Is there any extra power to be had from removing the accessories or swapping out a heavy flywheel?

The data we have from this series of tests is actually broken down into two distinct types: parasitic loss and moment of inertia—through there may be some overlap.

ls engine on a dyno for test run
Have you ever wondered how much power accessories are worth? What about heavy flywheels? What the heck…let’s test both! (Image/Richard Holdener)

Test Number One: Accessory Drive

For test number one, we decided to illustrate the power consumed (and therefore gained when removed) by the accessories. It should come as no surprise that running convenience items such as air conditioning and power steering both cost power. In effect, it takes power to drive what are essentially pumps.

The same can be said of the water pump (after all, it’s right there in the name) and alternator. This is especially true if an electrical load is placed on the alternator from high usage items like an electric fuel pump and/or thumpin’ stereo system.

For our test, we did not impose a load on the alternator, we simply free spun it, but the water pump and power steering pump were both pumping at full capacity, though the pump was not hooked to a steering rack. The test procedure was to first run the (stock truck) accessories, then replace them all with a Meziere electric water pump. In essence, we were outsourcing the power to drive the pump (from the dyno batteries), thus eliminating any losses.

The question before us was, how much power was it worth? A secondary question might also be is this something you could actually employ at the track?

Test Number Two: Flywheel

Instead of driving pumps and an alternator, test number two involved comparing a heavy flywheel assembly to a much lighter combo. The reason for the combo description is that, on the dyno, the flywheel was combined with a drive plate. The splined drive plate hub was used to transfer the power from the motor to the input shaft of the dyno (similar to the input shaft on a transmission).

The question before us was does the heavy steel flywheel and matching steel drive plate hurt power production?

To find out, we compared these hefty components to much lighter substitutes. Unfortunately, we didn’t have access to a lightweight, aluminum flywheel to test, so we did the next best thing. As luck would have it, all the junkyard LS motors we get come with something we usually just throw away. Since 99.5% of truck LS motors were attached to automatic transmissions, the motors all came with flexplates.

Lucky for us, we could easily attach a drive plate to the significantly lighter flexplate. For this test, the flex plate was merely doubling as a light flywheel, so save the comments about heavy torque converters (they don’t apply here).

After combining the 5.5 pound flex plate with the 11.2 pound aluminum drive plate, the combo tipped the (shipping) scale at just 16.7 pounds. This compares to 45 pounds for the 24 pound steel flywheel and 21 pound steel drive plate. All told, the changeover dropped 28.3 pounds, all of it reciprocating weight. It is also worth mentioning the O.D. of the aluminum drive plate measured 11.375 inches, while the steel drive plate checked in at 13.25 inches.

In the basic Moment of Inertia calculation (MOI = Mass x Radius Squared), the distance from the centerline being squared means changes in size can have more of an effect than changes in weight (mass). The aluminum drive plate changed both!

The Test Motor

To put these things to the test, we needed a suitable test motor. To that end, we reached for our 4.8L junkyard warrior. The iron block LR4 had started out life with a damaged cam, lifter, and rocker (let’s throw in a couple of pushrods just for good measure), but, was brought back to full (near that at any rate) health with some spare components left over from fellow junkyard warriors.

For this test, the LR4 was configured with a Truck Norris NSR cam from Brian Tooley Racing, a set of 1-7/8 inch, long-tube headers and Magnaflow mufflers, then finished off with a Hi-Ram Holley intake manifold. You might question the Hi-Ram (we would) on this application, but the motor was set to receive a (what else) junkyard Eaton M90 blower that required use of a Hi Ram. We figured why not get all the testing we can while the motor was up on the dyno, so the little BTR NSR cam got a big Holley intake. Holley also supplied the HP engine management system, while the 80 pound injectors came from the guys at Accel.

You can read all about that 4.8L Blower test in this article.

All testing on the accessories and flywheels was run on E85 fuel—again, we wanted to start with the same fuel we would be running the blower with. This combo did not need E85 fuel, though it did make a few extra horsepower when we swapped over from the 91 pump gas to the pump E85.

Test Results

The first order of business was to configure the 4.8L with the heavy flywheel, heavy drive plate, and (nearly) full accessories. Doing this allowed us to go step by step and minimize swap time and duplication.

Run first with the factory water pump, power steering pump and alternator (just free spinning), the 4.8L produced 385 hp at 6,600 rpm and 336 lb.-ft. of torque at 5,300 rpm.

The short(er) runner Hi-Ram intake allowed the mild BTR-cammed 4.8L to make power fairly high in the rev range. After running the accessories, they were replaced by a simple Meziere electric water pump.

The result of the removal of the accessories increased the power output from 385 hp and 336 lb.-ft. of torque to 404 hp at 6,700 rpm and 348 lb.-ft. of torque.

Now it was time to swap out the heavy flywheel and drive plate, since this run had also established a baseline for the flywheel test. After replacing the heavy steel flywheel and drive plate with the light flexplate and aluminum drive plate, the peak numbers jumped once again, this time to 411 hp at 6,800 rpm and 355 lb.-ft. of torque at 5,300 rpm.

The big question here is (unlike the accessories), did the flywheel test actually improve power, since gains only showed up under an acceleration (and not stationary load) test? Also, is a heavy flywheel sometimes beneficial to help launch a vehicle, since the rpm applied on the line was not yet being used to accelerate the vehicle?

