I can't comment on the fuel distribution issue, but have helped an individual with a BBC and bored out big injector TBI. Ran fine but pulled vacuum at WOT due to insufficient air flow. This even with the bigger bore TBI. Cam and heads were very mild too.

run 2 or 3 tbi's,problem solved.

I have always wanted to try and run two TBI's on a dual 4 barrel intake.
You would have to be experienced with tuning, or know someone that is.

My stock 5.7 EFI Merc has the big block TBI on it. They adapted it to a tiny 2 barrel intake. We will see if it still has vacuum at wide open throttle on my 383. If it does maybe next year I will try and run two of them.

There were several variants of EFI on Merc motors so the nomenclature is causing confusion. Around 1994 the Merc EFI motors began in ernest, with the 350/454/502 Mag EFI/MP. Those motors had a dual stage progressive throttle body with 8 individual port (hence the Multi Port name) injectors. In addition a little lower in the line were the TBI motors, which were the two barrel 'carb looking' throttle body with a large injector in each bore. After that things changed drastically as some throttle bodies became large mono blade pieces with port injection etc. I don't know of any 4 barrel style TBI Merc motor. There might be one, but I quit following all the nuances a while ago.

So this is a project/Topic i have a special interest in and have been doing tons of research on it. I'm a firm believer that EFI is far superior to a carb any day and all day. but i can save that lecture for a different day.

thread i have started on it
EFI Stock TBI maps for a 350

gear head EFI will have stock truck bin files and the matching .xdf files


for a stock merc 260hp 350Cu.in motor the 2x 1-11/16" should be fine.

The unmodded TBI units should be (i can post the math if needed)
5.7L SBC 2x 1-11/16" ~=490 CFM
7.4L BBC 2x 2" ~= 645 CFM

a 350 @55000 @ 0.85 VE =~474 CFM

No such thing as zero vacuum at WOT in a NA engine. Each piston on it's downward stroke with intake valve open, creates vacuum to pull the intake charge in. Equal pressure on both sides of intake manifold creates NO air movement, it's physics.

Holly assumes a 1.5" pressure drop across the carb, so you should not see "0" vacuum.

you can run it on a dual plane, you just need a spacer or a way of balancing the vacuum pulses between the plans ( milling or cutting the interior wall)

Engine Masters ran into the issue you were talking about.

My Innovation 540 CI 600HP engines had Holley TBI and they were GREAT!