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5 minute(s) of a 612 minute read
10-6-2011
1. Basic Principles:
Say you have installed Bilstein and were happy at first but now want to go to the next step, firmer, stiffer, more like a GT3. What to do? Besides the first and important step of adding R compound tires, this leads to discussion of further modification of the Bilstein, stiffer springs. Note that nothing comes free and this "upgrade" WILL sacrifice comfort for handling. Anyway, IMHO, here are the steps for a "Turbo to GT3 conversion":
1. R compound tires: Don't ever underestimate the difference a tire could make. This is a key component of imitating GT3: you want GT3 handling, you need R compound tires such as Michelin Cup, Pirelli Corsa, Toyo Roxes R888, etc. No way around this. Why? Not just increased traction, but more importantly the stiffer tire sidewall that hugely influences suspension behavior (makes the car stiffer with less body roll).
2. More negative camber, starting numbers: -1 front, -1.6 back. More as needed.
3. Stiffer and adjustable anti-roll bar. For adjustment of understeer/oversteer, body roll, and ride comfort. Combine this with rear Tarett Drop Link Here for even better feel of tightness and control - you will love it, guarantee.
4. Bilstein basic or with stiffer springs: A basic Bilstein with R compound is outstanding and already transforms your Turbo to something that is very fast. However, if you want to take the GT3 in tight corners & curves, and if you want to duplicate the subjective feel, IMHO even stiffer springs are needed. This is where the beauty of using coilover comes in: it's designed with that in mind, the ability to change springs to your need and taste.
2. How to change springs for Bilstein:
Keep in mind there is a limit as to how much you could increase the spring rates. Per Swift, the normal rule of thumb is that you can go up to +/- 4 kgf/mm (224 lbs/in) in additional/reduced spring rate from the coilover's stock rates. What this means is that since the Bilstein starts at 336 front/560 rear, you could go up to about 564 front and 784 rear without having to re-valve your Bilstein shock absorber. For reference, a 911 professional race car would run in the 1000-1200 spring rate range (anyone pls correct as needed).
First some basic technical information for the Bilstein springs - they are weird in that their ID is 70mm, not industry standard and making it difficult to find replacements. Second, don't worry about the helper springs (in the Bilstein these do not contribute to the rate; they are "helper," not "tender" springs), all you need to change are the main springs:
Front: Helper 80 lb/in spring rate, 60mm length; main 336 lb/in rate, 151.5 mm length; ID 70mm for both helper and main springs.
Rear: Helper 80 lb/in rate, 60mm length; main 565 lb/in rate, 200 mm length; ID 70mm for both helper & main.
There are two places you could get springs that fit Bilstein: Hypercoil and Swift Springs. The springs you want to look for is 70mm ID, 6 inches long front, 8 inches long rear. The process is very easy and cheap - labor should be around 300-400 per axle.
Swift Springs: http://www.swiftsprings.net/products...r-springs.html
Hyperco: http://www.hypercoils.com/PDF/70mm.pdf
Currently my car is set at 448 front/600 rear - versus Bilstein's 336/560.
The front spring is Swift Spring part # Z70-152-080: Internal Diameter 70mm, free length 152mm=6 inches, spring rate 8 kgf/mm=448 lbs/inch.
The rear spring is Hypercoil part # 8P0600: Internal Diameter 70mm, free length 8 inches, spring rate 600 lbs/inch.
The springs are cheap and directly replaceable for the OEM Bilstein springs. If you are into more advanced level of suspension tuning, the springs are the place to play around and have fun with. It's an easy process and the change will be easily noticeable.
The stiffer springs make steering response more immediate (no free play) and precise. Soft/mushy steering IMHO is one of the biggest problem of the stock Turbo compared to its GT2/GT3 sibling. The car is definitely on the firm side but has extremely fast reflexes and minimal body roll at sub 100 mph speed. This is the best all around car that I've ever driven: for me, perfect mixture of handling vs. comfort, precise steering, precise suspension response. I love every second that I am driving my Turbo baby! 
