Brett.
I agree with you that a 100% 13B-MSP RENESIS is limited to somewhere around 450WHP, but not because of the size/shape/flow/placement of the side exhaust ports, I strongly feel it would be due to the 10:1 compression rotors. The elevated CCP/CCT/EGT/Blow-by created under boost is just too overwhelming on the hard/soft seals, rotors, housings, plugs, etc. Even 9.7:1 compression rotors have significantly less CCP/CCT/EGT/Blow-by under boost which is why they're the rotor of choice in almost all of the pro drag motors, many running 80+psi of boost. Granted those are all peripheral exhaust ports but the exhaust ports aren't "as critical" in terms of their effect on reliability & power as the actual compression & combustion events are. And though numerically it may seem like an insignificant increase in compression, the effects are exponential. I admit I went super conservative with the 9.0:1 rotors as it was unexplored territory for me running them in an MSP-ONLY motor, but now that I have more empirical data the next step would be to test 9.4:1 and 9.7:1 rotors. Not sure if/when that will ever happen though.

Now on to my reply about the dyno results, RotaryMachineRX nailed it already on the rear gear.
Copied my reply from FB but applies here as well:

"The lazier than normal pulls and late/narrow power band with such a large frame turbo and small displacement motor is exacerbated by the ultra low 2.73 rear gear. We expected this, it's not exactly rocket science which is why we designed the setup to be nitrous-assisted but it hasn't been activated yet. The TPS was over-volting on these pulls (explains the throttle modulation you hear in the video) so we didn't get a chance to use it. If we were running a 4.88 or higher rear gear ratio that alone would significantly improve that graph to a much more "usable" power band, but our mph potential would go from ~200mph (w/ 2.73) to ~160mph (w/ 4.88+). Also keep in mind the turbo is sized more appropriately for our Phase 2 billet 3-rotor, so we're in no way implying that this is the optimal combination to achieve these HP levels, we know we can more efficiently do that with anywhere from a 66 to 76mm turbo. Anyways, the base tune is obviously far from done and we're more than stoked with these results at such an early stage in the tuning process, and more importantly with such promising telemetry. With this chassis, weight and aero combination we only need ~450WHP to 500WHP to go 200mph (Phase 1 goal) so the initial results have far exceeded any of our already high expectations of this "MSP-ONLY" #RENESIS."

Also, the motor is as solid as ever based on the increasing HP/TQ numbers per pull, post-dyno compression check, absence of any hairline cracks on the side housings and absence of any bearing material in the oil or filter.
We’ll know we’re nearing the flow limit of the motor or ports when increasing the boost doesn’t net any positive gains, or worse, we start losing power. The turbo certainly isn't going to be the limiting factor.

 

Thanks for posting here. Hope you don't mind the analysis ... I feel it's worth discussing and don't mind you proving me wrong again . I accept that the low compression rotors have had a major positive affect on the maximum possible whp result . What I also see though, is while it's a high peak number for a Renesis ..... it's not even close to a good result if compared to a pp exhaust engine. So you have to ask the question ...why ? I still strongly believe that comes back to the side exhaust port inefficiency as detailed here on page 1. Elliot has suggested it's to do with high IATs ... I can't see a 100whp lost due to that alone.




Were you in a1:1 gearbox ratio ?
I felt that they didn't go full throttle till the last 1000rpms ...why was that ?
My feeling about the narrow powerband is that the turbo is way too big for the engine and it's operating very close to the surge line. At any lower rpm it possibly wont hold full boost due to this. When you add nitrous to spool the turbo earlier you may just end up forcing the turbo into full surge ... will be interesting.

 

You’re comparing the dyno results of a handicapped setup (low rear gear ratio, big turbo, no nitrous) that has an unfinished tune to what the average dyno results would be of a fully tuned pp exhaust motor that is optimally setup (4.88+ gearing, big turbo, IC, etc.) and concluding that the MSP dyno is “...not even close to a good result if compared to a pp exhaust engine”.

You do see how that logic and therefore conclusion is clearly flawed right?

One is currently an apple and the other is an orange, but you’re treating them both as equals and drawing a conclusion between the two.

