Harvey wrote:
Max torque 230lb/ft with 300hp? Maybe I'm missing something, but that seems terrible to me relative to a decent turbo setup. Not trying to rain on the parade, but what's the advantage to the Rotrex setup?
Depends on what you value. The greater low RPM boost from a turbo does not come without a price. Instant low rpm boost in a turbo means a nearly open exhaust (loud), small to medium sized turbo, lots of octane and relatively high base compression. I had a setup like that about 10 years ago with an FM kit running a GT2554R and open exhaust. Instant boost that quit at about 230whp on our 91 pump gas. Most turbo guys want more peak power so a larger turbo is used with lower static compression, increasing boost lag at low rpm. Personally, I hate any lag in TPS vs torque. As a a race driver, I'm fine tuning slip angle on the way into, through and past the apex by modulating torque. With a Rotrex car, it's just like driving a big N/A engine. Deviate feels about like an E36 M3 engine. Seamless power everywhere that builds with RPM. A turbo OTOH, is much more difficult to get anywhere near that linear TPS vs Torque relationship. Usually involves TPS and RM based 4 port boost control that takes some level of expertise to get dialed in without spikes or creep. Not impossible, just not PnP easy like the Rotrex.
Some folks don't mind that lag. I know a few friends that actually like that Vtec like hit of torque that comes whenever it's darn good and ready. To them, that "feels fast". The seamless, instant torque delivery of a good Rotrex build is fast, just doesn't feel as violent. That said, Deviate fairly leaps if you jump on the throttle at anything above 4500rpm.
Having been down the road of building a turbo setup that will survive sustained track use with a fast driver in summer heat, I know it isn't cheap or simple. A Rotrex build OTOH, is pretty much bolt on out of the box. Easy to tune, bombproof. No custom V-band manifolds, heat shielding, boost controls, water lines, high pressure oil lines. A good friend after driving Deviate said "5 bolts and you're a hero". Not quite that simple but you get the idea. His daily is a 300whp NB with a GT28 sized turbo BTW.
In any case, the 100lb increase in torque at the stock torque peak of 4800rpm, available instantaneously is "enough" for most drivers to put a smile on their face. Peak OEM torque is matched at 1500rpm. Look at the plot again. If I didn't state "supercharged" there most would assume a 3.0L high revving N/A motor.
There will always be the internet readers who are deeply disappointed if any Miata power adder doesn't exceed the power to weight ratio of a ZR1. I'm OK with that.
Keith Tanner wrote:
I thought the Rotrex planetary box was just a different way of dealing with the internal gearbox found on every centrifugal. Does it actually change gear ratios as the thing speeds up? It was my understanding that the only way to build boost faster on a centrifugal was to overspin it and then bleed off pressure at the high end.
Correct. Most folks are familiar with the Vortech centrifugal. Basically a big, low rpm belt driven turbo. They usually operated at a 2:1 drive ratio using helical gear, thus the noise. Being so slow, the compressor had to be huge to push any air. These types of compressor don't really become efficient until the tips of the blades are moving fast. This is why Vortech are know for having a bit of a dual personality, nothing at all down low then whoosh. The Rotrex using a gearless planetary arrangement (traction drive), driven at 9.49:1 (I'm not making that up) in the case of the C30 series. Being spun at 120k rpm, the compressor wheel can be tiny, about the size of a Garret 2560 give or take. It's also equally efficient. Almost no noise from it because there are no teeth meshing. So it is still technically a centrifugal but without the stereotypical inefficiency of a slow turning Vortech style blower. I think of the Rotrex as a belt driven turbo. You trade boost down low for instant response, greater thermodynamic efficiency, simpler and more reliable hardware. Not everyone's cup of tea but for some people, it's spot on perfect. Now if we can just figure out a way to drive one with electric motors at whatever speed we want..
