Smackdown: Engine dyno vs. chassis dyno

Smackdown: Engine dyno vs. chassis dyno

Real-World Showdown: We put Don Hicks’ Ford 393-Cleveland Stroker Motor to the Test on Westech Performance’s Engine and Chassis Dynos. The Results May Surprise You.

a blue car parked in front of a building © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

How much power do you lose through the drivetrain? The “experts” claim about a 1517-percent power-drop “at the wheels” with a manual trans compared to flywheel numbers; as high as 27 percent with an automatic and loose converter. But many other factors and variables come into play, as we discovered when we ran Don Hicks’ newly rescued 393 Ford Cleveland stroker on both an engine dyno and—in his 1973 Mustang—a chassis dyno as well. 

Smackdown: Engine Dyno vs. Chassis Dyno

What’s the real-world difference between running an engine on an engine-dyno, or, as installed in the car, on a chassis-dyno? Recently, we were able to conduct an engine dyno versus chassis dyno smackdown using the Ford 393-Cleveland stroker-motor from Don Hicks’ 1973 Ford Mustang. Regular HOT ROD readers know that over the last few months we’ve been busy fixing the ailing 393 that initially arrived at Mark Sanchez’s Advanced Engineering West (AEW) with a flat cam lobe. Initially Sanchez replaced the dead flat-tappet hydraulic cam with a Comp Cams Mutha’ Thumpr hydraulic-roller cam and associated valvetrain components (Part 1, March 2020 print edition, and online at HOTROD.com. But that was only half the fix. A previous machinist had botched the adjustable valvetrain conversion on the motor’s rare Ford Australian 302C-2V Cleveland heads to the extent they were beyond economical repair. In their place went a set of modern, free-flowing Trick Flow Specialties (TFS) 225 aluminum cylinder-heads. To keep up with the new hot cam and heads, we swapped out Hicks’ existing low-rise Edelbrock Performer intake and Brand-X small four-barrel economy carb for an Edelbrock high-rise Air-Gap dual-plane intake topped by Quick Fuel’s SS-750 double-pumper carb (Part 2, May 2020 print edition, and online at HOTROD.com).

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But the proof of the pudding, as they say, is in the eating. We wanted to make sure Hicks’ engine was really and truly fixed, and of course the new combo would inevitably need some fine-tuning. With the engine already removed from the car for its original repairs, we could go for a twofer: Westech Performance has both a SuperFlow engine dynamometer as well as a SuperFlow chassis dyno. For a single four-barrel carbureted engine Westech gets $800 a day, which includes (as needed) initial break-in, valve-lash adjustment, carb tuning, setting the timing, and recurving the distributor. When the engine leaves, Westech’s Steve Brul says its ready to go into your vehicle or boat (barring any major defects or repairs, of course). Fuel and other consumables are extra, or you can bring your own. The chassis-dyno is $550 per half-day for a single four-barrel applications; it includes engine tuning and optimizing the carburetor and ignition timing. Whether engine or chassis-dyno, Westech charges more for multiple carbs, forced induction, power-adders, and/or EFI.

Engine Dyno: Accurate Measuring of Torque and Power

First, the engine dyno, which measures torque and power at the flywheel. To set the stage, you’ll recall in its repaired form after installing the small-chamber TFS heads on the 393’s existing 0.030-inch bore x 3.85-inch short-block, static compression came in at 10.08:1. Comp Cams’ 291THR7 Mutha’ Thumpr hydraulic-roller cam specs out at 0.567/0.552-inch valve lift and 224/249-degrees duration at 0.050-inch tappet-lift. The TFS heads’ intake ports themselves flow 313 cfm at 0.700-inch lift, an important gauge of a normally aspirated engine’s power potential.

a close up of an engine © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Despite its provisions for all sorts of granular fine-tuning, Quick Fuel’s SS-750, 750-cfm, double-pumper needed only slight jet changes to perform well: 71 primary/77 secondary on the engine dyno; 71/71 “square” in the car.

