I found a pretty cool thread on the topic. Where one guy had to modify his car with a hammer to fit these 680 tires. Lol. So, then the question is, will the intercooler duct be the limitation in the rear wheel well. Or actual metal work?

http://rennlist.com/forums/porsche-c...tire-size.html

 

Both the street car (all models) and race car (all models) have it's ride height measure from the same points, very same way. It's just how the chassis "sits" on the subframes. Only visual cues can be found from side skirt vs ground alignment and how deep the wheels are in the wheel wells.
This leads to shock lenght issues. Cup rear shocks have quite small free movement, so in order to center the shock when car's at standstill some calculations and measurements must be made.

Yes, Cup tires are like balloons, they have lots of dampening built into them compared to similar OD street tires. Actually you need to use either 19" or 20" wheels to find tires big enough (in OD). Then the sidewall is almost nonexistent. Springs and shocks must be adapted accordingly. Just look at 991 GT3 or GT3 RS.

 

I get that. But, if the chassis sits closer to the subframe then the common measure point has no relevance in reference to the amount of wheel well space when comparing one to the other. Unless I mill my subframes to cup specs. I just want to know how much wheel well room I have to work with.

 

As I've posted a number of times above, compared to a TT, the Cup/GT3/GT2 subframe will be 7mm closer to the body of the car due to the thinner solid bushings. It's simple math. The Cup comes with more wheel well space from the factory than the TT. Additionally, the cup has no fender liners which in and of itself gives it a lot more wheel well room. Your other major limiting fact on the TT that precludes you from running anything close to a Cup size tire is the intercooler hose interference of the TT.

 

I have read and understand your previous comments about the differences. I am not arguing that point. I do not think their is anything wrong with me looking into a solution for this from another view point. I get your point. I get your math. I understand the limitations. Running the exact same tire sizes as the cup may not be the answer. I am not looking at it from the angle of that. I am looking at this at the angle of all possible solutions. Don't get frustrated reading into my posts. I believe you and Pete on your findings on the subject. I just think you and I have different goals on our cars and cannot assume that what you are doing is what I want to do. I am not going to be running standard street gt2/3 ride height. I understand that. But I want to calculate where I want to be and what is needed to accomplish that.

 

No worries. We are just trying to help you out. That's what the forums are about. As I alluded to in my first post, converting a race suspension to a street driven car is usually a tricky proposition and can leave you with less than desired results especially when you are installing shocks that have very limited travel. If not done right, you will have very limited rebound or compression travel and your performance will be downright scary on anything less that very smooth roads. I am assuming that you are converting your car to mainly track use with this project.

Up till now we've talked about fitting the rear shocks but what are your plans with the front cup shocks? I am assuming you are planning mounting the front shock higher up in the upright? If so, I would make sure there is no way for it to slip free under load as I would not solely rely on the upright pinch bolt. Maybe have a collar welded onto the shock body to make sure it can rest on top of the upright and take up some of the load. Front Cup shocks are set up to run at an 86mm ride height (with a comparable 25.2" front tire) which is a full 1.4" (37mm) lower than GT2 height. A ride height of 86mm will have your front tires tucked up under the fenders quite a bit and will limit the steering angle of the front wheel to a certain extent unless you are running Cup spec camber which is -4.5º. I am at 98mm ride height with -3.3º and had to roll my front fenders to have adequate clearance. Just more stuff to think about...

 

No worries. Your help and input is appreciated. My goal will be to bolt in the shocks sans springs on the front and aft and with a jack I can check the shock travel to determine droop versus ride height and compression. What are you seeing for droop versus ride height with your setup?

The cup shocks have a travel of 106mm front for the front and 111mm rear. This is taking into account the stock 70mm bump stops. So add 70mm to each for total travel of ~7". Speaking to ZF Sachs engineers, the bump stops can be shortened if needed to 30mm if using a stiffer bump stop. This will not help the droop obviously. If the goal is to mount the shock so that you have equal extension as compression than I can see that being a difficult goal. The bottom line is i need to bolt them on and check travel. I think with lower than 1000lbs springs that the travel could be a real issue. I may go higher than 1000R/900F too if needed. This car will not be daily driven so ride quality is not priority.

 

I have about 2.5" of droop in the front and a little more in the rear, probably 3". I can get an exact number for you next week as my car is at the shop right now but I'll be working on it on Monday. I would definitely want at least 2" of droop at a minimum for the street. If you are primarily setting your car up for the track than I would go stiffer on the springs. Even with 1400/1500 I find the car a bit soft when pushed hard. You have to remember that our cars are about 600-700lbs heavier than a Cup. Running stiffer springs will get you closer to what the shocks are set up for to begin with as the factory springs are 1370/1484. Additionally, the stiffer the set up will be, the less of an issue you will have with bump steer.

 

I measured the wheel well ride height on a 996cup for you today. With Pirelli 245/645/18 (25.55") and 305/680/18s (26.9") the front measures 24.0" and the rear 24.5" from the ground to the top of the wheel well arch with the car aligned to spec.

 

Awesome!!! Thanks for sharing! My friend who has a cup seems to think we should be okay with a 1" spacer as long as we don't go too low on spring rates. But, he thinks I need to test travel without springs.