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I'm still referring to the flatter torque curve powerband. That C38-74 would have a crazy powerband like Sav showed a page back.
What do you think the C30-94 would do on a fully developed Whammy engine? I'm not sure how a higher flowing engine behaves as you up the Rotrex sizing. I'd expect some movement of the powerband up revs as the Rotrex flow increases.
The C30-74 made around the 300whp with a nice powerband peaking around 6500. Do you think the C30-94 would hit the 280ft lbs and close to the 400whp mark but peaking another 1000 up the revs?
Who did the c30-74 make 300 for. Didn't it make like 240 on Mobius' CNC head built engine.
My engine at last dyno, with the cnc head / stock 10:1 pistons but with stock muffler unfortunately, made 217whp/166wtq. I will point out that my C30-74, during the entire time I owned it, seemed to make somewhat less power than expected compared to other C30-74 dyno plots. Something about my C30-74? Maybe my local dynos read lower than others (dyno dynamics and dynojet)? Dunno. Trackdayhookey had a rotrex build that made much much more than 300 but that was not a C30-74, it was a larger compressor.
Originally Posted by Joe Perez
This is just one of those situations in which I acknowledge that I am not a guru-level source of subject-matter knowledge, and instead ask myself "Which OEMs or professional racing teams are using this technology, as opposed to some readily-available alternative?"
Centrifugal superchargers were not uncommon in OEM automotive applications during the period of the 1930s-1950s, a time in which they were also a dominant technology in military aircraft.
In subsequent years, advances in precision machining, metallurgy and lubrication technology enabled other methods of forced induction to become dominant.
So I ask, right here in 2015, which top-level professional racing teams are using centrifugal superchargers? Which OEM automakers are using them?
Or have we just latched onto them because they seem esoteric and unique, and it's tempting to believe that we've discovered some hidden secret?
The thing is, Joe, as Emilio pointed out earlier, you just don't get it. Your analytical stance re: the rotrex is comparable to the analytical stance of many re: the Miata in general. "It has no power! Why the **** would I buy that?" The answer is that the Miata just works, and does what it was intended to do, which was to deliver an awesome driving experience. How does one objectively quantify "awesome driving experience?"
One can't.
Similarly, the Rotrex just works. It takes the awesome driving experience delivered by the Miata, and improves it. It introduces only minimal complexity, and places minimal stress upon the drivetrain. There is zero chance of boost creep or overboost. The throttle response is razor sharp. There's no chance of things melting under the hood. There's no additional water & oil line connections to possibly leak. The Rotrex oil lines might leak, but that's not going to kill your engine.
I flogged mine for 6 years. Other than an initial belt alignment problem, I changed belts once and changed the rotrex oil once in 34,000 miles, 3k of which was on track. Essentially zero maintenance. No studs backing out, no cracked manifolds, nothing melting under my hood. I like to work on my car, and I like to drive my car, but I absolutely ******* hate working on my car when I'm supposed to be driving it. The Rotrex was perfect for this. I'm only switching to an EFR now because 1) life circumstances afford me an opportunity to look at a clean slate build; 2) my engine is balanced with an ATI superdamper, which has a larger crank pulley diameter, which makes upgrading to a C30-94 problematic because it has a lower RPM limit than the C30-74, and a balanced engine is too sweet to simply replace that damper; and 3) TSE has released their manifold, and I know Andrew has spent the last several years of his life working 24/7 to create a manifold & turbo solution that addresses every issue that kept me from going turbo initially. No pressure, Andrew
What makes a Rotrex a great solution for a number of people has nothing to do with the area (or lack thereof) of the torque curve produced. It makes a Miata better, period. Is it the best solution for everyone? No. Of course not. But for someone who has owned and driven Miatae, you seem remarkably resistant to the argument that sometimes things are better solutions than a numbers-only analysis might suggest.
You're stereotyping, adjusting the facts to suit preconceived notions. I don't anyone has ever stated a Rotrex was best at anything.