Brule installed the engine with Westech’s standard set of Hooker Super Comp Cleveland dyno headers: 1-inch primary tubes dumping into 3-inch collectors and finally (good for pumping up the torque curve) out to atmosphere via 18-inch collector extensions. After a problem-free 20-minute break-in, tuning began, with the engine responding best on the engine dyno with 35-degrees total advance. Quick Fuel’s SS-750 double-pumper needed only a simple jet change, from its factory-installed No. 74 primary/65 secondary jets to a 71/77 combo. All runs were on 91-octane unleaded “California” pump premium gas, corrected using the SAE J607 performance-industry standard factor (roughly equivalent to the old-school muscle car “gross horsepower” rating method, it generates numbers roughly 4-percent higher than SAE J1349 net hp, the rating method in use for late-model cars).

© Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Hicks’ existing PerTronix breakerless distributor has a built-in adjustable rev limiter. There’s an LED light (arrow) on the module—the number of times it blinks equates to the rpm limit, in thousands. For instance, to set the limit to 7,000 rpm, turn the adjusting screw clockwise so the light blinks seven times.

Turns out, thanks to TFS, Comp Cams, Quick Fuel, Edelbrock, and Hicks’ pre-existing stroker short-block, the final, fully tuned-in engine dyno run shows—at least in small-block terms—we had spawned a real torque monster: a peak 500.8 lb-ft at 4,600 rpm with an overall torque curve flatter than the Texas panhandle from 3,500 rpm (the beginning of the sweep) all the way through to 4,800 rpm. Torque never fell below 497 lb-ft throughout that range, and output was at least 400 lb-ft or better all the way up to the 6,200-rpm test-range high-point. At power peak, the 393 churned out 512.3 hp at 5,800 rpm; it exceeded 450 hp from 4,800 rpm on up and made over 500 hp from 5,6006,000. Average output throughout the 3,5006,200-rpm test-range was 481.4 lb-ft and 476.9. Average torque peak-to-power peak numbers, important for establishing best e.t. shift points for max controllability in a car geared strictly for competition, came in at 482.5 lb-ft and 476.9 hp. I like the taste of this engine-dyno pudding!

© Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Clearing the big RobbMc fuel pump requires replacing the Cleveland’s standard FL-1A oil filter with a slimmer Motorcraft FL-400S, which is OE on 4.6L V8 mod motors installed in many early 2000’s F-150s, as well as many Ford and Chrysler econoboxes back to the 1980s. It has the same stud mounting threads and sealing surface as the classic filter but holds -quart less, so an oil and filter change on the 393 with its Milodon pan is only 6.5 quarts instead of the normal 7. We ran Westech’s standard dyno electric fuel pump on the engine dyno, but the RobbMc unit on the chassis-dyno.

Pretty darn good for a street-driven small-block. But (there’s always a “but”) Westech has an optimized, sealed dyno cell with blow-through fans (you can barely open the cell doors with them on), plus strictly controlled coolant and oil temps. Typically Westech’s engine dyno runs start at a 140-degree F coolant temp and (at worst) end with the temp no higher than 160 degrees. Great for generating accurate and repeatable numbers (Westech routinely recalibrates its dyno), but—let’s face it—engine dynos aren’t truly representative of real-world street conditions, in a hot day-time environment, with varying vehicle weights and inertia effects. In other words, how we drive our hot rods in the real world. We were about to find out how well these great engine dyno-generated numbers stack up against in-vehicle numbers generated “at the wheels” on a chassis-dyno.

a close up of text on a white background © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Corrected to SAE J607 and running on 91-octane pump gas, on the engine dyno at the flywheel the 393 developed 500.8 lb-ft of torque at 4,600 rpm and 512.3 hp at 5,800 rpm. Note the flat-as-a-pool table torque curve. The only parasitic drag was the engine-driven mechanical water-pump. 

