mkd

iTrader: (8)

I'm sure this question has been asked before, but let me ask it again: Why did Mazda decide to use a two-rotor twin-turbo?

Even before I owned one of these things (and before getting a taste of turbo troubleshooting and the rat's nest) I remember thinking that an NA 3-rotor would be SO MUCH simpler and more reliable.

One theory would be "Twin Turbo" marketing.
Another would be tunability (doubt it).

What do you think?

newtgomez

I definitely wouldn't rule out a marketing scheme, but it definitely has to go deeper than that. Turbo technology back in the 90s wasn't as good as todays stuff so I'm sure they wanted a good powerband with low end torque to help around town and the sequential twins do a great job at that. Compound set ups back then weren't that good and a single turbo solution probably wasn't feasible with technology back then for making power big enough to keep up with other modern advances. An NA 3 rotor would have been a blessing, but I'm sure they have their reasons. Personally, I think they did the right thing even with how complicated the system was and it also gives the car a ton of character because of it.

j9fd3s

iTrader: (3)

i don't know, but you could ask Koby at sevenstock..

here is what we do know, we do know that the FC was making 215hp, and they wanted another 50. the goal was 10lbs/hp. the FC turbo could not do that, and they could have gone with a larger turbo and made the HP, but then it would have been too laggy. you have to remember that every road test of a rotary points out low little low end torque the rotary has, its like how every road test says Italian cars have personality, or german cars are well engineered, its just something that is in every road test.

so they did the sequential turbos. which have both, and its cool, and its unique (the JC Cosmo in 1989 had nearly the 1st sequential setup, and satellite navigation, and steering wheel controls for the HVAC, and phone)

from a marketing angle the turbo race cars were also twin turbo, although not sequential....

gmonsen

Most of the reasons have been noted already. The 993 Porsche turbo used the same sequential setup a few years later and for the same reasons. They ought to know given the on-off character of the 930 turbo. I owned one and they were scary.

I've talked to Koby and others who were involved. Especially as to why they didn't go with the NA 3 rotor. The responses I've gotten over the years don't work. They essentially said that the 3 rotor wouldn't fit and would change the balance of the car in terms of weight distribution. I was the first person to install an NA 3 rotor and the motor weighs less than the twin turbo and fits fine and in no way changes the handling. They were just wrong and their blindness to what they probably should have done may have been because they were engineers and they wanted something very sophisticated. An engineer with a budget is like a hammer. Everything looks like a nail.

dguy

iTrader: (8)

I don't know if I agree with you there, gmonsen. While an NA 3 rotor is a very cool rig, and I respect what you've built, it really isn't where they were taking the RX7 and rotary platforms since the inception of the turbo 1st gen or what the public (Japanese) were salivating for. Boosted JDM supercars.

That said, was the sequential aspect of the recipe incorrect/overly complicated? Yes I think so. However what we also tend to forget is that when development started and was probably rubber-stamped turbo technology and tooling for such a low volume of weirdly sized turbos was still stuck back in the 60s (that may be an exaggeration ). It's not like they had access to the EFRs of today at a volume or price point that an OEM needs.

So I guess yeah, the engineers got carried away with their complexity and of course I agree with the saying 'An engineer with a budget...' however in my estimation the decision to produce a turbo rotary sled was not, on the whole, driven by engineers, the execution was.

WANKfactor

iTrader: (1)

How comparable would a Stock-ish port n/a three rotor and a stock REW be in the drivability, fuel economy and cost to build departments? I'm guessing the three rotor would have been difficult to get the required output and torque curves out of while keeping noise and emissions resonable at the time. Also, it was the 90's everything had to be turbo. And the Japanese manufacturers had that "gentleman's agreement" at the time where they had a self imposed 276ps power limit which basically meant they were all going for nice torque output.

quichedem

iTrader: (4)

I was always led to believe the twin turbo was for, first, low-end torque, and, second, for weight balance and packaging. These were the days of "money is no object" one-upsmanship in the Japanese auto manufacturing industry. A complicated, and emissions-compliant, twin turbo system on an engine with notorious low-end torque deficit and already poor emissions performance (and relatively poor fuel economy to boot)? The Japanese were all about engineering their way around it!
So, to say that the engineers got carried-away, when they were given carte blance by management to go ham, isn't really fair. I imagine that a 2-rotor with some form of a turbo system was always the plan. Likely, turbo technology for the time was found to be sufficient and within the cost envelope and the rest is history.
I like the idea of a 3-rotor as much as the next person, but I don't think that adding rotors is always the answer. Fully-funded auto manufacturers have a staggering amount of resources at their disposal, and I would fully believe if someone in the program told me that the 3-rotor engine changed the dynamics in a less-favorable way from the 2-rotor.

