Quest for an
Electric Porsche Boxster.
Select email exchanges between members of REVOLT Custom Electric during the conversion process:
3/15/2008
I am interested. I will be needing a new car in about a year. About how much "ballpark" would you estimate the cost of converting a decent luxury sedan or sports coupe? I am thinking along the lines of a Porsche, something light and sporty, perhaps a BMW roadster. I would like to find a great deal shopping for the donor car. I would be open to suggestions. Range will need to be at least a consistent 60 miles/chg up to at least 80 mph. I would like to consider the
AC Propulsion system as well.
Rob
For a basic conversion, start with about $13,500 in parts including converted air conditioning, plus batteries and labor (I'll get to these in a moment). This will give you a decent, if not thrilling level of performance. For high street performance and/or racing applications, costs go up, but there are a few cost "thresholds" beyond which significant performance increases can be had for little additional money. While range can be a physical limitation, power and top speed are limited in a practical sense only by your budget.
For your desired range, and most likely the handling level you'll expect from a Porsche or BMW, you will want to use lithium batteries, which unfortunately carry a price premium today. The more economical choice is lead-acid, but it is very difficult to design in more than 45-50 miles of range with these large, heavy batteries. A pack of decent ones will cost you about $2500-3500 and typical range is 35-45 miles.
Currently our prices for lithium batteries (safe, non-burning lithium iron phosphate) are around 5-8 times that of lead-acid per watt-hour of storage. So that same 35-45 miles of range will cost you about $15,000, will weigh less than half and take up much less space, and will last at least twice as long. They'll also be immune to the sagging performance of lead-acid in the cold, and will remain brisk and powerful until they're empty due to their flat voltage/discharge curve.
At 65mph, I would roughly estimate a midlevel luxury sedan like a
BMW 5 series would use about 350 watt-hours per mile at 65mph. Smaller cars (BMW Z4,Z8, P. Boxster) can be from 250-350, and of course
midsize SUVs can be well over 400. At the moment, our lithium battery prices are right at $1 per watt hour, fully configured with battery management systems (electronic circuitry that watches and protects the batteries from abuse). You can figure your battery budget and desired range from that. (There are complications in the math with
lead-acid batteries, but lithium is pretty simple). Drag-inducing add-ons like ground effects and spoilers can reduce your range, so many sleek-looking sports cars can actually have poorer efficiency than you'd expect at high speeds, but of course these features can be modified and there are other aero improvements (belly plate, grille closure) that can help.
DC systems offer comparatively high horsepower per dollar. This makes them suitable for budget builds, and cramming as much horsepower as possible for racing applications.
Drag racing especially benefits from
DC.
AC systems can be powerful, are generally more sophisticated and as it turns out are particularly well suited to autocross, but the economics are different since the design requires many more discrete
silicon power switching transistors, one full "H-bridge" of transistors per phase (typical drive systems are 3-phase). A DC system in contrast has only 1 single switch.
Comparing drive units as directly as we can at the "high end": while a top-of-the-line DC controller for racing runs around $5k and puts out around 350HP (competitive with 500+HP in a gas engine) and a good dual motor configuration will set you back about 4k, plus DC/DC converter and 12kW charger for another 4k or so, in contrast a 200HP
AC Propulsion AC150 drive package (which contains all those parts) costs about $27k plus about $1k for the coupling (ACP wants to supply the couplings for us for the time being instead of having us build them, they're pretty adamant about it).
AC has its benefits. Regenerative braking is almost nonexistent in DC controllers, but it's a standard feature in AC drives. It will get you about 10-15% additional range in stop-and-go and hilly driving. Just as important, it will make your brakes last a very long time (often as long as the car) and will keep brake dust off your wheels. Finally, the AC150 specifically has one really cool feature -- the rotor is stable up to 13,300 rpm. You can't shift a transmission with this motor unfortunately, but with that speed range you don't need to and you won't want to. You can leave it in 2nd gear, remove the shifter, and get fantastic torque at the wheels, uninterrupted, all the way to redline.
