Rotrex C38-71?
 

Hello...I wanted to get an extra opinion on whether the C38-71 is a good size for the Renesis. I've seen very few Rotrex'd RX-8s around the web which often use a C30-94 or smaller. Imo, anything in the C30 range is too small. Here is the compressor map for the C38-71. I also did some math and plotted where the Renesis would be on the map (only the purple line matters). Below is the spreadsheet that gave me the data points for the graph. I used 92% VE across the board cuz I was lazy lol. I assumed the crank pulley diameter to be 146mm (5.75") since that's roughly what I measured in the dark last night (plz let me know if this value is incorrect). These Rotrex units come in pulley diameters ranging from 70mm to 110mm in 5mm increments. I'm leaning toward the 110mm pulley since rotaries have fairly high redlines. The smaller pulleys would overspeed the compressor. Redline may also have to be reduced to ~8k due to the amount of boost being made. Otherwise, this setup would end up at ~20psi @ 9k rpm.

Overall, I think centrifugal superchargers naturally complement the n/a rotary's powerband. Though I admit, a turbo would surely provide more hp/tq and area under the curve but w/e. On a side note, I wanted to point out that Rotrex superchargers spin much faster than other centrifugal superchargers (e.g. Paxton, Procharger, Vortech, etc.). For example, the C38-71 has a step-up ratio of 7.5:1 while most Vortech units have a ratio of ~3.5:1. The Rotrex spins at least twice as fast, which is why their compressor maps are very turbo-like. But yea, I just wanted to ask if I picked a good compressor/pulley combo. Or, would a different Rotrex & pulley combination be more ideal? Please let me know if my math doesn't check out. Oh, and does anyone know the diameter of the stock crank pulley?

https://cimg7.ibsrv.net/gimg/www.rx8...36315c26b1.png
https://cimg4.ibsrv.net/gimg/www.rx8...a004baa6bd.png

Btw, here are the maps (PDF) for the other Rotrex units in the C38 range. Do you guys think any of these would be a better fit? (C38-61, -71, -81, -91/92*)

*I heard that C38-91/92 units tend to break at the shaft when bouncing off the rev-limiter, especially on setups using toothed belts. I've seen it happen on Hondas (S2k, Civic, etc...), and is due to the higher mass associated with the larger compressor wheels of the -91/92 units. Apparently, Rotrex didn't bother beefing up the shaft to compensate for the heavier wheel. Thankfully, that issue isn't present on the C38R (their largest compressor) but then again, a C38R is far too large for our application.

there’s nothing wrong in your basic calculations, but rather a shortcoming to comprehend why an equivalent turbo would be so much better in every way regardless of the things you’re trying to avoid going this other route. A small displacement engine with a short crank/eccentric torque lever and a wide, high rpm powerband is not complimented by a supercharger. Just the opposite; another shortcoming of comprehension.
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Volumetric efficiency jumps to over 100% when the APVs kick in at 6000ish and is close to 100% below that . Something like 98before 103 after seems about right. Plugging that in will drastically alter your curve. You also should factor in losses from intake/pipework/intercooler which increase exponentially as flow increases.
In principle though, I agree with Team .... a well sized and setup turbo will make any supercharged Renesis look mediocre.

Umm, nowhere did I claim that a s/c was going to be superior to a turbo setup, Team. I'm well aware that a turbo would be a better choice "overall". But I am not after a lot of power nor am I trying to break any records. I mostly just find it fun exploring my options; a Rotrex being one of them. Though, I will most likely end up going the turbo route anyway. Why does it always have to be a turbo vs s/c debate though? Can't we just appreciate both? That's how I see it anyway.
Edit: I hope you're aware that I was talking about centrifugal superchargers specifically, and not positive displacement superchargers since your blanket statement regarding superchargers did not specify which you were talking about. And the 2 types of s/c have very different powerbands. I agree w/ your assessment when it comes to PD s/c but disagree for centrifugal s/c.

Do you just ballpark this figure? Like, say, 2-3psi? Or do you have a specific way of calculating these losses?

Btw, does anyone know the exact diameter of our crank pulley? The larger of the 2 diameters. Or actually, knowing both diameters wouldn't hurt lol.