gmonsen

Again... We all know that the twin turbo was the cool thing at the time. I'm not saying the engineers "got carried way", but that they were out to build a very fast car that showcased their engineering skills and resulted in one the most iconic sports cars of the past several decades. They could have produced an NA 20b-powered car for a lot less effort and cost and the result would have been a better car. It is no sweat getting 325 whp out of an NA 20b compared to 220-230 whp from the 13bTT. (Mine is making about 380 whp and has made 397, call it "400", whp.) The NA 20b weighs the same or slightly less and fits. It would have cost them a lot less to build, as its just a Cosmo motor without the turbos. I have no idea what the miles per gallon it would have gotten, but it would not have been unreasonable. I knew at some point, but forgot. Didn't care. As importantly, an NA 20b motor would have been dead reliable and all problems the Rx7 has had with its image, it's "lack of reliability", over the years would not have been an issue. Further, they could have brought out a turbo 20b version at some point to offer even more performance. So, they just made a bad decision.

hadokenny

iTrader: (4)

I think if they put the same effort building an NA 3 rotor engine as they did building / engineering the twin turbo 2-rotor set up in the FD, the 20b rotor would be superior in reliability and power delivery. Emission and torquet output are the only things I can think of they a twin turbo set up would have an advantage.

Narfle

iTrader: (16)

The emissions compliant 3 rotor wouldn't have made much power. 50% more than the s5 13b give or take. Kindof apples and oranges to compare the 13brew to an aggressively ported 3 rotor.

BLUE TII

iTrader: (9)

Well, lets just thank God that Mazda did decide to make the sequential twin turbo 13B-REW FD instead of an NA 3 rotor.

The tuning potential was huge and allowed the FD to make 350rwhp+ with bolt on mods and become the go-to Rwd tuning platform for racing in Japan.

Look at what a dud the RX-8 is in the tuning industry despite its superior chassis to the FD (and most any other performance car). Just much harder and less streetable to try to make power out of an NA rotary.

Back in the sunset of the FC RX-7 Mazda brought an NA 3 rotor FC for the journalists to play with- Mazda undoubtedly learned a lot from that experience.

scotty305

iTrader: (2)

At one of the Sevenstock events when the RX8 was relatively new (possibly Sevenstock8 or Sevenstock9) I got a short moment to chat with Mr. Kobayakawa who was responsible to many of the decisions on the FD. I explained that I have an FD, and that I enjoy it a lot, and I asked if there was anything he might have done differently given the 10-15 years hindsight he had at the time. He said he might have installed a 2-rotor non-turbo if he could have made similar power as the Renesis engine. It's been a while, but I think he suggested that FD RX7's could have been more popular if the cars were cheaper and less complicated.

DaleClark

iTrader: (55)

Interesting question and lots of interesting answers.

First, I think the Mazda Japan engineers have always liked a turbo rotary. Also, Japan in the 80's and 90's was turbo obsessed - they put turbos on everything, big TURBO stickers, etc. The rotary engine responds very well to a turbo, the big exhaust ports make a LOT of energy to spin up a turbo.

As far as why 2 turbos, I think this goes back to the FC. The 87-88 FC turbos had an actuator in the exhaust manifold to block off flow to one of the turbo exhaust paths to increase spool up. Then, the 89-91 turbos had "dual scroll technology" where each rotor had it's own path through the turbo with its own wastegate port to increase spool. Going twin turbo - one turbo per rotor - is a logical next step in the evolution.

I think the marketing angle is a LOT of it - all the manufacturers in the 90's were doing twin turbos. Also, I've always felt that the fact you can feel the transition on a stock car - the 10-8-10 boost pattern - is pure marketing. A PowerFC can easily be set up to dial out that transition dip so there's no technical reason for it to be there, having a smooth powerband makes more sense. But, when you go on the test drive and "feel" that 2nd turbo kick in, that will sell you on the twin turbo concept. I think Honda's VTEC having a push in power was also a selling point, you can change the VTEC RPM to have a smoother powerband but it's not good marketing on the test drive.