For other examples of converted
EVs around the world, our nonprofit organization (AustinEV) hosts a site for the benefit of the EV community:
http://evalbum.com
Chris
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I looked at Edmunds and I think a Z3 roadster might be a great, light chassis for the buck...
http://www.edmunds.com/used/1997/bmw/z3/724/specs.html#
I was going to suggest the Z8 (
http://www.edmunds.com/used/2003/bmw/z8/100187551/specs.html), but it turns out these all-aluminum cars go for $70-90k, if you can find one :o)
I really like the
S2000 (I'm a big honda fan). Compared to the Z3 it's a little longer, a tiny bit shorter vertically, a little wider, and a bit heavier:
http://www.edmunds.com/used/2007/honda/s2000/100800929/specs.html
It also has a much more powerful engine, meaning the transmission is more likely to be stout enough to handle a decent electric motor.
Another thought is the
RX8, a much heavier car but with true seating for 4:
http://www.edmunds.com/used/2007/mazda/rx8/100793780/specs.html
The Mercedes SLKs are pretty nice, though you'd have to go back a few years to find one at a similar price, and they're also a bit heavy to start out with:
http://www.edmunds.com/used/2004/mercedesbenz/slkclass/100331689/specs.html
Like the 300Z before it, the
350Z is a very heavy car for its size:
http://www.edmunds.com/used/2007/nissan/350z/100846273/specs.html
But you also have a larger, heavier V6 engine to pull out of it. Still, I think the only reason to go with this one is if you have an particular liking for the style.
The
Boxster is actually a lot lighter than I'd thought .. and of course it's received rave reviews from just about everyone. You do have to go back a few years to find one in the same $20k price category though:
http://www.edmunds.com/used/2003/porsche/boxster/100183117/specs.html
Of these, more popular cars like the RX8 or the S2000 may have less expensive upgrade parts (suspension, etc) than the european imports.
What are your thoughts on these so far?
C
hris
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Of these cars, the best deal will probably be a 98 Boxter or 97-98 Mercedes SLK convertible, probably favoring the Mercedes. I like the hardtop convertible idea if it is reliable.
BMW 3 series is another possibility. I will look into both cars over the weekend. The other cars are too steeply priced. I think the maximum cost of the whole project including the donor car needs to be closer to $45-50k for me to stomach the risk.
Rob
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http://visforvoltage.org/forum/2702-hands-test-48-volt-20-ah-lifepo4-pack-ping-battery
http://www.diyelectriccar.com/forums/showthread.php/evdl-best-bang-buck-battery-14029.html
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
Later in the above thread, Thunder sky may have a more reliable LiFePO4 option available now... Would it be sacrilege to get a bid from them for our project?
http://www.diyelectriccar.com/forums/showthread.php/lithium-vs-lead-great-cost-debate-14035p5.html
http://www.diyelectriccar.com/forums/showthread.php/lifepo4-group-purchase-14074p5.html
http://www.evpower.com.au/-PROJECTS-.html
Rob
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The problem with these is that they're not really EV-duty cells. Given that "C" is a proportion of current from a battery to its capacity such that a 20Ah battery has a C value of 20, these cells are rated at 1C continuous discharge, with a burst to 3C. Our cells are rated at 4C continuous with a burst of 10C for 30 seconds, and 20C for 10 seconds. (The higher power cells I was referring to can do 50C in a 10 second burst, great for drag racing.)
You could make an EV pack out of these bicycle packs, but it wouldn't be very powerful, and worse, it would be operating at very close to the edge of its performance envelope, which will shorten its life. The further your normal operation is from the cells' maximum capability, the longer they'll last and the cooler they'll be in operation.
Looking at it another way, here's how you'd figure your pack with these modules. Each one is 36V and 20Ah. To reach a desirable target voltage of 200V, you'd need either 5 or 6. Let's choose 6 for now, so our voltage is 216V. A single string of these modules at 216V will contain 4.3kWh, so if you're looking for 16kWh, you'll need 4 in parallel. This will give you 80Ah, so a total of 17.28kWh of storage.
However, you'll only be able to deliver 240A of current, at a peak (and they don't mention how long that peak is). The 1K Zilla is capable of 800A of input, and so much of its capability (and the capability of your motor) will be wasted. Nominal current will only be 80A, which at 216V is 17.28kW continuous, or about 23HP. Take out about 10%
power conversion plus drivetrain loss, and ultimately a flat level cruise at 65mph (approx 20HP) will again put you close to your pack's continuous current limit.
If you rearrange the voltage and current (higher or lower pack voltage, corresponding lower or higher current), the numbers will work out similarly.
Chris
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We think the 9 should be sufficient for what you are trying to do. The 11 is heavier and more expensive. We might want to get you to take a ride in Mark's MR2 when he gets back from his trip on the 10th so that you can feel the acceleration there. He has a 9" motor in there at the moment with a z2k limited to 1000 amps so it would be similar to what this setup would do. If you want to get the 11 it would be $2325 instead of $1450 for the
Warp 9.
