I am thinking of buying an RB 6-port manifold and holley 600 carb from a friend. I looked at it tonight and the manifold has been cutt at the top were it mounts to the carb. The normal holley flange has been cutt away around the circles so that it lookes like a merged racing collector. I has a point in the middle and the walls curve down in the middle and are knife edged. It is as though you are looking at a race header collector when you look down through the carb inlet path. Why is this done? Has anyone heard of this being done? Does this damage the velocity of the ports at the top of the manifold, right under the carb. I would think that this would slow flow at the venturies.

 

I recently did that to my setup, Apparently im not the only one.



The problem is, the RB manifold is an Isolated Runner(IR) design. A vacuum secondary Holley does not work well with an IR. period. Reason bieng is the vac signal is referenced from both primary barrels. And the reversion of one primary runner cancels out the vac signal generated in the other primary runner. So the secondary diaphragm sees little or no vacuum.



So the goal for me was to modify the IR manifold to a plenum-ram type manifold to equalize the vacuum under the carb, control reversion, and achieve good drivability. I did this by smoothly merging the runners to smooth airflow and to eliminate a plateu where fuel can pool up. I also used a 1 inch "open carb" phenolic spacer to increase the vloume of the plenum. Soon another 1/2 inch will be added with a nos cheater plate but thats another topic.



Why not just use a mechanical secondary carb? I tried, and I think 600+ CFM is too large for a rotary at low RPM(street driven car). Extreme accell pumps and cams are needed and to me its just "putting a band-aid" on the problem.

 

Carbs should have their own forum.

 

[quote name='RONIN FC' date='Jul 28 2005, 08:34 AM']I recently did that to my setup, Apparently im not the only one.



The problem is, the RB manifold is an Isolated Runner(IR) design. A vacuum secondary Holley does not work well with an IR. period. Reason bieng is the vac signal is referenced from both primary barrels. And the reversion of one primary runner cancels out the vac signal generated in the other primary runner. So the secondary diaphragm sees little or no vacuum.



So the goal for me was to modify the IR manifold to a plenum-ram type manifold to equalize the vacuum under the carb, control reversion, and achieve good drivability. I did this by smoothly merging the runners to smooth airflow and to eliminate a plateu where fuel can pool up. I also used a 1 inch "open carb" phenolic spacer to increase the vloume of the plenum. Soon another 1/2 inch will be added with a nos cheater plate but thats another topic.



Why not just use a mechanical secondary carb? I tried, and I think 600+ CFM is too large for a rotary at low RPM(street driven car). Extreme accell pumps and cams are needed and to me its just "putting a band-aid" on the problem.


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What about the RB holley carb that they message and say is so cool. The one that came with the carb is a mechanical secondary (with no return spring included??) This carb and mani will be going on a mildly street ported S4 N/A block with TII intermediate plate and rotor housings. I need to make peak power around 7500 or less. Will this manifold cause a huge loss in low end torque compared to the stock FI??

 

The carb RB originally had on there was vac secondary, one built specifically for them. It wasnt working so well for everyone, so now they sell the 6 port kit with mech secondary carbs.



The lack of 6 ports is gonna have an effect on low end power. But top end makes up for it.



I know when I had a mech double pumper, taking off from a stop light was chalenging. The accell pumps ran out before the intake velocity was enough to supply 4 venturis. Causing it to bog. I would immagine porting would make that worse. But im still on stock ports in the N/A, so..

 

I just recently did this mod and it works good for drag racing. It actually works much better than the IR setup. I just can't figure out this damn 3K rpm stumble.

 

[quote name='ArmyOfOne' date='Jul 28 2005, 01:17 PM']I just recently did this mod and it works good for drag racing. It actually works much better than the IR setup. I just can't figure out this damn 3K rpm stumble.


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So if I wanna run my stock crank angle sensor and ignition system, can I ditch the TPS sensor or do I still need that?