Manifold Swapping for Roots Blower - 134 HP With One Bolt-On!

Since we will be putting this engine into a vehicle with a standard belt arrangement, we changed the water pump to the passenger-side inlet, standard-rotation pump. An easy way to tell the difference is by looking at the water ports on the back of the pump. Round ports indicate reverse rotation, and D-shaped ports indicate standard rotation. Fortunately, the Boss has an early timing cover that accepts both pumps. This is handy when you want to hook the lower radiator hose to the passenger-side radiator outlet.

The Boss does not come with an alternator bracket, so we used this trick piece from Millerspeed. Made of polished billet aluminum with stainless fasteners, it works with the blower and the early pulley system. For naturally aspirated engines, Millerspeed has cool aluminum Gilmer drive pulleys as well.

Our supercharger was equipped with a 6.00-inch drive and a 3.75-inch driven pulley, so it is 60 percent overdriven. To figure it out, we took the drive pulley, divided by the driven pulley to get the drive ratio and the percentage (6.00/3.75=1.60:1 or 60 percent overdriven). Overdriven simply means the blower is spinning faster than the crankshaft. Underdriven means the blower is spinning slower than the crankshaft. Think about it in terms of the number 1. If your drive ratio is 0.750, the blower is 25 percent under the number 1 and underdriven. If the drive ratio is 1.250, you are 25 percent more than the number 1 and overdriven. Simple, no?

The 174 was set up to produce 5.80 pounds of boost at 8,533 blower rpm. Using the pulley ratio math, that is about 5,400 engine rpm. Weiand puts the margin of safety between pump gas and race gas at 8.5:1 static compression ratio and 6 pounds of boost. The Boss has a static compression ratio of 9.0:1, so we were right in the ballpark.

The 174 is designed to fit around either the factory large-cap distributor or the small-cap aftermarket jobs. For this test, we used the MSD billet distributor that includes the steel gear you'll need for the Boss roller cam. All the tests were run with 32 degrees of total timing on 91-octane pump gas. We recommend using the fastest timing curve available and having it all in by 2,500 rpm.

On the first run, we used the 650 HP from last month's tests to see how it would work. We ran it out of the box with 69/70 jets and it made a peak of 465 hp at 6,800 rpm.

With a blower, if you can't get the air in, you can't make power. Weiand recommends a 750 cfm for virtually all blower engines up to 454 inches. We jetted the secondaries on a Holley 750 HP carb to 80/87 and gained an easy 13.7 hp for a total of 479.5 hp.

The 750 HP also had a boost-referenced power valve that the 650 did not. Boost referencing allows the power valve to open the enrichment circuit even though there is still a vacuum signal under the carb trying to hold it closed. By plugging the vacuum port on the baseplate and drilling a hole into the vacuum chamber that operates the power valve, this modification allows a vacuum hose to be run to the manifold under the blower where a zero-vacuum signal will allow the power valve to open at signals below the valve's rating.

We used 1 3/4 Hooker headers for the dyno test, but we've seen 15/8 headers do the same job without a significant loss of power. Superchargers force air instead of relying on exhaust scavenging, so header size is less critical. Use the headers that fit your car and are recommended for your engine size.

The Red Zone
The supercharger was working as advertised at 6 psi until we approached 6,000 rpm, where it really took off. With nearly 9 pounds of boost at 7,000 rpm on pump gas, you'd expect the engine to explode, but the cylinder pressure (BMEPs) didn't spike along with boost. High-rpm restrictions in the cylinder heads backed up the boost in the intake manifold, causing a reading that was inaccurate. With larger heads or a larger engine, the boost reading would have gone down and the horsepower would have gone up.