Chassis Dyno: How Much Power Do You Lose Through the Drivetrain?

a blue car parked in a parking lot © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Most chassis dynos extrapolate an engine’s power and torque based on vehicle inertia. Many variables can affect chassis dyno results. It’s a great tuning aid, but Westech’s Brul maintains “you can’t directly equate the reported torque and power numbers back to engine dyno results, or even the results obtained from two different chassis dynos.” Operator experience is critical for obtaining consistent results from small incremental changes.

When it comes to chassis-dynos, over the years, we’ve heard opinions ranging from a “flat 60hp loss to the rear wheels” to, more accurately: “Expect up to a 25-percent drivetrain loss to the wheels compared to flywheel numbers.” According to Brul, the 25-percent rule applies to typical old-school automatic trannys and loose performance torque converters; stick-shifts may lose slightly less because there’s no converter slippage—according to some sources, about 1517 percent. “I’ve seen a 700 flywheel hp engine lose 200 hp—nearly 30 percent—at the wheels because they have a 4,500-rpm stall-speed converter and 16-inch-wide rear tires,” remarks Brul. “Wheel weight, tire size and weight, and how fast you are accelerating that tire/wheel mass can drastically change a chassis-dyno’s recorded results. Then there’s airflow through the engine compartment, oil and coolant temperatures, inlet air temps, air cleaner size, the effect of engine-driven accessories, airflow quality through the engine compartment with the hood shut, how tight the dyno’s tiedown restraints are, and on and on we go.” Keep this in mind as we evaluate the chassis-dyno tests.

a close up of an engine © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Not obtaining full throttle has tripped up the best of us. AEW’s Sanchez modified Hicks’ aluminum-plate bracket left over from the previous low-rise intake/econo-carb installation. “I had to redrill the holes and change the angle, so the cable points and pulls 100-percent straight.” It bolts to the existing boss on Edelbrock’s Air-Gap intake. 

Initial Chassis-Dyno Tuning

Sanchez installed the 393 in the Mustang and drove the car back to Westech. In the car, initially the engine had difficulty making a full chassis-dyno run; it was lazy over 5,400 rpm, down nearly 160 hp and 77 lb-ft compared to the engine dyno results. Working together, dyno operator Ismael Candia and Sanchez determined Hicks’ electronic ignition wasn’t receiving a full 12 volts needed for its proper function. Adding a relay cured the problem.

a circuit board © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Hicks’ breakerless distributor needs a full 12 volts in run—which it wasn’t getting due to voltage drop in the coil’s ballast resistor wire left over from the OE points ignition. Installing a 30-amp relay from Westech’s spare parts bin was a quick and easy solution. When triggered by the old resistor wire, it sends full voltage to the coil. A second relay sends 12 volts to the carb’s electric choke. The Parts List lists functionally identical Hella universal ISO weather-resistant relays.

On an engine dyno, the ignition advance is locked-out and only total timing is important. On a chassis dyno, with a road-driven vehicle, the total ignition curve becomes important. By tweaking the distributor’s advance curve, Candia found as installed in the vehicle the engine made as much power with just 7 degrees of base timing and 30 degrees of total timing (base plus centrifugal advance, as read at the crank) as it did with higher total advance. Candia speculates it may be temperature-related, as the ambient air around the chassis-dyno is significantly hotter than the controlled engine dyno cell. As a rule, small-block Fords seem to need less timing than the General’s classic small-block. In any event, less timing is better than more timing if it doesn’t cost any power: The greater the advance, the more the engine is theoretically working against itself because the piston is still rising towards Top Dead Center. And theory aside, 30 degrees offers detonation insurance if Hicks ever gets a bad load of gas.

a close up of an engine © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Quick-Fuel’s SS-750 double-pumper carb is one of the few premium carbs that still retain a functional electric choke combined with high-end features like replaceable air-bleeds and screw-in PVCR restrictions. Note the vacuum-advance connected to spark-ported vacuum and fabricated G&J stainless steel dual-feed fuel line.

