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STF ZX3 Build

61K views 233 replies 29 participants last post by  Woundup7 
#1 ·
Guess I'll just use this as my introduction. I'm new to ZX3's, but not to autocross. I started autocrossing in 2005 and have owned/autocrossed a lot of cars since then. My most recent autocross car was a 2005 Scion TC built for RTF (HS index.) Finished mid-pack with it at nationals, which was about where I'd hoped to end up with a non-Mini Cooper. I've considered prepping the car for STF, but after more research I'm realizing there are better cars for the class. I loved the torque of the Scion's 2.4L for corner-exit, but the weight was the real concern for me. In RTF trim, I got it down to 2776 pounds (race weight.) Which isn't bad for a starting curb weight of 2905 pounds. However, I think a full STF build would only get the car down to around 2700 pounds. That's still pretty heavy for a car that can only run 225's on 7.5" wide wheels. Mazda3's have similar power and slightly less weight, but are still a little on the heavy side. Enter the 2.3L Duratec ZX3... 2600 pounds and 149hp, stock. In STF prep, it seems reasonable to think it could get under 2500 pounds and over 160hp, (based on dyno charts I've seen.) 2.3 Liters means the car should have fairly decent torque. Also, the dimensions are pretty good. Somewhat short in height and length, and also pretty narrow. Operating costs for STF should be pretty low, and the car has enough power that it could still be fun on a road course. All of these things together, have really sold me on this car.

I've got a line on a very low-optioned D23 ZX3, and if all goes as planned, I should be picking it up next week. I'm pretty excited to build this car, as I think it could be a very strong contender in STF. Especially on larger, power courses.

I'll hold off on sharing my build plans until the car is actually in my driveway. At that point, I'll probably be asking a lot of questions on this forum. I'll also try to get pictures up once the car is officially mine.
 
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#97 ·
Good question. Most likely something custom. GC is making plates for the Mazda 3. The mounting studs are further apart on the Mazda3, but otherwise the strut mounts are almost identical. So it might just be a matter of relocating the studs. I won't know for sure until GC posts pictures. The other option I've considered, is modifying the stock strut mounts to function more like K-mac's. Where you basically have a move-able bearing plate sandwiched between two other plates. When you snug it up, everything stays in place.
 
#99 ·
Due to a lack of reasonable options for camber plates, I've decided to fabricate some on my own. The range of adjustment won't be huge, but it will position the strut exactly where I want it, (max negative camber AND caster.) Unfortunately, they'll be steel, (no tig welder,) so they'll be a little heavy.

I'll try to get some pictures posted as I make more progress.
 
#107 ·
Finally... another update.

The car has been out of commission for a bit while this was getting taken care of:



Also installed poly bushings in the dogbone mount, and finally decided on front springs. I've been pretty open about most things on my build so far, but for the sake of "competitive advantage," I'd prefer to hold off on sharing spring rates for now. I'm also adding these slick Hyperco flat-wire helper springs to keep things in place when the suspension is unloaded. They're 4" free length, but compress to about 1/2" under load. I'm not entirely sure that they're necessary, but with the 6" springs, it gives me peace of mind. I'm expecting the dual spring spacers that sit between the springs to show up in a few days.

 
#108 ·
Your spring rates aren't hard to figure out. EDIT: nevermind. bigger picture shows tape over spring rates. damn phone.

How did you do that to the Clutch?!?!? Did you just do a stock replacement?

The poly mounts helped alot with the wheel hop. So when is your next ProSolo?
 
#109 ·
How did you do that to the Clutch?!?!? Did you just do a stock replacement?

The poly mounts helped alot with the wheel hop. So when is your next ProSolo?
Not sure how the clutch ended up like that. There was a TSB for the OE clutches delaminating. Maybe that's what happened? Yeah, replaced with stock clutch. No allowance for performance clutches in Street Touring.

I'll be going to Lincoln in May for the ProSolo and Tour.
 
#111 ·
Time for a battery swap...



Old battery: (36 pounds)



New battery: (7 pounds)



"Installed" pictures:




Almost 30 pounds saved. I'm very aware of the lower CCA's of this battery, but this is very common in autocross. It might not be a great idea for your daily-driver, but it's a great way to save weight for autocross. I did a test-start shortly after install, and it sounded very strong. Also, you can see the bracket I made. Steel strap and aluminum centering block. It bolts down using the original bolts for the battery tray.
 
