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Redline with my 3.63 and the BMW 6 speed is a breathtaking 150mph in a car that feels really small at that speed on the track. I would not want to go that fast in it on the autobahn or any other open roadway, and I consider mine to be very stable. It stops being about stability and starts being about survivability at some point.
I agree!
Being geared for 200 is sort of cool but...
Actually being in a Miata above 150 is a bit on the "redneck dangerous" side of the equation.
Above 120 you really need downforce that your car doesn't appear to have.
Above 150 downforce (a **** load!) is a REQUIREMENT!
At these speeds the car will be drag limited well below its gearing...
The BMW transmission has too wide a ratio for this application.
The first two gears are too low...
SIx, are the first two gears too short on the 3.63 ratio?
3-6 look fine for 3.63 IMO.
Redline with my 3.63 and the BMW 6 speed is a breathtaking 150mph in a car that feels really small at that speed on the track. I would not want to go that fast in it on the autobahn or any other open roadway, and I consider mine to be very stable. It stops being about stability and starts being about survivability at some point.
Originally Posted by technicalninja
I agree!
Being geared for 200 is sort of cool but...
Actually being in a Miata above 150 is a bit on the "redneck dangerous" side of the equation.
Above 120 you really need downforce that your car doesn't appear to have.
Above 150 downforce (a **** load!) is a REQUIREMENT!
I have no idea what you guys are talking about.
My current redline is 7500, and I can hit redline at 5th, which gives me 150 mph.
It is not breathtaking (I just hold my breath), and the car seems very stable.
I lift and the speed comes down to 120 mph or so, that's when I open my eyes.
In all seriousness, this car will get a very stealthy 3" exhaust, and a complete flat bottom with venturi tunnels, Ferrari style.
The flat bottom will have two NACA ducts on either side of the transmission, and air will exit at the center rear after cooling the diff, aided by the venturi effect of the exhaust outlet.
(The exhaust tip will be slightly recessed to get that effect)
We have modeled my car on the computer, and there is a pretty detailed virtual wind tunnel program. I can play with ride height, rake and such on the screen at any speed.
I have to retain the stock look of the car, so no wings, nothing that will attract undue attention...
One very cool thing to mention is the amount of help I am receiving over here.
Being a TV personality (40 episodes in total on TV) plus weekly live youtube programs where I talk about technical stuff has a wonderful advantage.
I also document everything I do on the car on a Turkish forum, and my threads are pretty popular - over 700,000 hits on the stuff I wrote in one forum.
People recognize me, and generously offer whatever means they have.
Someone I know from that forum owns a factory - they manufacture PET bottle production plants, so he has impressive machinery at his disposal - and we will do all CNC work over there, for example.
It will be a scary car come spring...
(I just need to finish restoring the Modena 360 and the 92 Miata I am currently rebuilding the engine and such before I can tear mine apart)
First gear is a little short with the 3.63 gears if I were drag racing but it is still very usable and useful for navigating parking lots and loading the car onto the trailer. I wouldn't gear it lower but slightly higher without going into the 2.xx range would be fine. 3.15 or so sounds good.
And I wasn't concerned about the car being light at 150 mph. Mine is stable. It just occurs to me at that speed how little structure exists around me and how quickly things happen. Thoughts of my own mortality tend to elbow and shove their way to the forefront like Braineak at a New Kids on The Block reunion concert.
After further reflection the 3.15/3.07 looks best for third through sixth.
Sixshooter is right...
The BMW 6 speed looks like a "normal range" box in these gears.
First and second are artificially low to allow good acceleration from a stop with the stock drive train and also achieve good fuel economy in the higher ranges by using a numerically low rear end ratio.
BMW wanted BOTH and didn't have enough torque to push a numerically high first/second gear.
The more torque you have the lower the gear you can push.
I concentrated too much on making 1st and 2nd work better, My bad...
SO use the 3.15/3.07 and have the 1st two ratios in the six speed altered.
This WILL make it more complex!
And limit you to 3.07 for your numerically lowest ratio.
BMW does not sell internal six speed parts in the U.S.A. unfortunately.
I have no idea about Europe.
Godless Commie might have the cojones and access to tooling to make this happen.
I don't...
IMO "cost no object" total gear ratios for a super high torque Miata like GCs would be
1st = 12.0
2nd = 7.5
3rd = 5.0
4th = 4.0
5th = 3.0
6th = 2.5
Final note: I've seen way to many lift vs drag charts that show significant lift on the rear of the car at high speeds to push one much past 120 without any down force devices.
I don't know if the Miata is the same but I'm betting it is.
I too, will try to make some type of diffuser/down force tunnel in the back of mine as I cannot stand the "Cessna wing on big tall strong brackets" look on a street car.
Splitters work far better on the front than any other devices I've seen. Little tiny splitter is worth enough gains that you HAVE to use huge wing (or really good diffuser) on back to "balance" the car.
And I am NOT planning to exceed 150 in my car due to the exact reasons Sixspeed mentioned. As I get older (56 now) I feel much more "mortal".
