Chicken or Egg: Dyno Style
Peter Stanwicks; Haddam, CT: I don’t have any muscle cars or hot rods, but really admire the technology and craftsmanship. I mostly do old sports cars and boats. My most recent project was building a Bristol 100D cross-pushrod engine. What is the procedure for an engine dyno test? Is a load put on the engine gradually as the throttle is opened to wide-open throttle (WOT)? Is the load then reduced to allow the engine to gain rpm? Another way to ask the question: Is the entire dyno test done at WOT and the load varied to produce the rpm?
Steve Magnante: Interesting question, and one with a couple of answers. In the old days, dynos were manually operated and had two control levers: one connected to the carburetor and the other to the dyno to manipulate load. The results could be influenced by the operator and the technique used. Dyno operators had to be well trained or damage could result.
But today’s dynos are mostly automated and have one handle (or dial) that controls the throttle. Outfits like Superflow, Dynamic Test Systems (DTS), and Land & Sea manufacture a range of affordable dyno units, helping the practice of dyno testing and tuning spread throughout the land. Before this, dynos were the sole province of Detroit automakers and only the most well-funded professional race teams and engine shops.
During a test on a modern commercial unit, the operator generally pushes the handle from idle to full open (WOT) in a steady move that takes just a couple of seconds. From there, the handle is maxed and the computer controller takes over as long as the operator keeps the handle buried (pulling the handle back aborts the run in an emergency). To answer your question, the carburetor/EFI throttle blades are wide open throughout the test, which usually takes place over 15 to 20 seconds. Of course, without a load, revs would shoot to the moon and parts would scatter. But because the crank is attached to a power-absorption unit, it’s safely harnessed and protected from destruction. And it is the power-absorption unit that is discussed next.
Most affordable dynos found in small shops are of the water-brake variety. Here, a rotational element (affixed to the crankshaft) churns inside a stationary housing (stator). It’s quite similar to a torque converter, but with the converter held still and using water instead of ATF. The amount of drag on the crank/engine is controlled automatically by the dyno computer.
A strain gauge connected between the non-moving stator and its rigid mount determines how much torque the crankshaft is spitting out. Then, since horsepower is a mathematical equation derived from torque, the horsepower curve/chart is calculated by the dyno’s computer—as well as torque. Other sensors monitor, record, and print out information pertaining to A/F ratio, brake-specific fuel consumption, fluid temperatures, and more.
More expensive eddy-current dynos rely on an electrical current instead of water. Here, a steel rotating element (affixed to the crank) spins through an electromagnetic field. Increasing the magnetism adds load against the crank, which is then measured as torque. The eddy-current dynos are far more costly, but offer greater load-control precision. They can hold an engine within 1 or 2 rpm versus the less precise water-brake’s 5 to 10 rpm capability.
The cream of the crop is alternating-current dynos, which use (essentially) a huge AC electric motor. These dynos can measure engine output (by absorbing power), but also have the ability to reverse the situation, bully the engine, and power the crankshaft. This is useful in replicating real-world, on-track conditions where downshifting, upshifting, and coasting are part of the horsepower-delivery picture. In these modes, the rear tires of a moving car can actually “motor” the engine due to vehicle momentum. While water-brake and eddy-current dynos are passive, AC dynos can be active and copy this sort of condition.
To further answer your question about the relationship between the engine and dyno during testing, every dyno is calibrated to keep a firm grip on the crankshaft as they arm-wrestle each other. If this grip was iron-tight, the engine wouldn’t be able to accelerate and no data would be gathered. So dyno power-absorption units must be set to allow the engine to creep up to the preset redline. This is called the acceleration rate and is generally measured in rpm per second. Most commercial dynos let the engine rpm rise at the rate of 300 to 600 rpm per second, but this can be manipulated.
Hopefully, this sheds some light on the dyno situation for you. As for your Bristol, there’s no law that says every car has to have a V8 to be quick and fun.
In-Car, LS Cam-Swap Advice
Dave Conant; via CarCraft.com: I have an LS3 in a 1969 Nova, and I’m thinking about swapping cams. I really don’t want to go through the hassle of pulling the engine and want to do the swap in the car. I’m worried about the harmonic damper and front cover. I know Gen III engines don’t use alignment pins or dowels anywhere but the heads and transmission. How do I install the front cover so the seal is centered before the damper goes back on?
