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Drivetrain Power Loss - The Brutal Truth

How Much Power Are You Losing Between the Crank And The Rear Wheels?

Photography by David Featherstone,

Everybody talks about horsepower, and the magazines play up to this with big-time flywheel numbers. But recently, chassis dynos have become very popular and are capable of measuring rear-wheel numbers well in excess of 1,000 hp. While it's common knowledge that you lose a little bit of power between the crankshaft and the rear tires, there's more to this equation than just the power it takes to spin that transmission and the rearend gears-a whole lot more!

We talked this situation over with Flowmaster's chief R&D man Kevin McClelland, and he sent us some very interesting data on two completely different power combinations. The first was a 357ci small-block Windsor Ford in Flowmaster-owner Ray Flugger's '63 Mercury Comet convertible using a late-model four-speed automatic overdrive transmission and 3.50 rear gears. The second involved tests on a '70 Buick GS convertible 455 engine belonging to Dave McClelland, Hot Rod TV host and the voice of NHRA drag racing. The Buick backed the big-block with a Muncie four-speed and a GM 12-bolt spinning 2.73 gears. Kevin built, engine-dyno-tested, and then chassis-dyno-tested each combination, giving us a unique perspective on the power loss that occurs when you bolt the engine in the car. The results illustrate how important even the smallest details can be in terms of producing the most power at the rear wheels.

FoMoGoThe 357ci small-block Ford in the Comet began life with a 302ci block utilizing a stroker 3.50-inch crank and 4.030-inch overbore using forged pistons, 10:1 compression, and a set of McClelland-ported Ford Racing aluminum GT-40 Y303 heads fitted with 1.94/1.60-inch valves. McClelland then slid in a relatively mild Crane CompuCam hydraulic flat-tappet camshaft measuring 216/220 degrees of duration at 0.050-inch tappet lift and 0.533/0.544-inch valve lift using 1.6 rockers. The intake is a Ford Racing Cobra EFI casting, while the headers were custom-built 151/48-inch pieces. McClelland decided to duplicate the 211/42-inch exhaust on the Comet with the same lengths of exhaust pipes and the same mufflers on both the engine and chassis dynos. The only difference was that the engine dyno used straight pipe while the chassis required several mild mandrel bends.

Once the engine was in the car, Kevin discovered that there was insufficient room between the radiator and the water pump for a large, electric cooling fan, which necessitated the use of a five-bladed, engine-driven flex fan.

With the engine in the car and everything hooked up to make it completely streetable, the car was placed on Flowmaster's SuperFlow chassis dyno to compare the flywheel numbers with the Comet's rear-wheel performance. At one particular data point, the little small-block lost a staggering 106 hp, which equates to an astounding 40 percent loss between the engine dyno and the chassis! This was the worst case, but overall the engine suffered from an average loss of over 36 percent across the powerband from 4,000 to 5,500 rpm (see Test A).

Kevin felt that the large engine-driven fan was a major culprit in this dramatic power loss, so he tested the little 357ci small-block again after removing the fan blade. The results showed that the fan killed as much as 22 hp at 5,200 rpm. More typical was a loss of between 13 and 19 hp between 3,600 and 5,900 rpm (see Test B). The accompanying charts reveal that the average power loss with just the simple engine-driven fan was 18 hp or an average of 6 percent across the entire powerband!

This is where the chassis dyno really begins to tell the brutal truth. When a little small-block Ford loses well over a third of its flywheel power, you need to start looking for where all that power is going. The combination of the automatic transmission and the torque converter absorbs much of it (see the "Where's It Going?" sidebar). Adding a manual trans like a T5, or Tremec TKO could reduce this parasitic loss by eliminating the slippage and pumping losses. The best you can expect to gain is another 10 percent, which is significant when you consider this is around 25 hp.

Buick BravadoTo confirm and compare this power loss, we also looked at a completely different combination, a 455ci big-block Buick Stage 1 convertible with a Muncie four-speed. The 455 employed a stock rotating assembly while Kevin used cast pistons, valves, and an oil pump (all from Hi-Tech) along with a custom-ground Crane flat-tappet hydraulic camshaft that specs out a 210/218 at 0.050-inch tappet lift with 0.432/0.448-inch lift and a wide 114-degree lobe separation angle. Kevin placed the compression close to the '71 engine specs at 8.3:1, increased the valve sizes to the 2.125/1.75-inch Stage 1 dimensions, and performed some bowl work. He topped off the engine with an Edelbrock Performer intake matched to the factory 750-cfm Q-jet and a Crane electronic-ignition conversion.

The cork in the system and what limits the peak horsepower on this combination are the stock cast-iron exhaust manifolds, which also meant that Kevin had to neck the exhaust pipes down to 211/44 inches to fit the manifolds. The exhaust was then routed to a set of SUV 50-series Flowmaster mufflers. Kevin also had to create a couple of 90-degree bends in the pipes in order to clear the dyno cell, which created a more restrictive system than what ended up in the car. What this means is that the observed differences probably would have been greater had the actual car exhaust system been used on the engine dyno.

