13B rotors in Renesis for lower FI compression?
 

I know a few people have run modified 13B rotors in their Renesis engines with mixed results, but honestly some of these builds don't get the proper parts/money spent on them so the results often have to be taken with a bucket of salt. Pettit offers this on their Stage II builds, but they likely don't sell them separate. Someone I ran it by thinks he can do it since he knows those rotors inside out as well as the Renesis. Which custom machining the side seal and balancing it is no BFD for him either.

I'm just unsure of there's any real benefit compared to the possible negatives with dropping back to a 9:1 PR. It seems like it might be easier on the engine at high rpm, but likely with a loss of response at low rpm. Renesis rotors are pretty thin too so I can see the potential for possible issues at heavy loads like high boost coupled with high compression. E85 fuel will help that situation out a lot though. Just not sure if I want take a chance on trying them out or not due to all the unknown factors like the side seals gumming up with carbon, though E85 fuel will have less potential for that, but eventually it could happen ...

:Wconfused

From the Pettit website


https://cimg3.ibsrv.net/gimg/www.rx8...e40646fd4d.png



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Bueller bump ...

I was briefly debating this route myself. If I pulled the trigger on doing a forced induction build, I would do Stage II or III. There is a certain appealing aspect about a turbocharged renesis, though. My original motor has 160k on it, maybe luck would hold out with a boosted application, hah. Though I may just pickup another FD.

Ok .....good to know !

Granted going with a lower compression ratio would allow for more boost essentially getting you back to equivalent PV = nRT. However doing more compression outside the chamber rather than in the chamber should allow for more intercooler benefit. Of course if you pressurize with no boost 10:1 that’s 14.7 X 10 = 147psi at compression whereas 9:1 is 14.7 X 9 = 132.3psi at compression. Basically boosting 1.63psi on 9:1 gets you to 10:1 assuming no pressurization losses. It makes me wonder if 9:1 is really aggressive enough to really offer much advantage of being able to cool that compressed air before it goes into the chamber. I think 7:1 or 8:1 would give you a lot more room to of course boost higher but also cool that air better with an intercooler. The utmost goal being to keep intake temperatures down as much as possible. Interestingly enough there are inter coolers that can chill below ambient using AC, but it seems like another thing that can break and detonate your engine when suddenly that air isn’t as cool as it should be. Of course E85 has a higher autodetonation temperature (most important for FI), will burn cooler (good for keeping exhaust temperatures down especially given more power through FI), and with less carbon deposits allowing for higher boost and margin of safety. It’s something I’ve been thinking a lot about if I would go FI when my motor wears out and I have to rebuild. I’ve looked at the Pettit engines as well.

it doesn’t really work that way, if I were to do it my focus would be to be to get all I could out of the chamber even if increasing the chamber size some is possible.

Because I was strongly advised not to try and increase the volume outside the chamber area by someone who’s advice I know can be taken to any rotary bank and easily be cashed. :suspect:

and there’s no need to get compression anywhere near that low; in the low 9ish range is plenty sufficient.

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Quote from Corky Bell "consider the 'squish volume' sacred ... do not tamper!"
In a rotary that would be any area outside the indented chamber.

In saying that .......... perhaps we could consider the area outside the trailing end of the chamber not part of the "squish volume" ..................

a mod is pointless though if in the end the potential net gain is offset by an equivalent net loss …

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That's what I'm saying ...there is no net loss if the material is removed outside of the effective 'squish area'.

that may be what you’re saying, but you obviously aren’t hearing what I was saying, just sayin’ … :suspect:
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:lol2:

Going no lower than a compression ratio of 9:1 seems to contradict recommendations from Racing Beat. Obviously I can’t check your assertions because they are anonymously sourced. That’s fine, I get there are trade secrets involved in a lot of this.

Relooking at the ECR equation it makes sense to me that a 9:1 should be sufficient although rather than safely limited to maybe 5psi of boost with a 10:1 you are limited to around 10 psi with a 9:1. So that makes sense to me. I guess with 10 psi of boost with no losses you could make an additional 60% more power or so assuming no parasitic losses.

You still have the same housing, intakes, and exhausts which are going to cause limitations on air flow and heat dissipation. Of course an engine can be reinforced through better dowling. The 13B-REW with peripheral ports of course can be ported to a greater extent, etc. If there is outsize advantage that I am missing with just using slightly lower compression rotors please identify.