Now if that apple was to somehow more closely resemble the orange with a finished tune, an IC and a 4.88+ rear gear, then you could draw a more accurate conclusion between the results of one dyno to the other. But since there are no plans to turn the apple into an orange they can’t be accurately compared.

 

Ok , I get your point ...It isn't finished . Plus you have a very specific goal that nobody else has ever tried for, so making comparisons is unfair to your build.
However, you still have to massively improve your powerband width to reach your goal. So as far as the technical side of it goes. My thoughts. FWIW
Lack of an IC isn't going to hold you back ..... you have charge cooling via the fuels chosen.
The final drive ratio isn't holding you back ...... gearboxes have gears(you don't have to use 1:1) and dynos have ramp rates.
Oversized turbo .... yes definitely a problem ... possibly insurmountable IMO
Nitrous ..... will definitely raise your peak numbers but will it solve your ultra narrow powerband issue? If you went down a couple of turbo sizes ......maybe.

 

it also has all 6 intake ports open continuously, probably doesn’t even start spooling until 6500 if not higher.
.

 

So .... I've figured out how to get the Renesis rotors down to 9.4:1 safely.
CNC setup and machining is underway and this will go into my next engine.
Edit : I'm not anticipating that this will unlock a lot more power TBH. My main objective is to allow 400ish whp on pump gas. But I will try and if it does that will prove your theory correct and mine............... wrong.

 

https://www.rx8club.com/new-member-f...9/#post4941284



.

 

yep....
although ..it's actually more like 9cc

 

I didn’t measure anything out as in actually parts, and I probably wasn’t even trying to be absolutely accurate, but let’s do that:


[displacement + combustion chamber volume ]/combustion chamber volume = Compression Ratio


for CR = 10:1

[displacement + combustion chamber volume ]/combustion chamber volume = 10

[displacement + combustion chamber volume ] = 10 combustion chamber volume

[displacement ] = 9 combustion chamber volume

displacement/9 = combustion chamber volume

**** theoretical combustion chamber volume = displacement/[CR - 1]


In theory a Renesis engine is 1.308L displacement

= 0.654L displacement per rotor

10:1 = 0.654L/9 = 0.0727L at TDC

9:1 = 0.654L/8 = 0.0818L at TDC

DeltaDisp = 0.0818L - 0.0727 = 0.0059L = 9cc


edit: so what I probably did was just roughly divide displacement by CR, but you have to subtract 1 to CR to get an accurate number.

however, I’m half asleep and maybe made a mistake. I won’t consider myself being belittled by having someone point it out, but would welcome having it corrected.
.

 

hmmm that's not the math I used ...lol . Not saying I'm right but if I do the numbers per what you worked out above and put them into the formula .

[displacement + combustion chamber volume ]/combustion chamber volume = Compression Ratio

0.654+0.0595/0.0595 = 11.99:1 CR (not 10:1) Am I missing something ?

Edit : was not trying to belittle ..... wasn't sure I'd done it correctly myself so wanted to make sure ...that's all.

 

No, I just can’t add or subtract correctly

combustion chamber volume = displacement/[CR - 1]

and you’re correct, it’s 9cc

good job!

 

Anyway , I've managed to pull out approx. 5cc with confidence the rotor wont implode, which brings it down to around 9.4:1 .

I should add : The idea to do this came from that post of yours plus the one from Jetset8 above.................. But I'm not gunna say thankyou (just yet) cuz it's costing me a packet.

 

Highly interested in that experiment.., If you need a guinea pig I'm open to validate your idea with my hybrid renesis setup

 

I'll be trying this out in a month or two is my guess so I'm already being the guinea pig. If you are interested in getting some rotors done for your project though...PM me.

 

tried to warn you and hope you’ll change your mind before making such an expensive mistake, in that particular configuration lowering the compression ratio is only going to make the end result worse …

which I know we discussed using lower compression REW rotors before, but that was non-hybrid and have since reversed my position on doing that after discussing it with Pettit Racing and putting more evaluation into it.

if you’re going to do this then sticking to a standard Renesis with it or just going with an REW swap is going to be your best chance for a successful end result.
.

