Ashyukun (Robert)
Ashyukun (Robert) PowerDork
4/15/21 9:35 a.m.

So, as part of the massive overhaul/rebuild effort on my DMC, I'm planning on swapping in a more modern and more powerful engine. One of the top contenders for me is the 13B Renesis out of Mazda's RX-8- but the potential of using that engine presents me with a conundrum: can the stock transaxle in the DMC (a Renault 369-08) handle the 8000 RPM or so redline of the rotary.

The stock PRV hits its max HP at 5500 RPM; a 6-port Renesis hits its top power at over 8000 RPM. Overall it's not an exponential increase in power (150HP for the PRV and 232 for the rotary) but I still don't want to grenade the transaxle when pushing the engine to its top power. Power-wise, the transaxle should be fine with the improved input shaft I have for it (generally regarded as the main weak point), and I will be overhauling the transaxle as part of the process of overhauling the car. There is some disagreement on the DMC board about this- some have argued that the transmission just wasn't made to spin that fast, while others have contended that you can transmit more power through shafts/gears when they're turning faster so it shouldn't be a problem.

Thoughts? Thanks!

(NOTE: as pointless as this note may be- please don't comment with, "You should really be using XXX engine instead," that's an entirely different discussion and I'd rather keep this remotely on-topic)

¯\_(ツ)_/¯ PowerDork
4/15/21 9:39 a.m.

Does that transaxle get used somewhere else?  Maybe a race car?  9k rpm isn't an unheard of RPM for a race engine, I would see if you can find somebody who races with one and ask them since they'll have way more firsthand experience with those sorts of input shaft speeds than the average DeLorean owner.

Pete Gossett (Forum Supporter)
Pete Gossett (Forum Supporter) MegaDork
4/15/21 10:06 a.m.

In reply to Ashyukun (Robert) :

Any idea how closely related that trans is to the one used in the Europa? BEC swaps aren't uncommon in those, so I'd expect they'd see similar RPMs. Although I don't actually know if they retain the stock trans for those swaps either. 

Kubotai New Reader
4/15/21 10:16 a.m.

It was always my impression that torque is what really is the limit for gearboxes.  So, is the peak torque for the rotary any higher than the peak torque of the PRV?  I would guess that they're about the same.  It would be hard for me to imagine that bearings in the gearbox that are OK at 5500 rpm would fail after brief trips to 8000 rpm.  I suppose you could argue that turning faster will result in more heat but are you really going to be turning 8000 rpm for all that long?  I think this is a street car build so mostly you would be running at some constant speed.  If you haven't changed the CWP then the gearbox is turning the same speed at 70mph with a rotary as it was with the PRV so there shouldn't be any difference in heat generated.  Seems like it would be OK to me but what do I know.

Driven5 UltraDork
4/15/21 10:16 a.m.

RPM doesn't break gears, torque does. Granted, I suppose that perhaps the higher RPM could be a potential for lubrication issues. The biggest issue with though, generally seems to be synchros and shifting at the higher RPM. Also with significantly increasing the RPM range, it's going to significantly alter the speed range in each gear, which may or may not have a desirable effect.

gearheadE30 Dork
4/15/21 10:29 a.m.

My understanding is that it usually comes down to forces generated when shifting and lubrication. There's a lot more load on synchros and dogs shifting at high revs because there is a lot more energy stored in the gears as rpm increases. No idea how to quantify if it is actually a problem, though. I suspect if you give the transmission a bit of time between gears instead of slamming the lever back and forth, it will still live a happy life.

I don't know about the lubrication side...what I would be worried about is that at high rpm, you could aerate the oil which makes it much less effective as a lubricant. Maybe a thinner oil and being very careful with oil level? I would think there has to be some  history of people using those transaxles in a race application.

As others have said, usually transmissions have a torque capacity limit, not a power capacity. I don't think that's a big risk with this swap.

Are you planning to change the final drive ratio to work with an engine that likes to rev almost twice as high? If not, it may end up feeling like the rpm gap between gears is pretty big.

Ashyukun (Robert)
Ashyukun (Robert) PowerDork
4/15/21 11:24 a.m.

In reply to Pete Gossett (Forum Supporter) :

Earlier Europas look to have used the same transmission as the Renault R16, but I can't find any details on it; later ones used the Renault 365. The DMC used the 369. Honestly I'm having a hard time finding a lot of info on the practical differences and interchangability of them...

Ashyukun (Robert)
Ashyukun (Robert) PowerDork
4/15/21 11:27 a.m.

Good point about the final drive ratio... I am going to have to look into that...

93gsxturbo SuperDork
4/15/21 12:44 p.m.