With the powertrain installed in the car, including the exhaust and the stock GS air-cleaner assembly, Kevin ran a series of chassis-dyno tests to compare the power numbers. As you can see from the results in Test C, the combination of the Muncie four-speed trans and 12-bolt rearend exacted a price, which turned out to be slightly more than expected for a manual-trans car. The worst occurred at 5,000 rpm, losing 62 hp over the engine-dyno numbers, which represented a full 25 percent loss. However, the rest of the curve saw rear-wheel power differences between 14 and 20 percent. The overall average power loss penciled out at 21 percent, but if you threw out the number at 5,000 rpm, the average loss drops to a more realistic 17 percent.

Interestingly, the Buick was also equipped with a clutch fan, which we knew from previous testing draws very little horsepower. This fan was especially "soft" for power, because later on the Hot Rod Power Tour, the clutch gave up completely, requiring Dave to buy a new replacement unit.

ConclusionsIf you take only one thing away from this story, it's that the little things can cost much more than you think-especially five-bladed engine-driven fans. The hot ticket will be either to dial in a clutch fan that only operates when the engine is hot, or switch over to an electric fan. Every little detail can make a difference in your rear-wheel horsepower.

For the Ford, once Kevin eliminated the engine-driven fan, the AOD automatic transmission-equipped drivetrain still ate up an average 33 percent power. This may be the price for using an automatic, especially an AOD. By comparison, the manual transmission-equipped Buick looks much better, costing only an average of 21 percent power. That's still a difference of 12 percent power. If you're making 400 flywheel horsepower, that's an amazing 48 hp. That's a bunch.

We've heard average estimates of around 15 percent for most street cars, but based on these tests, that might be a bit low for an average. It appears that this average-power-loss number will change based on the specific automatic transmission used, with a greater percentage loss for the heavier, less-efficient units. This probably deserves a closer look into the efficiencies of different automatic transmissions. The bottom line is that there's rear-wheel power to be gained by paying attention to those details.

Test A
Small-block Ford 357 ci in a ’63 Mercury Comet with an AOD automatic, a 9-inch rear, and a 3.50 gear.
RPM Flywheel Rear Wheel Difference Differential
Power Power w/o fan
TQ HP TQ HP HP % HP %
2,500 384 183 - - - - - -
3,000 410 234 - - - - - -
3,500 420 280 270 180 100 26 - -
4,000 426 325 304 230 95 41 85 37
4,500 413 353 297 254 99 39 83 32
5,000 389 371 277 265 106 40 88 33
5,500 344 361 254 266 95 36 76 29
  Avg. 99 hp 36.4% 83 hp 32.7%
Test B
Chassis Dyno Fan Comparison
RPM HP HP Difference
w/fan w/o fan HP %
3,600 222 235 +13 6
4,100 281 298 +17 6
4,700 310 329 +19 6
5,200 321 343 +22 7
5,900 299 318 +19 6
Avg. +18 hp 6%
Test C
Here are the results of our engine-dyno versus chassis-dyno tests on the Buick 455. Drivetrain: Muncie four-speed and GM 12-bolt with 2.73 gears.
RPM Flywheel Rear Wheel Difference
TQ HP TQ HP HP %
2,250 435 187 367 158 29 18
2,500 448 213 378 179 34 19
3,000 467 267 392 223 44 20
3,500 454 303 398 265 38 14
4,000 428 326 371 283 43 15
4,500 384 329 327 280 49 17
5,000 329 313 263 251 62 25
  Avg. 42.7 hp 18.3%

Where's It Going?The basic fact is that any drivetrain uses power to transmit power between the flywheel and the rear tires. The biggest culprit in this power-loss chain is the automatic transmission. An automatic will consume power in two ways-slippage in the torque converter and line pressure. Slippage is an inherent function of any torque converter (excluding lockup converters) with a slippage factor of between 5 and 8 percent. This does not mean you lose 5 percent power, but anytime that converter gets hot, that is flywheel power that is lost to slippage. In addition, all automatics must drive a pump to create the hydraulic pressure required to apply the clutches and bands. This eats power. Heavier transmissions, like the GM TH400 or Ford C-6, for example, also require additional power to spin those heavy clutch drums. The quicker you accelerate, the more power these units eat.

But let's not forget the manual transmission or the rearend. If a gearbox gets hot, that's energy that's lost before reaching the rear tires. Gear multiplication (meshing of two gears) will always require power to complete because of the friction it generates. High gear in a four-speed, for example, is the most efficient gear since it merely connects the input and output shafts together. It is this 1:1 relationship that eliminates the slippage of a torque converter and makes a manual trans more efficient than an automatic.

Rear axles also eat power because of the sliding action of the hypoid gear design. The Ford 9-inch consumes more power than a GM 12-bolt because the 9-inch's pinion is placed lower on the ring gear for better gear contact. This makes it stronger than the 12-bolt, but it also requires more power to turn the gears because of increased gear overlap.

These are the main sources of power loss. The more attention you pay to these details, the more power will end up twisting the rear tires.

SOURCES
Griffin Racing Radiators
100 Hurricane Creek Rd.
Piedmont
SC  29673
Be Cool Inc.
310 Woodside Ave.
Essexville
MI  48732
Flex-a-lite Consolidated
7213 45th St., Unit E
Fife
WA  98424
800-851-1510
Vintage Air
10305 I.H. 35 North
San Antonio
TX  78233
800-862-6658
Fluidyne Racing Products
4850 Airport Dr.
Ontario
CA  91761
Spal Advanced Technologies
Des Moines
IA
Flowmaster
8-00/-544-4761
www.flowmastermufflers.com
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