I once wanted to try for 1 liter speed records with an early Honda Insight. Someone else had this idea too. It all came apart when he was close to 200 and the resulting mangled tiny car solved the "itch" for me
I once wanted to try for 1 liter speed records with an early Honda Insight. Someone else had this idea too. It all came apart when he was close to 200 and the resulting mangled tiny car solved the "itch" for me
[Scary Honda Insight RUD]
I'm still blown away Brian made it out of that car every single time I watch that video, many props to whoever built the cage in it.
GC, don't do that, but love what your building here
It's too bad the change between 2.XX and 3.XX requires a housing change.
It's too bad they only offer 2.56 and 2.93 in the 2.XX housing.
This is the least desirable housing due to limited gearing options.
Is one housing physically bigger/heavier than the other?
Bigger is better (she said!.)
Do they use the same sized bearings?
Might not matter if they do
Probably best to build the 3.XX set up first and see how that does.
Few takers for the 2.xx setup if you want to sell it later.
Until this thread I've never looked at 2.XX as a reasonable ratio for a hot street car.
2.XX ratios are far more suitable for a LSR car.
From your first post the 3.15 is geared for nearly 300kph which is FAR faster than you should push a Miata anyways...
You COULD always build BOTH (for science!) and see which you like better.
We would WATCH!
Six is right regarding loading onto trailers and navigating parking lots, especially if others drive the car (wife/girlfriend)
All my serious street cars have been "less than friendly" in parking lots.
The 3.15 and the 3.07 are so close to each other that either will work about the same. Only .08 difference between the two and I'm guessing the 3.15 will be easier to find.
The first two gears will still be too low IMO. Live with it...
If you can get a fully functional 3.15 just try it out in the car before you go through a full rebuild. Make sure you don't want to go lower before you spend the bucks (or what ever the currency is over there).
Can both housings fit in the same mounting configuration?
I hope they can, most of the BMW rear ends are very similar regarding mounting but some German car specialist might chime in here.
I can tell you from a feel point if view, my friend's Z4 3.0i (same trans, 3.07 diff) is pretty spot on. It has torque anywhere you want it and the gearing matches nicely.
Btw, I also have the same car (Z4 3.0i) and I actually went 3.23 instead of 3.07, but the car makes 200ft lbs at 3000rpm, not 250ft lbs AND the miata is way lighter, AND it has smaller tire diameter.
Hard to tell, 3.07 might be pretty good. At the end of the day, every ratio has it's own (little) advantage over the other, but I'm sure you can get used to 2,93 or 3.07 with all their pros and cons
My plan is to retain the PPF in this car. I have a ton of reasons for wanting to do that.
It is also pretty obvious trying to make the PPF work with a Bimmer diff and trans will be a ridiculous exercise in futility - the PPF is shaped uniquely to work with Mazda components designed for this purpose.
So, I will have to make my own PPF. Pick up points, attachment, clearance, etc are a non-issue with a custom made PPF.
The plan is utilizing a Warren Truss with uprights design, and making an inverted U shape and bolting the fourth side from the bottom to completely enclose the driveshaft once the shaft is installed.
I have experience in load distribution, support placement and weight saving strength adders from years of model aircraft design, so there is some level of confidence.
Plus, I have access to FEA analysis to a certain extent.
My initial idea was using 4130 chromoly tubing, but that can't happen. Too difficult to find over here.
@gesso gave me the next best idea: angle metal...
Much easier to work with. Can be bolted in place, and the whole thing can be moved over to the bench to be welded once fully assembled in place.
Considering the fact that the stock PPF is a 6 mm thick C shape 6061 aluminum, I figured a box shape would require less wall thickness.
I have come up with two alternatives so far for the said box shape:
40x40x4 mm angled aluminum with 30x3 mm flat bar braces, or 25x25x2.5 mm angle iron with 20x2 mm flat bar bracing.
(The whole thing will be media blasted and powdercoated before installed in the car.
I am definitely not a mechanical engineer, but everything I have read up so far points me in this direction in terms of material choice.
So, if you were to fabricate a PPF for your turbo Miata, what material would you use? Am I way off?
Porsche 944 and 982 use a big *** aluminum tube for their PPF. The driveshaft runs inside it.
Makes it a PIA to change clutches as the entire transmission (in the back) has to move approx 12" rearward to allow access to clutch (in the front, under a bell housing).
Pilot bearing changes are especially odious.
Pirate a light pole?
Maybe see if the driveshaft will fit into the center section of a GM truck (Suburban) with the giant sized aluminum drive shaft.
Just suggestions....
I'd want it stout for your car.
What I want to know is your reasons for keeping it.
I'm not arguing to abandon it.
I just want your "take" on keeping the PPF on a high horsepower Miata.
One other bit...
Porsche uses "guibo joints" for the driveshaft in these applications.
These have less vibration than u-joints and pretty much everyone has slowly changed over, even GM.