From our calculations, we are thinking that a roughly 20 kWhr pack would get you what you want with the boxster. The price from K2 includes the raw cell cost (1920 cells) and the module assembly (48 modules). The total blade (module) cost from
K2 is $15,468.00. There would also be shipping involved from
China to them for module assembly and then to us that they estimate at about $600 or so. The BMS only adds $30-40 per module. We will have a better idea as to the exact cost of this once we are done with the Mazda.
The AC is in there. It is the Masterflux Sierra compressor. We have achieved outlet temps of around 53 degrees with the Saturn install with this compressor. That was at the low end of the range for the controller and your pack voltage would be higher. We will be testing the Saturn to see if the voltage to the controler matters. Masterflux is producing a higher capacity compressor (the Alpine) for release in mid 2009 and we can get an engineering sample (for around $1000 vs around $475) if you end up being unhappy with the Sierra's capacity.
Aaron
Oh, I meant to say that we are having K2 quote us on their 26650EV cells for these modules. We have a high enough voltage and several series strings in parallel so we are confident that we don't need to call for their more expensive performance EV cells.
Aaron
If you end up choosing the 911 we may need to redo the pack requirement a bit. I think those are a bit heavier. Though, a hard top would probably be more aerodynamic.
This pack size should do fine for performance. The voltage is around 200 and we are paralleling modules so we don't have to pull as many amps from each individual cell to feed the
motor controller. If you want more range, we would need to add additional strings. If you end up wanting a small incremental increase in performance, the limiter would be the controller and the motor. Upgrading to a Z2K for example would add an additional $2000. I think the 9 and the Z1K coupled with the current pack would provide a very fun, high performance car, but if you want blistering performance (REALLY want the drag race type horse power), we may want to consider going with the high performance cells from K2, the Z2k and the Warp 11. That will be more expensive.
I'll write up a description of the 9 vs 11 tradeoffs for you when I get back in from the garage this afternoon.
Aaron
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My reading last night indicates the Warp9 is limited to a Max of less than 192V before you risk flash damage. So current will be the limit for
max power, 192 kW max, probably less to the motor. I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW). I think we want the
Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier. We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
Rob
Actually, on the battery side, the maximum input current to the Z1K controller is 800A. A 200V nominal pack will sag under this much current, so peak input into the controller is going to be somewhere around 150kW or so. Figuring for controller, motor and drivetrain losses, this equates to about 150 peak HP at the wheels (rough estimate).
However, even though it seems like we have a recognizable figure (peak HP), we're still not really comparing apples to apples. I think you'll find this level of performance to "feel" as strong or more so than the 230HP engine in a stock 2004
Boxster (for example), and the reason for this is torque.
Horsepower does not push the car; torque is the force that causes acceleration. Peak horsepower really only describes the extent of velocity at which you can still expect that accelerating force to be available. At freeway speeds you should be well within the envelope in which torque will be more important than peak horsepower.
The stock 1997 Boxster engine produced 180 ft-lbs of torque; the 2004 produced 192. The 2004
Boxster S topped out at 250. At 1000A, a Warp 9 should easily be able to do 250-300 ft-lbs. AND it will do so, *not* at a peak only achievable within a narrow RPM band. With an electric motor, that peak torque is available from zero RPM and holds flat until back-EMF from the motor reaches the vicinity of battery voltage as speed increases. This one factor makes a huge difference. More area under the torque curve = better overall acceleration with less peak horsepower. You will want an upgraded clutch, as even on sportscars, stock clutches tend to slip.
With our recommended configuration, it's true you will most likely not be able to continue accelerating as briskly as stock at higher speeds. A 200V pack will still be fun when you mash it at 55mph, but it's not going to be quite as thrilling at 70. If you really do want to be able to keep up with a stock Boxster when accelerating beyond the speed limit, then you will want at least the larger controller. Of course as a bonus, you'll then be able to easily embarrass the stock Boxster, and most other cars on the road, at lower speeds with essentially double the available torque. You'll also be at significant risk of damaging your transmission and driveline, if you can maintain traction. A multi-disk racing clutch will be a must.
I think the continuous power from the controller will be closer to 300 amps with Z1K, so continuous power will be closer to 50 kW).