As far as a 3 rotor I think the reduced fuel economy made it a no-go for the US. History again, the FC was almost hit with a gas guzzler tax and Mazda went to a lot of lengths, including reducing a lot of weight, to avoid that. It's impossible to say what the EPA test numbers on a 3-rotor would have been but it does use more fuel for sure and would probably have had highway numbers in the teens instead of the 20's. US emissions would have very likely been a lot tougher to make it pass, and these are all things manufacturers need to think about - you can engineer an amazing car, but if the EPA says it doesn't pass emissions you can't sell it, and that's the end of that.

3 rotor NA's are SUPER cool, but we are seeing it from a "remove this engine and put this engine in" perspective. If the car came stock with a 255 hp NA 3-rotor, that would be a lot of fun, but the modding potential as it sits would be very limited. Intake and exhaust, make another 10-15 hp, and you're done with the easy stuff. As stated this has been a big issue with the RX-8, mod wise it's really strung out from the factory and you can gain very little with bolt ons. Mazda may not have considered aftermarket modding as much when designing the car but we are fortunate it's a turbo car.

Dale

gmonsen

I think Dale makes so many good points. I remember going on a test drive in 1994. I was awestruck by the acceleration and the smoothness of it. Didn't really feel the 2nd turbo kick in, but was overwhelmed by the pull all the way to 8,000 rpm. Porsche and BMW (and the Corvair) had brought out turbo cars and they were considered the leading edge of engineering prowess. Mazda being an engineer-driven company had to have and market what was leading edge.

[I just want to say that, while an NA 3 rotor might have presented issues with emissions, I am so sorry no one here has driven an NA 3 rotor (other than myself). People can theorize about the specifications, but have no visceral sense of the motor. If Mazda had brought our an NA 3 rotor making 250-300 hp, I don't really see why the performance potential of modifying it wouldn't be there. Mine just has large (some say monster) street porting and ITB's. People would also take the NA 20b and stick turbos on it.]

scotty305

iTrader: (2)

While trying to find how to spell Mr Koby's actual name, I came across this reprint of a 1992 Motor Trend article about the FD.

https://www.motortrend.com/news/1992-mazda-rx-7/

It seems like the magazine writer was impressed by the broad powerband, which might have been difficult to achieve with a non-turbo 20B. It also seems Mr Koby and others on the team were mostly concerned with suspension and weight, so they might have thought it preferable that the weight from twin turbos is closer to the rear of the car compared to the extra rotor and housings of a 20B. The team went to pretty extreme measures to reduce weight, as evidenced by the barely-sufficient swaybar mounts. The team certainly wasn't perfect either, in hindsight I think the wiring harness has some odd

Personally I'd love to drive a non-turbo 3 rotor even at just 250 whp, I think the improved engine sound and response would be worth it. Around 2005 or 2006 I drove my stock twin-turbo FD and an LS1-swapped FD at the same autocross event, the the instant torque of the V8 RX7 was fun and the handling wasn't noticeably worse. GIven today's technology, I suspect a 3-rotor with aluminum housings should equal or beat a 1994-spec 13BREW in terms of power vs weight. You might even be able to make it run cleaner given modern EFI and catalytic converter technology. If you play the modern-tech thought experiment, an aluminum-housings 13B with a single modern turbo might make even better power-to-weight and keep that weight further to the rear, plus smaller displacement should have an even easier time with emissions.

arghx

iTrader: (3)

It’s not clear a 3 rotor would have been positioned in a way that would meet crash standards and all other engineering targets.

gmonsen

You are right about the instant torque and also the fact that you don't double the hp in 1 second. The power is far more controllable in a corner. You never want to floor a big turbo FD before getting pretty much out of the corner.

Also, the NA 20b as installed in the car weighs about 75 pounds less than the 13btt. You add a rotor, a housing, and a plate, but you get rid of the turbos and downpipe and intercooler.

BLUE TII

iTrader: (9)

Would have been a cool option to have a "driver's car" version of the FD with an NA 3 rotor and some short gearing making it engaging to drive.

