Engine Brains Recommendation :)

[deppmobile]

Not Mopar, but 1970s big block supercharged Chevy

My own knowledge is now spent on this - new MSD ignition (definitely needed) rebuilt carbs (definitely needed.) Just can't get it to run right.

Left by the side of the road today, with massively leaning out mixture under light throttle, despite it running like a dream for 20 minutes before.

I'm going to give Dave B a ring tomorrow about trailering it up there for him to cast his eye over, but that's 200 miles away - which I'll do as i trust him.

But just in case, any recommendations down south for someone who knows classic big block Chevys, knows their way around a Rootes blower and might help?

Thanks!

[Pete]

Are the Carbs "Boost referenced" ones or just normal ones?

[deppmobile]

This is where the challenge is, Pete, they should be, but I'm working with someone else's work and I'm at the limit of my knowledge.

Doesn't boost reference only affect high rpm?

[Pete]

I think they affect Power Valve operation, so off-idle and initial throttle, not high in the rev range, hence my question.

Often people lob on standard carbs and then can't tune them effectively.

[Pete]

When I was going to go carbs and Blower, I bought an MSD device that took out timing in response to boost as a safety precaution, but ended up going Methanol and mechanical Fuel injection....

[Dave999]

no expert no experience but in my mind you would boost reference a carb, if you have a blow through setup.


if it is set up
air filter--> carb ---> forced induction device---> manifold ---> head.... Then you have a draw through set up

not a blow through set up

Draw through....fuel goes through the blower with the air..

in a draw through set up the carb only sees the difference between normal atmosphere and the vacuum caused by the SUCTION side of the blower., if the vacuum is considerably higher than normal operation you might want to seal up the throttle shafts but in theory they provide a minimal leak

the carb just see's the vacuum created by the blower below it looks in vacuum terms like 1 BIG plenum
i.e you do not need solid floats you don't need sealed throttle shafts and you don't need to expose the "outside" of any diaphragm or piston based valve or pump to boost pressure to keep it a step above what it sees from the pressurised inlet
the Carb not being subjected to above atmospheric pressure and it doesn't care what provides it with vacuum in order to meter fuel and air

other way round .. blow through, more complicated and boost referencing is necessary

the forced induction device followed by inter cooler or FAT PIPE into a carb hat, sealed to the air horn on the top of the carb instead of a filter

then you are expecting the carb, a device built to only ever see vacuum, to put up with the boost pressure in all inside parts of it.

in which case everything exposed normally to atmospheric pressure from the outside of the carb needs to see Boost pressure instead or be completely sealed off

i.e a power valve needs to see boost pressure on both sides not just the one that is exposed to the carb bore
the fuel pump diaphragm or piston needs to see boost pressure on both sides this avoids it blowing up like a balloon or shooting out
the fuel pump needs to be able to pump at desired fuel pressure + boost
the floats need to be plastic honey comb not brass to avoid being crushed.
the thorttle shafts should have bushes replaced with double sealed bearings fitted with "bearing fit" locktite
and the PCV connector, and any other vacuum fitting needs to be threaded and blocked with a grub screw

unless the carb is completely enclosed in a box which you pressurise. i.e it sees boost on the inside of the carb and the outside but how do you connect your throttle cable ? involves levers and bearings and well....

or you run fuel injection
in which case the pump and/or the fuel regulator need to be boost referenced and the throttle body needs some of the carb fixes in place. i.e shaft and vacuum fittings

for blow through set up it is easier on the engine side to implement with LPG, a proper performance Gas carb is already sealed and the regulators that control the gas can be boost referenced with a small hole in the right place plumped into the manifold. Propane has a RON of over 100 and an LPG turbo set up can be done on standard CR motor provided you use an LPG cam to save you from valve seat problems...complexity in this case is tanks and plumbing and getting it all signed off as safe.

seen it, seen it work, read about it, watched the videos.... never done any of it, hence i might be talking rubbish

Dave