High Compression + 8374 , how far would you push it?
 

There's those that live dangerously, those that play it safe, and those that say "What if".
I fall under the latter and decided to build a Stock port REW with scalloped Rx8 Rotors and slapped an 8374 to the side of it. Mainly out of curiosity on how the combination would effect the midrange. Now that I'm into the driving / street tuning / slowly working out the kinks it makes me wonder how far others would be willing to push the boost on it.
I'm currently sitting at around 8 psi (literally just getting into the turbo efficiency area). I'm really pleased with it and the boost response on the street. Under load (3rd / 4th gear) I've seen 8 psi by 2500 RPM with cool air, light load (2nd gear) normally hits 8 psi around 3-3400 RPM. With how my exhaust is setup and routed (3.5" DP into 3" back - Passenger to Driver's side under the trans back to the muffler that then exits on the passenger side) to it has enough back pressure to be able to run off the Wastegate pressure of around 5psi.

TLDR: What PSI you think it'll go boom on straight 93 ?

You should be able to take it up to 12-14psi without any issue. To really get something out of this turbo, you'll want to invest in better fueling like water/meth and push around 18-20psi. You're still in the high 200hp range at 8psi roughly.

My stock, cat gutted RX8 knocks pretty good nearly everytime I floor it because Im cheap and running 87 octane in it.

So, I say if it hasnt blown up yet at 8psi on 93 octane leave it there. I wouldnt raise the boost though.

Long Term this will also get tuned on E85 as I have full FlexFuel capabilities. Due to the poor availability of E85 in my area and this being a true street car (95% street driven), I don't want to add Water/Meth. I'd rather run less boost safely.

Looking through my logs I'm not seeing any knock that makes me concerned. I'm running a very conservative tune while I work through all the fresh build bugs and nuances before I have it tuned by a pro. I'm just a curious weekend warrior.

I'm pretty sure the differences between an REW and MSP are going to be substantial enough to not be an Apple to Apple comparison. MSP intake ports are optimized for the for the slight difference in the side seal location on the MSP rotors. They sit a bit more outward which makes my weekend warrior mind believe it will effect efficiency by reducing overlap / duration being used with REW stock ports.

1. i'm not sure i follow this. how would the side seal itself affect port timing?

2. however, assuming your theory/explanation is correct, wouldn't you also be countering the loss by introducing the peripheral port exhausts?

The chamber is encapsulated by the Apex, Corner, and side seals. With the MSP Rotors having the side seals closer to the Rotor face they open the intake later and close it earlier.

I personally believe this is why the folks that swapped from S5 NA to MSP didn't see the gains they were expecting from the bump in compression.

Edit: For the second part it'd be based on the Keg, in this case I'm using an REW so exhaust would be untouched and remain OEM specs.

at the risk of taking your thread off topic a bit, you're still saying the side seals have an effect on port timing, and i still don't see it.

i'm with you up to this point. yes, they all work in tandem to basically seal off the chamber for the engine to hold compression and go through the cycles.

this is where you're losing me.

they did this because they were able to basically streetport (a relatively large one) the engine from the factory. they got even a little more timing by adding the bevels to the rotors. they moved so the seals wouldn't "push/fall" into the port, like they would if you opened one of the older engines too early.

i really don't know what to make of it, i've read a little on the subject here and there, and my understanding is vague at best. it sounds like efficiency starts to take a hit at around 10:1 and completely nosedives after 12-ish. i don't know how much of that is theoretical or tested - especially when it comes to everything between 10 and 12 since 10 is the highest i have seen. i remember RB used to sell "higher compression" rotors back in the day, but i honestly don't remember the number.

point being ... i don't know enough about the subject to hazard a guess as to why the 9.7s perform better, but i have noticed they do, just like anyone else paying attention.

i know. :) i only posed the question because you said the MSP rotors (side seals) decreased your overlap and i was saying using the peripheral exhaust (REW rotor housings) would increase it.

by the way, i dig your experiment. i'm definitely tuned in to see whatever progress you make and the results it yields.

