tall IDA intake manifold porting?

[Jim Ratto]

Looking inside so many tall IDA manifolds, even if they are match ported @ head flange, they still NECK DOWN in between the area under carb and the "match ported" area (meaning just about area where the 'Y' of the manfilold converges).
Looking at cross section drawings of almost any competition-minded engine, none of them have this "squeeze down" in the inlet tract. The VW's big brothers, the various air cooled Porsche engines (take your pick...904, 911, 917, 908 etc), ALL of them show a mildy tapering I.D. from carb butterfly or injector stack to intake port.
Why do people not port VW manifolds accordingly?

[Jim Ratto]

I should elaborate. A friend dropped off his Cb wedgeports and "matchported" tall IDA manifolds. The manifolds match the intake port @ flange, and they do carry the profile up into intake maybe 1.75" up, but then they do the "neck down" right in area where the Y spreads out, and then flare open to butterly area.
What happens once rpm goes above level that gives best efficiency for that "smallest" ID within the manifold. I bet power falls off and engine rpm stalls.
More thoughts?

[Rune]

There is power to be found by opening up the manifolds, that's for sure. Runner length will also change engine characteristics.. There seems like manifolds are kinda an overlooked part in the jungle of of the shelf hi po parts

[Lids]

Don't the manifolds also perform part of the mixing process (fuel/air) if you mess with the manifold to much would the air/fuel mix start to separate and run down the inside of the manifold?

[Jim Ratto]

There is power to be found by opening up the manifolds, that's for sure. Runner length will also change engine characteristics.. There seems like manifolds are kinda an overlooked part in the jungle of of the shelf hi po parts

I know some headshops/tuners have welded up entire manifold, top to bottom and ported throughout. Why not offer manifolds that are cast to accomodate opening them (though I understand down near 8mm holes, there is only so much that can be cast in before there is no clearance for nut)

[Jim Ratto]

Don't the manifolds also perform part of the mixing process (fuel/air) if you mess with the manifold to much would the air/fuel mix start to separate and run down the inside of the manifold?

Yes, but i think that has more to do with temperature and atomization. Along with turbulence as well, but why bother to port to such @ degree @ head flange (like Wedgeport) only to have it choked down about 1.5" under butterfly, then flare open again? Then the manifolds may be smaller than actual venturi or throat of port there 

[Rune]

mounting bolts in the heads can be moved a bit wider apart..

[Lids]

the internal diameter of the manifold would have an effect on rpm range, and as i understand needs to be matched to the cam!  Perhaps the ones available are trying to meet the needs of too many cam choices.

it all has something to do with the return wave pulse.

[Lids]

http://www.team-integra.net/sections/articles/showArticle.asp?ArticleID=466

[Jim Ratto]

cross section of 917 4.5L  and typical 48IDA intake ports

[Bruce]

 Perhaps the ones available are trying to meet the needs of too many cam choices.

it all has something to do with the return wave pulse.

You're thinking too much.  The reason the manifolds choke in the middle is because the head porter was LAZY!

[Rune]

Would that mean the CNC programmers at CB are lazy too? Or is it not possible for the CNC machine to reach deep enough into the manifold?

[louisb]

I seem to remember Gene Berg talking about porting the manifolds all the way up in either the catalog or the blue books. I think it required a special bit maybe?

--louis

[Jeff68]

I have Berg Ported 870 style 044 heads (42 X 37.5) that were CNC and hand ported.  I also have the 590 manifolds for IDA's.  The manifolds were match ported to the head flange AND up tp the carb flange.  In other words the intakes were ported from the head to the carburetor flange.  Not all porters are lazy......

[Jim Ratto]

Would that mean the CNC programmers at CB are lazy too? Or is it not possible for the CNC machine to reach deep enough into the manifold?

The manifolds that go with these CNC heads are not CNC'd.

ou can see from the pic of my manifolds (done by Berg) that they were not "gone thru", it is easy to see the cross section getting tight shortly above mounting flange, especially on "fan shroud" side of runners. I remember when I had Berglar apart @ my parents' house for valve job/springs its Skat Traks were ported to oval shape all the way well up into the "neck" of the manifold. When that car had its compression, I'm sure it ran like an ice block down a grassy hill. With the 7.5:1/FK87, it was a waiting game. Good thing it had the close 5 speed.

[RFbuilt]

jim, i was wondering..

how bout having the tall manifolds (or any manifold for this matter)  'bored' out, to match the ID of the flange side of those IDA's ? or slightly bigger (for anti reversion) whichever theory one sits behind on... 

maybe bore it down 1-1.5inches  deep ,   myt this be of help when porting the manifold from the head side flange? easier to connect the two sides (carb side and head side) of the manifold...

this just a thought, going by the post u made of the cross section of the 917 engine and ida.. 
looks like 1-2inches down after the IDA it has the same bore/area as the butterfly of the carb.. then tapers down to port size half way thru the manifold?

[Jim Ratto]

jim, i was wondering..

how bout having the tall manifolds (or any manifold for this matter)  'bored' out, to match the ID of the flange side of those IDA's ? or slightly bigger (for anti reversion) whichever theory one sits behind on... 

maybe bore it down 1-1.5inches  deep ,   myt this be of help when porting the manifold from the head side flange? easier to connect the two sides (carb side and head side) of the manifold...

this just a thought, going by the post u made of the cross section of the 917 engine and ida.. 
looks like 1-2inches down after the IDA it has the same bore/area as the butterfly of the carb.. then tapers down to port size half way thru the manifold?