Rotrex owner: "hai gays, building a rotrex, wat injectors???"
Perez, Scott, 18psi, et, al: "trubos are better becuz torques!!"
Every damn thread
Next time you're in California, I'll give you a ride in a Miata with a Rotrex. You can drive it too. Careful, it tends to break the tires loose when leaving lights if you get more than about 40% TPS even at fairly low revs. Then you can write a dissertation on how unrelentingly miserable and unfulfilling the experience was.
My engine at last dyno, with the cnc head / stock 10:1 pistons but with stock muffler unfortunately, made 217whp/166wtq. I will point out that my C30-74, during the entire time I owned it, seemed to make somewhat less power than expected compared to other C30-74 dyno plots. Something about my C30-74? Maybe my local dynos read lower than others (dyno dynamics and dynojet)? Dunno. Trackdayhookey had a rotrex build that made much much more than 300 but that was not a C30-74, it was a larger compressor.
The thing is, Joe, as Emilio pointed out earlier, you just don't get it. Your analytical stance re: the rotrex is comparable to the analytical stance of many re: the Miata in general. "It has no power! Why the **** would I buy that?" The answer is that the Miata just works, and does what it was intended to do, which was to deliver an awesome driving experience. How does one objectively quantify "awesome driving experience?"
One can't.
Similarly, the Rotrex just works. It takes the awesome driving experience delivered by the Miata, and improves it. It introduces only minimal complexity, and places minimal stress upon the drivetrain. There is zero chance of boost creep or overboost. The throttle response is razor sharp. There's no chance of things melting under the hood. There's no additional water & oil line connections to possibly leak. The Rotrex oil lines might leak, but that's not going to kill your engine.
I flogged mine for 6 years. Other than an initial belt alignment problem, I changed belts once and changed the rotrex oil once in 34,000 miles, 3k of which was on track. Essentially zero maintenance. No studs backing out, no cracked manifolds, nothing melting under my hood. I like to work on my car, and I like to drive my car, but I absolutely ******* hate working on my car when I'm supposed to be driving it. The Rotrex was perfect for this. I'm only switching to an EFR now because 1) life circumstances afford me an opportunity to look at a clean slate build; 2) my engine is balanced with an ATI superdamper, which has a larger crank pulley diameter, which makes upgrading to a C30-94 problematic because it has a lower RPM limit than the C30-74, and a balanced engine is too sweet to simply replace that damper; and 3) TSE has released their manifold, and I know Andrew has spent the last several years of his life working 24/7 to create a manifold & turbo solution that addresses every issue that kept me from going turbo initially. No pressure, Andrew
What makes a Rotrex a great solution for a number of people has nothing to do with the area (or lack thereof) of the torque curve produced. It makes a Miata better, period. Is it the best solution for everyone? No. Of course not. But for someone who has owned and driven Miatae, you seem remarkably resistant to the argument that sometimes things are better solutions than a numbers-only analysis might suggest.
So what you are saying is that there will be a used Rotrex system for sale soon.
And what frustrates me even more, if I'm honest, is that both Emilio and mobius seem to be of the mind that I am anti-Rotrex.
I'm not. And if one does a little thread-archaeology, they'll see this. I'm not anti-anything except for absolutist opinions founded on fallacious or imaginary data. I personally don't understand the appeal of the rather unique power-delivery curve characteristic of centrifugal superchargers in general, and I haveactually driven one car equipped with one (an older Mustang equipped with a Vortech unit). So I asked a fairly simple question about practical applications, and that was taken to be a biased indictment.
Anyway, it's clear that I can contribute nothing more here, and I'm obviously not going to change anyone's preconceived notions of my internal biases, untrue as they may be.
Does anyone other than the three or four for people writng these long posts in the debate care? No.
You don't understand why people like them. Even though it has been explained over and over again by people who have them. That's it. It is really quite simple. You may not ever understand. It is no one else's duty to explain it to you, though we have tried.
