by Hib Halverson
© December 2012
So, what were the results of our Holley DP tuning sessions? The benefits were snappier throttle response, improved drivability around town and better performance at wide-open-throttle. As for fuel economy? The BBfH's highway mileage went from 15.9-mpg to 18.4-mpg.
Once I was through changing things, to validate my work, I took our 850 to the Carb Shop and had owner Bob Verbancic run it on his 540-cuin. Big-Block Chevy carburetor "test engine". After making a couple of pulls with my 850 on his test engine, Verbancic commented, "Not bad." then added, "For an 850 without a lot of the mods a high-end carb would have have, your fuel curve at wide open throttle is pretty good. On our carb test engine (540ci.), part throttle was a little lean, but I'd expect that. Also on a 540, at WOT your carb was generating 2-in. of vacuum which means it's a little small for a 540, but...it's probably ok for a 460 with your camshaft and the rpm you run. If you were to install modern aluminum heads and a bit more camshaft and extend the rpm range to 6,800, you'd be better with a 950."
Here's a list of our final calibrations:
|throttle plate bypass holes: 0.043-in.||throttle plate bypass holes: 0.043-in.|
|idle feed restriction: 0.0150-in. (0.0205-in. wire in IFRs)||idle feed restriction: 0.0345-in.
intermediate circuit orifices: 0.042-in.
|main jet: 68||main jet: 67|
|power valve: 7.5||power valve: 3.5|
|power valve channel "jet": 0.073-in.||power valve channel "jet": 0.063-in|
|total fuel flow area: .01592 sqin.||total fuel flow area : .01570 sqin.|
|accelerator pump: 30-cc.||accelerator pump: 30-cc.|
|accelerator pump discharge nozzle: 28||accelerator pump discharge nozzle: 28|
|accelerator pump cam : black||accelerator pump cam: pink|
While the drivability-enhancements covered in this article apply to Holley Double-Pumpers on a wide variety of engines, it is important to remember that your final carburetor calibration and results will likely differ from what you've read in this story. The above calibration chart is simply a starting point. The Big Block from Hell's carb tuning specifications will be close only with engines similar to ours, ie: 450-500 cu.in., 2500-6500 rpm nominal operating range, roller lifter cam with short duration (234/244° at .050) and high-lift (.567-in.) dual-plane, high-rise intake, heads with modest port volumes and big (but not huge) valves, 9.5-10.0:1 compression, headers, high-performance ignition, a manual transmission, 8.5:1 final drive ratio in first gear and a curb weight of about 3400 lbs.
Engines which are either more aggressive or less-aggressive than ours or Vettes with different transmissions or curb weights may need a different carburetor calibration and in some cases, vastly different.
by Hib Halverson
© December 2012
Robust ignition goes hand-in-hand with properly calibrated, high-performance carburetion. Firing lean AFRs at part-throttle with minimal misfire as well as being able to perform at 6500-rpm can be a tough test of an ignition system.
Before taking on the Double-Pumper tuning challenge, make sure your ignition is up to snuff.
Full Story: It's Fuel AND Spark!