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Why do Rotrex suck at torque?

 
Old 12-12-2015, 05:50 PM
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Default Why do Rotrex suck at torque?

I dont know what else to add other than this is from my experience of looking at dyno graphs and seeing them make 230hp and 170 torque.

How can an engine make 120hp over stock and only pick up 70 lb-ft?

I just dont understand it. Or course everything I know about this stuff comes from and article in Hot Rod magazine What is Torque vs. Horsepower - Engine Power Delivery Explained - Hot Rod
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Old 12-12-2015, 06:35 PM
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Torque is twisting force, that's it.

Engines produce a certain amount of torque at whatever RPM you want to measure it at based on how much air/fuel it's burning at that RPM.

So if you can shove a TON of air in the motor at 2,000 RPMs, it will make a ton of torque at 2,000 RPMs.

The rotrex setup is terrible at low-rpm boost compared to pretty much any other forced induction method. Thus they don't make a lot of torque compared to the other forced induction methods.
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Old 12-12-2015, 10:07 PM
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compare a rpm vs boost trace of a typical turbo miata and a rotrex. Lets say they both make 10psi of boost peak. The supercharger is always increasing in boost as RPM increases but the turbo has a bit of lag then hits 10psi by 4k and holds it there till redline.

Lets say peak TQ is at 4500 RPM. At 4.5k the supercharger might be at only 5 psi and the turbo is at 10 psi already. Therefore the TQ and HP gains are greater for the turbo lower in the RPM range.

Near redline, where peak HP typically is, both the turbo and the supercharger are making about 10 psi. Therefore they both move similar amounts of air and can make the same TQ and HP at that RPM.

Turbos have better powerband due to their ability to build boost quicker and have control over it with things like electornic boost control solenoids.
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Old 12-12-2015, 10:41 PM
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Basically rotrex sucks and turbos don't. I've driven a 240hp rotrex and it was a gutless bore. Sorry Richard.
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Old 12-12-2015, 11:37 PM
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Originally Posted by patsmx5 View Post
Torque is twisting force, that's it.

Engines produce a certain amount of torque at whatever RPM you want to measure it at based on how much air/fuel it's burning at that RPM.

So if you can shove a TON of air in the motor at 2,000 RPMs, it will make a ton of torque at 2,000 RPMs.

The rotrex setup is terrible at low-rpm boost compared to pretty much any other forced induction method. Thus they don't make a lot of torque compared to the other forced induction methods.
So how does an engine like a rotrex 1.8 BP burn so much air and fuel at nearly 7000rpm to produce 230hp but only 170 lb-ft?

If the rotrex comes on late to boost and doesnt produce it until later in the RPM range why does the torque not follow the HP closer?

Maybe I just need to bone up on my maths and undestanding of engines and basic crap.
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Old 12-12-2015, 11:50 PM
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Originally Posted by cyotani View Post
compare a rpm vs boost trace of a typical turbo miata and a rotrex. Lets say they both make 10psi of boost peak. The supercharger is always increasing in boost as RPM increases but the turbo has a bit of lag then hits 10psi by 4k and holds it there till redline.

Lets say peak TQ is at 4500 RPM. At 4.5k the supercharger might be at only 5 psi and the turbo is at 10 psi already. Therefore the TQ and HP gains are greater for the turbo lower in the RPM range.

Near redline, where peak HP typically is, both the turbo and the supercharger are making about 10 psi. Therefore they both move similar amounts of air and can make the same TQ and HP at that RPM.

Turbos have better powerband due to their ability to build boost quicker and have control over it with things like electornic boost control solenoids.
I dont claim to be a genius hence my simple questions. I dont understand why a turbo can produce say 230hp at 10psi and 200 lb-ft where the rotrex will be at the same power and pressure levels but produce only 170 lb-ft.
Maybe I dont have enough info or understanding. Maybe a boost graph would help.

Or is it that the rotrex just doesnt build pressure early enough?
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Old 12-12-2015, 11:57 PM
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Horsepower is dependent on torque. A turbocharged car and a supercharged car could make the same peak horsepower, but different max torque, assuming torque peak somewhere before power peak.

The reason blowers suck at torque is because they are run directly off the torque the engine already produces. They are an additional load on the engine. Turbos do not have the same effect since they run off of exhaust gas (waste).