And its not two posts. Your first objection goes back several years. But it really doesn't matter. You remain one of the vocal stawarts that enter Rotrex threads and pollute the discussion between those with direct personal experience.
Edit: A vortech at 2:1 drive ratio using helical gears is not to be confused with a Rotrex running 8~10:1 ratio using the gearless planetary drive. Rotrex has a small turbo like compressor wheel spinning at turbo like speeds for better than turbo efficiency.
I'm still referring to the flatter torque curve powerband. That C38-74 would have a crazy powerband like Sav showed a page back.
What do you think the C30-94 would do on a fully developed Whammy engine? I'm not sure how a higher flowing engine behaves as you up the Rotrex sizing. I'd expect some movement of the powerband up revs as the Rotrex flow increases.
The C30-74 made around the 300whp with a nice powerband peaking around 6500. Do you think the C30-94 would hit the 280ft lbs and close to the 400whp mark but peaking another 1000 up the revs?
Short answer, yes.
With the bigger compressor, you're trying to stuff more air through the straw. Resistance (boost) goes up with the commensurate reduction in efficiency. Double edged sword. The compressor might have its most efficient island at say 2.5:1 pressure ratio so it's heating the air the least there but the engine doesn't flow well enough to take all the CFM with it backing it up, raising boost. Raise boost too much and you heat the air more, which reduces efficiency. Deviates motor was well matched, being capable of 270CFM (28in hg) and 8000rpm with a blower shoving about 41 lb/min (537CFM) at a 2.8:1 pressure ratio. We assumed the NB2 head, even with big valves, cams, CNC porting would be hindrance so we were expecting 14-17psi. We were surprised to see barely 10psi and a whopping 300whp on our 5th pull. It flowed so well that the blower didn't have to work hard to deliver max CFM. That P/R however, was at the bottom of the C30-74's best efficiency island. That lead me to think that a C30-94 on a similar engine could be even more efficient with its boost. That was the kernel of the idea for John's car which is mentioned below.
Tom (K24madness) has a similar build to Deviate. Less head work, similar cams but an NB1 so no VVT. He's making a bit less on top at a slightly higher boost level because his head flows a bit less. Probably in the 255CFM range I'd guess. Peak tq comes a bit higher in the rev range. That fits the pattern.
Gordon's (trackdayhookey) head was basically a stock NB1 so 204CFM head on his 430whp build. Relatively poor head flow and a giant C38-74 blower (73 lb/min!) ended up having a torque plot that was a straight line that barely tapered on top. That's a combination of the blower being way off the bottom of its best efficiency island and the head not flowing so well. Really hard to drive with the ECU set on kill. Nothing then whoosh. Deviate actually made more torque in the midrange but was easier to drive, a ***** cat really. Like driving a small Z06.
I think a mildly worked NB2 head with a C30-94 (52 lb/min) is the sweet spot to generate the most torque than can safely be fed through an AZ-6 on pump gas. John, the engineer here, has a stock NB2 head, forged bottom end and a C30-94 JR race kit proto on his car. We're still fiddling with it but the plan is to gear the blower so it his max compressor speed at a low 7000rpm. On paper at least, it should make something like 200lbs tq at 3500rpm, instantly, rising to about 250lbs at around 5500 (about 700rpm past OEM torque peak). We'll bleed off boost on the top end on pump gas to keep it down to a very conservative 270whp. On corn, full boost should net around 330whp.
Anything past about 280lbs tq while entertaining, is academic until a readily avaliable PnP transmission solution surfaces.
The OP asked why Rotrex's suck at torque. It's simply because centrifugal blowers build boost with RPM. Knowing that you can optimize the motor to make the most of the situation.
Emilio took that knowledge and created a whammy motor that has a great power. Real flat torque curve over a broad usable range (for track mostly). I think VVT played a big role here. My torque curve is pretty flat as well but comes in later due to lack of VVT. Again another example of what can be done with a Rotrex C30-74.
