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Old 11-15-2012, 06:08 PM
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SBC 392 or 396 fueling for economy advice please

Hello hotrodding gurus,

I'm looking for some advice and guidance on what to do about improving my SBC setup please. I acquired the car already modified, and the seller didn't have all the details about what had been done, as it was all done back in the early 1990s. What I have (some of this might be inaccurate, but it's the best info I've got):

1985 Trans Am with replacement engine & original T5 manual box & axle.
SBC 392 or 396 ci 4 bolt mains
Dart II aluminium heads 2.2" inlet, 1.6" exhaust
Unknown camshaft
Wiseco forged flat top racing pistons
Manley steel rods
Stroked steel crank
Trap-door sump.
Edlebrock inlet manifold, possibly Performer or Torker.

Was originally the sister build to a race engine with a 1050 carb and appropriate cam, but this was too wild for the street, so the cam was changed to something less harsh, and the carb was replaced with a 750 modified to something like 925, by Demon we think. Now running a standard Holley 3310 750.

It seems to have plenty of power and goes well, but I'm getting pretty terrible miles per gallon from it, and it's not very smooth to drive below 2500 RPM. I'd like to get better fuel economy for general street use - fuel costs over 8 US dollars per US gallon over here I am happy to swap to a bigger carb for occasion visits to the strip.

Assuming the engine needs 750+ CFM to achieve its full performance, will it do any harm to run a much smaller carb for economy on the street, and would that even work?

Am I right in assuming a spread-bore carb will give me better economy with light use of the gas pedal, but still have plenty of fueling capability if I put my foot down? If so, how much better economy would I get with a spread-bore carb compared to the same CFM square-bore?

I suppose it is probably a good idea to measure the compression ratio and cam profiles.

Can anyone recommend a suitable cam, carb & inlet manifold please?
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