Couple things at play - 

Torque and HP - either it will take it or it wont.  Can limit the issues here by using skinny street tires and an organic clutch to act as the driveline fuses.  A wise man once said "with a 2 step, a twin disc, and drag radials you are gonna find the limits of your driveline real quick" in regards to my supercharged C5.  And I did find those limits...again and again and again.

Shifting smoothness - There is a reason that people don't swap a 9 speed Eaton Fuller into their street cars.  Yeah they will take a ton of torque and horsepower, but all the big bits used to make them durable also mean they shift slowy and just straight up won't shift past a certain RPM.  Lotta guys look at truck transmissions like the NV4500 as a poor man's Tremec since they already have a Chevy bell pattern, are rated for decent HP and Torque and are 5 speed, but in addition to being huge, they dont really shift that well since they are designed to be shifted slowly in a big slow moving truck, not shifted quickly in a race car.   So even if this trans will in theory hold your HP and Torque, it may suck donkey dick at shifting at 9000 RPM because it was never designed with syncros, dogs, shift rings, etc that had that design intent.  Thats also part of why a Tremec costs real money - triple disc carbon syncros, etc are much more expensive than your big single brass syncros, but they package better, weigh less, and shift better at high RPM.   

Heat dissipation/lubrication - Increased RPM and HP is gonna increase the heat.  At the very least make sure you have good oil and all the oil passages are deburred and as good as you can get them, if you are still burning up your oil or bearings you are gonna need to look at an oil cooler.  Much more important for track use than drag strip or street use where you dont have long duration high load situations.  

gearheadE30 Dork
4/15/21 12:44 p.m.

hmm, the Europa comment reminded me of a comment I heard about Esprits. Don't the V8 Esprits use the same (or very closely related) transaxle? Some of those were good for 7400 rpm or something like that. I think the 4 cylinders actually didn't rev as high.

Strange things can happen with torsional vibration at high rpm, especially with high inertia, not inherently balanced, and/or with engines that have power pulses spaced further apart. Are rotaries considered a severe case for torsional vibration? My gut feel is probably not, and they are probably fairly close to natural balance, which may help with high rpm durability.


Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) SuperDork
4/15/21 12:56 p.m.

I have yet to break a transmission from rpm and I feel like I have tried (downshift full load from 3 to 2 sometime). 

The speed of the shift at high rpm may be what gets you. We modify all our syncro transmissions internally to make this better but depending on the transmission design who knows if it's enough or not. It will be one of those "will see when we get there" scenarios.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/15/21 1:09 p.m.

Zero practical experience with that trans, aside from doing a clutch in a DeLorean and discovering the joys of about five thousand M7 bolts...

Bearings are usually good for more RPM than an engine can do.  Bushings (like tailshaft bushings on RWD transmissions) not so much.  The synchros WILL not like shifting at high RPM.  Hell, Mazda transmissions do not like shifting at high RPM.  Fortunately, the rotary's broad powerband means you can just short-shift and save the high RPM for the final gear before a braking zone.  Or just acquire some patience, and don't expect high RPM shifts to be anything but slow and deliberate.  Patience is always on backorder so I short shift smiley

Ashyukun (Robert)
Ashyukun (Robert) PowerDork
4/15/21 2:55 p.m.

In reply to Pete. (l33t FS) :

I'd have to be short shifting it a LOT though. Unfortunately it sounds like there aren't many real options (at least reasonably priced) for changing the final drive ratio on the transaxle- nor on any of the other rear-engine transaxles that could be used as well. The PRV making max power at 5500 RPM would have the car going about 35MPH... the Renesis at 8000 RPM would have the car going like 54MPH. Of course that would raise the theoretical top speed from 145MPH to 210MPH!

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/15/21 3:10 p.m.

In reply to Ashyukun (Robert) :

That gearing sounds pretty good for a lightweight car, IMO.  Lighter cars are less tolerant of short gearing, acceleration in 1st gear gets too close to the engine's ability to rev itself.  BTDT


'Course, I have no idea how much a DMC weighs...

californiamilleghia SuperDork
4/15/21 4:41 p.m.

there is a lot of info on Renault and other mid engine gearboxes here



Ashyukun (Robert)
Ashyukun (Robert) PowerDork
4/15/21 4:53 p.m.

In reply to Pete. (l33t FS) :

Roughly 3000lbs, at least with the stock engine. Should be a bit lighter with the Renesis.

noddaz UltraDork
4/15/21 5:07 p.m.

Does Kennedy make an adaptor for a Porsche transaxle?  I am guessing that a Porsche transaxle would be stronger than the stock transaxle.

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