Alright everyone, I'm definitely feeling the draw to the dark side. Thanks for the input thus far. I'm looking at about 6 months until the new engine is in the car and running. I'll be spending that time researching options and lurking build threads. Stay tuned for some porting and polishing on the head as well as a full aftermarket valvetrain.



I posted about that concept a few pages back, and it still peaks my curiosity. Except since it had to do with rotrex no one elaborated why "it wouldnt work" or why is was "fail." As long as the charger can support a high rpm, and the wastegate was set to prevent over boosting I feel like the centri unit could match or come close to a turbo power curve.

The only problem I have with that is its like trying to make an ugly girl hot instead of just getting with the dime in the first place.



I would assume jet skis require the same amount of torque as say a sport bike. That could be why theyre big on the skis, I know centris have been used on bikes where the stock toque levels are low as ****.

 

Here's one reason why the wastegate concept on the rotrex is fail.

On a turbocharger, you're using energy that would otherwise be wasted: exhaust gases and exhaust heat. With a supercharger, you're using energy that is being sapped directly from engine output to turn the charger.

With a turbocharger, the wastegate that keeps boost pressures in check essentially just lets that air and heat go by without creating more boost. No harm done -- it was wasted energy anyway, and letting it go by doesn't hurt efficiency.

With a Rotrex, you're still using precious energy to spin the charger harder and harder. You're going to take that perfectly good energy and wastegate it away? Why? It's a band-aid for an inefficient setup that, as you say, is like trying to make an ugly girl hot.

I hope I've communicated what's in my head here.

 

My thoughts too.

Their response will probably be something along the lines of "how much energy does it take to turn a rotrex, probably not much"

And to be completely honest, after all this rotrex bashing, I gotta admit that if I was concerned about track reliability, and tracked my car full time, I'd heavily consider a rotrex.

But I'm not, so I keep hating on them

 

But somehow it is being done in unlimmited drag racing, where they are competing with the best turbo builders and where ultimate power is King.

It is a simple concept really, size the housing and impeller so you have adequete power down low, and then bleed off the excess force up top. I know making it happen is not so easy.

Now that I am speculating, it seems that engineers could come up with a way that as the impeller reaches its maximum boost ouput the blower is called upon so to do, the blower does not increase drag as the rpms increase. If anything, maintains a constant level of drag, or is this already the case?

 

Pretty sure in drag racing nothing besides the last 2k of the powerband really matters.
And no one's arguing peak power figures here. Just area under curve for a small and mediocre 4 banger

 

I get you Vlad, but I do not think you get my point.

Turbofan stated that putting a wastegate on the Rotrex would fail, and I believe that to be the case with the current Rotrex housing offered to the Miata community.

I am suggesting a larger housing that gives you the power down low, and then bleed off the excess up top. That is what I was trying to convey.

 

Gotcha. Well I'd like someone to try this, but seems like most people get very discouraged with being the "guinea pig" with 5k worth of built engine and 4k worth of f/i equipment, and at that point decide to go with a proven power path.

Kinda looks like OP is headed that way too.

*EDIT: and while I think it (your idea) may help out with lowend, ultimately I think it will still not even come close to what a turbocharger would do down low. Its still very much dependent on RPM vs just "spooling up" from like 100 compressor speed to like 80,000 rpm compressor within 500 engine rpm or something (all just guesstimates for illustrative purposes lol)

 

Because always full throttle. Because water drive - there's no sticky mechanical coupling of the drivetrain to the drive surface, as with our cars, so the jet ski engine and drivetrain can essentially spin up to full thrust immediately regardless of the speed of the jetski itself.

But it's not going to sap all that much power. Sure, it's going to move more air when the wastegate opens, but it's not pressurizing it, so it's not going to take that much energy. It's what my rotrex does already when I lift the throttle. Or at cruise, for that matter - it's blowing a lot more air than the engine can use, and it recirculates. It's no big deal. The compressor is spinning, but there's no real load on it. Same for the wastegate case - there's no significant extra load to move that air around the loop compared to the 10/12/whatever psi of compression already going on. Yes, sure, there is some energy needed - shut up, those of you that want to drill down and obsess over tiny details.

I looked at doing this, but the plumbing was more than I wanted to mess with, and it breaks my "keep it reliable stupid" for this car. For my car, since I'm running the MAF, both the recirc valve and the wastegate would have to loop the compressor post-MAF. I did a bit of searching but didn't find any dual-signal-source valves that would open either under vacuum, or at a set PSI, so I'd have to plumb some sort of Y. Meh.

But if I could hit 10psi and stay there ... hmm. And I did by myself a new welder for Christmas ... hmm.

I don't have the math/engineering chops to lay out the equations for what I'm about to suggest. But it is my belief that the amount of extra work required for my engine to spin my compressor at full speed / full load via the belt drive is less than what would be required to spin the same compressor wheel at same speed/load via an exhaust-driven turbine. There are secondary advantages that contribute to this as well, the belt drive isn't creating any exhaust backpressure which the engine must push against and which inhibits scavenging of the cylinders. It takes a *lot* of thrust to move a mechanical wheel with a stream of air. Turbines are not particularly energy efficient. They have great advantages in packaging size and in the amount of power that can be produced in that size package, but they're not particularly efficient.
If anyone has actual data on this I would love to see it. Maybe I'm wrong.

 

Rotrex C38-91

make it happen.

 

Unlike a turbo, to get from 3500rpm to 7000rpm your compressor speed has to double. Will a normal compressor wheel operate efficiently enough at both 75k and 150k rpm for example?

 

I'm not interested in 3500-7200. I am interested in 5000-7200. You just want to bleed off the high end boost, roughly over the top third of the compressor's RPM.

Now I want to go draw lines on compressor maps. I'll have to think about how to draw them, though, since there is air recirculating, the actual pressure ratio involved is less than if it was drawing in straight from atmo.

 

I apologize if this has been addressed already, but is this build CARB legal? I assume not as I haven't found a damn thing CARB legal for a '00+ Miata but I thought I'd ask as I see your in Cali too.
Thanks.

 

Lol, no. I'm pretty sure KW does not have CARB certs for any of their Rotrex products.

Jackson Racing, however, has a EO for a Rotrex kit on a Honda CR-Z Hybrid AND the 2006-2011 Honda Civic R18

 

as of now the car is carb legal with the exception of the lack of charcoal canister. but obviously with a built engine and standalone ecu... its not going to be anywhere near carb legal haha

 

Got the booboo valve guides out. Replacing all 16 with Sealed Power guides. While I wait for the new seats and guides to come in I touched up the exhaust ports a little bit. Attacked the ridge in the casting on the intake and exhaust side aggressively but everything else is just going to be cleaning up the rough castings etc.

 

Got a flowbench at the school?

 

Sure do! I'm leaving some ports unfinished for the time being to get a baseline.

 

on snap, that is really cool.
in for results

 

Props will be reserved for when OP delivers

But seriously, that's awesome

 

In my build thread, I posted stock miata head flow numbers, various honda flow numbers, and flow from my head ported the first time, and then ported a second time.