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

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.

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.

Basically rotrex sucks and turbos don't. I've driven a 240hp rotrex and it was a gutless bore. Sorry Richard.

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.

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?

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:

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.

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.

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.

2 words.

Power Delivery :bigtu:

Interesting, has anyone ran a setup like this on a Miata?

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:
https://www.miataturbo.net/attachmen...ine=1450017575

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.

^^^ 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.

Math

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.

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...

Can't you salve all this by driving the compressor with an electric motor? :giggle:

Why drive the compressor with electric when you could just get an electric supercharger?!

I like where this is going. More complexity, more weight, less torque, less reliability. It's all good.

Have the supply of satire gone dry ? :facepalm:

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.

I commented on how to improve midrange torque on a Rotrex. Never said it could match that of a turbo.

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

A CVT drive setup inside the supercharger would work better I bet.

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.

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.

Planetary gears and clutch plates (2 speeds)

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.

https://www.miataturbo.net/attachmen...ine=1450051787

Rotrex was working on it?

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).

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.

Or

Procharger's i-1 Variable Ratio Supercharger - Intelligent Design

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.

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

Ummm. No it doesn't flow as well, and never will. Check out these ports.

Mazda B series.

https://www.miataturbo.net/attachmen...ine=1434649947

Honda B16

https://www.miataturbo.net/attachmen...ine=1434985778


Wonder which will flow better.

You need to read about what HP and torque are. And reread my post after you have a clear understanding of what each are, and how they are mathematically calculated or measured. Torque is measured, it's twisting force. HP is calculated by the equation HP = (torque x RPM) / 5252 if memory serves me correct.

Sure stock, but a highly developed head looks nothing like those cross sections. A BP-05 fully developed can flow close to a BP-4W fully developed. The starting points are much further apart, but the BP-05 has a lot more meat in the head to be worked.

The ports aren't the only part of the equation. Because the piston sizes for all 1.8L engines are all around the 84-85mm diameter, you can only fit 4 valves of a certain size in there. Valve lift, duration and ramp angle are restricted by mechanical boundaries which are the same on all engines (ie piston positioning) so as long as the head flow isn't the bottleneck and can supply a clean laminar flow the engines will perform similarly.

I think the advantage a Honda has is less to do with the flow through the head and more to do with how well it can rev. That's harder to match with a BP engine.

Anyway, my point is... A Rotrex is a good match for a highly developed BP race engine, as it is an application where torque is not required as much as power.

I always try to equate everything to water and how it flows. It makes sense to me as I can visualise it better! You can do this with electronics too.

So Torque is basically how big your bucket is. It's a measure of strength and how much someone can lift. Whilst power is how much water you can move in a minute. It is a combination of how fast someone can run multiplied by how much water they can carry.

A Rotrex BP hitting 300whp @ 8500 will only have a torque rating of 185ft lbs. So it can only carry a small bucket of water (18.5L) but can sprint the water forwards and backwards rapidly (ie 8500 rpm), so it can fill the tank to 300L within a set time period. (replace 'L' with your preferred unit of choice)

A turbo BP hitting 300whp @ 4500 will have 350ft lbs of torque. So the turbo car has a much bigger bucket (nearly double), but it can't move as fast. It takes longer between trips but can fill the tank to the same 300L mark as the sprinter.

A diesel truck can hit 300whp @ 2000 and has 780ft lbs of torque. It has a massive bucket but a low cyclic rate... but a couple of trips will fill the bucket to the the same level.

Each of those options, if they can operate at their peak power will be equally as fast as each other at filling the end tank. The end tank is a representation of total power, which is equal to acceleration.

Where it gets complicated in a car is that the torque curve isn't flat, ie the bucket size isn't constant. Also the gearbox isn't a CVT so the engine must cycle through the revs. The Rotrex BP will have close to a flat torque curve in the upper RPM range (ie it's bucket stays the same size) but it's peak power will be somewhere very high in the rev range. The engine needs to stay as close as possible to it's peak power to maintain it's work rate. The further away from that peak, the less water it can move per second (ie the person with the bucket has to sprint forwards and backwards to move the water because they have a small bucket. Any slow down in the speed also slows down the rate of water being moved).

The Turbo BP will also have a flat curve and peak torque in the midrange somewhere. This means there is a far larger rev range for the power to be generated over. ie, the person with the bucket can jog backwards and forwards and it matters less if they slow or speed up a bit, as the size of the bucket stays the same and can move lots of water in one go. It does take a while to get going though which is a downside of a turbo, ie turbo lag... the first trip the bucket is small, and gets bigger with the first few goes.

Not sure if that helps...

I also use the bucket analogy to explain this.

I talk about how if you have a weak handle you need to lift half as many rocks, at double the speed to do the same amount of work to stop the handle from breaking. But if you have a set of forged handles you can fill the bucket to the brim with rocks and do it at half the speed for the same amount of work done, or the same speed for double the work.

I use the amount its filled to represent torque, the handle to represent internal strength, and the amount its filled multiplied with the rate at which you fill and empty it to explain the work being done over time, which is HP.

Dann

or install a $20 boost control solenoid on your turbo setup...

Actually, the rods are subject to more force from RPM than they are from torque (as long as you arn't deting or pinging). Doubling the torque only increases stress on the rods somewhere between 20-50% (forget the exact number and don't feel like mathing) but the relation on rod stress to rpm is a 3rd or 4th power relation.

So a better metafore would be that you can load the bucket with a weak handle with a lot of rocks, if you move it very slowly. But a stronger handle will let you move the same rocks faster.

