Andy Hollis
Andy Hollis
1/13/07 5:42 p.m.

Its been a year now since we started the STS2 Miata project, and we are thrilled with the results we achieved in our car's first season, but more importantly with the growth of the STS2 class. This was our stated goal way back in the first installment of this blog series and why we have shared all of our development experience here. It looks very much like STS2 has hit the sweet spot for many drivers on fun vs versus $$$ spent. 2006 saw full fields at almost all national events ensuring great competition, and early indications are that 2007 could add at least another 50% to those fields. Amazing.

This installment will serve as a summary of everything we've learned so far on setting up a Miata for STS2 competition. It is organized in such a way to maximize performance (and fun) for any level of dollar investment.

Buying a car

Most of you will start with a car you already own, but some will make a purchase specifically for autocross useage. Here's some info for those folks. In general, any 90-97 Miata is a good starting point with the big decision being between the 1.6 and 1.8 cars. The pros on the 1.6 are the available VLSD, 100 lbs lighter weight and 4.3 final drive. The 1.8 is a bit heavier, and runs an open 4.1 diff, but makes significantly more power, especially in the mid-range.

Once you decide on a car, you'll want an option package that fits your desired usage. The lightest are the base model cars, but most folks using the car on the street will want a/c and a radio. Power steering is another thing present on many of these cars and has a quicker steering ratio which trades off against some extra weight and power drag on the motor. ABS is also present and desireable, but rather rare. Since it requires a package with p/s, it incurs some weight and power hit. For regional use, though, any early Miata is a fine starting point. For more details check out this table from Miata.net

Pre-build

Before starting any build-up, its important to take stock of the platform and fix those things that could significantly affect performance. It's a good idea to replace the alignment bolts and eccentrics since they tend to get messed up over the years and won't hold under autocross stresses. Check your suspension bushings for major slop and your frame for any major damage or misalignment. Rust can also be an issue in certain parts of the country. This is also a good time to verify that the car has not been molested in ways that are contrary to the rules allowances. For example, missing trim parts and undertrays are common.

The other big thing to baseline is the motor. Do a compression check as described in the service manual to make sure yours is in solid condition. If not, investigate further and fix as necessary. Do the appropriate maintenance as described in your factory service manual. Fresh plugs, oil/filter, fuel filter and air filter are all important. Check your resistances on the plug wires and coil pack to see if they need replacing. One freebie is to bump the static ignition timing to 14-16 degrees BTDC, depending on the quality of your local fuel. Its also nice to put the car on a dyno and compare to dyno plots of others with similar mods (examples exist all over the web) to get a feel for overall condition.

Tires

At this point, we've found the Falken Azenis RT-615 to be the best available tire in either the 205/40-16 or 205/50-15 sizes. Each has pros/cons, with the 16 accelerating and slaloming better, and the 15 being better in sweepers and keeping the motor off the rev-limiter. The Kumho MX in the 15" is also good, especially in warmer climates since that tire actually likes excessive heat build-up! Finally, Hankook's 225/45-15 is another great alternative. All of these tires are a bit squirmy when run full-tread which is fine for regional competition. For more serious efforts, consider shaving to 4/32nds.

Wheels

For the 205/50-15 tires, a 15x7.5" rim is optimal but a 15x7 is perfectly fine for regional use. The 225/45-15 really wants a 7.5" rim. Since the 205/40-16 Falken has a narrow tread width, it can run either 7 or 7.5" rims with little difference. Keep your offset as near to the factory specs of 40-45mm as you can to keep the zero-scrub radius feature and narrow the car. From there, it all depends on your budget. 15x7" wheels that weigh 13 lbs can be purchased brand-new for around $100 each designed for Spec Miata racing. A lighter 15x7 option is the Enkei RP-F1 at just under 10 lbs for about $200 each. For super-serious competitors, the ultimate are 7.5" wide options in both 15" and 16" diameters from SSR, but they are very pricey at well over $300 each.

