBCs. Apparently Mercury Marine sold/sells engines with rectangular port manifolds on oval port heads and engine builders have found major power losses by smoothing the transitions. Clearly something else is going on...

[Troy Patterson]

Consider that the primary air stream or flow isn't spread uniformly across the port area, a majority of the flow occurring in a much smaller area and taking a path something like a string pulled through the port would take. Where the port mismatch exist may have a very small effect on the primary area flow area.

Troy Patterson TMPCarbs.net TMP Carbs

I should add that the new larger runner area upstream will affect and change the flow path.

Troy Patterson TMPCarbs.net TMP Carbs

[RW TECH]

"The port mis-match is gynormous but the engine makes every bit of power"

So are you saying the port on the intake manifold is larger than the port in the head and there is a step at the head in the airpath? Just curious. Sounds like you are confirming what Troy said in the previous post.

That's exactly what I'm saying.

[Boz-Race Engines]

ive found a mismatch works quite well on certain applications also

[tmoss]

Consider that the primary air stream or flow isn't spread uniformly across the port area, a majority of the flow occurring in a much smaller area and taking a path something like a string pulled through the port would take. Where the port mismatch exist may have a very small effect on the primary area flow area.

Troy Patterson TMPCarbs.net TMP Carbs

there is physical cross sectin and there is effective cross section - which we see using velocity probes. I also think the increased runner section is an extension of the plenum, it's just closer to the depression source and more "available".

[Bob Hollinshead]

I've read about this rectangular port intake on oval port head deal at least a dozen times but have never seen any dyno testing results to prove the phenomina. Neat stuff but way above my thought process. My machinist uses oval port intakes on rectangular port heads and makes good power.

[CREngines]

i would really like to see this discussed more in detail. does any body have a link to any of the old threads? my search was unsuccessful.

[talon]

Here is one - the last post on this thread


viewtopic.php?f=15&t=5326&hilit=phil+martin&start=15


There are othere threads. I have seen this on my little 10" bench with SBF manifolds runner #4,#5 and have asked others with bigger benches to try just moving the manifold .03-.05 and see if they see the same thing.

Also seen this on a SBC vortec - this was really picky though. I actully put a little chamfer on the head and put the mis-match in the manifold - .05. But again this was on my little 10" bench( toy ) - you guys with the big benches try this.

listen to Phil Martins CD sold here on speed talk

The little I dealt with this - the more snakey turns the air has to make to the center of the piston bore - the more you may see benifit from this. The pockets of turbulent and slow air that form from this extra room will thicken up though as FPS pick up, so as with anything this is not a blanket statement. I could be talking out of a orifice that was not designed for talking??

Then there is the whole pressure wave deal happening. I aint got no way to see that and relate to that though - so that is not something that I can visualize like air when I am sailing a string or putting in a pitot tube in the air stream.

Jim

[tmoss]

I agree with preferring convergance to divergance. I think the opinion in that thread that the air column "slams into the flange face" ignores the points of velocity profile, and "effective" cross section for intakes with less than a straight shot runner as well as the benfit of additional plenum area - which is also a proven adder of dyno power and track improvement to a point. Experimentation discussed on this site has shown that adding plenum adds power on the dyno and track but past a certain point, despite continued power addition on the duyno, the shift recovery and ET on the track gets lazy.

[CREngines]

It was definitely interesting to note as Madbill did earlier that when this is witnessed on a BBC oval head Rec manifold that altering the head flange to a radius resulted in a power decrease. the velocity at the interface point would have to be substantially lower in a large mismatch application like this... Perhaps it has to do with lowering the velocity at the head entrance or if the MCSA is there due to a pushrod pinch it lowers the velocity at the MCSA? Im going out on a limb with that one. any thoughts on that gentlemen?

[CHT]

I've rarely seen it lose out. I have got Phil's interveiw and I commend anyone on here to buy as many of the CDs Don sells as you can, including his nascar book. This is not a shamless plug either IT'S WORTH THE COST.

[cjperformance]

The big mismatch made the oval port head think it was feeding from a larger plenum manifold? The bigger runners on the square port heads would make the manifold have more volume.

kwilliams,

When you say big mismatch,,,,, did you leave the intake oval port shape oval ?? or did you possible grind a funnel shape(head side oval to square) to match the square port intake shape ??

I have seen this done before and wondered if this would help or hurt the air transition from the intake into the port.

Thanks for looking

Ron

I was commenting on Ted's post above mine. I have an old Offenhauser book that says to do funnel shape the opening of a Ford BBF head to match their big port intake for a performance increase.

To me this topic is interesting.

Yes the old Offy books and most of the old performance engine build books say to open up the BBF intake port to suit the Offy or SCJ intakes, it makes the ports look nice! , but definetly does not make them work nice. With stock port openings and the big intake manifold ports which in the 460 amounts to a 1/4inch plus mismatch. In any application/use that suits the port and bowl size in the head, they work better all round with the mismatch in place. Funneling the port out to suit the intake dose nothing to boost performance, yet fitting the big intake on the small port with mismatch in place out performs the same head port with a matching size intake!, so this is probably saying that any of the intake manifolds available that do match the head port are just not up to the flow requirement. If more performance is required than the head port can manage then the entire port must be reworked, not just the first inch or so of the port.
In all but the highest rpm applications, a mismatch seems to go a long way toward cleaning up un vapourised fuel and helping mid range torque without hurting or even increasing hp.