Why do Rotrex suck at torque?
#25
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.
#26
Former Vendor
iTrader: (31)
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 15,442
Total Cats: 2,099
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.
The Rotrex does several things very well, but arguing that it can ever match the midrange power of a turbocharger is really silly.
#28
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
#30
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.
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.
#31
SADFab Destructive Testing Engineer
iTrader: (5)
Join Date: Apr 2014
Location: Beaverton, USA
Posts: 18,642
Total Cats: 1,866
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.
Scratch that it might work harder if you had a pre charger restriction.
#33
Former Vendor
iTrader: (31)
Join Date: Nov 2006
Location: Sunnyvale, CA
Posts: 15,442
Total Cats: 2,099
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.
#35
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).
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 07:27 PM.
#36
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.
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.
#39
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.
#40
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