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OHSCrifle
OHSCrifle GRM+ Memberand HalfDork
10/25/15 9:16 p.m.

Just wondering.. to solve the Renesis fuel economy woes.

I figured this crowd would hear about it if anyone has done it.

Driven5
Driven5 Dork
10/25/15 10:04 p.m.

Not that I'm aware of, but I've seriously thought about doing so on more than one occasion...And maybe even with a Focus Duratec instead of the Miata MZR.

kb58
kb58 Dork
10/25/15 10:37 p.m.

I wouldn't due to the higher weight of the RX8, unless you plan to boost it.

Sil80redtop
Sil80redtop Reader
10/25/15 10:49 p.m.

You'd get 30mpg with the V6 out of the Camaro.

Knurled
Knurled GRM+ Memberand UltimaDork
10/25/15 10:58 p.m.
Driven5 wrote: Not that I'm aware of, but I've seriously thought about doing so on more than one occasion...And maybe even with a Focus Duratec instead of the Miata MZR.

The second time I drove an RX-8, I started wondering what it would be like if it had a 2.0 Ecoboost in there. Theoretically, it might even bolt in with NC parts.

My RX-7 by the numbers makes maybe 240hp or so. If an RX-8 makes 240hp then a lot of those are reedy thin, abused horses that you hear about in animal neglect cases. That or my numbers are off and I'm making way more than 240hp and 170ft-lb, which I suppose is possible but would do damaging things to my opinion of my engine building/tuning skills.

kb58
kb58 Dork
10/25/15 10:59 p.m.

Swapping in a new engine is a fairly big project, about the same amount of work regardless of choice. Why not start with something that you get excited about driving, instead of creating something which has lesser performance than either donor?

Driven5
Driven5 Dork
10/26/15 12:37 a.m.

In reply to kb58:

It seems to me that if the subframes/mechanicals are a direct bolt-in affair, it should save appreciable time and cost. And even though I know it's not as simple as it sounds, so too should (to the best of my understanding) being able to more-or-less reuse the critical donor electricals. The idea of doing essentially a 1:1 powertrain exchange between a not-necessarily-stock NC Miata and an RX8 would actually result in two products that I could certainly get 'excited' about driving...Each for totally different reasons.

Flight Service
Flight Service MegaDork
10/26/15 5:30 a.m.

Ok, so you are doing it for fuel economy. I could not think of a reason to do this.

honestly I woud go with a 2.3/2.5 Duratec before I went with the Miata engine for this.

Ecoboost RX8 anyone?

KyAllroad
KyAllroad SuperDork
10/26/15 7:47 a.m.

IMO putting weaker motors in things for "mileage" gains almost never works because the motor ends up being flogged all the time in an attempt to keep up with traffic. A bigger, torquier, more "relaxed" mill should usually net you the better economy than the one being thrashed.

alfadriver
alfadriver GRM+ Memberand UltimaDork
10/26/15 8:04 a.m.
KyAllroad wrote: IMO putting weaker motors in things for "mileage" gains almost never works because the motor ends up being flogged all the time in an attempt to keep up with traffic. A bigger, torquier, more "relaxed" mill should usually net you the better economy than the one being thrashed.

If it were really a heavy car, you would have a point. But the RX8 weighs less than most cars that have the MZR/Duratec N/A motor installed in it. And cars like the Fusion do get close to 40mpg on the highway in real conditions.

Seems to me, a 2.3l from a Ranger, but using the intake/cams/controller from a Fusion or something like that would deliver a pretty nice package. Not RX8 performance, but not bad, either. And with all of the performance parts available for those engines... well.

The 4.444 final drive should be lowered a little, though. Or at least a lot of gear matching should take place.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/26/15 8:37 a.m.

Yes, it's been done. The MZR bolts in with a Miata subframe. Ppf requires some cleverness and you'll have to do some electron shuffling to get full function of all gauges, etc.

Duke
Duke MegaDork
10/26/15 9:50 a.m.

If you want economy, why not some sort of Asian 4 cyl diesel?

alfadriver
alfadriver GRM+ Memberand UltimaDork
10/26/15 10:02 a.m.

I just remembered the Solstace cars- where the turbo got better fuel economy than the N/A.

The reason behind that was that the gearing was changed a lot to take advantage of the torque- which meant the turbo performance was not as much as one would think- but the system was better.

