This is my intake and vacuum line routing..
Jasper, I believe you are thinking of the old (+6 yrs ago) TWM now Borla system? The later version has located the injectors below the throttle plate and they are angled directly at the intake port, thus the fuel rails run along the centerline between the throttle bodies. They have been using a cast-in plenum chamber for the MAP sensor and the Idle Air Control for quite sometime, which I believe they pioneered and Roush used their stack system on their "IR" engines. Good luck with your hack.I always hated the fact that the Borla's sprayed directly onto the butterfly plate.. can't be good for fuelling accuracy.
I had to dig up some of my old build pics. But here are a few pics that should answer your question. I just saw that Borla Induction has another system out. Mine works pretty well as-is. You can see the smaller plenum is for the IAC, the larger plenum is the vacuum manifold and the MAP sensor resides on top next to the capstan. All sensors, with the exception of the upgrade Penny-Giles Hall-Effect TPS, are GM sensors so not proprietary and easy to replace, if ever needed.Thanks Tom, do you have any pictures on the 'new' system you could share? I havent been able to find any meaningful information about what you're describing. (w.r.t cast plenum or injector rails now being part of the 'Borla' system?)
Randy. . . I don’t think you’re going to find enough leakage to give you idle problems .. . .
Yes there certainly are some aftermarket carbs and throttle bodies that do not have very good fitment of the throttle plates - most that I have seen were caused by a misalignment during assembly - usually pretty easy to address. You will not get an airtight seal under normal conditions and you must remember the expansion rates of the materials used lest you run into a binding issue when hot. Some Holley carbs use exceptionally thin throttle plates that are not milled to match the bore taper.Randy
I disagree with your comment. I had a 1st Gen Borla EightStack system that was very similar to this. Could not get a low idle (wanted to stay at 1500rpm) and it was caused by poor-fitting throttle plates. Borla reworked the system for me and was able to get the idle down into the low 900 range. And it is all caused by poor-fitting plates. It is amazing how much air is pulled thru those openings.
In a perfect world, they would seal 100%, you would hold them open ever so slightly with the idle stop screw and your Idle Air Valve would vary the makeup air to have a low, smooth idle. Reality, that's not going to happen. I purchased a compound used on Porsche & Honda systems to the openings even more however it is a royal PIA to apply and make work. Not sure if I will tackle that or just be happy with a 950 rpm idle. Back in the day, there were similar issues with GM tripower and they used a product to 'seal' the throttle plates. Same issue - you're trying to stop the airflow.
Look at the edges of the throttle plates. Are they tapered to match the wall of the bore? If not, that's your first issue. Snapping the plates open/closed repeatedly will begin to show the high spots - but you need to be real careful if you try to take some off. You could be making it worse. Scraping the point may help, using any type of a power tool is a disaster in the making. The fit is really that close.
Use google and search the terms "seal throttle plates' and you'll find a lot of references to this. Here is a link to wet your appetite: DAG-213
Keep searching, you'll get there