 

What's your reasoning here ?

 

It’s not “my” reasoning, it’s because the more intake-exhaust overlap an engine has, the lower the dynamic compression ratio it will have compared to it’s static CR.

It’s why piston race engines with high-overlap cam timing run 12:1 - 15:1 compression ratio. Except on a rotary it’s generally accepted that 10:1 CR is the highest practical CR due to the rotating bathtub combustion chamber limitation.

as I attempted to explain in the past (and was chided over no less), a hybrid Renesis already leaks like a sieve for the reasons I’ve posted about numerous times. Lowering the compression ratio will only make a bad situation worse. Go review the KMR/Mazdatrix dyno graph again and then take away more low-end output and spool response. For somebody toodling around on the street in their RX8 like Mr Youtuber there, it will really be annoying to generally drive around in and a dog that can’t hardly get out it’s own way most of the time it’s being driven sanely on the street.

As I also stated before, the only way I see a Renesis hybrid configuration having any chance to perform any better (a mediocre chance, it will never be great compared to the alternatives imo) then it’s going to need a big turbo with high flowing turbine housing to provide a lot more CFM and less emap. Lowering CR will be counter-productive to that end goal. Or rather, require even more of same.

Not going to apologize for being frank or hold back about it at this stage of the game going on over these ideas; most of these things are really basic combustion engine dynamics, with the exception of the Renesis’ zero-overlap intake-exhaust cycle. For someone who has a solid understanding of these principals it’s fairly easy to recognize when somebody else doesn’t.
.

 

For an N/A engine I agree entirely . But for FI ...not at all. You are way overstating the losses and downplaying the gains from lower CR......... IMO.

https://www.rx8club.com/series-i-maj...9/#post4944507

 

I wouldn't use Kyle's setup as an example here, his engine was way too ported, all 6 intake ports open (maybe for racing it makes more of sense) but definitely it not ideal setup to take it as reference why the spooling time/power came so late...

Not much overlap will be added by using the GSL housings without any porting. You can always port the exhaust port up, following the same position as the side port if bigger exhaust port is needed. The way you talk about the drivability of this sounds worse like a semi PP setup (where the overlap is worse).

Low compression wise, as Brett mentioned for FI it makes sense, hybrid or side ports will help benefit from this.

 

So my summed up understanding here is the below shortcomings of the REN 6 port. Do you think the Kaboom is coming from the recompression no overlap cycle detonating or the following ignition event detonating with the EGR/leftovers?

• Short apex seals, machining for rx7 depth seals already exists.
• Siamese port sharing and the cross sectional area being too small. This restriction inevitably prevents gases from leaving out the exhaust. Only fix here I see would be a custom sleeve. (more research needed on what's possible)
• No overlap, remaining gases are being recompressed and carried over. Couple thoughts here for the solution. There was the patented small peri exhaust port you tried. Or the bridgeport route. One thought I had was blocking the APV on the upper intake and bridging the APV port. This volume would really act as a pressure relief to lower the comp ratio during recompression. Maybe the timing isn't early enough or too small to matter. Adding overlap i'd think should help prevent detonation on recompression, but still recycles gases. I will read more on EGR affect on detonation in boosted engines.
• High compression rotors, I always thought the reason people ran lower comp rotors was due to the robust nature of the T2 rotors being chunkier and wont implode. If anything I could section cut an rx8 rotor if we want to see how possible it is to reduce the comp ratio. Otherwise its Lowcomp & HighBoost vs Highcomp & Lowboost. The high comp lowboost should spool faster correct, but can you explain why one would be safer vs the other?
• Complex intake volume and valving (4 port may be an improvement)
  • SSV - 3500-4000rpm opening
  • VDI - disabled (could make a plug)
  • APV - disabled on undersized(greddy) turbo, but enabled & restricted on larger turbos. (doesn't exist on 4 port)

 

Both
Siamese : forget it...
APV : interesting idea but clutching at straws somewhat ... IMo
Rotors : I've actually reduced compression on the Renesis rotors in my latest build - yes it can be done .... up to a point.
APV blocking : 6 port is still far superior to a 4 port ...even with the APVs restricted.

