Build #2, No huge car progress...but...cool new toys! Options

[jd74914]

Hello all.

I figured I’d throw up a progress thread for my car’s second rebuild. For those who don’t know me my name is James and I’ve been a 914 addict since age 13 (now 26) and used to hang around here a bunch more. It seems like a good start to this thread would be some background information. Its first build took place when I was in high school (age 13) and ended as a freshman in college. When I originally purchased the car it didn’t run and had some pretty serious external rust problems. I rebuilt the motor, fixed tons of electrical issues, replaced all of the rotten metal with new (all hand-formed since I didn’t have the money to pay for reproduction pieces), and repainted. Everything was done in my garage with the exception of turning/balancing the flywheel and I learned how to MIG weld and paint from my dad, some books, and through a lot of practice.

After reassembly, it was my daily driver for 3.5 years during my undergrad degree. I pretty much drove it hard and put it away wet for the entirety of these years and it never saw a garage. Something about getting a mechanical engineering degree, dating a few girls, working throughout the year as a design engineering intern, and finding FSAE cars really limited the amount of time I spend on my own car. By the end of my undergrad degree there were some pretty rough spots, mostly in terms of the suspension/brakes and a pesky ignition switch (replaced 3 or 4 times and it kept failing), which pushed me to taking it off the road and fixing everything correctly. I thought it might take about a year-that was 3.5 years ago! (IMG:style_emoticons/default/wacko.gif)

Just like after the first rebuild, life got in the way and the car sat as I went through a master’s degree, worked full time, continued to play with FSAE cars, and starting working on friend’s real racecars. Now I’ve finally finished my MS (and know way too much about fluid dynamics and heat transfer (IMG:style_emoticons/default/laugh.gif) ), am applying to schools for a Ph.D., still haven’t stopped [advising] FSAE design, and really want to drive her! The play was to start and finish rebuilding the suspension last summer (I saw Chris Foley-Racer Chris in the grocery store one day and told him this), but I got carried away and a bit behind. This thread is to chronicle the build back to the road.

We’ll start with a few pictures from when it was originally completed in 2006/7 (well, it's missing the plates and still has the original windshield but...).

[jd74914]

I picked up a gearbox off Craigslist this week. It's out of a 2012 WRX and has ~30k miles (the car was wrecked). It bench shifts smoothly and is super clean so I'm hoping that it isn't messed up inside, but I guess we'll see when the center diff comes out.




I've got feelers out now for a broken central diff in the local area so I don't have to ruin this one making a locker if it's in good shape. Good ones seem to go for $200-350 on NASIOC so selling this one would go pretty far in offset the overall transmission cost. Anyone here have a broken diff they'd be willing to let go for cheap? (IMG:style_emoticons/default/laugh.gif) (IMG:style_emoticons/default/biggrin.gif)

The 2008-2014 WRX transmissions seem to be pretty stout units. Some threads on the Subie forums show their gear widths are greater than the early WRX transmissions and the case castings seem to be a little beefier too. Unfortunately, around here at least they are a bit more expensive than the older push-clutch Legacy, Impreza, etc. stuff. Right out of the box the gear ratios appear pretty well matched, though even with the 3.900 front diff first gear seems a little bit low. Ideal 1st gear is from an STi RA but those gears/main-shafts are super rare and priced accordingly.

The plot below shows tractice force, or the actual power put through the tires into the ground, vs. vehicle speed for the 08-14 WRX transmission. It assumes a perfectly stock '05 Legacy GT power output (shown in the second plot). In terms of pure acceleration and drive efficiency, you want this curve to have a parabolic shape. The parabolic shape means that power to the ground is constant and at its maximum. As mentioned before, 1st is a little bit low, but it's going to stay that way forever probably. (IMG:style_emoticons/default/laugh.gif) The easiest way to make the in-gear traction numbers fit the overall traction parabola better is to shift the engine powerband up slightly and increase torque in the 4100-4600 rpm range. With some AVCS and boost tuning this shouldn't be too difficult. (IMG:style_emoticons/default/sunglasses.gif)

One other thing to note is the red "max tractive force" line. This line is the most force the tires can handle and still retain traction. It's a pretty simplistic calculation just using CG, wheelbase, and an assumed single tire coefficient of friction which then calculates dynamic driving wheel weight and from that max sustainable tractive force. The number isn't correct since I'm just guessing CG, weight, etc. The tire friction coefficient is pretty questionable since I've never seen one for street tires (only slicks) and the fact that it's not taking into rotation speed or any lateral forces added by the suspension kinematics. It is useful to basically know first gear and the start of second are really just for burnouts! (IMG:style_emoticons/default/laugh.gif)

The spreadsheet used for these calculations is attached is anyone wants to play with it. No guarantees on accuracy, though I believe everything is correct. The format was borrowed from something posted a long time ago on Pelican Parts by B. Smith (IIRC, unfortunately I never wrote down the source), but has been modified extensively. You can add different gear ratios in, I just only put in some common stuff. Check out the last page for some Subaru transmission information if you want to play around with different gears not listed in the list or personalize it to your transmission.



Stock dyno plot (not mine, just from a local eddy current dyno)