Steve Magnante: Being a total, clean-sheet design, the Gen III small-block gave Chevrolet engineers a rare opportunity to eliminate some to the previous small- and big-block’s more frustrating traits. Back in 1955 when Harry Barr, Ed Cole, and the guys whipped up the original 265ci mouse V8, many competing American V8 engine-block designs featured bulk-adding, deep-skirt crankcase extensions. By slashing the block down to the crankcase centerline, more than 50 pounds of dead weight was eliminated from the block alone. Then by making the intake manifold do double duty as the tappet valley cover, even more weight was saved.
But the resulting oil pan rail/timing cover was a sealing nightmare. With its dual cork side gaskets and semicircle end seals (one-piece by 1986), achieving a leak-free assembly was rare. By contrast, the Gen III takes sealing to a whole new level. All mating surfaces are flat and uninterrupted, and a new wave of gaskets—with integral silicone beads and foolproof, anti-crush shims—are the norm. They’re easy to seal up and rarely leak a drop of fluid.
That said, your observation regarding the Gen III’s lack of front cover-alignment pins is correct. The goal is to position the timing cover so there is equal distance all around the crank snout and seal before tightening it down. It’s not a big deal—if the engine is on a stand and out of the car. There, you can eyeball things and get by. But with the engine in the car, unless you’re a giraffe, it’s about impossible.
Predicting this hassle, GM service departments were issued special tools and fixtures to aid in-car servicing. Tool subcontractor Kent Moore Tools offers what you need. Get the front and rear cover-alignment tool set (PN J41476) and harmonic-damper installation kit (PN J41665). These tools and fixtures are easy to use and yield a leak-free job.
Speaking of yield, don’t forget that the factory damper bolt; like all fasteners on the Gen III V8, it’s a one-time-use, torque-to-yield item. The factory guide says to use the Kent Moore damper-installation tool to seat the damper after your cam swap. Then it says to take the old bolt and use it to crank down the damper to 240 ft-lb. Then you’re supposed to remove the old bolt and replace it with a fresh one. This bolt gets tightened to 37 ft-lb and a second pass with a rotational-angle-type torque wrench spins it another 140 degrees past the 37–ft-lb mark. Doing all of this in the car is a hassle. You can skip it by switching to an old-fashioned bolt from ARP (PN 234-2503). It’s even reusable!
More Info
ARP Fasteners; 800/826-3045; ARP-bolts.com
Kent Moore Tools; 888/220-8350; ToolSource.com
Some AMC Weirdness
Steve Magnante: Nobody asked for this one, but the aluminum-block construction seen in this month’s Bristol and LS3 engine discussions reminded me of some cool AMC engine history. Did you know AMC offered an optional aluminum-block, six-cylinder engine in 1961–1964? Manufactured for AMC by the Doehler-Jarvis division of the National Lead Company, it was essentially an aluminum-block version of Rambler’s iron 195.6ci OHV six. The alloy mill saved 50 pounds and was only offered in the midsize Rambler Classic—never the compact American (bummer).
AMC also toyed with an all-aluminum V8 that never saw mass production. Based on Rambler’s first 250, 287, 327 V8 family (1956–1967), only prototypes were made. Alcoa (the Aluminum Company of America) was in charge of casting the T356 sandcast blocks and heads. I discovered a 327ci version (in parts) in the hands of hardcore Rambler historian and collector Larry Daum.
One final bit of AMC engine trivia concerns the 1983 2.5L Jeep inline-four-cylinder engine. A replacement for the Pontiac-sourced 2.5 four used previously, the new 2.5 was expanded into the 4.0L inline-six in 1988. At AMC’s Kenosha, Wisconsin, engine plant, the four- and six-cylinder heads were machined on the same line for efficiency. But since the four-banger head was shorter than the six, it was cast with a dummy extension that allowed it to fit onto the same drilling, decking, tapping, and so on stations as the longer 4.0 head. After the final machining operation was completed, the stub was simply sawed off. The circular shaved area that accepts the thermostat housing on the 2.5 is where the stub used to exist. If only engines could talk—the stories they’d tell!
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