Weiland 6-71 Blower powered 383 Small-Block

There’s a lot going on in this photo. Let’s start with the pair of Holley boost-referenced blower carburetors. These Holleys use special metering blocks that add power enrichment when referenced to manifold pressure below the blower. There’s more—the Weiand linkage and fuel-line kits. The fuel lines were a breeze to install. The linkage kit took some time to finesse but was not difficult. The key is to open the carburetors evenly with the least amount of linkage travel.

There’s a lot going on in this photo. Let’s start with the pair of Holley boost-referenced

Power Curves
First of all, bang the gong and call out the Marines—we made 706 hp with a little 383ci small-block with a venerable 6-71 and only 14 pounds of boost. That's pretty cool. We also made an impressive 603 hp with barely 6 psi of boost. But let's look beyond the “more boost!” peak horsepower curves to see what else our test reveals. Comparing the first two peak horsepower tests, the difference is less than 2 psi of peak boost pressure, increasing from 5.8 to 7.6 psi. Despite this small boost increase, the engine gained 39 hp at a peak of 642 hp. Now let's look at the change in horsepower between test 3, with 11.7 psi (694 hp) and test 4 (706 hp), with 14 psi. This is a similar boost increase of 2.3 psi, yet the power gain was only 12 hp. The explanation for the smaller increase at the higher boost level is the enemy of all superchargers: heat. While we didn't sample the intake-manifold temperature, if we had we would have seen a major increase in heat with the higher boost levels of 11.7 and 14 psi. While we did add boost, and power did increase, you can see in the horsepower graph that the increase in power between tests 3 and 4 isn't nearly as impressive as the gain between tests 1 and 2. At the higher blower speeds and boost levels, the supercharger makes much more heat because it's working harder. Anytime you heat the air, it becomes less dense. So even though the engine sees a higher boost level, the increase in inlet air temperature reduces the density (oxygen content) of this volume of air. Oxygen is the key to making more power, and with less dense air, we have less oxygen. That's why the power improvement from test 3 to 4 isn't very good. This results in less performance for the addition of the same amount of boost. If we had kept going, eventually we would have seen that 16 or 18 psi of manifold pressure would not make as much horsepower as would 14 psi. More boost is not always better.

This straight-on view of the blower gives you a good idea of how pulley ratios work. The larger the crank (drive) pulley and the smaller the upper pulley, the faster it will spin the blower (driven) pulley. For the record, underdrive is the exact opposite with the crank pulley smaller than the blower pulley.

This straight-on view of the blower gives you a good idea of how pulley ratios work. The l

One way to improve this situation is to somehow cool the charge coming out of the supercharger. This can be done with an intercooler, which is unwieldy with a Roots blower. Another way is with water injection, which works very well. A third method is to use E85 fuel. This 85-percent-ethanol fuel enjoys a very high latent heat of vaporization, which means it absorbs a lot of heat, reducing the blower discharge temperature and increasing inlet air density without affecting boost. Plus, E85 has a nominal octane rating of 105, which makes it an outstanding fuel for superchargers. You will need special carburetors, an E85-compatible pump, and fuel lines to move the additional amount of fuel needed when using E85, but it's a great idea that works extremely well.

Good Gas
Because we were starting with a high static compression ratio of 10.9:1, we needed to use a high-quality, high-octane fuel. Rockett's best fuel is a 118-octane blend that mixes 120 RON with 115 MON fuel to deliver that 118 R+M/2 octane rating. The key is the high MON number when it comes to blower engines. If we'd had access to a set of E85 carburetors, it would have been fun to see how Rockett Racing's race E85 fuel would have performed. We're betting the power would have jumped at least another 20 to 30 hp. With the Rockett race gasoline, we exercised restraint with only 28 degrees of total timing. A lower static compression ratio would have allowed for more timing and perhaps more power.