Shocks/Springs/Coilovers

This is the heart of the build and also where the streetability compromises come in. Do the wrong thing here and you can make your car a real pain (literally!) for street use. We like Koni shocks for a whole lot of reasons (quality, value, performance, support). The basic off-the-shelf Koni is a fine low-cost starting point (~$500), but will limit how far you can lower the car before hitting the bumpstops especially in the front. It will also limit you to springs rates not to exceed about 450 lbs. Next step up, at double that price, are the RACE-valved Konis, which are shorter and stiffer than the OTS variety.

Some folks are finding they can get by with the RACE shocks in the front only, since the springs are usually stiffer there. From there, you move into the realm of custom-built, custom-valved Koni 2812 and 30-series shocks. These will maximize suspension travel, allow for any spring rate and are threaded-body to directly provide ride-height adjustment without coilover sleeves. Pricing starts about $2500 a set, so these are are only for the hi-end competitor looking for every advantage.

Coilover sleeves and springs are available from many sources. Some have found good success with the cheap stuff available for $50-$100 off eBay, though it can sometimes be tricky to verify the spring rates. After that, you move right into the "good stuff" with real racing springs from the likes of Eibach and Hypercoil. Both Koni and Ground Control make good quality coilover sleeves. A GC kit of sleeves and springs is around $400.

As for spring rates, that depends a lot on your intended use and level of compromise. In short, stiffer springs require stiffer shocks to control them, which hurts the street ride. 550/350 is probably the stiffest you want to go for a car that sees a lot of street use. 400/250 is a really good compromise, while 700/450 seems to be the high-end for ultimate autocross-only performance.

One key ingredient is the use of alternate spring top mounts to provide additional suspension travel when lowering. Flyin Miata sells some very clever top mounts that extend travel by .75" but they are quite pricey at over $250 a pair. A similar effect can be achieved for about $50 total by using late-model 99+ Miata mounts modified as described earlier in the blog.

Sway Bars

We like the Racing Beat 1.125" hollow front bar, but a 15/16" solid bar will provide a little less roll stiffness at lower cost, and slightly more weight. Others have had success with a 7/8" solid bar. In the rear, the basic balance of the car can be tuned between the following: no bar, 12mm stock bar, or 5/8" Racing Beat adjustable bar. Racing Beat has a great chart showing the rates. Expect to pay around $100-$150 for any bar. See previous installments of this blog for installation details and tips.

Ride Height, Alignment and Corner Weighting

We run our car's ride height at about 12" front and 11.5" rear, measured from the fender lip to the center of the hub. To run that low, you need shortened shocks, upper mounts that provide extra travel and stiffer springs. You'll have to run closer to 13" both front and rear if using standard length shocks, OE mounts and softer springs in order to stay off of the bump stops. Bump stops can also be cut down to provide more clearance, but hitting them becomes more severe on bumps.

Once a ride height has been established, corner weights should be altered to establish similar balance turning both right and left. Check out the Dial-In section of the blog for details.

Finally, a good performance alignment will tie it all together. Front and rear camber should both be between 2-2.5 degrees negative and front caster around 4 degrees. You'll want to go to the high side of camber when running on grippy concrete with a car that rolls a fair bit and on the low side for lower grip asphalt and a stiff car. Front toe-out should be around .25" in the front for competition which you can dial in with a full turn of one tie-rod end at the track and back out again for better tire wear on the street. In the rear, something between 0 toe and 1/8" total toe-in is best. Use less with stock rubber bushings and more with hard plastic or urethane.

At this point you have a really great-handling car that can be quite competitive at regional events, even with a totally stock motor. Once you get used to this, adding some power will help further reduce your times. But money spent here will have less overall effect than the money you spent on your suspension.

