So basically an open loop GM 80's TPI applied to a BBC

2 fuel channels, 1 spark channel

Yeah....merc gets by with it by running rich and running the timing far before MBT......but I would think that would lead to high EGT and be murder on exhaust valves.

Exactly, tuned fat AF so between low flowing imbalanced injectors over time and lean cylinders it dont blow up. Funny thing is a 500efi tune double fires injectirs, a 502 mpi tune doesn't, i havent dynoed a mpi 454 or 502 yet w stock cam so not sure if it would be better or worse to double fire them with stock cam, it definately helped on the 560 hp 502 i dynoed w 500efi cam and modded intake/throttle body

Not a big boat...but one of my toys is a low buck twin turbo 460 jet boat....I have a microsquirt ECM that has 2 fuel channels and 4 spark channels (no individual cylinder control, waste firing GM LS coils).

The intake is an old tunnel ram that I converted to EFI with injectors in spacers between the upper and lower parts of the tunnel ram. I used the carbs as throtte bodies because I am too cheap to buy aftermarket ones, i put a TPS on the rear carb.

I run it at about 15.5:1 AFR up to about 80 KPA.....then slope to 12.5:1 at 100 KPA....then at 150 KPA I target 11.5:1

I too noticed some driveablity improvements (especially low speed) by firing the injectors multiple times/cycle......I do not have an individual cylinder EGT on it....but the plugs look dead even...im trying to post a pic....but my work computer is fighting me.

I think some of the old school tricks for distribution on carbs intakes is the modify the bottom of the manifold to "direct or block rivers" until you get even plug readings.

However, this would only work for one RPM/load case region in the operating range.

These issues makes it easy to understand why OEM carbureted auto engines typically had an exhaust heat cross over for manifold heat


My intake set up is far from perfect......but it does a great job for the application and resources put into it.

Smitty,
Excuse my ignorance, but are all O2's equidistant? Could some cylinders be burning more or less in the pipe? Or, is 1 cylinder just scavenging more from the other, or fuel distribution from the carb? How tall carb spacers are you using? I have some 3" and 4" spacers Jim designed I could send you. You obviously you have the technology to monitor, what would a colder plug do in those high cylinders more or less burn?
Just throwing out thoughts. Probably some dumb ones. lol
(Now we are all worried about our builds after your findings)

you look at the plugs?do they show anything,color,piston.......

Id think if the o2 readings, correlate to the egt readings, that would make sense to me. I know I have seen some engines with big differences in egt temps across the cylinders. Like 200, 300+ degrees in some NA builds with a single 4 barrel single plane.

Heres a video talking about this issue

#7, 8 after a couple lean initial pulls were very lean, slightly sand blasted looking porcelain, strap heat discolored to ring, definately lean. As mentioned top, swapped to dual plane, lean cylinder moved to #3, number 7 and 8 went to middle of range.