Also read this:
https://www.rx8club.com/series-i-maj...9/#post4944507

 

APV: yeah I think the small bridge may open too late or not be large enough. Or atleast if you haven't had luck with the mini peri exhaust build, the apv probably wont be enough.
6port vs 4 port what makes up the improvement? The additional apv flow at higher rpm? I am not sure how much porting a 4 port secondary could take, if the APV volume became a coolant jacket.

The comp vs boost vs power seems interesting. Based on those it seems lower comp exponentially lets you run more boost. Combined with the other graph shows comp and boost vs power being linear. Meaning lower comp high boost will have a very slight benefit for overall power ~1%. Going off off the 9cr20psi vs 10cr12psi numbers one chart I found shows that they do not have matching or close effective compression ratios.


The below image I marked the example below for 10cr12psi vs 9cr16psi (matching effective cr)....I mean really its only talking about a 0.25% power gain in favor of the higher compression. Whether any of these graphs is actually more or less relevant in our case .


The last graph you listed there seems to show that the most important thing is intercooler efficiency (intake air temp mods) out of anything. Seems that the efficiency here may be worth a couple psi, thus more important than the 0~1% benefit of changing comps. Or I am overlooking other pros/cons when it comes to cr vs boost for detonation. (maybe excessive EGR favors lower cr) It seems like cr just affects your turbo choice and boost threshold.

From the Rx7 forums 9.0cr 400-450hp@15psi....500hpWM at 19psi.................You have achieved ~412@14psi which seems pretty on point if you were 9.0cr, but for 10cr power theoretically should be higher.

If EGR is our issue compared to REWs maybe a larger combustion chamber wont help considering the additional volume that can be recirculated. It really seems like we just need an EGR dump peri exhaust port during the no-overlap timing. I should look back through your mini peri port thread, which was only ran NA right?

 

That compression chart above is not that helpful IMO. It doesn't take efficiencies of compressor + intercooler into consideration which is why it deviates by comparison to the colour chart I posted. The point being ... much better to compress the air outside the engine then get it nice and cool before it goes into the chamber. With a higher CR engine you don't have that luxury - you heat the air more by compressing it more internally - and the only cooling you get is from fuel vapourisation.

4 vs 6 port.... the APV port stays open way longer which is why it can only be opened at higher rpms . there isn't room on a 4 port iron to go up as far as the apv goes - without filling in the water gallery.

 

Right, that makes sense. WM is really the only way to cool internally, but it wont favor hiboost lowCR vs lowboost HighCR I dont think. Thus a lowCR higher boost seems to be the move.

EGR - Thoughts on whether the flamefront could be too slow and being cut off by the peaky rx8 exhausts. I included the good ole REN vs 13b graph below. The rx8 has a short timed and peaky exhaust compared to the long duration 13b exhaust. Perhaps the 13b peri is just safer because they have a large target where that flamefront cant be cut off before it is done.....Then to top it off the overlap prevents recompression during combustion. Could the leading and trailing timings for the rx8 is on a knife edge between too aggressive ign timing causes typical detonation, and too little timing causes a slow incomplete flamefront that gets cut off and recompressed/recycled?

I have been working on a full CAD model for a 13b (my brother got a cnc machine, so in the distant future I may try making plates). Are there any metrics that you think would be helpful? I can remake ports and probably calculate cross sectional area per degree timings. Might help compare the REW vs REN ports. Then I could simulate adding things like your mini peri through the bolt hole. Basically redo this graph with the ability for us to modify the engines/porting compared.

 

To be clear ..... If you ran a fuel that will never detonate ..... the stock Renesis engine would still come to a point where the exhaust ports became choked and make no more power.
To dramatically improve this via porting is virtually impossible with the side exhaust port design.
Jetset proved that using low CR rotors, E85 and a huge turbo can mitigate the inherent design issues (and make a lot more power) but to my mind that was just tying a giant turbo shaped Band-Aid around it.
Which is why I decided to concentrate my efforts into getting the best possible outcome within the constraints of a relatively stock Renesis.