If your Boxster requires any more than 30KW or so to maintain 70mph, I'll be surprised. Continuous horsepower is only important in making sure you can cruise on the highway without heat continuing to build up in the motor. It will definitely be limited more by the motor than by the controller, and a motor's continuous horsepower rating can be increased by increasing airflow, either by ram-air or a blower, or both. The Warp11 does have a higher continuous horsepower than the Warp9, but if you already have more than you need to maintain cruising speed, then the larger motor doesn't help for this purpose. Sustained cruising at very high speed may justify a Warp11, but it will also significantly impact your driving range. (Horsepower to overcome aero drag increases proportional to velocity cubed.)
BTW, continuous power is also something that's routinely ignored by some people in building lead-acid powered cars, since lead-acid typically doesn't provide enough range to overheat a mildly overloaded motor. Driving for only 30 minutes at a time allows you to exceed the motor's rating, and get away with it.
I think we want the Boxster to have at least the equivalent power of its stock engine. I want the car to be as peppy as a stock boxster, maybe a little peppier.
Remember, peak horsepower is only meaningful when needing acceleration at high speeds. Comparing gasoline-powered cars, people toss around peak horsepower as a number that more-or-less represents how well the car will accelerate. Even though some cars have better torque than others, the HP figure is meaningful for comparison because
gas engines have roughly similar torque curves. An electric motor is radically different, so you have to abandon the convenient number and get into more detail to get an idea of how the car will behave.
I would be *thrilled* to put a Warp11, a Z2k and 200V of lithium in your car. It would turn your project into an important demonstration of our services, and ultimately it's exactly the sort of project we'd like to focus on as our core business in the future. You'll have more peak horsepower than stock for fantastic acceleration at high speeds, and at legal speeds your car will have entry-level supercar performance. Your car will also be heavier, less efficient, and more likely to break something when driven hard, since you'll be so far outside the drivetrain's torque specifications. Netgain has already received reports specifically about Boxsters breaking axles with a Warp 11 at 2000 amps.
I've been recommending the Z1K and Warp9 because it seems the most in line with your stated goals and intentions for the car (or as far as I've understood them), and because the 11-inch motor is twice the price for far less than double the torque per amp of current. Ultimately it's your choice, and I hope we can gather enough information that it will be reasonably well-informed.
We should shoot for 0-60 in less than 6 secs and sustain 85 mph without overheating the controller. That is not drag racing, but may require the upgrade. Hopefully you guys can figure this out, because I am not sure what will work and reading about it is confusing at best.
I think we may be able to achieve close to 6 seconds with 200V, 1000A and a Warp 9. I'll see if I can get some numbers to substantiate that; it's notoriously difficult because Netgain does not publish performance details for their motors at high voltages and current. (I talked at length with Netgain yesterday and have asked for some specific performance information that they're going to try to get for me.)
If you would like to maintain 85mph continuously and your car is based on lithium, a Warp 11 and a Z2K may indeed be the safer bet; the components will certainly run cooler. However, your range will be disappointing, and again if your car is based on lead-acid batteries, you probably won't be driving long enough to build up enough heat to damage the lesser components anyway.
Torque and power are definitely confusing subjects, even more when you realize that the way we use these figures in conversation about cars is based on assumptions that no longer hold true when dealing with such a different animal as an EV, especially one based on a series-wound
DC motor. I hope my attempts to explain haven't made the water even muddier than before.
Chris
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After thinking about our discussion yesterday, we might want to look at using the Warp11 with the Z1K. It will be easy to upgrade the Z1K later, but not the
Warp 11. The Warp 11 may give better durability, run cooler, and gives
more efficiency per amp. Granted it weighs more which might offset those benefits, but 70lbs extra doesn't seem like a lot in a 3000lb car. Looking at the torque curves makes it look like it gets about 10% more torque per amp from the battery. Thermodynamically, this makes sense as it produces less heat and must be converting the energy into mechanical power.
If the torque per amp is 10% more than the
Warp 9, and the cost of the upgrade is much less than 10% of the total project, then it makes sense to go with the Warp 11. Of course I am just guessing about the math at this point. You would think Netgain would have this all over their website comparing these motors
price points, etc.
Rob
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I seem to remember that the 11" is not very efficient at high current due to large end turn ratio, so I would be leaning toward the 9". But the Boxter is a tank at 3000# and so will be a challenge. Much will depend on the voltage and power capability of the batteries.