The 917 was actually mechanically injected, with no butterflies... this baby had slide throttles and Bosch plunger injection. But if you look, the ID of the intake tract starts @ stack mouth and slowly tapers into area behind intake valve. The ideal port? The gradual taper generates air speed. Same with Cosworth ports, though those are different, as the ports bificurated for 2 sep intake valves. This business of big hole under carb, then squeeze down and then open back up for head port... doesn't make sense to me.

[RFbuilt]

yes sirooo that is what i was trying to get at (bad english haha)

lets say the ida manifolds have the same inlet as the outlet of the 48ida's (butterfly side)
maybe boring the manifold down (deeper) and maintaining this ID ,  then porting the head flange side.. to open up to where the 'bored' section is (above)     thus giving it a slow gradual taper..

i too dont get the, taper down.. choke then open up again to lets say "wedge" ports..   

wont it be restrictive? i mean its like a 2nd set of venturi's

[Jim Ratto]

yes sirooo that is what i was trying to get at (bad english haha)

wont it be restrictive? i mean its like a 2nd set of venturi's

I'm sure it wil be 

[RFbuilt]

well i guess.. when i always thought..  from opening (vstack) to end (back of valve)
it was always (in my understanding)  big, then taper , taper , taper, then the bowl gets a lil more room (depending on portwork/application)   to try to stack up the charge?

hmm would be awesome if we get a few head guys to chime in?   to shed on some light..

maybe UD or Kroc?

[Jim Ratto]

I'd hate for one of the experts to give away their secrets though. I did some work to my manifolds over the weekend, focusing on getting rid of the "hourglass profile" on the intake tract. Using a 37.5 exhaust valve, I opened the chokehold up until the valve would just slip through. Sorry no pics, our camera died.
Will report if the car runs better or worse.

[jamiep_jamiep]

Thiking out loud here... but the join between head and manifold is basically part of a bend isn't it? So the cross sectional area of that particular point would/should be larger than the cross section of the straighter part of the tube further up?

Here's a quick (crap!) sketch I've done to illustrate what I'm trying to say:

[jamiep_jamiep]

For reference, from one side of the port to the other at the top of the manifold in the diagram above it is 51mm in its original scale whereas from point to point at the join between head/manifold is 66mm. Obviously these figures only relate to my rough and ready sketch, but illustrate what I'm trying to say better than I can type it...

[Jim Ratto]

For reference, from one side of the port to the other at the top of the manifold in the diagram above it is 51mm in its original scale whereas from point to point at the join between head/manifold is 66mm. Obviously these figures only relate to my rough and ready sketch, but illustrate what I'm trying to say better than I can type it...

Hi, I understand your point here, but that is assuming that the only "curve" is where intake port turns sideways @ flange area and that the intake is 90 deg to head. My point isn't so much where the "oval mouth" flares open, but farther up within intake, maybe 2" below where carb bolts down. This seemed to be the point where the port diameter squeezed way down. My understanding is that for ultimate cylinder filling, the intake tract should slowly taper from throttle plate to just behind seat.

[jamiep_jamiep]

Wonder if port volume plays some part in the reaasoning for it.... increasing the volume down closwer to the valve to maybe basorb some of the reversion action and kind of choke it off from hitting the carb.... or potentially as mentioned, just some people don't bother to do it.

You can probably tell that I'm no expert, but always interested to know a bit more about stuff like this.

We could really do with an experience porter to chime in here!

[RFbuilt]

I'd hate for one of the experts to give away their secrets though. I did some work to my manifolds over the weekend, focusing on getting rid of the "hourglass profile" on the intake tract. Using a 37.5 exhaust valve, I opened the chokehold up until the valve would just slip through. Sorry no pics, our camera died.
Will report if the car runs better or worse.

i agree... not to share secrets, but maybe insights..

coming from an inline4 background..  tells me.. the intake is or should be just the reverse of the exhaust or vice versa..

so  having a choke kinda interest me why its there 2inches down the entry of the manifold..  or is it.. just the "no time/lazy" result..

i always thought (as i port inline4's for a living)    intake tract should be.. big, taper , taper , taper and expand a bit (bowl) accordingly.. to stack up flow behind the valve when its closed (bfor opening)

knowing our flat4's are totally different beast, id like some insight on that... hehehehe

[Jim Ratto]

car is back on the street. The power curve has shifted up by 500-700rpm. Definitely "uncorked" things, but the need to downshift a bit sooner has arisen too. More later...

[Peter]

Thats cool Jim,
do you like it better like that?

[Jim Ratto]

Thats cool Jim,
do you like it better like that?

for the most part, yes. Only a few commuting miles since the carbs went back on. 105F yesterday so didn't "jump on it", but in slow traffic, up grades, doesn't pull from as down low as before in stock 3rd. Which, yes, is ok by me. Now with light throttle even, it comes into its own @ 4200+, more willing above that to 6000+, but seems it wants to be shifted higher. I like peaky. If it cools off any this weekend, I'll put it through its paces.

[RFbuilt]

soooooo 

u essentially turned ur motor more into a "cammed porsche  reving into cam"  characteristic?

thats one interesting data Jim!   good stuff right there..


now i wonder...   howsit gona be if u topped the compression a tad bit more... and would that 'suck' more and prolly give a stronger signal thru the carbs.. give u more of what u kinda lost?  wonder if it would gain even more top end

interesting..

a snappy midrange,peaky motor... hahaha thats one hard combo to do