#114 ·
Now why'd you have to go and post that? [:0] (I'm starting to feel a bit of "lightweight battery" envy.)

Not sure how it works. I had a 160cca battery in my old STX BMW, which started the 2.8L I6 just fine for almost 2 years. Most of the people who use the ETX9, use it on smaller engines in the 1.6L - 1.8L range. So I might be pushing my luck with the 2.3L, but it was cheap. So I figured it was worth a try.
 
#120 ·
I've been super busy lately, but here's a quick update:

I installed the suspension, wheels/tires, and a race seat all in the last two weeks. This last weekend, we had our first two local events of the season. Back to back events on Saturday and Sunday at the same site, but different courses each day. So the big question... Can the car be competitive?

YES!!! My hotshoe co-driver and I finished 2nd and 5th overall out of 135 drivers on Saturday, and 1st and 6th overall out of 146 drivers on Sunday.

Results here:
Saturday: http://www.mnautox.com/results/2014/mowog1_pax.htm
Sunday: http://www.mnautox.com/results/2014/mowog2_pax.htm

In-car video from Sunday: https://www.youtube.com/watch?v=4QOv3sSPPfs

We have two other nationally competitive STF cars in our class, locally. Those two cars are both fully-prepped. My Focus still has a long ways to go. I have a lot of parts that I didn't have time to install yet. So I'm pretty excited for the future of this build.
 
#121 ·
Congrats!!! I gotta think this thing is going to be VERY competitive, a Mazda 3 won last year and this is just a lighter version of it.

Is that tire rub in the video?
 
#122 ·
Worse. Wheel rub. Just barely makes contact under load. I've got a solution for it though, so it should be fine at the next event.

My car can still lose another 70-100 pounds. I should still be able to find another 20hp or so, too. My only power mod so far is the cold-air intake. I have a header and custom exhaust ready to be installed. Just haven't found time yet. Then I'll need to do some dyno-tuning. I'm just happy all of my research wasn't in vain.
 
#130 ·
Yup, already run ARP extended wheel studs on my car, so a thin spacer is certainly a simple solution.

I think the larger problem is that so few folks run 15" wheels on these cars with these arms that there may be a larger problem that those that run larger wheels do not see. That needs to be addressed first and I think that's the line of thought Justin is going down. I haven't had chance to even switch over to my street wheels yet but hope to check it out this weekend.
 
#133 ·
Nope, my rules allow only for factory pieces and bushing are free, looking into spherical bearings which are allowed for my next upgrade. The ability to move to these make the build in my class the real final move to r u crazy spending all this, yep this is my 10/ 10ths build and the really most important item needed is driver talent which is suspect here.:)
 
#134 ·
Hmmmmm, finally got to take the 15's off and put the 17's back on. I don't think its the arms, nothing felt loose. I think its such a tight tolerance, that the flex in the rubber bushings is enough to make it rub.

Problem: this was the rub you heard in my videos from my last autocross. Both rear wheels look like this.



Not the best pic in the world but it was rubbing here



Here is a pic with the wheel on showing there ain't much space between the wheel and the LCA.



The good thing is a 3mm wheel spacer should solve the problem.
 
#135 ·
Hmmmmm, finally got to take the 15's off and put the 17's back on. I don't think its the arms, nothing felt loose. I think its such a tight tolerance, that the flex in the rubber bushings is enough to make it rub.

The good thing is a 3mm wheel spacer should solve the problem.
My wheels are +45, but they were rubbing in the exact same place. Maybe a difference in alignment?

I found the play in the arm by jacking up the car and leaving the wheel on, then having someone shake the tire at the 12 and 6 positions while I watched the arm. Once the wheel was off the car, it was a bit harder to identify play in the arms. IIRC, the bolts that go through the bushings aren't fully threaded. So I'm thinking maybe going to a fully-threaded bolt will take-up some of the play where it passes through the bushing. Not sure yet. Hoping to dig into it soon though. I still wonder if there isn't a little flex in the wheel too.
 
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