It would be interesting to put a turbo 2.0l GTDI into that. And gear it a lot different. Really hard, but interesting.

1988RedT2
1988RedT2 PowerDork
10/26/15 10:08 a.m.
Knurled wrote: If an RX-8 makes 240hp then a lot of those are reedy thin, abused horses that you hear about in animal neglect cases.

QFT. I can still remember how disappointed I was when I drove the RX-8. Nice feel, and I wanted to like it, but it was really, really sad in the power department. I was ready to love a N/A rotary that made 240 HP, but next to my Turbo II, it felt terribly lame.

So maybe the Miata engine is a good fit if lame is okay. But for me, the RX-8 needs another 100 HP over stock.

Duke
Duke MegaDork
10/26/15 10:13 a.m.

So LSX/6-speed swap? With that l-o-n-g-t-a-l-l 6th gear, you should get plenty of economy out of a lighter car like an RX-8... if you have right foot discipline.

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
10/26/15 10:21 a.m.

That's basically the same as an lsx swap in an NC Miata.

Expect terrible economy around town and good on the highway. Imaginary fuel savings are not the reason for that swap.

STM317
STM317 New Reader
10/26/15 11:29 a.m.

The MZR/Duratec engines seem kind of tall. Any clearance issues with the low hood on an RX-8?

pres589
pres589 UberDork
10/26/15 11:41 a.m.

F2T and a cowl hood scoop to deal with the height. Improved turbo and fueling should get you 40mpg on the highway and literally 2x the stock torque. Not sure how much fun is involved in getting the transmission and engine together with a working clutch in the middle...

Desmond
Desmond Reader
10/26/15 11:46 a.m.

Why a Miata engine tho? The BP engine is not a very efficient design in my experience. Despite its small size and feeble power rating, it still manages to only bring me 22 miles per gallon average, at best. If I'm highway cruising, maybe more like 26 or so, but seriously this engine does not do as well as you would think.

Storz
Storz Dork
10/26/15 11:49 a.m.

S54 + RX8 =

aircooled
aircooled MegaDork
10/26/15 11:58 a.m.
Desmond wrote: Why a Miata engine tho?..

Maybe the electronics will match up?

I would think a Speed 3 motor would be nice (if you could turn it and make the electronics work).

Duke
Duke MegaDork
10/26/15 12:02 p.m.
Keith Tanner wrote: Imaginary fuel savings are not the reason for that swap.

alfadriver
alfadriver GRM+ Memberand UltimaDork
10/26/15 12:13 p.m.
Desmond wrote: Why a Miata engine tho? The BP engine is not a very efficient design in my experience. Despite its small size and feeble power rating, it still manages to only bring me 22 miles per gallon average, at best. If I'm highway cruising, maybe more like 26 or so, but seriously this engine does not do as well as you would think.

(ignoring the fact that I've gotten 30mpg from my '99 BP Miata)

One does not need to use a BP for this swap- the MZR/Duratec seems to be a better swap. Plus, if the Miata was geared more for fuel economy instead of non-downshift acceleration- it would get a lot better mileage.

The 2.5l Swap would work really well, up to a certain acceleration rate.

Knurled
Knurled GRM+ Memberand UltimaDork
10/26/15 12:23 p.m.

Yes, the BP engine would not be the one to use, the MZR or Duratec (or Ecoboost, hint hint) would be the ones.

I don't know how well a Ford engine controller would speak with the Mazda electronics. The RX-8 uses an EEC-V but wiser heads than mine would know what CANbus nightmares or synchronicities exist.

Engine swaps for fuel economy never make economic sense unless you're REALLY in it for the long haul. 20mpg vs. 30mpg would take a long time to save you $5k or so.

alfadriver
alfadriver GRM+ Memberand UltimaDork
10/26/15 12:36 p.m.
Knurled wrote: Yes, the BP engine would not be the one to use, the MZR or Duratec (or Ecoboost, hint hint) would be the ones. I don't know how well a Ford engine controller would speak with the Mazda electronics. The RX-8 uses an EEC-V but wiser heads than mine would know what CANbus nightmares or synchronicities exist. Engine swaps for fuel economy never make economic sense unless you're REALLY in it for the long haul. 20mpg vs. 30mpg would take a long time to save you $5k or so.

Really? Which one?

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