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      10-31-2012, 06:44 AM   #317
Lieutenant Colonel

Drives: 08 E90 M3, 99 E36 M3 Turbo
Join Date: Feb 2011
Location: Concord, NH

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Because the planetary gear drive blowers like the Rotrex, HKS, and ASA spin about twice as fast as the Vortech, Paxton and Procharger single step up ratio gear drive blowers, the centrifugal effect begins a little sooner and boost build a little sooner. I think "a little" is the right description. The downside is that, at least thus far, none of these blowers have had the top end of the Vortech. Their small impellers are less efficient at high flow volumes and their complex gear drive system does not allow them to reliably be spun any faster.

So thus far, it has been a compromise between a little better midrange with flattening out on top or a little less midrange and no loss of flow up top. ESS used to use the ASA, but changed to the Vortech. ESS has previously stated their testing did not show any dramatic differences in power curves or reliability, but that it was simpler for ESS to use one blower in all of its kits and only the Vortech had the flow range for that broad use.

Whoever said they want to feel instant boost needs to study centrifugal superchargers. A centrifugal that makes 9 psi at 8400 rpm will make roughly 3 psi at 4200 rpm. It is just the nature of the centrifugal. The planetary gear drive blowers may improve on this a little -- would 4 psi at 4200 make you happy -- but not dramatically and they sacrifice some flow on the top end. If you want instant boost, the closest you will come is a positive displacement blower like a twinscrew of Eaton 4 lobe, but no kits are available for these cars. Or a turbo sized a little on the small size.

One of the car specific problems with instant boost is that the S65 is an 8400 rpm motor and high rpm motors just don't make much power at low rpm and this is also true when they are fitted with any form of forced induction. Another car specific problem is the 12.0:1 compression ratio. You simply cannot run much boost in the natural torque window (peak cylinder pressure) on pump gas. The centrifugal is actually well suited to this engine for these reasons -- its boost follows the engine's power curve and causes no real issues. It is also much much easier to tune than a turbo.

I know Rotrex has come out with the C38-91, which is higher flowing than the previous top of the line Rotrex models and that ASA is also working on a higher flowing blower or may already have it. But the inevitable larger impeller means it takes a little longer to get flowing, which diminishes some of the low to midrange advantage. Time will tell on how these work out. AA has already shared dyno tests of a built motor running a C38-91 at way more boost than Evolve is running and the power was not exceptional. It was on a development tune, so maybe a little more timing (always risky with 12.0:1 CR on pump gas) will help. ESS, on the other hand, has run the Vortech Si Trim to about 650 rwhp.

Exciting news in the world of centrifugals is coming from Procharger, which has developed a variable speed transmission that is user adjustable to alter the flow patterns. It can spin the blower faster at lower rpm to make boost sooner than any centrifugal on the market. Should be released later this year. It has a big body to house the transmission and may be best suited for large displacement V8s that have the torque to spare to drive the blower fast at low rpm. Time will tell.