I've just used the calculation sheet from Borg Warner to estimate this ..... for a rough estimate though, assuming you use quality IC and a decent intake, I'd just throw in 3-4 psi at the top end if you are aiming for around 350-400whp.
BTW ...in efforts to date on a Renesis, centrifugal SCs have performed worse than positive displacement SCs.

The only centrifugal SC I've seen available was the HKS kit which used a Rotrex C30-94, which imo is too small. I'm not surprised the HKS kit underperforms. Are there any other examples of a centrifugally supercharged Renesis btw? Cuz I couldn't find any besides the following.

https://www.rx8club.com/series-i-maj...d-apex-180886/
https://www.rx8club.com/series-i-maj...-rx-8-a-60059/

Edit: Regarding the positive displacement SCs...both the Pettit & Stillen SC kits had actual R&D done to create said kits. On the other hand, the HKS kit is obviously just a rebranded Rotrex slapped together with some plumbing. While I have no proof, I'm confident that HKS did not put in anywhere near the amount of thought and care that the 2 PD SCs enjoyed. So it makes sense that the PD kits perform better. If only the HKS kit used something from the C38 range, but alas it is what it is...

Edit2: Are there any examples of Renesis with non-Rotrex centrifugal SCs? Like Vortech, Paxton, Procharger, or etc...

https://www.rx8club.com/series-i-maj...7/#post3158241

Check blue line on second chart down . That was a procharged 8 running 10psi

Team, you're an asshole. Every post you make (and you seem to feel the need to respond to every. other. goddamn. post) is trashing someone else's idea in an especially asinine way without even pretending to bring evidence. It's always something like "you're stupid because of things I 'know' but I'm just too cool to tell you." Stop. You are singlehandedly the worst part of this forum.

I’m the first to admit having a lot of bad ideas too. From that comes experience and knowledge. For the sake of this thread I’m deleting my rebuke of the prior post. And I’m deleting the former post that triggered it. Not that there was anything in either one worse than the first post that remains.

The evidence is all over the forum. The truth of it is despised and not sought, because the errors and untruths that were made are then exposed for all to see. You reject anything doesn’t meet a preconceived expectation. An expectation that you desire more than the truth itself. Until you come out from that darkness you’ll never have the eye to see it or the ear to hear it.

Yet it’s right in front of you jumping up and down, screaming and flailing, you still swear it doesn’t exist.
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He can be odious, but I think he has a point that should be considered re: super/turbo charging.

I thought his post was pretty low key honestly.

And yes, he does like to embellish with his "Philosopher King of the Rx8 Club" statements.

He has "gotten better" in the last few months in my opinion. More signal, less noise.

Yea...if it's running at 10psi, that 10psi would only be had at redline. 6k rpm would prolly see ~5psi, and 3k rpm maybe ~2psi or so. It might even make a smidge less power than stock n/a at sub 3k rpm lol.

Which do you guys think would be the more "stressed" setup? A Turbo Renesis @ ~10psi or a Rotrex Renesis @ ~15psi? Assuming the turbo setup is moderately sized and reaches peak boost in a reasonable fashion.

Apart from Team virtue signalling 5 times and insulting twice every 3 words he types, he has good intentions.
Besides the issues pointed out already I'd like to add the extra stress which the front stat gear would have to take due to the belts feeding any supercharger. That bearing will get munched quick...

exactly, those are not even very good turbo setups compared to now either, it gets stomped on everywhere and all the belt tension required to avoid pulley slip due to the high ratio/rpm requirement takes out the main stationary bearings in the process


https://cimg7.ibsrv.net/gimg/www.rx8...580736472.jpeg


mock me as you will, but you only mock your own ignorance
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Good question ...that is getting to the crux of it. To make the two setups equally quick the SC needs more boost than the turbo. One has high back pressure ...the other higher IAT and higher combustion pressures to deal with. It would take a lot of expensive experimentation to give a definitive answer. Maybe it's been done in the piston world ? :dunno:

To put some perspective on what Team was alluding to re the improvements in turbo setups ...my current one would outperform all of those above running only 10psi.

In addition, which setup do you guys think can more safely run a higher redline? Turbo or Rotrex? What is the limiting factor? Taking into account the fact that a centrifugal SC needs to be ran at higher boost than a turbo to achieve similar performance, ofc.