As installed in the vehicle, the carb ran a little rich on the top-end, so Candia leaned out the secondaries by six jet numbers, from the engine dyno-happy No. 77s to No. 71s. But other than carefully adjusting the carb’s four-corner idle-mixture screws, there were no other fuel metering changes needed.

Rear-Wheel Power Losses Analyzed

After sorting out the basic tune, Candia made runs without an air cleaner but with all engine-driven accessories hooked up and functional. At the wheels the car made a best of 379.8 rear-wheel hp at 5,362 rpm. Torque peak occurred at 3,657 rpm, with 427.5 lb-ft at the wheels—a drop of about 73 lb-ft and 132 hp compared to the engine dyno. In other words, about a 15-percent torque loss and 24 percent horsepower decline. Hicks was running a T5 manual trans, so according to tribal knowledge that seems a little higher than expected—what was burgling the top-end power?

Analyzing the data later, we homed in on Hicks’ original Ford mechanical steel flex fan as one power thief. Back in the ancient pre-internet days we ran extensive engine-fan tests. These tests established that a bottom-feeder stock, no-clutch, mechanical fan robs about as much as 50 hp on the top-end (no bull). Even the best steel-bladed flex-fan still costs upwards of 20 hp. The higher the rpm, the more a mechanical fan drags the engine down and the greater the loss. Without the fan we’re looking at a 112.5 hp drop (22 percent loss).

a close up of a motorcycle © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Don Hicks stock Ford flex fan: Even the best flex fans cost up to 20 hp, many much more (for the ultimate fan horsepower-loss test, see my article “Save Power and Keep Cool with an Electric Fan,” Car Craft, print edition, May 2000). If running a mechanical fan, a standard (not heavy-duty or towing) thermostatically controlled clutch-fan has the least horsepower drop.

The Mustang’s massive 9-inch Ford rearend might be another: While a fully-built 9-inch Ford is probably the strongest rearend out there, due to its greater hypoid distance (the distance from the center of the ring gear to the center of the pinion gear) it’s less efficient then a 12-bolt Chevy or Dana 60. Years ago, Car Craft logged the power to the tires on a Chevy 12-bolt versus a Ford 9-inch. The Ford lost about 2.6 percent on average from a performance engine that made a peak of 326 rwhp at 5,800 rpm. For more on the 9-inch Ford’s great strength, see my article on HOTROD.com.

On the other hand, the late-model T5 has smaller internal gears lubed by slippery auto trans fluid, so it’s theoretically more efficient than a legacy four-speed. Head-scratchin’ time, again.

Then there’s a chassis-dyno’s typical 3040-degree higher engine coolant and oil temps to consider, parameters that the standard dyno correction factors—based as they are on atmospheric pressure, humidity and air temperature—don’t really account for.

In place of Westech’s 1-inch engine-dyno headers, Hicks runs Ford Powertrain Applications (FPP) 1-inch primary-tube headers secured by ARP fasteners that exit via a 3-inch dual-exhaust system with a crossover pipe and DynoMax mufflers—though FPP owner Stan Johnson maintains, “In a Cleveland motor, running under 7,000 rpm, that -inch primary tube size difference makes little or no difference; 2 to 3 hp at most.”

On the other hand, theory holds that—except for torque converter flash on an automatic trans-equipped car—the peak torque and power rpm points should be the same on a chassis dyno as they are on an engine dyno. But in these tests on the chassis dyno the torque and power peak rpm points spread out wider (had a greater rpm separation) compared to the engine dyno numbers and they also occurred at a lower rpm. This could be because of smaller header tubes plus a full exhaust system, or because a chassis dyno extrapolates engine rpm from wheel speed and sometimes lags engine speed (somewhat analogous to a slow tach) or both! (See what we mean about all the variables?).

