How To Tune Your Carburetor - What A Drag, Part 4

When we introduced our street/strip '72 Nova SS project in the February '11 issue, it had received a new Total Cost Involved tubular front clip (consisting of a tubular subframe, control arms, coilover shocks, rack-and-pinion steering, etc.). The TCI front clip has enabled the Nova to steer/drive as nice as (or better than) a late-model performance car.

1 Here's our time slip from the one and only baseline pass (left lane) with the Nova—it threw the alternator belt during the run. This baseline pass can be viewed at superchevy.com by clicking on Videos. With only tuning and stickier tires, we feel easy 10.8s are possible. In this story, we'll apply proven tuning tricks to add power, throttle response, and any efficiency we can to the thirsty Dart 509.
1 Here's our time slip from the one and only baseline pass (left lane) with the Nova—it t
2 With the Nova strapped down to Tune Time Performance's Mustang dynamometer, we were anxious to find out the baseline power and air/fuel ratio. We would be looking for a safe A/F of 12.5:13.1 (peak torque to peak hp A/F) during each dyno pull on this carbureted, naturally aspirated engine. If the A/F is below 12.5, it's too rich, above 13.1 it's too lean. Rewards of a proper A/F will be gains in performance and economy. Notice the prop rod (OK, shower curtain rod) holding open the fiberglass 2-inch cowl hood and its unfinished underside. This is what was on the car when it was purchased.
2 With the Nova strapped down to Tune Time Performance's Mustang dynamometer, we were anx
3 For the baseline, the Proform 950 carburetor featured #80 primary and #86 secondary jets. Total timing was set at 36 degrees (we didn't change the timing during dyno tuning). We previously installed the lightest advance springs that came with the MSD Pro Billet distributor for an aggressive timing advance curve—total timing was all in by 2,400 rpm. The Proform 14x3-inch air cleaner with its cool carbon-fiber solid lid was also part of the baseline package. Notice the absence of a carb spacer. There was not enough hood clearance to add one.
3 For the baseline, the Proform 950 carburetor featured #80 primary and #86 secondary jet

In the last episode (September '11), we installed a Performance Automatic Turbo 400 transmission, a Currie Enterprises F9 9-inch rear, and a Driveshaft Shop 3.5-inch aluminum driveshaft. We wanted a tough team of drivetrain components to reliably handle all the power the 669hp 509 Dart motor would dish out.

4 We were expecting better numbers, but the Mustang dynamometer is known to show 13-percent less than a Dynojet or SuperFlow dyno. Still, 406 rwhp and 392 lb-ft torque isn't too shabby considering the mild hydraulic roller cam (0.540/0.560 lift, 242/248 duration @ 0.050). On this pull we found out the A/F mixture was too lean and unsafe at 13.5:1. Routinely, Tune Time will let up during a dyno pull if the A/F goes above 13.2, but your author was driving and we just had to know for our baseline. Having too lean of an A/F ratio put us on a jetting mission to make more safe horsepower with less chance of engine-damaging detonation.
4 We were expecting better numbers, but the Mustang dynamometer is known to show 13-perce
5a In preparation for dyno testing, we brought along a box of jets (Summit-Holley jet kit, Summit PN SUM-210230) to make adjustments to the fuel mixture.
5a In preparation for dyno testing, we brought along a box of jets (Summit-Holley jet kit
5b As lean as the A/F was, we decided to move up four jet sizes, from #80 to #84 on the primary jets. The larger jets will help to richen the A/F mixture ratio (the lower A/F number, the richer). To prevent damage to the jets, we used our trusty old Moroso jet driver (Summit PN MOR-62293) for all of our jet changes.
5b As lean as the A/F was, we decided to move up four jet sizes, from #80 to #84 on the p

For the baseline strip test, we drove the Nova to Atco Dragway in Atco, New Jersey. On 10-year-old, dry-rotted BFGoodrich drag radials we rolled it out for a soft 1.67 60-foot and 11.06 at 123 mph elapsed time (see superchevy.com, click on Videos to see the run). This turned out to be our only pass since the 509 threw the alternator belt and partially cut it. We felt fortunate we were able to reinstall the belt and drive home. High 10s were possible if we chanced making another pass, but we wanted to drive the Nova home and not be embarrassed by having it towed.

