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!