You may be able to hit six seconds with a Z1K and a high voltage pack, but in your case I would use the Z2K-HV or -EHV depending on your pack voltage.
I don't know the wait time, Mandi can get back to you on that.
--
I hope this helps,
-Otmar
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Just wanted to clarify about the motor efficiency comparison. The short answer is, yes the
Warp 11 is more efficient than the Warp 9, enough so that it may be justification enough to go with it, especially if your driving habits don't involve a lot of in-town driving where the motor's higher static and dynamic masses will impact efficiency in ways not accounted for in the official specs.
The *reasons* why the Warp 11 is more efficient are a bit different though. In fact, in most cases a larger motor is *less* efficient than a small one, for a few reasons. First and foremost, a larger motor has a larger brush contact area, stiffer brush springs and a larger commutator diameter. This creates a higher continuous
friction loss, and the difference is easy to feel. The motor is physically harder to turn by hand. It also has a larger fan which moves more air; this creates a higher windage loss at high RPMs.
Second, while it is true that a larger motor produces more torque per amp, there's no free lunch, and "torque" and "amps" are only components of total power, not the whole picture. If there were nothing else to the equation, a massive dishwasher-sized motor making vastly more torque per amp would somehow magically produce more shaft power on the output than the electrical power you put into it. The reality is that there is a tradeoff -- "back-EMF" (
voltage drop across the motor) rises with RPM faster in a larger motor. To cause the motor to draw a given current at a given shaft speed, the controller has to supply proportionately more voltage to the larger motor. Thus, the power input (volts x amps) grows to match the power output (ft-lbs x rpm). Greater torque from the larger motor has zero effect on efficiency.
Third, while a larger motor may run cooler at a given power throughput, this is in part because it has a larger surface area (internally and externally) from which to dissipate heat. Though *temperature* may not climb as high (and this is good in terms of avoiding overheat and insulation failure), total heat dissipation may be as high or higher in a larger motor. "Heat" is a measure of
thermal power, "temperature" is only one component of heat.
The Warp11 is indeed more efficient than the Warp9, because the 11's design was more thoroughly dictated by Netgain with fewer concessions to any existing Warfield design. It's simply a better-designed motor for EV use, though the difference is not earth-shattering. The Warp 9 is primarily an industrial motor, with a few changes to make it physically compatible with the 9"
Advanced DC motor, and a few other changes to make it better able to handle an automotive operating environment. For what it's worth, Netgain has been working on producing a variant of the
Warp 9 that incorporates some true EV-oriented modifications, and they actually have a prototype now but it will not be available on the market for a few months at least. For those who aren't racing, we usually recommend the Warp 9 to save money on capacity that's not needed, and because in heavy stop-and-go driving the extra weight may more than cancel the 11's inherent efficiency advantage in steady-state operation.
Be aware that you're not just adding 70lbs to the car. Some of this is static mass, but some of it is spinning mass, which implies a much higher impact because you need to not only accelerate it forward, you also have to get it spinning; this is why we usually lighten flywheels when we're doing a conversion. All the energy required to spin up the heavier armature is lost as heat during braking and upshifting.
I've had several conversations with Netgain about documentation and making information available to customers, or at least to their vendors. They realize they have a problem, and are working to address it.
Again, I would be totally happy to install a Warp11 in the Boxster, and leave enough room in the design for a future Z2K upgrade. I'd be thrilled to show off what we're doing, and I think it would make a very impressive statement about the sort of conversion work we can do. As an added advantage to us, Netgain will be able to ship an 11 to us a month sooner than a 9", due to their production schedule (9s won't be available until October). I just want to make sure you're making this decision on the basis of accurate information and realistic expectations.
Chris
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I really don't know what to do on this one. So, we should probably go with the most experienced opinion in the group, which ain't mine. Otmar seems to favor the Warp 9 and maybe going with a Z2K. But in his reply, he acts like he doesn't know the max voltage on these motors is 172-192V. Why else would he offer up the Z2K EHV?
Did you guys have any pull in getting the controller faster? Aaron said he thought Nov was the delivery date, but that email says 6mos!
Rob
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Be aware that battery voltage does not equal motor voltage, and the battery voltage limit does not need to be influenced by the motor voltage limit, with the exception that it cannot be higher.
So, it would be perfectly reasonable to set a 175V limit on motor voltage for example, and run the controller from a 200V or 300V, or even 350V pack. The additional voltage gives the designer flexibility in adding more energy capacity, by simply making the string longer. That flexibility isn't as important with a lithium pack where the cells are smaller and module design is more flexible, but it still can be helpful.