6 The jet swap netted us 4 rwhp and 8 lb-ft torque and brought the A/F down to 13.3. Even though the A/F was too lean at 13.3, we were on our way toward safer power with less chance of damaging detonation should the engine be running a hot engine temperature of over 200 degrees in the summer.
6 The jet swap netted us 4 rwhp and 8 lb-ft torque and brought the A/F down to 13.3. Even
7a Next we moved up to #88 jets at both primary and secondary metering blocks in our aim to square-jet the carburetor. On this spin of the rollers, power was up by 2 hp and the A/F was at a maximum perfect of 13.0 to 13.1 during the top of the pull (5,000 to 6,300 rpm).
7a Next we moved up to #88 jets at both primary and secondary metering blocks in our aim
7b Now with the carburetor square-jetted (same size jets all around), later we could change the idle/cruise air bleeds for better fuel economy while cruising.
7b Now with the carburetor square-jetted (same size jets all around), later we could chan

With an established baseline, we were interested in how much power was going to the Nova's tires. To find out, we drove it to our friends at Tune Time Performance in Toms River, New Jersey. After learning our baseline numbers, we could tune for added performance, driveability and economy. Using Tune Time's Mustang chassis dyno with its wide-band O2 sensor will aid us to tune-in a safe air/fuel (A/F) mixture ratio while returning better performance, throttle response, and lower e.t.'s. Follow along and see how the right timing, jets, a carburetor spacer, an air cleaner, and air bleeds can make a rewarding difference when it comes to achieving maximum safe power and engine longevity.

8a We've had success using breathable air filter lids over solid lids in previous testing. This Spectre hpR filter element (14x4-inch, Spectre PN 880138) and 14-inch filter lid (PN 47612) with offer more airflow than the 14x3 paper-element, solid-lid air cleaner we used for the baseline. We only reused the 1-1/4-inch drop base from the 14x3 Proform air cleaner. The Spectre 14x4 filter with the 14-inch filter lid showed us 2 hp and 2 lb-ft torque at peak.
8a We've had success using breathable air filter lids over solid lids in previous testing
8b Air filter lids will show a few more ponies at the bottom of the power band than at peak along with a crisper tip-in response.
8b Air filter lids will show a few more ponies at the bottom of the power band than at pe
9a After jetting for a safe A/F along with the air cleaner swap, our peak gains were 10 lb-ft of torque at 5,100 and 8 hp at 6,000 rpm. The shocker was the average gains of 21 rwhp and 15 lb-ft rwtq during the pull.
9a After jetting for a safe A/F along with the air cleaner swap, our peak gains were 10 l

9b That's real power that you'll feel at every rpm. Unfortunately, for our 30-minute ride home we had to reinstall the 14x3-inch air cleaner due to not having enough hood clearance. Still, we felt that newfound power at our right foot.
9b That's real power that you'll feel at every rpm. Unfortunately, for our 30-minute ride
10 In order to use a 1-inch carburetor spacer and the 14x4 air cleaner, we needed a 4-inch cowl hood to replace the 2-inch cowl hood that came with the Nova. Here, Chad Pirchio and Dave Niz at Charlie's Auto Collision in Tuckerton begin to hang the high-quality Harwood 4-inch bolt-on cowl hood (Harwood PN B-14104). Notice the Nova's old 2-inch cowl hood in the background.
10 In order to use a 1-inch carburetor spacer and the 14x4 air cleaner, we needed a 4-inc
11a This all-important trial-fit sticker comes stuck on every Harwood hood. After making fitting adjustments at the hinges and latch, we only found interference fit in the cowl area.
11a This all-important trial-fit sticker comes stuck on every Harwood hood. After making

11b Luckily we didn't need to trim the edges at any other area of the hood. It fit and lined up very well.
11b Luckily we didn't need to trim the edges at any other area of the hood. It fit and li
12a A small, flat 3-inch sanding disc on an angle die grinder enabled us to get in there and trim the cowl edge.
12a A small, flat 3-inch sanding disc on an angle die grinder enabled us to get in there
12b After trimming, there was plenty of clearance to lift and close the hood without any other part of the hood touching the fenders or cowl.
12b After trimming, there was plenty of clearance to lift and close the hood without any

13a Underneath, the Harwood hood sports a show-quality finish—a noticeable difference from the 'glass hood that came with the Nova at purchase. Looking at the photos on the first page of this story, you'll notice the unfinished underside and the prop rod holding the original, inexpensive hood open.
13a Underneath, the Harwood hood sports a show-quality finish—a noticeable difference fro
13b The flimsy think couldn't support hood springs or it would have cracked in the area where it bolts to the hinges. Not so with the well-built, thicker Harwood fiberglass hood. We used the recommended hinge springs (Harwood PN 200). Say goodbye to that curtain rod we used as a prop rod!
13b The flimsy think couldn't support hood springs or it would have cracked in the area w
14a Here's a good before and after look of the hood swap. We'll attend to painting the new 4-inch cowl hood and adding the trim after more dyno tuning, an exhaust swap, rear suspension mods, and strip testing.
14a Here's a good before and after look of the hood swap. We'll attend to painting the ne

