The guy I mentioned wanted 165 whp for his junkyard engine build and he easily got that with the C15-60. Since you are starting with a much better base engine, another 20 whp would be cake. He had a map that made more than 185 whp, too. All that extra stuff for the C30-84 doesn't seem worth it, to me.

https://forum.miata.net/vb/showthread.php?t=374599

 

Larger compressor wheels typically create less heat in the air charge than smaller ones at lower volumes. They aren't working is hard to move the same volume and remain in a higher efficiency range. So I second your choice to use the larger unit. Shedding heat is difficult enough to do in our cars. And efficiency in operation is golden.

 

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In your shoes I would wastegate the pressure side of the rotrex with 4 port solenoid controlled by MS3. I'd run the pulley necessary to get 185hp around 6k RPMs. Then use wastegate to bleed boost to keep you there till 7500RPMs. That's should be doable with 93oct. You'll likely need a 100mm+ rotrex pulley through.

 

Thicker head gasket to reduce comp?

 

not usually a good way to reduce compression..
Quench change can make detonation worse not better..

 

I am using a C15-60 Rotrex on a stock BP-4W engine for track driving and can make my class-limited 165rwhp target with an inlet restrictor and de-tuning at the top end. 185 would be very easily managed. Not sure how much challenge you would face with the higher compression ratio pistons, but you could always source a nice used stock engine for the project.

 

Man, you are still running that thing on the engine you swapped in all those years ago? That is awesome.

 

I think I've decided to play it safe and run e85. Just having spent lots of $$$ building the motor at the start of the year, I'm not very interested in pulling it down again to change pistons, and e85 gives me the headroom and peace of mind I need to not have to worry about too much timing at the power levels I'll be running.

I plan to use a wastegate (or probably set n forget MBC) to keep a lid on the psi, and get it tuned unrestricted first (for curiosity sake) and then wind the boost down on the dyno till I get to my power cap. (easier than changing inlet restrictors till I get the desired result)
Would I even need to change the (MAP) tune from the unrestricted one, as my assumption is that 140kPa at 7000rpm is the same regardless of whether that is achieved by a wastegated 100% throttle or an unrestricted 80% throttle.

This would give the opportunity to run full power with a simple MBC change on the one event a year that isn't a championship event (and so power cap isn't necessary) and run it restricted at every thing else.
Then I don't need to have to remember to swap tunes each time, and my boost gauge will tell me if I have forgotten to turn the MBC back down... (and it wont hurt anything even if I haven't).

 

Should be no need to retune, just tune the whole map the first time around.

 

Cool, thanks. It made sense in my head, but glad to hear I was on the right track...

 

Yes! Fingers crossed

 

So, rotrex is in and running.
Ended up doing a simple inlet restrictor rather than mess around with plumbing/controlling a waste gate, dealing with potential over boosts, etc.

It hasn’t been to to a proper dyno yet, cos COVID-19, but virtual dyno spat out 222whp and 177lbft.
Fitting a 43mm restrictor in the intake chokes it down to 177/149 which sits just nicely under the 185whp cap.

I’ll leave my brother (rascal) to tell the rest of the story cos it’s actually his car. Plan was for him to upgrade to a ND and I’d buy this off him and rotrex it, but the gearbox issues scared him off getting a ND so he kept it and ran with the supercharger idea. I might still get to share the car on track down the road.

 

Glad to hear there was a fairly simple solution and you (him) were able to get close to the cap!

 

Virtual Dyno plot of 177/149 numbers.....

Wont be super accurate since its just virtual dyno extrapolation from a couple of road pulls, so affected by potential wind/gradient differences, but will be putting it on a proper dyno hopefully soon so as to get real figures, though these are close enough atm for my purpose..

Also note that Aus dynos read ~11-13% lower than US dynos, so I added 12% to get the below plot for comparison sake. (The 185hp target figure was set based on our class limit of 125rwkw/167rwhp on AUS dyno)


What it most interesting is the 43mm restrictor starts chopping boost off from 4000rpm..

 

I think that would make sense though, as a restrictor is a static device that would only start to be noticed when the flow is -restricted- beyond the normal hole. Rotrex doesn’t move enough air for the hole to become restrictive until 4K, and then it does! Nevertheless, cool build - I look forward to seeing dyno sheets to see how close you came (if that’s a plan eventually)

 

I expected the drop off, but not so early. Thought it would get to mid 5K before it started tapering off. (looking at other plots using restrictors) Not that it matters, just a curiosity..


Yes, the plan is to get it dynoed on an actual dyno so I get true figures.
I have a intermittent wiring issue that needs sorting before I can get it on the dyno though, hence why I haven't done that yet.

You can see the AFR flatlines at 4200 on the red plot, and disappears completely from 5400 on the blue plot. I get this scenario, as well as losing sync from the ECU log on an all too frequent basis. (also why I couldnt get a full pull right to 7000, rather than 6676) as the log drops out just when I need it. I can tootle around at low revs for hours and dont seem to miss anything, but as soon as I get into it, it will drop 3-4 secs of logs, either the start or the end of a run. Very annoying, since it doesnt do it when running in the shed where it would be much easier to troubleshoot..

Once I've found the wiring gremlin, then I can also get it tuned properly on the dyno. It's currently running a start from scratch road tune by yours truly (I'm just an IT nerd not a tuner) which is light on for timing, and fat in fuel so its safe to use on track till then. There would be a fair chunk of extra power with a proper tune..

 

so not 100% sure I've found the electrical issue, though I did find one of the toyota ignition coils seems less than ideal.. I'll remedy that as soon as I can find some suitable replacements..

Anyway, got it on a proper dyno this week. It was a hot day, temp was 32oC (90F) which wasnt ideal, but not a show stopper..
It made 239/181 (unrestricted) at 8psi on e85 which I'm very happy with given the soft timing/fat fuel tune.
Run was done in 5th (1:1 in 6speed), which is 25kmh/1000rpm. peaked at 177kmh = soft cut at 7100rpm.

Below is the dyno readout. Note Australian chassis dynos read on average about 12% lower than US ones, so I've factored that into above figure. (239whp -12% = 213whp/159kw)