Big-Block Power Tune
Getting The Most Power From Your Classic Big-Block
From the November, 2006 issue of Corvette Fever
By Steve Dulcich
Those Corvette enthusiasts...
Those Corvette enthusiasts lucky enough to own a classic big-block can maximize the enjoyment of these machines with a perfect tune. Some dyno work on this big-block-equipped '66 had it at its best.
Some enthusiasts are adamant about performing every aspect of wrenching on their own, from a ground-up restoration to the more mundane maintenance chores that surround the experience of owning a classic vehicle. While we admire such resolve, and can be considered to have such tendencies ourselves, sometime a little help can go a long way. When it is all said and done, and your prized possession is finally road-ready, it is the small details that can make all the difference in what enjoyment the driving experience will bring. A good example is a diagnostic chassis dyno test and tune session. Here, the process will identify problems with the fine-settings of the engine in ways that scarcely any of us can achieve on our own. The sophisticated dyno instrumentation will monitor and record vital engine functions, and point the direction for finessing the combination for optimal power and drivability.
This vehicle was factory equipped...
This vehicle was factory equipped with a Corvette icon: an L72 solid-lifter, four-barrel 427, rated at 425 gross flywheel horsepower. The engine was rebuilt and features all stock major components.
Such was the case with Rick Stoner's fine '66 L72. This Corvette was restored true to the original form, and no detail was spared in the complete rebuilding of this classic machine. The aim of the effort was to make it as road-worthy as a factory fresh big-block Vette rolling out of a dealership lot all of those years ago. In addition to being an avid Corvette fan and collector, Stoner is the proprietor of Westech Performance Group, one of Southern California's top dyno facilities. Naturally, a buildup would be punctuated by a complete dyno session, with an eye towards drivability, economy, throttle response, as well as the more empirical measures of outright max power output. Rick's L72 big-block had the advantage of a full dyno test session prior to the engine's installation in the car, however, massaging the final installed package on the chassis dyno definitely proved worthwhile. We followed along with the experts to see what was involved in the test and tune session to allow this big-block to flex its muscle.
| Difficulty Index ::: |
|ANYONE’S PROJECT | no tools required ||N |
|BEGINNER | basic tools ||NN |
|EXPERIENCED | special tools ||NNN |
|ACCOMPLISHED | special tools and outside help ||NNNN |
|PROFESSIONALS ONLY | send this work out ||NNNNN |
Fouled plugs are a common...
Fouled plugs are a common problem with these older engines, leading to a host of frustration resulting from a poor running engine. Here it pays to step-up to premium plugs and ignition components. We used Denso's premium quality Iridium plugs. Though costlier than typical replacements, the spark quality makes them worth the added expense.
From the factory, these big-block...
From the factory, these big-block Corvettes came with a transistorized ignition distributor, which was very advanced for the time. These units should be left to specialists for rebuilding, but can function on par with modern distributors when set up by an expert.
A good clean air filter is...
A good clean air filter is a must. A dirty or restricted element will cost airflow and power, and can also cause drivability and fuel efficiency plagues. Though mostly resto-stock, this car runs a high-flow K&N filter element in the original filter case.
Hidden out of sight behind...
Hidden out of sight behind the factory ignition shields, items like the plug wires, distributor cap, and rotor are often neglected. Such was not the case with this big-block, which was rebuilt with all-new, high-quality ignition components.
SuperFlow Chassis Dyno Tested at Westech
|Rpm ||Base ||Tuned |
|2,200 ||283 ||299 |
|2,400 ||297 ||314 |
|2,600 ||309 ||327 |
|2,800 ||316 ||331 |
|3,000 ||316 ||329 |
|3,200 ||321 ||334 |
|3,400 ||327 ||339 |
|3,600 ||328 ||341 |
|3,800 ||331 ||342 |
|4,000 ||327 ||338 |
|4,200 ||321 ||331 |
|4,400 ||317 ||324 |
|4,600 ||311 ||321 |
|4,800 ||310 ||318 |
|5,000 ||305 ||316 |
|5,200 ||264 ||285 |
|Rpm ||Base ||Tuned |
|2,200 ||119 ||125 |
|2,400 ||137 ||145 |
|2,600 ||153 ||163 |
|2,800 ||168 ||177 |
|3,000 ||182 ||188 |
|3,200 ||196 ||204 |
|3,400 ||212 ||220 |
|3,600 ||225 ||234 |
|3,800 ||239 ||248 |
|4,000 ||250 ||257 |
|4,200 ||257 ||265 |
|4,400 ||264 ||273 |
|4,600 ||273 ||282 |
|4,800 ||284 ||293 |
|5,000 ||289 ||301 |
|5,200 ||261 ||282 |
It is surprising how often...
It is surprising how often a neglected PCV valve goes unnoticed, creating problems such as a rough idle, plug fouling, oil burning, exhaust smoke, and erratic engine operation. Replacement is quick and easy.