Btw Brett, how long has your Renny been turbo'd now, if you don't mind me asking. (in time or mileage)

That makes sense considering how long ago that graph was made. The Procharged setup is just as sub-optimal as most of the turbo setups (i.e. Greddy). But, it is the only 1 on the graph to hit 9k rpm. Not sure why the "NA+boltons" line wasn't revved out to 9k rpm tho. :scratchhe

At a guess I'd say the SC would run a higher redline due to the lack of any backpressure to limit exhaust gas exit.
My chassis has been turboed since 2007 and for over 200,000 kms but there have been a few engines on the way . Not fair to judge on that as I've been testing boundaries for much of that time.

thank you for your insight Brettus 🙇‍♂️

but it’s a jump to say the PC is also sub-optimum, because it’s still the same more or less while turbo technology has made significant design advances.
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I marvel at how long the unsupported shaft is outboard of the front bearing. It seems a SC set up would have to have an opposing pulley, tensioner, and belt to equalize. Not sure if it has ever been done that way, or if there would be room.

that’s a lot to marvel over; more complexity, cost, and frictional losses. Wonderful idea!

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The benefits of the supercharger over a turbo is mainly the packaging and that there's no need to fabricate a manifold plus under bonnet temps will be a lot lower along with the exhaust ports having an easier time on the Renesis engine.

The Rotrex suits high compression engines and is used a lot on high end NA retrofits due to keeping the original character of the NA engine, the charger I have came off a Civic breadvan with a stock K24 bottom end which made 480bhp with the Rotrex for 15k miles then made 430bhp with a Pulsar turbo shortly before the engine let go.

Was intending on doing the swap last year but it's on the backburner whilst I try to find an adaptronic dbw board.

I'll be using an E&J stationary bearing support to replace the Torrington bearing which should prevent the damage from increased belt tension.

I'm debating on going electric water pump so could delete the waterpump which would reduce the pulley tension though the Rotrex doesn't need as much tension on the belts as other supercharger designs due to the traction drive employed inside.

Still haven't decided the best placement, may move the alternator downwards so it uses the same belt as the aircon then with the waterpump deleted mount the Rotrex where the alternator was originally.

https://cimg9.ibsrv.net/gimg/www.rx8...95073f9413.jpg
https://cimg9.ibsrv.net/gimg/www.rx8...1edc9a8417.jpg

Meant to put last night that the Civic made 480hp on the Rotrex whilst at the same psi the turbo made 430hp (both mapped on standard super unleaded (E5) in the UK).

The best way with the Rotrex is to run an external wastegate before the throttle body as then you can set the max boost so if you do overshoot it then there's no chance of damaging the engine.

There was a revision a couple of years back to the C38-91/91 with a thicker shaft to counter the breakage issue though it only seems to occur when a savage rev cut for the limiter is used.

Pretty sure this is an ESS pulley from one of my old RX8's and it measures up just under 12cm and looks to have five ribs, ideally you would want 6 or more for the supercharger belt.
https://cimg9.ibsrv.net/gimg/www.rx8...992e216e7e.jpg

That really doesn't prove anything without more detail. Possibly only that the turbo setup was below par (Chinese knockoff turbo is a clue) and the SC probably had a higher redline.

480 whp in a piston engine is only low-mid 300ish whp of flow in a rotary engine,

lowish 300 whp is about the limit of anything we’ve seen out of one, but again there are factors with using one on a torqueless rotary wunder-engine that are not going to be in it’s favor to reliably produce an equivalent result.
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The Honda's are equally torqueless in NA form, it made 480hp due to the limitations of the car rather than the supercharger (stock engine, piggyback ecu, E85 not being available in the UK).

If I were supercharging the Renesis with a clean sheet I'd use the Rotrex c38/91 as it makes over 600 wheel horsepower on built K22/24 engines which would equate to around 400 in a rotary, there's also the C38/R which flows 30% more but it's hard to get hold of.

It was a GT35 rep which was meant to offer over 500hp but couldn't go above 440hp without encountering detonation and had to use a few psi more boost than the equivalent power being made with the Rotrex, torque down low was better and it did indeed have a lower max rpm which was engine limited for safety.

well by jeepers, what are you waiting on for then? :suspect:
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