Before you spend $400-500 on bolt-ons, give thought to doing a quick refresh on the motor itself. This may actually yield more performance for the dollar, depending on what state it is in. If the bottom-end is solid, a simple valve-job and head-shave to service specs might yield more relative performance.

Crank Pulley

While this may seem like a weird place to start with engine mods, it is absolutely the best performance value for power in this case. The highest quality piece is from Unorthodox Racing, priced at around $200 but OBX makes an eBay knock-off for half that which many people have had success with. Make sure and get the right one for your year/model as there are three variants.

Intake

Once again, Racing Beat is high on the list with their product priced just under $200. For the low-ballers, various eBay knock-offs are available for $50-$100 and perform quite well compared to the stock system. We use the Spec Miata intake system from SP Induction.

Exhaust

Other breathing improvements will do little if the stock exhaust is choking them. This mod is as much about sound quality as it is about performance. The lightest and best-flowing configurable system (mid-mounted Supertrapp) has to be choked down quite a bit to make it tolerable on the street which affects street performance. Giving back a little on weight, you can have a nice streetable system for about $200 from any decent muffler shop using a Walker Turbo muffler and 2.25" aluminized tubing. Use of a mid-mounted resonator is useful to tame the drone these cars tend to make on the highway at 3000 RPM. Racing Beat, Borla and Jackson Racing all make good cat-back systems in the $300-400 range which are hi-quality polished stainless. Though heavy, they make good power and their flashy looks will last forever. The ultimate full exhaust would be custom built from 18-gauge stainless using mandrel-bent components from Burns Stainless and TIG-welded by a competent local shop. Expect to pay through the nose, though.

Header

Most of the headers we've tested give similar gains. Each has its pros and cons, and final performance will depend on the rest of the mods, as they all have to work together. Jackson Racing and Racing Beat both make good products with the JR piece providing the better value at $350-375 and the RB piece the best performance at $425. Once again, eBay knockoffs can be had for $150-200 but be careful about build quality and longevity. Another low-buck alternative is to take the stock exhaust header (yes, its really a tubular header) and smooth out the internal weld build-up at the ports and collector with a die-grinder. You'll get half the value of an aftermarket header replacement and its free!

Once you've done all of these things you've got a real performer, both on the street and the autocross course. There are other allowance you can take advantage of, but they deliver much less of a performance. For example, there is very little weight to be saved (10 lbs total) with the new seat allowance and a wee battery will make life on the street less fun. Also, unless your original suspension bushings are worn out, the gain from stiffer bushings is minimal.

Here's a sample low-buck build-up:

$400 15x7" wheels $400 205/50-15 Falken Azenis $100 EBay coilovers $500 Koni Sport $50 Late-model Miata upper mounts $100 15/16" Front sway bar (re-use stock rear bar) $150 Performance alignment and corner weight $0 Modify OE header, bump timing


$1700 Total

A lot of this is the wheel tire combo and some judicious surveying of the online forums will typically yield wheels for half this, sometimes with appropriate tires. Used springs and sway bars are also a good value. Used shocks can be a risk, though. At the end of it, you've got a solid regionally competitive car.

$50 OBX crank pulley $200 OBX 4:1 header $50 OBX intake $200 Exhaust (locally made)


$500 Additional ($2200 Total)

These things will up the ante a bit and get you closer to the national guys. But to play in the big leagues you'll need to upgrade as follows:

$500 Koni RACE-valved shocks (price differential from the Sports mentioned above) $150 Eibach springs, 550/350


$650 Additional ($2850 Total)

While this won't likely win you a national championship, it will put you in the mix for a national event trophy, assuming your driving skills match the car's prep level. Not bad for a total expenditure of less than $3000 on a car that typically also costs less than the same amount.

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PseudoSport
PseudoSport HalfDork
7/27/10 2:46 p.m.

Thanks, I’ve owned it for 10 years and I still haven’t had the time or money to fix it up. If you look at the other pictures of it you can see why it was $800.00

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