Also, be aware that Otmar is a racer and a performance junkie -- like I am, but with a greater tendency to let it influence his recommendations. :o) He's right; you may or may not be able to do 0-60 in six seconds. But it will be close, 0-30 is going to be awesome, and the rest of the way up isn't going to be too bad.
One thing is for certain; Zillas are scarce and will continue to be for an unforeseeable amount of time. This means they maintain their resale value really well, should you choose to buy a Z1K now and upgrade in the future.
Chris
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I thought we had all this worked out, but it keeps grating me to do more research... Looks like we will be going with a Z2K HV, and it will be available closer to schedule. But the motor decision is still difficult.
Based on everything you are telling me, and everything I have read, I have made some rough conclusions (possibly wrong):
The
Warp 11 will be much easier to damage the transmission and clutch, but give drag racing performance off the line. It will then lose power and acceleration at freeway speeds. It will use more battery to maintain higher speeds if higher rpms are required, thus less range. But could gear shifting overcome this?
The
Warp 9 will be less hard on the tranny and still go very fast off the line. It will also accelerate faster at higher rpms and possibly have the fastest top speed and longer range.
I guess if I had to choose priorities, I would like to not destroy the car, require expensive upgrades, or easily lose control and wreck the car and/or myself. I would prefer to accelerate faster from 30 to 70, than from 0 -30. I would like the car to be more efficient at 70 mph if one motor is superior to the other at 70mph constant speed.
I am thinking the Warp 9 will be the better choice, especially with Otmars comments. I did see some other forum comments about the Warp 11 using more amps compared to the Warp9 at highway speeds. If that is true, we should probably use the Warp9. I find it troubling how little commentary there is out there about this application and what works best. In fact on the evalbum pages, there are few finished cars using either a single Warp9 or Warp11 with a Zilla which quote great performance, typically using lead batteries. I see several using two 9s with Z2k, but they are trying to compete at the 1/4 mile.
Best wishes,
Rob
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All series-wound DC electric drive systems (motor + controller) exhibit the characteristic of torque output that remains relatively constant until a certain RPM, after which torque decreases with increasing speed. That RPM "knee in the graph" is partly dependent on the motor, but mostly dependent on battery voltage. Both the 9 and the 11, and any other series
DC motor behaves this way. The numbers are obviously different, but the curves are the same shape. All electric drive systems will exhibit better performance at low speeds, because below the RPM threshold the controller will be able to deliver maximum motor current.
Under 2000A, the Warp9 still poses a threat to your driveline, though not as significant as the Warp11.
The Warp9 will not necessarily provide better performance at high speed, unless you have insufficient battery voltage to continue driving the larger motor at the same current. Provided your battery voltage is high enough (200V should be), the opposite will be true.
You don't need to worry much about "damaging" your clutch; the main concern with the clutch is the nuisance of realizing that it's slipping, and needing to dismantle everything and upgrade it. This is why we specify an upgraded clutch at 1000A, and a racing clutch at 2000A.
Neither motor puts you at an unusual risk of losing control of the car, over and above the risk inherent with any powerful sports car. With 2000 motor amps either motor will provide enough torque to break traction. This is not special or unique to electric motors and as with any high performance rear-wheel-drive car, care must be taken not to apply too much power around corners without the skill to execute a controlled drift. Due to the inertia of the armature this will "feel" a bit different in an electric and will require some getting used to. Remember, you have a finite amount of traction available, and the force vectors for propulsion/braking and cornering add together to approach that limit. For example if you're coasting around a corner at the very edge of your tires' traction, any throttle at all will break the rear end loose, and vice versa.
Here's my take on the situation: In an ideal world my recommendation of 1000A and a Warp9 stands; though it may not quite deliver 0-60 in 6 seconds it certainly would come very close and would provide very entertaining performance on the street and highway. We have the problem of delivery date for the controller which has caught us by surprise, and so you've decided to pay the extra for the Z2K. You will certainly get what you're paying for, and this upgrade alone is going to give you incredible performance regardless of which motor you choose. Again, the power of the car is determined far more by the controller and batteries than the motor.
So between the Warp11 and the Warp9, the 11 will give you a modest amount of additional torque (which you don't need), and will give you higher continuous horsepower (which might be helpful if you like to cruise at 85mph). Netgain's dyno test data shows the Warp11 to be a little more efficient in steady-state operation, though they have not been able to test it across its entire performance envelope and you have seen reports from others that seem to contradict their data.