When I spec'd out the motor for my build I focused on building low end cylinder pressure. High compression and early intake valve closing. I kept seat duration low so I could build good cylinder pressure in the lower RPM's. The upper RPM's got help from the Rotrex. The resulting power curve looks like a high strung NA motor. Just what I was after. If I could change anything it would be VVT head instead of 99 head.
So if you like the Rotrex for all of the characteristics but want more torque just turn up the boost. That may mean different pulleys or different blower. Want more torque way down low? Custom cams, high compression and VVT will help. Oversizing the blower and bleeding off boost up top will also help. None of it will get you V8/turbo style torque hits though.
I hope this helps those who came to this thread for info on how to help build torque with the Rotrex. Those that want to discuss optimizing turbos let's do it in another thread. I've got waaaay more experience tuning those and managed to pull off some great results with my other twin turbo car.
Tom (K24madness) has a similar build to Deviate. Less head work, similar cams but an NB1 so no VVT. He's making a bit less on top at a slightly higher boost level because his head flows a bit less. Probably in the 255CFM range I'd guess. Peak tq comes a bit higher in the rev range. That fits the pattern.
I think a mildly worked NB2 head with a C30-94 (52 lb/min) is the sweet spot to generate the most torque than can safely be fed through an AZ-6.
Anything past about 280lbs tq while entertaining, is academic until a readily available PnP transmission solution surfaces.
I ended up with 9.5psi of boost on the top end. We were running out of fuel and had to cap RPM at 7,700. I geared the blower for 8k though. Whatever the case I think you ended up with a slightly better combo that I did. I kick myself for not going VVT.
I've solved the fueling problem and plan to retune it soon. I may swap blowers to the 30-94 before that happens. I need to reach out to JR and talk about options.
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.
When I spec'd out the motor for my build I focused on building low end cylinder pressure. High compression and early intake valve closing. I kept seat duration low so I could build good cylinder pressure in the lower RPM's. The upper RPM's got help from the Rotrex. The resulting power curve looks like a high strung NA motor. Just what I was after.
This is the sort of discussion we should be having! Thanks for the info.
I don't quite follow how this works. How does a shorter duration increase cylinder pressure? Is it because at lower revs there is potential for the boost to flow backwards out the intake valve whilst the piston is returning. So you're filling the cylinder and closing the valve whilst the piston is down and letting the piston compression work. At higher revs there is more flow so less chance the air can get back out the intake valve whilst the piston is moving upwards. The VVT would let you do this with higher duration cams so you don't get losses higher up in the revs from less valve duration.
I've been trying to research ideal cam duration for my engine and it's hard to find any information at all on cams for Rotrex and how they effect the powerband. My head is the BP-05 so there is no VVT but it does flow well enough to support 200hp N/A. I'm happy to lose all my low end torque but it needs to pick up around 5000 and push out till 8500. How do you spec a cam to work in this case? I assume big lift is still preferable with a duration around the 264 - 274 mark? I'm not worried at all about fuel economy but wasting boost due to overlap I figure would angle the torque curve in the wrong direction to what I want.
My plan is to run the c30-94 blower which will already push the torque curve higher, and then add a non VVT head into the mix and it's going to suffer a lot down low. Maybe I just have to spec much lower duration cams to balance this out.
I've been trying to research ideal cam duration for my engine and it's hard to find any information at all on cams for Rotrex and how they effect the powerband. My head is the BP-05 so there is no VVT but it does flow well enough to support 200hp N/A.
The cams you have will probably work fine. To get 200whp N/A out of a BP05 requires the equivalent of EP (E Production) cams. I'm guessing yours are 290-310° seat, .500, 520 (13mm+) lift. Rotrexes respond to N/A mods so tune it that way. If the powerband isn't what you like, at least you will have a data point. FWIW, no one in this country runs cams that big in a BP except for SCCA production class road racing. Those make about 195whp with heavily restricted engine rules. Cams are open so they run giant cams. Everyone else bolts a blower on or sticks to a class that requires stock cams.