Luckily we can make that now and dont have to follow emissions or noise laws or even a budget- so it can turn out a lot faster, lighter and look cleaner than Mazda could have made it.

scotty305

iTrader: (2)

I'm sure there would be more 3-rotor swaps if it was cheaper & easier to do. I haven't looked into it lately, but last I checked the big cost items were the engine itself (and likely rebuild), new subframe or Defined Autoworks mods to fit in the OEM subframe, standalone ECU and wiring harness, and new exhaust manifold. I don't remember if the 20B will mate directly to the RX-7 transmission. I already have the standalone ECU and could handle the wiring myself, but still can't justify the cost and labor that would be needed for better sound and throttle response.

TeamRX8

iTrader: (2)

It was my understanding that a factory 20B is approx. 100 lbs heavier than a 13B. Now you can build one with alloy parts, tubular header, etc so maybe that’s true, but wrt what you’d expect for a factory OE 20B I’m doubting it’d be lighter, not by a long shot.

gmonsen

Blue TII... Complete agree. I ran 433 gears for a while and it made the car extremely quick. Actually ended up going with 390 gears for more time in gear at expense of accel.

Scotty...I think you could cobble together a 3 rotor from a 13b's housings and rotors, plus another set, and different plates. E-Shaft is about $4500 out of Australia. Mates perfectly to stock tranny.

TeamRX8... If they are saying the stock 20b weighs more than the 13btt, then that is because the stock 20b was a twin turbo. Take the turbos off and the NA 20b weighs less than the 13btt. My motor down to the collector weighs about 75 pounds less than the 13btt.

dguy

iTrader: (8)

Sure but I think that's a pretty disingenuous comparison when we're talking about what an OEM would have done had they implemented an NA 3 rotor option in 1993. You've removed pieces on both sides of the engine (manifolds) and replaced them with thin wall tubular pieces when historically the construction of those things to keep production costs in check were cast, and cast heavy. I believe a more reasonable comparison would be a modern manifolded single turbo setup, or even twin itty bitty EFRs for that matter, so long as its a tubular designed manifold.

gmonsen

dguy... So, instead of being 75 pounds lighter, the NA 20b from the factory might just weigh the same as a 13btt? The cast manifolds cannot weigh 75 pounds more than the thinwall manifolds. Again, the differences are that the 13btt has 2 turbos, necessary piping, and an intercooler with piping that you take off and then add a rotor and housing to get to a 3 rotor. There would probably be heavier cast manifolds, but their weight wouldn't add 75 pounds. Its all a bit irrelevant in any case. I only know of 2 other 3 rotor NA cars. Most who want 3 rotor motors are going the turbo route for more power. I remember Peter Farrell giving a talk at a dinner in 1996 where he had just finished the 3 rotor Phil Hamlin. He was saying they would get to 900 whp soon. A turbo 3 rotor was the thing. Still are damned cool.


I'm stuffing an old video of my NA 20b here. This is why I did it.

dguy

iTrader: (8)

I'm saying that you're not comparing an apples to apples build in packaging or execution, and that I believe you're omitting some things that may have been removed and simplified for your build that would not fly if it were a factory backed product. The weight that you were able to achieve would not have been feasible given the budget, production, amentities, and emissions constraints that the dream 20b we're discussing would have been under. Further as you must know you're not adding 'just a rotor and housing' when going to a 3 rotor, the extra thick iron, multi piece e-shaft design, extra gear and bearing among other things add up to a not so insignificant amount of weight beyond what you describe. I say all these things with confidence as I've built a 3 rotor car (tube chassis) that has since gone from supercharged to turbocharged and I'm literally about to dyno this thing in the morning:






And while its far from stock, I also have all of the gear we're speaking of and have been handling it as we've been having this discussion off and on (thin wall 3 rotor manifold, more than likely half the weight of the cast 2 rotor NA manifolds):





Last but not least I'm not dissing your build in the slightest, this discussion is based on what Mazda could have produced as an OEM in 1993 with production locked down in ~1991 or earlier, not the awesome work that you've been able to take on and refine.

TeamRX8

iTrader: (2)

Agreed, that’s more or less what I was trying to convey too.

Back to a twin setup, packaging aside it’d be kind of cool to see a 13B with dual BW S252-SXE turbos (uniquely a 7070 sizing) that has the combined potential of 108 lb/hr at 30 psi. Heard through the grapevine that an aftermarket v-band inlet turbine housing with higher AR is coming for it. Currently you can only get that in 0.83 AR. Pretty much a rainy-day dreamers idea though.