How would you increase the chamber size considering the walls already not being very thick with all the oil and coolant passages? Or is there some margin there? Would you do billet housings with differently machined passages? Clearly Billet housings are going to be incredibly more costly.

Maybe you just need to get those 16X blueprints and spend a few million iterating through it…a

a lot of what you read in RB is from a loooooong time ago. Now with EFI, ECUs, AI, shatterproof apex seals, and better equipment overall you can accomplish things better and easier now. Depending on where you live then E70 fuel can be bought at the pump and rivals race gas in many ways at lower cost.

go spend some time searching and reading at RX7Club. There are people making big power on pump gas with a 13B now that rivals some past race engines. Of course there’s a difference between a street car and a race car beating on it hard lap after lap, but things are different now than in the past is the main point.

I was referring to the combustion chamber pocket on the rotor faces. We are talking about machining that surface area as opposed to the entire face. Or I was any way. Due to the thinner casting on the Renesis rotor it’s likely to be limited.


https://cimg9.ibsrv.net/gimg/www.rx8...b49e79b397.jpg
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That makes sense. I forgot about the rotor pocket. As I understand keeping the RX-8 rotors would be preferred as they are lighter than the lower compression RX-7 rotors. I’ve talked with Pettit briefly through email but might give them a call as they suggested. I would like to build up another engine for FI, but I also need to gather much more information. Right now I’m climbing the mountain of learning. Noted to scour the RX7 forums. Still in research phase. I want to understand all this inside and out.

I would be happy to target 300WHP while not sacrificing too much in engine reliability for a car that is mostly street use with occasional leisure track use in the future. I am willing to invest in engine improvements and support mods. If this isn’t attainable then I stick to NA. Fortunately I don’t live in California either…

If FI is viable for me, I wonder if it’s a cheaper proposition to buy a worn out core and have Pettit rebuild it to level II or III or just have them straight up build a new off-the-shelf engine. Obviously a worn out core is still going to require new housings, housing seals, rotor seals, and any other custom machining. Perhaps more depending on if engine was subjected to prior FI. I do expect to spend tens of thousands ($20k, $30k, etc) for engine + FI + supporting mods, but I still want to be cost conscious about what I do, because these projects can quickly balloon up in cost.

Agree that it's important not to machine any part of the rotor face that creates the 'squish' pre the combustion event. However (unique to the rotary engine).....AFTER combustion there is an area of the rotor face that isn't involved in this. Perhaps you have forgotten that Mazda recognised this and put some scallops on the side of the Renesis rotor to improve exhaust gas exit.....................

you still deaf? Let try again in simple language:

DO NOT MACHINE ANY ROTOR FACE SURFACE OUTSIDE OF THE BATHTUB!!! FRONT, REAR, ETC.
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What benefit would that serve to lower compression? Seems like none and a whole lot of other problems. As I understand compression is only happening in the pocket or at least that’s where everything is compressed into.

I believe TeamRX8 wants to remove material to lower compression and a side benefit of slightly reduced rotor weight. Outside the pocket doesn’t work towards that goal.

what he’s saying is wrong, just use your brain and think it through

it would more or less be true on a piston engine, but not on a rotary engine

because squish and the combustion flow process is not the same between them.

it goes back to my previous comment about net loss vs net gain, which clearly is not understood
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Team obviously has a closed mind on the subject and refuses to even think it through so no point engaging with him.
Removing material from outside the pocket does indeed reduce compression but where it is done is crucial. It would be really bad to do it anywhere on the leading portion of the rotor as that area is what squishes combustion gases into the 'pocket'. Mazda did what I'm suggesting by putting scallops on the trailing outer edges of the rotor but their main reason for doing so was to improve exhaust gas flow, not to reduce compression. The fact that they did do it and saw no negative side effect is what is interesting to me. I believe there is scope to remove more material in that area within reason.
Between that and taking a minimal amount out of the 'pocket' as well I have calculated that compresssion can be taken down to around 9.3 - 9.4 :1 which would have the effect of allow ing another 3-4 psi of safe boost on pump gas.

I don’t have a closed mind, my mind was opened to the truth presented by a rotary engine expert, which you and I aren’t.

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