This is important: High power (300whp+) Rotrex BP builds for track use are rare. Maybe a dozen or so ever so the knowledge base is thin. The build you propose is uncharted territory so you're not likely to get definitive answers here or anywhere else. I'll say to you what I often tell customers with nutty builds "I don't know the answer and probably no one else does. Build it and you will become the worlds expert and tell me the answer"
Deviate ran relatively small and short cams, 264° seat, .425 lift. Combined with VVT and we have peak power a few hundred RPM below the 8000rpm rev limiter and peak torque at 6300rpm.
If your head/cams flow well enough, you actually won't make lots of boost with a C30-94 but will make a bunch of power. Max P/R of the C30-94 is only 2.52:1. Makes it easy to get ambient IAT's without needing 5' of I/C across the nose. If running race gas, I'd suggest 11.0:1 if 100 octane, 12.0:1 if running 110 octane or more.
The 1000whp+ Rotrex K20 Bonneville motors on C16 (116 octane) run something like 14.0:1. They run twin C38-94's I think. Mental!
The cams you have will probably work fine. To get 200whp N/A out of a BP05 requires the equivalent of EP (E Production) cams. I'm guessing yours are 290-310° seat, .500, 520 (13mm+) lift. Rotrexes respond to N/A mods so tune it that way. If the powerband isn't what you like, at least you will have a data point. FWIW, no one in this country runs cams that big in a BP except for SCCA production class road racing. Those make about 195whp with heavily restricted engine rules. Cams are open so they run giant cams. Everyone else bolts a blower on or sticks to a class that requires stock cams.
This is important: High power (300whp+) Rotrex BP builds for track use are rare. Maybe a dozen or so ever so the knowledge base is thin. The build you propose is uncharted territory so you're not likely to get definitive answers here or anywhere else. I'll say to you what I often tell customers with nutty builds "I don't know the answer and probably no one else does. Build it and you will become the worlds expert and tell me the answer"
Deviate ran relatively small and short cams, 264° seat, .425 lift. Combined with VVT and we have peak power a few hundred RPM below the 8000rpm rev limiter and peak torque at 6300rpm.
If your head/cams flow well enough, you actually won't make lots of boost with a C30-94 but will make a bunch of power. Max P/R of the C30-94 is only 2.52:1. Makes it easy to get ambient IAT's without needing 5' of I/C across the nose. If running race gas, I'd suggest 11.0:1 if 100 octane, 12.0:1 if running 110 octane or more.
The 1000whp+ Rotrex K20 Bonneville motors on C16 (116 octane) run something like 14.0:1. They run twin C38-94's I think. Mental!
SCCA E Production Miata
What are the current cam options for the BP-05 cylinder head? Short of Tomei is grinding/welding the only option? With the use of SUB's is there anything to be gained grinding the factory cams?
What are the current cam options for the BP-05 cylinder head? Short of Tomei is grinding/welding the only option? With the use of SUB's is there anything to be gained grinding the factory cams?
Billet cams can always be made to any spec.
Going down in base circle (e.g. 33mm, using subs and a bit longer valve stems) will make regrinding a stock cam an option, but I don't have data on what the limit in duration/profile is (unsure how much extra I was able to get based on Mazda blanks).
With a blower you don't need the extreme cams so a cheap regrind of a spare stock cam will give a data point.
Going down in base circle (e.g. 33mm, using subs and a bit longer valve stems) will make regrinding a stock cam an option, but I don't have data on what the limit in duration/profile is (unsure how much extra I was able to get based on Mazda blanks). With a blower you don't need the extreme cams so a cheap regrind of a spare stock cam will give a data point.
That was my thought as well. In combination with the Ford YS4 shimless buckets.
Im not aware of any reasonably affordable billets available.
12-15-2015, 10:25 PM
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