One of the best things you...
One of the best things you can do for your classic Corvette is invest in a competent dyno tune. The information gathered will help identify any problems and point the direction for the cure. Here, Westech's Steve Abbruzzese securely strapped the Vette to the chassis dyno's power-measuring rollers.
A key instrument for data...
A key instrument for data gathering is a wide-band lambda tester, which can quickly and accurately read the engine's air/fuel ratio. The arrangement here uses a remote sensor apparatus, with a probe going into the factory sidepipes.
At the Westech dyno, information...
At the Westech dyno, information gathered from the wide-band lambda is fed into a display unit, showing air/fuel ratio; it is also fed into and recorded by the dyno computer. Here we have a crisp 13.6:1 at a 750-rpm idle.
The Holley carburetors fitted...
The Holley carburetors fitted as original equipment to big-block Corvettes are sensitive to the fuel level in the float bowls. Before any tuning is done, the float level should be checked and adjusted as necessary.
Before any power pulls are...
Before any power pulls are done, the base carb adjustments are made, starting with the engine idle speed and mixture, which are simply adjusted via the idle mixture and idle speed screws. The accurate tach and lambda on the dyno ensure a perfect, true setting.
A remote control panel displays...
A remote control panel displays data and controls the dyno's function from inside the car. In preparation for a power pull, the car is run through the gears, and then brought to full throttle in top gear, and run through the test rpm range.
We had a chance to talk to Rick Stoner about this particular machine to gain some insight into this car and Rick's enthusiasm for the marquee. As Rick says, "It's a ground-up restoration that's been done for a little while; we went through every nut and bolt. I wanted to bring it up to a high-quality Corvette; I'm not into the national Corvette show-car stuff, but wanted a nice car and an all-numbers matching car. I wanted it to use all stock stuff, to make it the way it was when it was brand-new."
We inquired about where Rick's interest in these midyear Corvettes originated. Rick says, "I've always liked the Corvettes. I bought my first one in 1969 from a friend of mine that was going into the service. I bought it for $3,000, and I still have that car. If you can find good buys on these cars, you won't lose with these cars. I have nine of them now. It may be a big investment in these things, but I can go out and touch them, feel them, and drive them."
With the new plugs, air filter,...
With the new plugs, air filter, and basic tune-up parts in place, the first full power tests were run. Grunting against the rollers, we found a baseline of 290 hp at 5,000 rpm, and a torque peak of 331 lb-ft at 3,900 rpm.
We inquired about the engine, and Rick informed us, "The engine is absolutely rebuilt from the ground up with all stock components. I didn't put [hardened] seats [in the heads] in this one, so it will have to run leaded gas or additives if you were to drive it very much. The latest one I did, I installed the hardened valve seats so I could run unleaded pump gas with no additives. I did knock some compression out of it when I rebuilt it, but I didn't change anything else."
Rick conveyed that these cars provide a unique driving experience, "For a guy who grew up with those cars, driving them just makes you feel like you are 18 years old again. I've always loved those old cars, and the value has always been good. I like the earlier cars the best, the midyears, and the old straight axle cars. I didn't particularly care for the Corvettes after 1967. It's surprising how well these cars still perform on the road. My little '64 roadster gets 21-22 mpg on the highway with a 300hp, single four-barrel engine. The 427 car has a lot of punch to it, but I'm not drag racing them anymore, so I'm not out flat-footing, seeing how fast it will go. I'm a little too old, and if I wanted to do that I would get a race car. This is just a cruiser, something to have fun with on a Sunday afternoon, or go to a car show in, or just go for a nice evening ride if I want to. It's relaxing, it's fun, it's enjoyable, and people, when they see it, appreciate the car as well."
Power tuning on the dyno comes...
Power tuning on the dyno comes down to the air/fuel ratio and ignition timing at full load and wide-open throttle. The numbers showed a lean mixture, which indicated a jetting change is required. The carb was opened for a step-up of two jet sizes.
These four-barrel 427 engines...
These four-barrel 427 engines used a vacuum mechanism to control the secondary barrel opening rate and timing. Additional test pulls revealed that a quicker opening rate could provide a benefit, an adjustment made by removing the vacuum pod and replacing the spring inside.
The most important aspect...
The most important aspect of dyno testing is crunching the numbers to analyze the results and determine the course of improvements. Here, the dyno tuner's skill and knowledge is key. The crew at Westech was thorough.
With the jetting dialed in,...
With the jetting dialed in, the next step is the ignition timing. Here, a trial and error process of adjustments, called a timing loop, zeros in on the precise ignition timing required for best output.
With some fine-tuning and...
With some fine-tuning and no major component changes, the dyno tune brought this big-block's peak output to just over 300 hp at the rear wheels with 345 lb-ft of peak torque for a nice power gain of 11 hp and 14 lb-ft.