Here watch this video:
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Old 12-13-2015, 12:22 AM
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Originally Posted by Guy Farting View Post
I dont claim to be a genius hence my simple questions. I dont understand why a turbo can produce say 230hp at 10psi and 200 lb-ft where the rotrex will be at the same power and pressure levels but produce only 170 lb-ft.
Or is it that the rotrex just doesnt build pressure early enough?
basically

With same boost (lets say 10psi) the turbo has the ability to make that same 10psi WHEN/WHERE THE ENGINE REALLY LIKES IT (highest VE), vs the brotrex makes it at redline and doesn't care where the engine likes it most.
I actually think a centri is just fine in really high revving/high flowing applications (like an s2000) or on higher revving v8's (like the 5.0 coyote) that don't really need lowend grunt of a twin screw or turbo. On a BP though....mehhhh
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Old 12-13-2015, 12:40 AM
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Originally Posted by 18psi View Post
basically

With same boost (lets say 10psi) the turbo has the ability to make that same 10psi WHEN/WHERE THE ENGINE REALLY LIKES IT (highest VE), vs the brotrex makes it at redline and doesn't care where the engine likes it most.
I actually think a centri is just fine in really high revving/high flowing applications (like an s2000) or on higher revving v8's (like the 5.0 coyote) that don't really need lowend grunt of a twin screw or turbo. On a BP though....mehhhh
that makes sense thanks for the time to answer my questions.
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Old 12-13-2015, 12:58 AM
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Also remember torque can never be higher than hp above 5250rpm. That 220/200 setup you mentioned is probably 220hp at 6500rpm and 200 ft/lbs at 3000. If you take a similar sized rotrex and turbo, making the same boost, they'll probably make the same torque figures at their max hp rpms.
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Old 12-13-2015, 02:00 AM
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Solving the lackluster Rotrex torque curve is simple. Install a much larger blower size so you make maximum boost at a lower RPM then bleed the upper end with a wastegate.
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Old 12-13-2015, 03:02 AM
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2 words.

Power Delivery
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Old 12-13-2015, 09:01 AM
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Originally Posted by k24madness View Post
Solving the lackluster Rotrex torque curve is simple. Install a much larger blower size so you make maximum boost at a lower RPM then bleed the upper end with a wastegate.
Interesting, has anyone ran a setup like this on a Miata?
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Old 12-13-2015, 09:39 AM
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The whole reason why Rotrex'es do not make torque is because compressor speed is tied to crank speed.

A turbocharger has a shaft which is driven by exhaust gasses so it is independent of crank speed. The Rotrex compressor is driven by the crank so its compressor speed is set by engine RPM.

Look at the following compressor map:


Here you see the red lines being compressor speed: as you can see the pressure ratio (and thus more or less boost pressure) is almost linear in the efficient areas of the compressor map when set out against compressor speed. Also note the compressor speed does not change significantly for changing airflow.

So a waste gate controlled turbo will run more or less a constant compressor speed through the rev range of the engine; providing a fixed level of boost providing high torque at low RPM.

The Rotrex is bound to max RPM of the compressor; at this point it provides max pressure at high efficiency. The max RPM of the compressor is set to the max RPM of the engine. So unless the engine is spun to max RPM; the compressor lacks the speed compared to a turbo and it can only provide the boost pressure at high RPM of the engine. You cannot make the Rotrex work at low RPM of the engine as you would destroy the Rotrex by overspeeding at high RPMs of the engine.
Attached Thumbnails Why do Rotrex suck at torque?-compressormap.gif  
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Old 12-13-2015, 10:58 AM
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^^^ Based on what you're saying, someone should design a Rotrex system with a variable pulley system. At low RPM's the pulley shrinks to increase speeds (sucking in more air). As the engine speed increases the pulley expands out delivering good HP numbers with risking an over rev of the supercharger.
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Old 12-13-2015, 11:04 AM
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Originally Posted by my97miata View Post
^^^ Based on what you're saying, someone should design a Rotrex system with a variable pulley system. At low RPM's the pulley shrinks to increase speeds (sucking in more air). As the engine speed increases the pulley expands out delivering good HP numbers with risking an over rev of the supercharger.
No. You shod install a bigger rotrex and bleed off excess pressure.....like was already said.
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Old 12-13-2015, 11:10 AM
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Math
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Old 12-13-2015, 01:20 PM
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Originally Posted by aidandj View Post
No. You shod install a bigger rotrex and bleed off excess pressure.....like was already said.
The bigger-rotrex-but-bleed-off-pressure approach wastes power compressing air and then venting it -- a variable pulley (if such a thing existed and was actually reliable) would be significantly better. I've never seen such a device, though, so it seems likely that it's not really practical to build. Given the power consumption requirements of a big blower (which I've read can be 50+ hp), you'd basically be talking about putting something like the transmission out of a Kei car between the crank and the blower...

--Ian
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Old 12-13-2015, 01:44 PM
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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 -- a variable pulley (if such a thing existed and was actually reliable) would be significantly better. I've never seen such a device, though, so it seems likely that it's not really practical to build. Given the power consumption requirements of a big blower (which I've read can be 50+ hp), you'd basically be talking about putting something like the transmission out of a Kei car between the crank and the blower...

--Ian
Pro-charger actually made (makes?) a CVT style pulley.. But i imagine the overall response was that it is too complicated for no real benefit.
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Old 12-13-2015, 02:45 PM
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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 -- a variable pulley (if such a thing existed and was actually reliable) would be significantly better. I've never seen such a device, though, so it seems likely that it's not really practical to build. Given the power consumption requirements of a big blower (which I've read can be 50+ hp), you'd basically be talking about putting something like the transmission out of a Kei car between the crank and the blower...

--Ian
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...
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