March 2011 Chevy High Performance Q&A - Performance Q&A

Under Pressure Round II

Kevin McClelland Feb 11, 2011 0 Comment(s)
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A: Austin Healeys are very cool cars. They have a very nice size engine bay in a small package and were built to have small-block Chevys swapped into them. You’re off to a great start with your Innovate wideband O2. I like your solid tuning foundation working out the primaries first and then moving on to the secondaries. With it going lean at higher engine speeds, we will assume that you have plenty of fuel pressure and volume and that your float levels are correct. Our first recommendation was going to be that you contact Quick Fuel Technology, but you’ve been there, done that. All the books you have listed are great reference material. They give you a clear understanding of the role of the air bleeds as a timing tool more than an enrichment tool, timing when the circuit will become active, and how much air is emulsified into the main well before going through the boosters. When you reduce the size of the air bleed, the circuit will come in sooner and be slightly richer.

To get you closerwithout having to change out the air bleeds more than necessarywe like to use either stainless safety wire or warranty tag wire (attaches most warranty tags to new parts). This is just a fine (0.018- to 0.025-inch stainless) wire that you can install down the main air bleeds and run under the air cleaner base to hold them in place. This will restrict the size of the air bleed and change the fueling. You can then see how sensitive the carburetor is to changes in the air bleeds. If the wire brings the jetting in line, work out the area of the bleed hole and the wire diameter. This will give you the final size of the air bleeds you need to install in your carb.

We know of no other shortcut than to cut and try. At least with adding a restriction to your factory air bleeds you can figure out the final size and change them once. Good luck with your metering and be safe on those track days. Watch out for the hero drivers!

Posi Minded

Q: I am 17 and my ’68 Chevelle currently has a 10-bolt rear, open-ended, with highway gears. It has a mostly stock 350hp, 327ci small-block up front with an M-20 behind it. I recently purchased a ’68-72 Chevelle 12-bolt rear at a swap meet; this rear is open-ended as well, and I believe it has 3.07:1 gears.

How do I figure out what series the rear is? I found CY 0618B on the axle tube and NF X 3917124 on the pumpkin. Depending on what series the rear is, what posi would you recommend for a street and strip application? I’m looking at putting 3.73:1 or 3.90:1 gears in the rear.
Jacob Hill
Via email

A: Identification lies in the ring gear carrier that the rearend is equipped with. This will give you the gear range that can be installed with that specific carrier. The two-series carriers were used for the rearends equipped with 2.56-2.90:1 rearend gears. The three-series ranged from 3.08-3.90:1, and the four-series was 4.11s and up. The series of the carrier is specifying where the ring gear flange is located. With a drop in gear ratio (higher numerically), the smaller the pinion gear gets in diameter. With this drop in pinion diameter, you must move the ring gear closer to the pinion or make the ring gear physically thicker. On the market, blanchard ground spacers can be used between a three-series carrier and four-series gears. We don’t like using spacers because of the chance of ring gear runout.


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