Feel free to chime in or go check out the full video on the Richard Holdener YouTube Channel.

engine dyno chart
How much power does it take to spin the accessories. How much does it take to spin up that heavy flywheel? This test shows the results of both on the 4.8L LR4. To get things started, we first equipped the mildly modified (BTR cam, Holley Hi-Ram & 1-7/8 long tube headers) 4.8L with the factory FEAD system from a truck. Though we did not use the A/C, we did have the water pump, alternator, and power steering hooked up—but no load was applied to the alternator. Run with the accessories in this manner, the 4.8L produced 385 hp at 6,600 rpm and 336 lb.-ft. of torque at 5,400 rpm. After removing the accessories and replacing them with a simple Meziere electric water pump, the power output increased to 404 hp at 6,700 rpm and 348 lb.-ft. of torque at 5,300 rpm. The final test was to replace the heavy steel flywheel/drive plate assembly with a lighter version consisting of a flex plate and aluminum drive plate. The change netted an increase to 411 hp at 6,800 rpm and 355 lb.-ft. of torque at 5,300 rpm. (Dyno Chart/Richard Holdener)
ls engine on a dyno getting prepped for a test run
Our test motor started out life as a damaged (bad cam and lifter) iron block, 4.8L LR4 that came from the same place most of our “test” motors come from, a local wrecking yard. (Image/Richard Holdener)
BTR Camshaft in a box
From previous testing, the LR4 test mule was equipped with a BTR Truck Norris NSR cam. The No-Springs-Required (NSR) cam offered 0.498 inch lift (both intake and exhaust), a 212/22X duration split, and a 107.5 degree LSA. (Image/Richard Holdener)
706 casting number on gm ls cylinder head
This 4.8L came factory equipped with 706 heads. Running the NSR cam meant we did not have to upgrade the valve springs. We eventually tried replacing the 706 heads with the higher-flowing 799 heads from the L33 and saw no power gain. (Image/Richard Holdener)
holley hi ram intake with cover removed
Because we would eventually be running an Eaton M90 blower on this motor, the blower adapter plate required use of a Holley Hi-Ram. Since we were already on the dyno, we decided to run the Hi-Ram for this series of accessory and flywheel tests. Besides, the Hi-Ram would allow us more rpm to play with. (Image/Richard Holdener)
holley efi throttle body installed
The Hi-Ram was combined with a matching lid and 102/105mm throttle body. The large throttle body was more than enough for the mild-cammed 4.8L. (Image/Richard Holdener)
close up of exhaust system on an engine on a dyno
The exhaust system consisted of a set of 1-7/8 inch, long-tube Hooker headers feeding collector extensions, 3 inch pipes and Magnaflow straight-through mufflers. (Image/Richard Holdener)
man at controls of an engine dyno
Rather than rely on the factory ECU, we dialed in the AF and timing curves on the 4.8L using Holley HP management system. (Image/Richard Holdener)
front of an engine attached to a dyno
To start the test, we equipped the 4.8L with (nearly) full accessories, including the water pump, power steering, and alternator. No A/C was used, nor was the alternator subjected to any load. A fully loaded alternator might add as much seven to nine horsepower, to say nothing of a fully engaged cooling fan! (Image/Richard Holdener)
LR4 LS engine during a dyno test run
Run on the dyno with the accessories as described, the 4.8L produced 385 hp at 6,600 rpm and 336 lb.-ft. of torque at 5,400 rpm. Now it was time to ditch the accessories. (Image/Richard Holdener)
meziere water pump installed on an ls engine
The entire FEAD system was replaced with this simple Meziere electric water pump. Elimination of the accessories increased the power output from 385 hp and 335 lb.-ft. to 404 hp (at 6,700 rpm) and 348 lb.-ft. of torque at (the same) 5,300 rpm. (Image/Richard Holdener)
flexplate bolted to an engine
After testing the accessories, we turned our attention to reducing the weight of the flywheel. Pictured was the 24 pound flywheel we normally run on the engine dyno. (Image/Richard Holdener)
flexplate installed on an engine
For our test, the steel flywheel was combined with an equally heavy (21 pound) steel drive plate. These were utilized in our testing with the accessories, so run with these components and the Meziere water pump, the 4.8L produced 404 hp and 348 lb.-ft. of torque. (Image/Richard Holdener)
Flywheel installed on an engine
To simulate a light flywheel (we didn’t have the budget to buy an aluminum flywheel), we simply installed one of the many flexplates that accompanied our junkyard core motors. (Image/Richard Holdener)
flexplate installed on an engine
The light (just 5.5 pounds) flex plate was combined with a lightweight (11.2 pounds) aluminum drive plate. Note that the drive plate was also smaller in diameter (11.375 inches vs. 13.25 inches for the steel plate), something that further improved the moment of inertia. (Image/Richard Holdener)
flexplate on table
Prior to testing, all of the individual flywheel, flexplate, and drive plate components were weighed on this shipping scale. The total weight change dropped from 45 pounds to just 16.2 pounds. If nothing else, who doesn’t want to drop nearly 30 pounds off the weight of their car or truck? (Image/Richard Holdener)
rear view of an engine getting installed on a dyno
Swapping the components after each test was a matter of separating the motor from the dyno and switching over the flywheel and drive plate. (Image/Richard Holdener)
LR4 LS engine being run on a dyno test
Run with the lightweight flexplate and drive plate, the power output improved from 404 hp and 348 lb.-ft.. to 411 hp and 355 lb.-ft. of torque. The question for readers, is this power gain real since it only shows up in an acceleration test? A separate question, despite the power change, would a heavy flywheel be a better choice to help launch a stock car at the dragstrip? (Image/Richard Holdener)

Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.