Having the intake ports open after top dead center like stock 32deg 2ndaries and 45deg after actually increases exhaust gas dillution because the intake stroke can only suck up exhaust from the exhaust port overlap because the intake isnt even open for 45deg of intake stroke (hence, no feesh air can enter engine).

However, this EGR designed in does actually buffer the intake charge for emissions and may help knock resistance as well.

Think along lines of the side seal actually opening the port and setting port timing rather than the rotor edge. Granted the airflow is limited during this time, but I believe it does have more of an effect than most people realize.

Good discussion is good discussion, I started this thread in hopes someone else was crazy enough to run a larger turbo and increased compression in an REW on pump gas like I am. Honestly wasn't expecting any legitimate responses but figured why not at least try. Forum responses are normally substantially better quality than social media.

okay. i think this is more a case of us seeing the same thing, but choosing different focal points to explain it. whenever i think of overlap, my mind goes straight to port apertures and timing specs. i thought of nothing but the rotor shoulders, opening edges and closing edges. the seals are an afterthought to me. there are times i may play devil's advocate, but this time i really thought i was missing something. :)

well, i've been giving it some real thought in the last year or so. i've had an S5 Turbo block sitting around about 6 years now, waiting to find the "correct" rotors for it, but if they're out there, i haven't seen them. so i'm thinking of just using a pair of the abundance of 9.4s i also have sitting around, and proceeding down that path. i don't have any intentions of running a turbo as large as yours though, but it naturally caught my eye when i saw your thread title. :) i'm definitely interested in what you're doing. i particularly like that you're limiting yourself to pump gas for now.

Side port opening line has to support the corner seal and side seals so they dont fall onto the motor.

This is why a bridgeport literally leaves a bridge for the cornerseal and side seal to traverse the port on.

Stock milled ports up to RX8 open minimum 32deg after TDC to support the seals.

Mazda hand ported (original Cosmo Sport) and common street ports allow the top sideseal to drop into the port and use bevel or scissor to make sure it returns to the side plate safely- these ports open minimum 24deg atdc. The outer sideseal still is 100% supported on the side housing.

If you bevel the sides of the rotors you can achieve earlier port openimg like the RX8 did.

The RX8 also moved the side seals so their path next to the port changed for earlier port opening and decided it was OK to shoot the outer side seal tips over thin air for a short distance at the very bottom of opening side of the port.

The RX8's scalloped rotors, moved sideseals and creative porting got Mazda to 0deg after top dead center port opening.

the zero overlap is the key on the Renesis; no gas dilution and as Blue mentions, the intake ports were rephased to open from 3 deg ATDC on the primary, 12 deg ATDC on the secondary, 38 deg on the APV, and then the APV closes at 80 deg TDC. So pulling clean intake air for 77 deg out of the 90 deg intake cycle above 6400 rpm. The early rephasing of the Renesis intake ports is also why adding overlap loses power; it dumps exhaust gas right off TDC immediately into the full port opening, over a longer timing period early in the intake phase, and totally fubars that design configuration.

Also why adding GSLE peri exhaust housings have not produced any successful results; it adds an immediate 52 deg of overlap under that same scenario, just in the unmodified OE peri exhaust port configuration. It’s also why 13B techniques generally result in a power loss on the Renesis.

Most people never sat down and fully evaluated what the differences were even though it was right in front of them. It was a whole new concept on how to make rotary power. It’s also why if the rotor seals start leaking everything goes to crap. The cutoff seal and rotor side machining also played strong roles in minimizing intake exhaust gas dilution, but that won’t be any benefit retrofitted to a 13B. It’s all relative to the side exhaust port configuration.


https://cimg3.ibsrv.net/gimg/www.rx7...37fbfcc58.jpeg

^ very eloquent post. i think it covers most bases.

Back to the original topic of how far to tune on pump gas vs the higher compression; I would still feel safe going to 1 bar, as long as you are running richer than normal( say 10.6 AFR). I would want dual egt probes to make sure each rotor is running within 75 F of each other however. A dyno really helps when it comes to new configurations, or unknowns. You can look at the power/gains drops off from the tune to determine what is and is not safe.

Good Info, I still have to install the dual EGT's. That'll happen over this winter.