Hello, can someone please help me out to understand what the reasoning and actual effect is of having the primary and secondary runners bridged together on the stock manifold?

I have an FB carb that I’m heavily modifying (a thread at a later date) and another SA manifold. I noticed that one has the primary and secondary (f/r) connected/milled open to each other under the nikki throttle plates. The FB had a similar front bridge and a hole between the primary and secondary on the rear that I assume has a similar flow for balance.

Is the reason to do this is just to have the primary and secondary ports drawing a vacuum across the primary venturis so they get a better signal while cruising? I was thinking the timing of the secondary port would elongate the signal as well. I’m probably going to mill in an even cut between the primary and secondary on my FB one. Just want to make sure it’s there for a good reason.

Before the nikki I had a holley; this wasn’t there at all. Going back to Nikki, for the first time for the last time. Just curious to know if this helps, hinders or just barely helps emissions or something else.

Thanks

Sa/s1 on the top, FB/s2/s3 below

 

My car seemed to like it when I opened up the rear with a channel like the earlier cars, but it's still something that's debated. In theory, the FB manifolds "shouldn't" be modified in that way because they've already been altered to flow "evenly" with the presence of the shutter valve. As I'm sure you noticed, there's a passthrough further down between the rear primary and secondary runners, but it's just a hole, and not particularly large at that. I opted to plug that hole, smooth out the transition a bit, and then cut the channel. So far so good and it's been there for over five years now.

As for why exactly it's there, I can't really say. The most explanation I've seen is "the SA manifold was better, so let's copy that". Supposedly the SA manifold had better primary runners, and the FB has better secondary runners. The exact differences I do not know, although I'm sure someone does (Jeff20B). My theory on the channels is that it allows some flow down all runners under most driving conditions; maybe it helps keep them cleaner, maybe there's a slight benefit to torque, I'm not sure. It's worth noting that the port timing between primary and secondary intake ports on carby engines is the same though, so I'd vote against that particular theory.

If anything I think the actual benefit came from tapering the channels a little so that they were not such an abrupt wall for air to collide with. Since I did both at the same time I can't say which made the difference, but overall the car seemed to approve of the changes. That same intake manifold has been used on a stock Nikki, hogged Nikki, blow-thru Nikki, and now a fuel injected turbo setup, so it hasn't let me down yet. Sorry I couldn't provide much solid information there, but perhaps it's at least some food for thought.

 

Might as well throw these up here. Was going to wait until after I fly cut the flanges… as you can see, I’ve gone down a bit of a rabbit hole.

 

 

 

 

The plan is to leave the primary runners at the stock size for now. They are turned to the same ID. When I have the intake in the mill again for fly cutting the carb flange, I can mill the primary to secondary slots to whatever size and shape. Keeping the brake booster vacuum signal on the front was probably over thinking that the rear rotor is usually the one to lose compression first.

What did you mean by the angle of the slot? The side wall taper or the bottom of it as it transitions/chamfers into each runner?

Curious about the timing, I don’t know what template was used for street port timing on this engine. Had it done for me back in ‘08. If a stock port primary and secondary have the same vacuum signal, would a normal-ish street port also be the same time? That’s where the thought of the longer signal came from.

I would do a few things differently next time. Less welding of silly unnecessary seams that a pro friend was nice enough to help with. The whole thing is going to get blasted again inside and out to get an even rough finish.

I could always just run this thing without the bridges between them and then cut them after the car is running to compare.

Using the SA intake with an added chunk to block off the small center exhaust port (s2, 81-83 12a) is also an option. Don’t know if those were around on the s3/fb 12a’s.

The most sane thing to do is run the normal block off plates and intake that I just traded with Legalizechey…