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327 Dyno Tune
Tuning a small-block '64 coupe for power, economy, and a good time
Dec 10, 2004
Westech Performance Group
Mira Loma, CA
JET Performance Products
Huntington Beach , CA
Holley Performance Products
Bowling Green, KY 42101
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327 Dyno Tune
00. Rick Stoner's immaculate '64 coupe was the subject of our attention. Built for the road, this Corvette captures the stock appearance, but has been modified with aftermarket components under the hood. The 327/four-speed combination makes for a nice driver, though poor fuel economy and rough running meant the tune was well off its potential. A Jet Performance Products-modified Holley carb and a day at the chassis dyno promised to make it right.
01. Before beginning the chassis dyno work, it was worth the time off the dyno clock to take care of the basics. We checked the ignition system and replaced the plugs with a fresh set of Denso's premium iridium plugs.
02. The 327 engine has been rebuilt with stock replacement components, with the exception of a mild Competition Cams XE262 camshaft and valvetrain package, and is supported by an MSD ignition and a Holley carb of unknown origin.
03. Firing the plugs; Stoner's car was carrying an MSD distributor, coil, and secondary wires.
04. Hidden low on the driver-side inner fenderwell was an MSD ignition box. With these ignition components, we were confident there was plenty of spark energy for this mild 327.
05. The ignition timing was checked and recorded, reading 42 degrees total timing (initial and mechanical advance only; vacuum disconnected) at the damper.
06. Tom Habrzyk, Westech chassis dyno operator and tuner, always goes through a safety protocol before running at speed against the dyno's rollers, including checking the knock-offs on this car to ensure they are secure.
07. Tom uses a remote load control to operate the dyno while the car is driven through the gears to speed on the dyno rollers.
08. As driven into Westech, the 327 twisted the rollers to register 208 hp at 5,200 rpm, which equates to about 240 at the crank by the dyno software's calculation.
09. Although the 42 degrees total timing seemed like quite a high setting, running a timing loop and adjusting the timing down to 38 and then 40 degrees found the best power at the original 42-degree level.
10. Air/fuel ratio is read by a wide-band lambda sensor, which works through a probe in the tailpipe. The readings showed the carb was too rich at wide-open throttle. Stoner had a new carb to replace the existing Holley, which had been problematic for some time. Rather than tune the present unit, he wanted it replaced.
11. To really dial in a carb, it's essential to have the required tuning parts on hand. Make certain the tuning facility being considered has an assortment of jets, power valves, pump squirters, and other small parts, or be prepared to bring your own.
12. The replacement carb, a 600-cfm Holley from Jet Performance Products, was bolted in place, and the tuning began. The sight plugs were removed with the engine running to check the fuel level in the float bowls.
13. Float adjustment on this Holley carb model is external. Loosen the locknut and thread the needle valve inward to lower the floats, and outward to raise them.
14. The float level is correct when the fuel in the bowl just trickles past the sight-plug threads with the engine running.
15. We then moved to adjusting the idle mixture: in for leaner and out for richer. Adjust evenly on each of the screws for the best idle quality. Set the idle speed, then check the mixture again.
16. We followed the basic tuning with another dyno pull with the new carb, and found the secondary opening rate was too slow. This carb is fitted with vacuum secondaries, and the opening rate is controlled by the tension of the spring in the actuation diaphragm, with a lighter spring quickening the opening rate. With the Holley Quick-Change diaphragm housing, spring changes are quick and easy. This style diaphragm housing is available from Holley as an accessory for vacuum-secondary carbs.
17. The carburetor was disassembled for a jet change, moving from a 70/77 jet combination in the primary and secondary metering blocks, respectively, to 64/70.
18. Wide-open throttle performance and outright horsepower are important points of a dyno tune; but for a street car, tuning the part throttle mixture is just as important. An eddy-current dyno like the unit at Westech can set a part throttle load to simulate road conditions, and the air/fuel ratio can be read on the display or recorded.
19. A sharp dyno tune will include optimizing the part-throttle enrichment circuits, such as power valves in Holley carbs or metering rods and springs in Carters. Tuning the part-throttle mixture can result in smoother transitions from part throttle, and sharper throttle response.
20. This time, the computer readout showed the mixture was right on the mark. Power moved up accordingly, with 231 hp at the rear wheels, which is about 263 at the crank. The midrange torque increase was even more significant than the top-end numbers.
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