I finally built a new engine for my car after it blew up back in '23. I did some mildly aggressive exhaust porting this time to try and get some more exhaust flow. But I also think I may have come up with a brilliant way to increase exhaust flow. After months of science and maths and flow dynamics I think I may have arrived at an interesting way to increase flow over and above what porting alone can accomplish.

If you reverse the rotors, the scallops actually open the exhaust earlier than then they would in oem orientation. Obviously you lose the extra open time on the intake but who cares when you are stuffing boost in there right?

If you think I'm crazy grab an old iron and a rotor and have a look for yourself. Has anyone given this a try before?

 

Haven't tried that but also ..... wouldn't want to. Opening exhaust ports earlier does very little for flow. The issue with the Renesis has always been the inability to keep the exhaust ports open for longer. This is why turbos struggle to make power past a certain level.
Also : porting right out to where the corner seal passes is a sure fire recipe for increased wear in that area and therefore a shortened life for the engine. If you do run those irons be sure to run plenty of premix on top of the omp. Premix alone wont cut it, you'll need both.

 

Engineering includes testing and validation. Until that happens, it's just GIGO...

 

Just as well it's only enganeering then!

 

In all honesty I was pretty nervous about assembling an engine, so I smoked a little hash and had a couple pulls from my bottle of wisers before I got started. It was going great until I went to put in rotor 2 and realized I had put the wrong one in R1 position. Kinda figured it was like Rx7's where they are interchangeable so I kept going. It wasn't until I did some reading a few days later found out they weren't.

Thing is, it runs great. Its built from old parts I had lying around my shop. I didn't want to spend thousands on brand new parts for a first time build in case I did something catastrophically wrong. So I'm gonna take it to the dyno when the weather turns nicer and see how she goes. Has about 4 hours of idling time and a few pulls up and down the street no insurance tho cuz its winter here.

I based the porting off pictures of the CNC porting that rx7 specialties in Calgary offers. I am a fabricator by trade so I have fairly steady hand with a die grinder. I didn't extend into the corner seal path (at least I didn't try to). But thanks for the heads up Brettus, my OMP is long gone I run 85:1 premix.

Edit: I would like to add it sounds really cool. Almost more like an old rotary. Still very smooth like renesis but just a hint of burble.

 

Won't you need to change ignition timing to compensate for the changed intake/exhaust timing?

 

They obviously don't understand the issue I was talking about. The area that wears badly is between the port and the housing, particularly when the engine is stressed. By making that area that the corner seal passes over... longer ...the wear becomes worse. Have seen this on numerous occasions.
Your heavy premixing will definitely help.
Area in red is where it will wear abnormally.....

 

Oh wow, ya that makes sense now. I guess like 1/3 of the corner seal hangs into the port? Now that you say I can totally see the wear path from that pocket on the corner seal. They've added some meat on the intake port in that spot for the same reason I guess. I didn't remove any material along the side edge there, but definitely at the bottom I have made it at least 4mm longer.

I will let you know my synopsis when the engine comes apart again. It's not a daily driver, or really high mileage vehicle so I was hoping to get a few summers out of it. I am really curious though, especially now I know its backwards. I will get my tuner to push it a bit on the dyno and we'll see how it reacts.

 

That area wears excessively particularly on stressed engines.
EG : turbo engines, engines with overheating issues, engines with omp malfunction, engines that spend a long time at very high rpms.

The omp does a good job of lubricating that particular area so when you switch to sole premix, you really have to use a lot to make up for it.
I went back to using omp for my turbo setup (upgraded for 30% more flow) once I pieced it all together. Only use premix for a track day etc.

 

That would be why the oil injectors are tilted to point at the ends of the apex seal I guess eh. I wont be returning to OMP on this setup for now, but I will keep that in mind for future. I think I had to use some of the ecu outputs that would've been for OMP on the drive by wire, that's why I ditched it years ago.

 

opening the exhaust port earlier on an NA Renesis engine yields little - nothing.

Not so on a boosted engine. It’s pretty common mod on the prior 13B turbo engines to prevent adding I-E overlap dilution, but there’s a limit between gains vs. losses. The issue on a Renesis though is the required feather-edge angled entrance due to water jacket location has limited flow potential.

The factory 13B exhaust port opens ~25° sooner than the factory Renesis as per the SAE paper diagram below (far left side)

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Update: I figured I'll report what I found so far. Unfortunately I'm looking at another rebuild, so it will be awhile til I actually get on a dyno to see what the power characteristics actually look like on paper.

I got about 900km on the motor so far and performance wise things were going really well. It's hard to quantify what the reversed rotors actually accomplished. The turbo repsonse was significantly better, but the caveat being I also did that exhaust porting porting. I have pretty large turbo (chinese version of the gt35 that comes on a 4L barra motor) and it was making .8bar(10~psi) boost around 3300rpm pretty easy. I think it was closer to 4000 rpm prior to the porting so I am very happy with that improvement. Cruising on the highway it would respond really quick and was a joy to drive. Plenty of power, especially once I cleaned up my tune a bit.

I was able to get a pretty nice idle around 1000rpm. I have my secondary ports stuck open so idling any lower was a bit too dodgy. It was still a very smooth renesis-esque idle but there was a noticeable burble that I reckon might've been from the reverse rotors. I didn't notice a real change in low end torque, but it was kinda rubbish before and still wasn't anything to phone home aboot.

While I didnt do a comp test, colloquially it seems just fine. Starts quickly hot and cold. When I first started driving it we were still getting negative degrees overnight so I did have to change the fueling a bunch in the tune for the cold starts.

Where am at now: I have sprung a pretty significant oil weep from between the front plate and first housing. Literally right next to the "m" in mazda on the housing. I am obviously a little heartbroken as I was getting really excited to do my first oil change and hit the dyno with my home built motor. I don't recall having any o-rings left over but my best guess is the one on the dowel either failed or was missed. Engine runs great but is undriveable due to the severity of the leak. Really fortunate I caught it before running dry and starving the bearings and whatnot. Gonna order a new soft seal set and have another go soon. At least weather is nice now, it was like -15 in January when I did it last time. Thank god for woodstoves.

I suspect part of my problem was my modified oil pressure regulators, I was getting pretty crazy oil pressure. To the point that when I would get into high rpms it was definitely blowing past the seals in the turbo. Almost like rolling coal, but blue, when I would do a hard 1-2-3 pull. Was running tons of 2 stroke as well so ive pulled back on that to aboot 100:1 now. Even at idle she would do like 40 psi. The rear regulator was crushed and I had 3mm or so of shim in the front one.

 

Check for a crack around the dowel location on the front plate.
If that is the case consider modifying the oil flow path such that it bypasses that dowel. Not an uncommon mod.