The original BMW installation had one in the front with a CV joint in the back.
Lots of manufactures use them for both.
Food for thought.
Porsche 944 and 982 use a big *** aluminum tube for their PPF. The driveshaft runs inside it.
Makes it a PIA to change clutches as the entire transmission (in the back) has to move approx 12" rearward to allow access to clutch (in the front, under a bell housing).
Pilot bearing changes are especially odious.
Pirate a light pole?
Maybe see if the driveshaft will fit into the center section of a GM truck (Suburban) with the giant sized aluminum drive shaft.
Just suggestions....
I'd want it stout for your car.
What I want to know is your reasons for keeping it.
I'm not arguing to abandon it.
I just want your "take" on keeping the PPF on a high horsepower Miata.
Believe me, I spent quite bit of gray matter thinking about a torque tube design. It just complicates things a bit too much - even for me - at the end of the day.
* I would need a new driveshaft designed with splined ends to slip in, as opposed to bolted on flanges
* Replacing the clutch or associated parts would require dropping the diff.
* Designing and machining the bell ends of the torque tube is a pretty big undertaking.
* No Suburbans for parts over here, and the light poles are a bit too hefty for a clean getaway during the, um, procurement stage...
Yes, I firmly believe in retaining the PPF for several reasons.
* It is a very long torque arm, and nicely deals with the up/down shenanigans of the diff nose.
* That long arm transmits some of the NVH into a large mas of metal, namely, the transmission and the engine, effectively "burying" and dissipating them.
* Connecting the drivetrain via the PPF distributes stresses and loads generated by the engine to the entire platform, mitigating fatigue issues.
* Isolating the diff by means of welding a tab on the rear subframe subjects the rear subframe to stresses and NVH that part of the car is not designed for.
* Also, a rear subframe mount for the diff nose would have to have a rubber bushing, and that would defeat the purpose by allowing movement. Alternatively, a solid mount would make driving a very unpleasant experience.
* A crossmember right between the seats to support the transmission could be a regrettable, buzzy preference when it comes to long trips.
* The PPF ensures the entire drivetrain - with solid connections - floats on just four vibration damping mounts located on the front and rear subframes, and all torsional and transversal loads are handled within that floating assembly.
* I personally believe the "PPF delete" thing seems like the easy way out, but introduces a ton of other problems over time.
I also believe power adders tax the stock PPF design, and may lead to cracks and even a snapped unit with the added torque. That's why I would like to make a stronger, box shaped (space frame) PPF.
It is just after 5 am over here. I may have missed a couple points for keeping the PPF, I can always edit this post and add more when I am thinking more clearly.
One other bit...
Porsche uses "guibo joints" for the driveshaft in these applications.
These have less vibration than u-joints and pretty much everyone has slowly changed over, even GM.
The original BMW installation had one in the front with a CV joint in the back.
Lots of manufactures use them for both.
Food for thought.
I was thinking about a custom driveshaft with CV joints, like a halfshaft.
A coaxial PPF system would be nice aesthetically, but presents some horrible problems if you ever need to do any maintenance or repair.
No reason why a tubular PPF running BESIDE the tailshaft could not transfer both torque and bending loads. After all, the tailshaft is strong enough to transmit all the torque, and it does not need to be huge in diameter to do it. Tubular PPF down one side, exhaust down the other....
so... my two cents on this....
I have two miatas, both with bmw gs6-53bz boxes and e36 diffs. one is v8 3UZ, other is with 1.8vvt turbo. none of them have ppf and I don't miss it tbh.
I find 3,15 ratio in my turbo car quite comfortable. it's quite long enough to daily the car and short enough to not call it a long ratio. It's the sweet spot to me. But it's the whole other story with the v8 - 3,15 is useless in 1st gear, had to change it to 2.56, now it feels fine too. keeping in mind that it has around 300Nm torque @ 1000rpm - such long ratio feels apropriate.
Ratios up to 3,07 are by 8mm closer to the input shaft, that's the only difference in the casings. both use same bearings,, same lsd clutches, same differential housing. same everything. So if you would ever want to change to a diesel ratio (longer than 2.93) you'd only need both gears and another lsd casing. I'd just get another differential in that case.
BMW diesels, among with lots of other cars are using a flex joint (#14) in driveshafts along with the rubber guibos.
when making driveshafts for both cars I have used this flex joint on the front side of the driveshaft and a U joint the rear. Flex joint makes it easy to remove the driveshaft as it provides length variance for a driveshaft to be removed. Made an adapter to fit a flex joint to adapt it to bmw output shaft and voila it goes.
No issues in both drivetrains whatsoever, they both hold ok. v8 car has this drivetrain for about two years, turbo car has it for a half year now. both gets more or less beating.
My advice would be: just don't bolt the shifter to the transmission tunnel to avoid excessive noise and tuck your differential as high as you could in the rear subframe as it get's quite tricky to get a good ground clearance with a 3" exhaust pipe.
01-12-2021, 02:29 PM
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