Why do Rotrex suck at torque? - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 12-13-2015, 03:55 PM   #21
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Can't you salve all this by driving the compressor with an electric motor?
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Old 12-13-2015, 03:58 PM   #22
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Can't you salve all this by driving the compressor with an electric motor?
Why drive the compressor with electric when you could just get an electric supercharger?!
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Old 12-13-2015, 04:26 PM   #23
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I like where this is going. More complexity, more weight, less torque, less reliability. It's all good.
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Old 12-13-2015, 04:35 PM   #24
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Have the supply of satire gone dry ?
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Old 12-13-2015, 06:24 PM   #25
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Originally Posted by codrus View Post
The bigger-rotrex-but-bleed-off-pressure approach wastes power compressing air and then venting it -

--Ian
Once you start venting you relieve the compressor of some of the work. In other words a C30-94 at 20 psi will require a LOT more force to drive than at 12PSI. While there will be some wasted energy it's nothing like a roots/twin screw style blower. All things considered the Rotrex is pretty efficient to drive.
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Old 12-13-2015, 06:36 PM   #26
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Originally Posted by k24madness View Post
Once you start venting you relieve the compressor of some of the work. In other words a C30-94 at 20 psi will require a LOT more force to drive than at 12PSI. While there will be some wasted energy it's nothing like a roots/twin screw style blower. All things considered the Rotrex is pretty efficient to drive.
On the flip side, the compressor efficiency goes into the dumps when you start to substantially increase flow without increasing pressure at the same time. I don't think I've ever actually seen someone successfully bleed pressure at high RPM, and it still won't match the powerband of a turbo.

The Rotrex does several things very well, but arguing that it can ever match the midrange power of a turbocharger is really silly.
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Old 12-13-2015, 07:17 PM   #27
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The Rotrex does several things very well, but arguing that it can ever match the midrange power of a turbocharger is really silly.
I commented on how to improve midrange torque on a Rotrex. Never said it could match that of a turbo.
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Old 12-13-2015, 07:47 PM   #28
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But it doesn't matter if you are wasting power when bleeding off air. The whole point of bleeding off pressure is to make less power...
Sure, but you're burning more gas, making more heat to get rid of, etc. It's much better not to make the pressure in the first place than to make it and then vent it.

I'm not saying it won't work, just that a variable-ratio pulley would be much better, if it could be made light and reliable.

--Ian
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Old 12-13-2015, 07:48 PM   #29
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A CVT drive setup inside the supercharger would work better I bet.
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Old 12-13-2015, 08:01 PM   #30
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Well, maybe a more efficient way would be to have a variable restriction on the intake side instead of venting air away? I'm thinking a salvaged drive-by-wire throttle body in front of the supercharger that would follow a boost target or in function of RPM could work.

This way, the compressor would stay more efficient than venting air away. The more I think of it, the more it looks like how a wastegate controlled via a EBC solenoid works for the turbos.
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Old 12-13-2015, 08:04 PM   #31
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Boost is a measurement of restriction. Less boost=less work. Doesn'tmatter where the less boost comes from. Venting before and restricting after are both forms of reducing the work that the supercharger does.

Scratch that it might work harder if you had a pre charger restriction.
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Old 12-13-2015, 08:06 PM   #32
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Planetary gears and clutch plates (2 speeds)
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Old 12-13-2015, 08:08 PM   #33
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Quote:
Originally Posted by Dalardan View Post
Well, maybe a more efficient way would be to have a variable restriction on the intake side instead of venting air away? I'm thinking a salvaged drive-by-wire throttle body in front of the supercharger that would follow a boost target or in function of RPM could work.
Probably just as bad as venting off the excess pressure. Maybe worse. Restrict the S/C inlet and the pressure ratio goes sky high, which drives the operating point out of the efficiency island of the compressor.
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Old 12-13-2015, 08:09 PM   #34
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Rotrex was working on it?
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Why do Rotrex suck at torque?-80-image_cfec66123fba43bb41a6e05a62b6277201f60ba6.png  
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Old 12-13-2015, 08:11 PM   #35
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Well, with the industrial centrifugal compressors I work with, a pre-charger restriction reduces the work done by the compressor. As there is less mass of air to move, the compressor effectively "free-wheels", exactly like how a vaccuum cleaner motor accelerates when you close the opening with your hand.

That, for sure, would be an interesting bench test to build. What is worse, uselessly moving air or uselessly building a high pressure ratio? As the goal with reducing the boost is to limit the power at the wheels, I would think efficiency is not a major concern except if it is for an endurance racing car that would be challenged with fuel consumption. Maybe venting air after the compressor is a better choice as the IAT would be lower (same energy injected with more mass of air = lower rise of temperature).

Last edited by Dalardan; 12-13-2015 at 08:27 PM.
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Old 12-13-2015, 09:22 PM   #36
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Trying to force a Rotrex to make mid power torque is the complete wrong approach. Use a Whipple or a roots if you want torque. A Rotrex is best suited to very high RPM track car that puts down controllable linear power between 6000 and 8500 RPM and does so without destroying gear boxes and diffs. Such a car has no need for any sort of power below 6000.

If ever I get around to it, that's exactly what I want to build. A 400hp+ Rotrex, 6 speed, 4.3 diff, powerband between 6250 - 8500rpm. To get that sort of power you need a very efficient intake/head/exhaust, a whack of compression, E85 and the C30-94 Rotex... nothing too difficult and I'm surprised there aren't more cars like this out there.
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Old 12-13-2015, 09:31 PM   #37
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Old 12-13-2015, 09:35 PM   #38
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Quote:
Originally Posted by Madjak View Post
very efficient head.....
Because that statement does not describe a BP motor.
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Old 12-13-2015, 10:13 PM   #39
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Originally Posted by aidandj View Post
Because that statement does not describe a BP motor.
Sure it does... I get 200whp N/A out of a 1.85L BP engine. That's not bad for an engine of a similar era and design (ie 16v, twin cam). You take any 1.8 / 2.0L Toyota, Ford, Honda engine from that same time and apply similar mods and you'll get a similar power range +/- 10% per L. Sure the Honda engine might rev a bit more, but you _can_ also make the BP rev to 9000 with a bit of work.
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Old 12-14-2015, 02:29 AM   #40
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Quote:
Originally Posted by Madjak View Post
Trying to force a Rotrex to make mid power torque is the complete wrong approach.
It's only the wrong approach because variable speed transmissions between the crank and the blower are complex/expensive/heavy/unreliable/etc. It's the same kind of compressor as you have on a turbo, and that makes mid-range torque just fine because the wastegate-controlled turbine is functioning as the variable speed transmission.

I would think a CVT between the blower and crank would work very nicely indeed, except it would probably be outrageously heavy.

Current F1-style turbos are the best answer, though. Motor-generator on the turbine shaft, you can spin the turbo up with the motor, then once it's making boost you use the generator function to control boost instead of a wastegate, take power off the turbine shaft, turn it into electricity, and deliver it to the wheels with another MG on the crank. Very cool, but I suspect it's a least a decade away from being available aftermarket.

--Ian
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