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.