14b We were surprised there's still good visibility with the 4-inch cowl. At high speeds, there isn't any flimsy-flex from the stronger, thicker, Harwood fiberglass hood.
14b We were surprised there's still good visibility with the 4-inch cowl. At high speeds,
15a Looking at the bottom-side of the Wilson Manifolds four-hole tapered spacer (Summit PN WSN-004110) helps you envision how it can maximize airflow through the carburetor and distribute airflow into each runner of the open-plenum type intake manifold.
15a Looking at the bottom-side of the Wilson Manifolds four-hole tapered spacer (Summit P
15b We've tested this particular spacer on many different engine combinations in the past 10 years with dyno results of 7 to 13 rwhp and average e.t. benefits of a tenth and 1 mph. Longer studs (Summit PN ARP-200-2408) are needed to accommodate the 1-inch spacer. We used an open-hole carburetor gasket (MRG-54C) under and a four-hole gasket (MRG-55C) on the top of the spacer (as seen).
15b We've tested this particular spacer on many different engine combinations in the past

16a Now with adequate hood clearance to test the 1-inch Wilson spacer and 1-inch-taller Spectre air cleaner, we headed back to Tune Time Performance. Right away we could feel the Dart 509 was responding with a noticeably smoother idle/cruise and snappier throttle response.
16a Now with adequate hood clearance to test the 1-inch Wilson spacer and 1-inch-taller S
16b After spinning the tires, we were pleased to find out the Wilson spacer was worth 7 ponies and 8 lb-ft torque (421 hp and 410 lb-ft torque)—that's an easy tenth at the strip. Remarkably, the A/F didn't change (13.0 to 13.1) during the pull from peak torque (5,100 rpm) to peak horsepower (6,000 rpm). At that we were able to leave our desired square jetting alone so we could attend to the air bleeds.
16b After spinning the tires, we were pleased to find out the Wilson spacer was worth 7 p
17a During dyno tuning, the idle/cruise A/F became too rich from jetting up to obtain that ideal WOT (wide-open-throttle), high-speed A/F of 13.0 to 13.1. We noticed the idle/cruise A/F changed from a baseline 13.3 to a pig-rich 11.6 during steady cruising on the dyno. An acceptable idle/cruise A/F on a carbureted high-performance motor is roughly 13.1 to 14.5:1. Not wanting to develop a flat spot in the transition of opening the throttle blades when going from the idle/cruise circuit to the high-speed circuit, we needed to be conservative with the size change of the idle/cruise air bleeds.
17a During dyno tuning, the idle/cruise A/F became too rich from jetting up to obtain tha

17b The 4150-type Proform carburetor has four idle/cruise and four high-speed air bleeds. There is one idle/cruise and one high-speed air bleed for each venturi. The bigger the air bleed size, the leaner the A/F mixture will become. We hoped our change from the #70 to #72 air bleeds (a one size change with air bleeds will make a bigger difference than that with fuel jets) would put us in the ballpark of a reaso
17b The 4150-type Proform carburetor has four idle/cruise and four high-speed air bleeds.
18a The total timing was set at 36 degrees and lightweight distributor advance springs were in place before our baseline strip and dyno test. Past strip testing has shown us that lightweight distributor springs and the proper timing can account for as much as 0.5 to 1 second in the quarter-mile. Still, after only changing the jets, air bleeds, carb spacer, and air cleaner, the 509 went from a lazy 406 hp to a lively 421.
18a The total timing was set at 36 degrees and lightweight distributor advance springs we
18b That's more efficient power to boot. While the peak power gains were 15 hp and 18 lb-ft torque, the average increase to the wheels during the pull (2,500 to 6,400 rpm) was 24 hp. For the next installment, we'll replace the Nova's compression-bent 3-inch exhaust with a mandrel-bent Pypes exhaust system (complete with X-shaped crossover) from Ground Up Restoration Supply.
18b That's more efficient power to boot. While the peak power gains were 15 hp and 18 lb-

SOURCES
Summit Racing PO Box 909 Akron OH 44398 800-320-3030 www.summitracing.com	Harwood 909-673-9800 www.eharwood.com
Tune Time Performance 931 Route 166 Toms River NJ 08753 732-349-7800 www.tunetimeperformance.com	Spectre Performance 1720 South Carlos Avenue Ontario CA 90761 909-673-9800 www.spectreperformance.com