Two hp here, 5 hp there: These numbers and variables start adding up and suddenly you’re 100hp or more down. And we haven’t yet considered the air cleaner.

a close up of a map © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

The best chassis dyno horsepower numbers were down about 22 percent versus the engine dyno. Interestingly, torque and power using a K&N air cleaner with -inch dropped base were slightly better over the full curve then with no air cleaner. Unfortunately, we had to install a 1-inch-drop base for hood clearance—which cost us another 14 hp on the top-end. 

But Will the Air Cleaner Clear a Mustang Hood?

Ah, the air cleaner. Real street cars need to run one, but not all are created equal. The rule of thumb is that there’s no such thing as too big an air cleaner. 19711973 Ford Mustangs are notorious for hood-clearance issues with the standard-performance flat hood—and Hicks now had a brand-new Edelbrock Air-Gap 351C dual-plane high-rise intake. Potential “da-oh” or “A-OK”?

a pair of shoes © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Hicks’ new Edelbrock RPM Air-Gap 351C is about 1.12 inches (call it 1) taller than his previous Edelbrock Performer 351C-2V. Is there a low-restriction air cleaner that clears a 19711973 Mustang’s flat hood?

K&N, the go-to air-cleaner guys, sent us a bunch of its Xstream Air Flow-series air cleaners. These are 14-inch-od open-element air filters with varying height filter elements and different amounts of base drop—plus the lid does double-duty as an additional filter element. Too much base-drop can restrict flow into the carburetor because the air must turn too sharply going into the venturis. Excessive drop can also interfere with the choke and choke linkage. In a perfect world, you want zero drop or even a raised base, but we knew going in that wasn’t in the clearance cards.

© Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

K&N’s Xstream series air cleaners have a unique “flow-top” filter-element lid for more filter area, which it says increases power and accepts more dirt without a performance loss. The Mustang needed 1-inch drop; K&N PN 66-3040 has that drop, a 3-inch-tall x 14-inch-od element, and a 2-inch overall height measured from the carb-neck flange. Summit Racing, the price source for most parts used in this project, sells it for $131.99 (as of February 2020).

On the chassis-dyno, a 3-inch standard-height filter element on a -inch dropped base had no overall restriction compared to running no air filter. At the peaks, this combination made 7 lb-ft more torque but 1.3 hp less than running with no filter. In terms of overall average numbers from 3,0005,750 rpm, it was higher by 2.9 lb-ft and 1.3 hp. That’s an insignificant difference on a chassis dyno—what the green-shade bean-counters call an accounting error. Unfortunately, it wouldn’t let the hood close.

We next tried a 1-inch dropped base, still with a 3-inch element. It gave us another -inch clearance, just enough to allow the hood to fully close while not interfering with the choke, but the dyno pull was down and more ragged. It cost 4.7 lb-ft at the peaks, and nearly 14 hp on top, with the engine peaking early at 4,989 rpm. We suspect the lower lid-height is interfering with airflow through the carburetor’s airhorn and possibly the air vents, causing an airflow restriction and some instability in the metering curve.

We also tried running a shorter 2-inch-tall element on the -inch dropped base, but the falloff was even worse. Without getting a Boss 351 NACA-duct hood or a custom bulged hood, this was as good as it was gonna get in the real world, in-car: 365 hp and 430 lb-ft.

a close up of a cluttered desk © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks a clock on a table © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

One way to determine air cleaner-to-hood clearance is by bench measurement. Measure the old filter assembly dimensions, including the filter thickness, the lid thickness, the base drop, and any hold-down protrusion above the lid. Don’t forget to account for the metal thickness.

© Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Verify your math in the using the clay method, especially (as in Hicks’ case) when dealing with a new intake and carb. Place clay lumps on the air cleaner lid’s highest point. Slowly and carefully close the hood—stop if you encounter resistance; otherwise, continue to “full-latch” closure. Open the hood and measure the compressed clay’s height—this is the amount of clearance. AEW’s Sanchez likes at least inch to account for engine rock. 

Is an Engine Dyno or Chassis Dyno Better?