What up the boost from 0PSI? Would need a turbo for that to help!

I know this is a turbo forum but for a track car a Rotrex is ideal. They are an easy install, will run all day, get the power where you want it, ie up high to protect the gearbox, are safer on the engine and have no lag.

They don't really work well on a stock motor that is driven on the street and the dyno results show that.

Look at the angle of the Honda ports. The head is much taller which allows for totally different entrance angles. You say look at a ported BP head, but what about comparing that to a ported B16 head.

all im suggesting is that you can more or less replicate the power output of a rotrex on a turbo setup. That's all.

still an easy install, run all day, and get the lack of power you want. just as easy on the gearbox and engine, but you'll still have the "lag".

The BP was designed to power the gasoline versions of these, not sportscars:

https://www.miataturbo.net/attachmen...ine=1450114580

you want a 1.6 B6 in your sportscare, amirite :likecat:

I've seen the car you speak of built. 430whp from a big unrestricted Rotrex. Redline was 7400rpm. My 350whp turbocharged car made more horsepower from idle to 6700rpm.

clearly, you've never been on track :giggle:

Response to Madjak:
The Honda doesn't rev better because it has a miracle bottom end design. It revs better because the heads breathe well and ours do not. We have engines with limited engineering $ invested in them. They are designed to be reliable, inexpensive, rudimentary, and make moderate mid range power. They are not designed to be performance engines.

Why do rotrex make more horsepower than torque? They increase speed linearly in relation to the crank speed and crank speed is integral to the calculation of HP.
Again, math.

It's exceedingly rare that I look at a thread on this forum and think to myself "you know, this thread really deserved to be locked, m.net style..."

The 430whp track Rotrex was built by, and resides in the garage of a good friend of ours. A few interesting notes:

It was by far, the most reliable 300+whp F/I Miata I have ever seen or heard of. Nothing else even comes close. He ran that car for over a year (50+hrs of flogging in California heat) at 400whp (detuned) on what was basically a stock NB1 engine. Forged 9.0:1 pistons, forged rods and that's about it. Stock head and everything else. He did at least one 2hr enduro with it. Ran on 100 race gas.

Because of the very conservative tune and stock long block, it did not make any torque anywhere. The one I built with a blower two sizes smaller but with a built Whammy engine made a lot more torque through the midrange. Enough to rocket off slow turns below 4500rpm on the edge of wheelspin. So a Rotrex build that's easily capable of shredding teeth of every gear in an AZ-6 is easily done. Just no one has built one yet.

The question anyone who cares has to ask themselves, is how much torque is needed and how do you want it delivered. The latest EFR based turbo kits like TSE's, with a big exhaust and good engine management deliver pretty close to the perfect powerband and boost vs TPS response. IMO, anything with an older non EFR turbo, or tiny GT2554R or crappy engine management, gives up total performance to a well tuned Rotrex setup.

It boils down to having more torque but having to wait for the torque to build after asking for it, then having to modulate if its too much for available traction, or with the Rotrex having less peak torque but all of it directly connected to TPS like a big N/A engine.

The guys who have most fun drag racing, playing on the dyno or hooning on public roads, tend to like turbos because of the big torque numbers, even if peak power is the same as an equivalent Rotrex. The Rotrex enthusiasts tend to be those who prefer canyon roads or race tracks and don't feel like working on the car to get it to stay together. Two different users.

I'm looking forward to driving a car with TSE's 6258 kit. FWIW, we have a street car here with a C30-94 and mildly built engine. We're aiming for 260tq/330whp on 91 at about 7000rpm.

Car was still very fast on track, but had to be driven to match its powerband.

The point is that they have pros and cons. They are not "ideal" for any application, same as a turbo is not "ideal" for any application. It's a personal choice to sacrifice powerband for response. This forum is full of turbo enthusiasts (myself included) who value midrange torque over N/A-esque response, so we all like turbos. Rotrex owners value the response over all else, and that's fine. They are polar opposite kits and I've never talked to someone who's been trying to decide between them for longer than about 30 seconds. You're either a turbo guy, or a Rotrex guy. Hence the endless :magna: on the topic.

As Emilio alluded to, my goal with the TSE EFR kit is to blend the two. Turbos have always had the edge on powerband, so all of my effort has gone into maximizing response.

May as well just shut down the supercharger subforum if threads can't go half a page without trolling.

There are some design limitations that can't be overcome with a BP most of which limit the max revs. Head flow is not the issue and can be made to work with good porting... it's in the vibration frequencies and valve train. A Honda motor revs well because the engineers designed the engine with revs in mind. And yes they do have a better bottom end.

The power curve on my engine is still heading upwards at 8300... it needs to rev to 9000 to hit peak power. It could probably get there a few times on a dyno but not constantly and risks floating a valve. I will be upgrading the valve springs to help stop the valve float and when I do I think 220whp is achievable with a BP-05... You don't get power that high with a head that can't flow. Its the valves, intake and exhaust that limit a ported head.

In terms of turbo vs rotrex, I know where I sit. There is something about driving a fast N/A car that fits with my driving style. Every turbo car I've driven on the track has left me wanting better response. They are just so hard to push at the limit through sweepers and on corner exits. Rather than balancing the car through corners you're left fighting the car itself.

A good example is the current F1 season. The turbo cars have lost the magic of the old high reving V8s. The low down torque, strangled exhaust, difficult setup and engine faults all highlight why I prefer F1 from the past. Sure the cars are still fast but at the cost of all the great things in the sport.