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Hey all! I've got a spare 12a ('82 I think?) that's going to be my winter project. I'm doing a rebuild on it and while I was in there I figured I would do a little porting. At the bare minimum doing an improved stock port, just cleaning up the factory casting marks in the runners. But I was toying around with the idea of doing a mild-medium street port. My biggest concern with that is my car is a daily driver in the warmer months, so I don't want to sacrifice too much low-medium end power. Thus I was wondering if anyone had ever don a half street port on a 12a, leaving the primary intake ports "stock" and opening up the secondary ports to a mild-medium street port. Has anyone done this before? Would it be a jarring change as the secondaries opened up? Would it be a rad way to keep drivability while adding more HP on the top end?
I'd love to hear what you guys think, and if this is either a really dumb idea or plausible a good one.
the later EFI engines do this, the primaries are small, and the secondaries are big. they were trying to keep the same port area with less overlap for better emissions and stuff, but no reason you can't do it to add port area
I guess my biggest concern would be
1. if the transition from primaries to secondaries would be to jarring, like an on and off switch
2. if the ported secondaries would cause any unusual air flow from the slightly bridged runners on the stock manifold. (FB style not SA)
Also this gets into a whole different subject, but is the nikki in it's stock form capable of supporting a mild street port? or will that require some changing of jets and air bleeds and stuff (I am by NO means experienced with carburetor modification)
From my reading it looks like the best middle ground would be the SA spec ports for overall driveability. Looks like it's just a mild street port.
The stock Nikki should work just fine but you're selling yourself short on not modding it. Even on my stock '85 I can tell that the top end is pretty choked out by the lack of flow. The bottom end should be improved with the normal 24mm iirc hogging and the mech secondaries give you fine control over when the secondaries open so you're not going to see a big transition unless your foot commands it.
I guess my biggest concern would be
1. if the transition from primaries to secondaries would be to jarring, like an on and off switch
2. if the ported secondaries would cause any unusual air flow from the slightly bridged runners on the stock manifold. (FB style not SA)
Also this gets into a whole different subject, but is the nikki in it's stock form capable of supporting a mild street port? or will that require some changing of jets and air bleeds and stuff (I am by NO means experienced with carburetor modification)
Even a mildly modified nikki can handle it. If /when my stock port engine in my SA goes south, I'll be porting it like this with large secondaries and larger exhaust ports. Keeps mpg reasonable and ups the fun quotient.
I'm sure Jeff will hop on this at some point. Here's what I've gathered over the years. It's an attempt to compile Jeff's recommendations into a single document. All of these threads can be found in the archives or cross referenced along the way. Read through these and you'll have a decent starting point for understanding what questions to ask for your specific goals. Good luck, if you find anything wrong in these, let me know, they are working documents, I'm still months away from getting my nikki working. I'll be doing something very similar to you, street port, NA on my 12a with a very stripped down manifold and no emissions carb. The trick is that most of the jet sizing is geared towards turbo applications, while they work with NA, I haven't been able to find a definite starting point for a hogged out nikki (or stock venturis with streetport) and which jets/drilled or not to use. I still have more to read/learn.
Also, there are factory service manuals for the Nikki at foxed.ca. Those are worth a read after you get the basics to understand the circuits.
I agree with the comments above about referencing all the work that Jeff20B and TJFarrell have done. You will likely need all their tweaks to make the car fun to drive on the street. I modded my Nikki for racing and you can see the results below. Not much difference down low, at least under dyno conditions. Significant improvement at higher rpm though. My experience is that you have to convert to mechanical secondaries because there is not enough vacuum for the secondaries to open normally and I think I would like to have the choke back, just to suck enough fuel into the carb on cold start-up. It doesn't need to stay on long, just long enough to get started. I may have an issue with my accelerator pump capacity (it seems somewhat sporadic) though causing that problem.
Without opening up the carb, there was essentially no improvement in engine HP with the mild street port.
09-14-20, 03:11 PM