If it were me, I'd get the Warp11 (we'll note I bought a Warp13 for my truck project).
On the other hand if we're talking about a sane customer not interested in racing, I'd get the Warp9 and save some money. I think it's a given we'll be force-cooling it with a blower, so it will have a little higher continuous horsepower than stock anyway.
(For what it's worth, we wouldn't have to force cool the 11, so a little less complexity, noise and energy consumption there.)
Either way, with the Z2K, the car is going to rock. Perhaps a little too much for its own good, but we do have the ability to detune the controller's output, to a "normal driving" profile that you can change to a "pull out the stops" profile at the flip of a switch.
--
Chris
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The difference in efficiency (and therefore range) with mixed driving will not be significant. It's hard to tell which would be better; both motors have pros and cons that would mostly cancel each other out. I believe the difference will not be sufficient to consider this an important factor.
Dual motors is a setup that's used by racers to improve horsepower. Though it seems to be something that would appeal to you, I've been hesitant to bring it up for a few reasons. Mainly, you're venturing completely into all-out electric supercar territory here, with more torque from two 8" motors than a single 11". You have the complexities of the mounting arrangement (John Wayland spent years and several revised designs trying to work out the vibration problems in his dual-motor setup, and eventually had the two motors fused together into one unit by custom motor builder Jim Husted.)
Second, to take advantage of the extra power, you'd need to raise your pack voltage, to at least 250-300V. You'd also need the series/parallel switching option on the Zilla, and the extra contactors and traction wiring that this implies.
As Aaron mentioned, this is usually the sort of setup that gets connected directly to the differential without the use of a transmission, simply because of the problems with putting that much torque through a transmission, and the absence of even high end multi-disk racing clutches that will operate without slipping. Unfortunately in your situation I think that all Boxster transmissions are of the traditional VW layout; the differential is integrated and cannot be used separately. Converting to a solid axle rear for direct drive would be an extreme undertaking, and would have effects on handling. Otmar dealt with the situation by retaining his transmission despite the use of two 8" motors, and his solution is to swap between 2 transmissions. One gets rebuilt while the other is in use.
On the other hand, dual motors would indeed get you higher horsepower at the top end. Essentially, you provide more output voltage than a single motor can stand by itself, but the voltage is split between the two motors because they're wired in series. As you speed up, the torque remains flat until a certain point, and it starts to drop off. When the Zilla calculates that it's the optimum time to do so, it cuts power for a fraction of a second, rearranges the motors electrically into a parallel configuration, and then each motor is fed full pack voltage. At high speed the motor can theoretically tolerate a higher voltage than it's rated for, and so suddenly you get a burst of additional torque. Sometimes a motor will flash over when treated like this; it's pretty unpredictable and is a risk inherent in the racing scenario for which this approach was designed. The flashover risk can be mitigated by variable brush timing, though this implies custom work on the motor (until Netgain releases their new Warp9 with built-in variable timing) and a control system for which no standard off-the-shelf products yet exist.
Another attempt at a summary:
Get the 9, save some money, have a lighter car that will handle a little better on corners due to a somewhat lighter rear end. Enjoy potentially better efficiency while accelerating, for in-town driving. Deal with some minor blower noise (which will be most obvious at a stop, when the air conditioning fans aren't running).
Or, if the extra cost isn't a showstopper, get the 11, have a bit more power and a motor that will run cooler and (according to Netgain's data) more efficiently when operated continuously on the highway. With no forced cooling necessary, the install will be simpler. Deal with some added weight.
Chris
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Here are a couple of good web sites on
K2 and A123
http://www.zeva.com.au/tech/K2/
http://zeva.com.au/A123/
I corresponded with Ian Hooper (the author of these reviews) last year on the subject of his testing, and his findings were definitely influential in our decision to go with K2 as our lithium supplier. We've built what I think is a great and promising relationship with them so far, and are in the process of negotiating some arrangements to move more volume and lower our prices in the future. Having visited their headquarters in Nevada and having seen the verification they perform on every cell during module assembly, seeing their heated and chilled enclosures for long term temperature-controlled testing, and knowing that they're actually licensed to legally sell LiFePO4 cells in the US (A123 is not licensed, and is currently being sued by Phostech and the University of Texas); I feel confident we've made the right decision.
Chris