If you want big, accurate numbers in a carefully controlled environment, the engine dyno is the way to go. Engine dynos directly measures true torque at the flywheel, so it will tell you how much power the engine makes in a perfect world. A properly controlled engine dyno cell, a carefully calibrated engine dyno, and a good operator is capable of test repeatability within 1 percent.

If you want to know how the engine will run in the real world as-installed in your vehicle and/or determine overall drivetrain performance and efficiency, then the chassis-dyno is the better choice. However, it doesn’t directly measure torque; instead it extrapolates output according to the chassis dyno manufacturer’s software logic. Also understand it’s harder to get back-to-back repeatability on even the same chassis-dyno, so expect at least a 5-percent variance in otherwise-identical back-to-back runs. There’s greater yet variance in results from different chassis-dynos then there will be on two different, but properly calibrated and corrected, engine dynos.

a close up of a device © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

The Fix is In

a close up of an engine © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

G&J Aircraft fabbed the -inch fuel plumbing from mechanical fuel pump to carb. Parts include dual-feed fuel stainless steel fuel log (A), inline fuel filter (B), Teflon-cored stainless steel braided hose (C), and miscellaneous 37-degree fittings. Don’t use rubber-core braided hose with today’s oxygenated fuels—they’ll rot from the inside-out (see “When Good Hoses Go Bad” on HOTROD.com). Debris can break off and clog the needle-and-seat or fuel-injection nozzles.

a close up of food © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

AEW and Westech use and recommend Lucas high-performance and racing motor oils—on the 393, Lucas high-zinc 30W break-in oil, and Lucas conventional 20W-30 Hot Rod and High-Performance Motor High-Zinc oil after break-in.

We’ve been saying this for a while now: The cost of modern aluminum heads is now cost-competitive with trying to recondition old, worn-out iron heads. In the Ford Cleveland line particularly, most heads have nonadjustable valvetrains stock, and for that engine family an adjustable valvetrain conversion is complex because the valve and rocker-stud centerlines cant in all three axes versus the cylinder-head deck. In this case, the botched conversion may also have caused the original cam failure, but, as Sanchez puts it, “The world may never know for sure.”

a close up of an engine © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Hicks wanted to retain a relatively stock look. Other than the cool-looking K&N air filter, you’d never know there was a 500-plus hp engine in there—until he starts it up.

The new Comp Mutha’ Thumpr hydraulic-roller cam really shined on the dyno and is performing great as a daily-driver in Hicks’ Mustang. We also like the Quick Fuel SS-750 double-pumper carb. Out of the box, it only needed basic curb-idle and fuel mixture adjustment, plus some slight rejetting. But it’s nice to know all those high-end granular tuning features are there if ever needed.

a man in a blue car parked in front of a building © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

Owner Don Hicks says, “It’s a beast! My wife won’t drive it. It’s too radical, you might say.” But wild horses couldn’t drag him away from this now-mean Mustang

a screenshot of a computer © Mark Sanchez,Courtesy Of The Manufacturers,Don Hicks

  • Engine dynos: best for accurate and repeatable measurement of an engine’s actual torque and horsepower numbers at the flywheel.
  • Chassis dynos: a great in-car tuning tool but can’t be relied on for highly accurate and repeatable power and torque measurements.
  • With an automatic transmission, experts claim a 2527-percent horsepower loss on a chassis-dyno versus an engine dyno.
  • With a manual transmission, experts claim a 1517-percent horsepower loss on a chassis-dyno versus an engine-dyno.
  • Many variables affect chassis dyno accuracy, but all these “little” things affect the results and the amount of power-drop compared to flywheel numbers.
  • A large, efficient, low-restriction K&N air cleaner with a -inch dropped base made about the same power as running with no air cleaner.
  • An air cleaner with a 1-inch dropped base caused a power loss but was needed for hood clearance on a 1973 Mustang.
  • Engine-driven mechanical fans cost power.
  • Free-flowing TFS aluminum heads may be more cost-effective then repairing old worn-out iron legacy heads and they make way more power, too!

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