Electric Porsche 944

Archive for July, 2013

Gear ratio’s and a possible transmission swap

by on Jul.15, 2013, under Electric Car

When starting an electric conversion it’s tough to know exactly what gear ratio will perform the best and feel the best when driving.  I was happy that my car had the turbo transmission installed since it would handle the extra torque from the 11″ motor, however I really didn’t know what the ideal gear ratio would be, but having a free box with 5 to choose from is a good start.

After driving the car for awhile on 300v I’ve come to the conclusion that my ideal ratio wasn’t included.  The car does drive beautifully and it’s quite quick, however I wouldn’t mind a few slightly “shorter” gears to choose from.

First gear is too short (too high a ratio) and 2nd is pretty close to ideal but could be a bit shorter for low speed driving and fun acceleration.  3rd gear is also close to ideal but is a bit tall for a 0-60mph run or spirited but not high speed driving.  What I feel might be ideal is a 2.5 gear, with a ratio about halfway in between my existing 2nd and 3rd gear.  As it turns out Porsche did make that transmission, it’s has the tough internals like the turbo transmission to handle the hp and torque that I have, but has my desired 2.5 gear.  It came in the later 944’s namely the 944S and S2.  It also has a 2nd gear that’s shorter than my 2nd gear and would provide slightly better acceleration without being too short like my existing 1st gear which is too far in that extreme.

944 Turbo
1st – 11.81    34.5mph @ 5500rpm
2nd – 6.949 58.5mph @ 5500rpm
3rd – 4.725 60mph @ 3830rpm
4th – 3.490 60mph @ 2830rpm
5th – 2.798 60mph @ 2268rpm (top speed 5500rpm 145.5mph)

As you can see from the above speed vs rpm vs gear ratio my existing 2nd gear is almost perfect for a 0-60 run, of course I would accelerate through 60 for the best time which puts the motor slightly above the recommended maximum RPM.  So for a safe 0-60 acceleration I need to use 3rd gear, which is a bit tall.

944 S2

1st – 13.56    30mph @ 5500rpm
2nd – 7.979 51mph @ 5500rpm
3rd – 5.425  60mph @ 4400rpm
4th – 4.007 60mph @ 3250rpm
5th – 3.015 60mph @ 2445rpm (top speed 5500rpm 135mph)

944_S2_transmission_brand_new

The S2 info is above, and as you can see a 0-60 run in 3rd gear would take me to 4400rpm which happens to be around where the power starts to taper off anyway.  4th gear would also provide a nice RPM for highway cruising keeping the fan speed fairly high for good cooling.

I have many other things to work on first, but I’ll keep my eyes open for a good used S2 transmission for a future swap.
In the mean time back to suspension and motor updates!

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The Weights are in and some Suspension Decisions

by on Jul.07, 2013, under Electric Car

Early on the goal was to have a conversion that came in at the stock curb weight with similar front/rear weight distribution.  When I got to the point when I knew the A123’s weren’t going to be the solution I knew the car was going to get a little rear heavy based on the new battery choice.  Once I switched to CALB CA60FI’s I estimated the final weight at 2900lbs +/- 100lbs, well it looks like I just squeaked inside that estimate.  In it’s current state the car weighs in at 2992lbs with a F/R weight distribution of 43.5% Front and 56.5% Rear, this is much more rear biased than the original 50.7% Front / 49.3% Rear gasoline powered 944.  It’s still far from the ~39%F/61%R of the 911, and lands quite close to the new Porsche Cayman which is 44%F/56%R.  I think it’s a good number for now, but I may try to move some weight forward, I’m working on a new water cooled charger, and a water cooled dc/dc converter.  The new dc/dc converter will be completely sealed and go up under the hood (vs the rear side panel) and share the cooling loop with the Soliton1.  If I can make the charger sealed enough it will also go up front, but the required fan cooling for the inductors might mean it has to stay in the back.

near final weight

Above is the current appearance, you can see the front ride height isn’t bad, but the rear is a bit low.

porsche1

Above is a pre-conversion picture showing the front slightly lower and the rear higher (but still not quite level).  The final goal is to have both the front and rear level at a height resembling the pre-conversion stance.

The options:

Reindex the torsion bars and tweak the final ride height with the built in adjuster.  This would likely correct the appearance, but wouldn’t address the fact the rear of the car has gained roughly 300lbs.  I also need to address the front end which has lost just over 100lbs.

Supplement the torsion bar system with rear coilovers, this would increase the load carrying capability while providing ride height adjustability.
Coilovers on the front would allow me to level the car and increase performance.
http://www.paragon-products.com/Coil-Over-Kit-Koni-Cup-p/koni_cup_kit.htm

Piece together an improvised coilover like system with add on adjustable spring perches.
http://www.paragon-products.com/Coil-Over-Kit-Front-p/944arhk-nr-xdo.htm

Replace the torsion bars with higher spring rate versions to carry the load, while setting the appropriate ride height.
http://www.paragon-products.com/Rear-Torsion-Bars-p/saw_944tb.htm

Initially the torsion bar looks like the most cost effective solution (simply reindexing the torsion bar may not be a solution, when considering the extra 300lbs), however I believe the front and rear shocks likely need to be replaced as well, and since I’m there replacing the front springs with a 1″ lowering spring could be worthwhile.

 

UPDATE:

Well as often happens with my posts, I don’t get around to finishing and publishing a blog post until things have changed.

I’ve already adjusted the ride height and I’ve purchased some larger torsion bars, and now I have a week off to play with the car, I’d like to get the new torsion bars in and the rear height adjusted to provide a suitable stance.  I’ve started to tear apart the ’83 944 which I bought strictly for the manual steering rack, but then replaced the engine and other stuff for the wife to drive it.  I had bought new strut inserts for that car so I’m simply going to swap them over to the EV.  They aren’t high performance struts, however they are basically new, and should let me evaluate the front spring rate better since the existing struts are starting to “bounce”.  This will help me decide what type of spring to put up front, or if I need to replace them at all.

I finally installed the manual steering rack today and it’s simply awesome!  A few short drives confirmed it’s got the feel and lighter steering weight that I’m looking for.  The stock steering wheel didn’t feel big enough to provide the required leverage on the depowered power steering rack, however it feels huge now and I could use a smaller wheel.  I’ve heard great things about the Porsche 968 Club Sport steering wheel, it’s a bit expensive so perhaps one day, although a nice Momo would actually cost less.

968wheel_1

 

I’m a fan of the Momo Tuner wheel which is similar to the Club Sport wheel above.

 

momo wheel

 

I’ve also done my best to level the car using the existing adjusters, and as it turns out it was lowered as far as it would go, so there was quite a bit of travel to bring it up.

near final weight ride height adjust

 

As it sits now before I change the torsion bars above, and to compare the pre adjusted level below.

near final weight

 

I’m happy to be at a point where car stuff is running in parallel with finishing the electric portion of this build.  The car itself is wonderful to drive and getting better every day as I address some of the normal car stuff, including the steering rack and suspension.

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