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Old 12-24-2007, 09:51 PM
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vacuum tuning question

finally had time to mess with the new carb on the truck today. got it to read 16" of vacuum at idle from the 11" it had before. seemed to make a nice difference in power and throttle response too!

i'm wondering 16" is about where it should be for the engine i'm running. i couldnt get it to go any higher without bumping the idle up higher than i want it to be.

here is my combo:

chevy 355
10:1 Compression Ratio
Speedpro hypereutectic flattop pistons
Comp Cams 270H magnum cam
Holley SystemaxII Alum. heads
Holley street dominator intake
Speeddemon 650cfm vac. carb
Harland Sharp alum. roller rocker arms
MSD Pro-Billet distributor #8361
MSD Blaster2 Coil
MSD 6AL Ignition box
MSD 8.5mm Super Conductor spark plug wires
Aeromotive Billet Fuel Pressure Regulator @ 6psi
Hedman Elite longtube headers

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Old 12-25-2007, 08:42 AM
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16Hg is plenty good......

for a street car, 14Hg is about the "practical" minumum to keep the fuel/air mix in suspension (all the way thru the longer intake runners) for a decent idle burn and no fouling (especially with colder ambient air temp)

once you have 14Hg+, then it's just minor tweeks of the base timing (1-2*) and carb versus idle rpms to find the setting your combo likes best

your "multispark box" helps alot at idle....you may be able to use a lower idle rpm's and only 13Hg and still have good throttle resonse

Last edited by red65mustang; 12-25-2007 at 08:57 AM.
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Old 12-25-2007, 01:00 PM
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Find the amount of spark timing that gives you the highest idle manifold vacuum with the least amount of throttle opening at the idle rpm you prefer. recurve the distributor to refect the timing required.
Something around 24deg at idle and 32-36deg at max mechanical advance @-/+ 3000rpm is good.
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