Light at speed

If you want to keep the current setup, I'd say aero, probably a splitter or canards to keep the front end down. Doubt a quicker steering rack will improve the twitchiness til you get the front end to stay down.

 

Just got home from a nice 70-mile drive in the country--hit 120mph a few times. I get scared at that speed because you can feel all the imperfections in the road. And, in the country the roads are not perfect.

Had her up to 160mph on a highway once and I swear my car was going to start flying....LoL....Not sure what vMax on a 355 is!

Maybe it's just my perception, though, because I know Ferrari engineers these things to handle high speed.

Maybe I'm just a wuss.

Either way, I do agree with what you are saying about the lightness at speed.

 

Point A) A car cannot be "perfectly stable" and light at speeds over 125 MPH.

The 255 front tires are part of the problem, 230mm larger than stock, giving the front end more grip, inducing oversteer. You can balance this by putting 295 or 305 rear tires (on wider rims with the proper offset.) Generally I recommend lowerng the rear for cars that are light at high speeds, and by lowering I men on the order of 1-2 millimeters, but, here, I think the problem is too much tire on the front.

There is a high speed instability under brakes when the fron end is too low caused by a shift in the aerodynamic center of pressure as the nose drops under braking. This is cured by raising the front 1-3mm depending on the roll axis inclination.

Thus; point B) we have insufficient information about your setup. What we need is:
Front ride height (left and right)
Rear ride height (left and right)
Front wheel load (pounds; left and right)
Rear wheel load (pounds; left and right)
Rear tire size
Front shock compression rate, and rebound rate
Rear shock compression rate and rebound rate

Can you describe the front end lightness:
does the front end wander around itself?
does the rear end wander and you have to find the rear with the steering wheel?
could you tell the difference?
Does it feel better under brakes or under acceleration?
Does it go away as you reach higher speeds?
How does the front end respond to steering input as you nose into a 100 MPH sweeper?
How does the front end respond at corner exit with power on? Does the car powerslide out of the corner if given enough throttle?
How much steering wheel angle is required to catch and hold the powerslide?

By lowering the car you have altered the underbody aerodynamics, altered the CoG, and changed the static suspension geometry. By changing the shocks you cange how the car reacts to dynamic inputs. Any of which could cause (or cure) your medium speed issue (high speed starts over 150 on these cars).

 

I am surprised, I thought the 348 was blamed to be some what twichy at high speed and the 355 really stable instead ....

 

I've been there. I, also, like the car setup to just barely oversteer (upon provocation) in a turn like the carrousel.

Have you looked up under the car and checked the status of the under trays? (clean and smooth? fully attached?) I saw a 360 at MSR Boney with the undertray detached, fluttering in the wind. Turns out the driver had been chasing a handling problem all day. I found it for him simply by watching.

Do you have a wing on the car? Too much rear wing can change the rake under the car at speed, this changes the aero center of pressure and upsets the balance. To compensate, you will want/need a splitter on the front. In my opinion, the splitter should be open in the center to feed the airflow under the car.

Are you running a tad of negative toe (like a street car) or a tad of positive toe (like a race car)?

What setting did you set on the shocks? And are you running stock/street springs. Stock springs with challenge bars require the shocks to be set on the firm side in both rebound and compression. This will over damp the longitudinal motions of the car and under damp the latter motions of the car; best comprimize you may have at this point.

While you are in there, this winter, changing the rack, you should look over the caster on the front hubs, and make sure that camber is equalized side to side.

So, lets say the under trays are just fine, and everything else checks out.

At this point, if you have a splitter, I would start by either lowering the front or raising the rear (depending on where they are currently set.) The rear end is much more sensitive to ride heights adjustments than the front--that is fewer rotation on the spring perch result in more change in overall balance.

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My car does this (wander at speed) to a certain extent, especially when I am not concentrating on holding my hands gently on the wheel. When I just barely touch the steering wheel, my car does not wander (well maybe about 10% of what it does when I am using the steering wheel to hold my hands and elbows up.)

Most of the time when the car wanders at high speed, I compensate for this by holding the steering wheel very lightly. Rational: Since there is nowhere to rest your elbows in the cockpit (unlike a Vette, or Viper), the undulation of the road surface can bounce your arms/elbows up and down, this leads to unanticipated steering inputs, and the car reacts accordingly. I like to hold my arms such that my muscles in the sholder are holding up the arms, with the hands resting on the wheel with no more than a half a pound of pressure (straight) listening to the front end through the sensors in my hands. This minimizes the unanticipated steering inputs from the driver.

 

The 355 is much more stable but the ride height and alignment have to be right.

 

After messing with tire sizes and suspension parts, did you make sure chassis rake is the same as stock?
With the design of the underbody, this is critical.

 

Its even a little more complicated than what you indicate.

At stock ride height, there is a particular rake (front to rear ride height difference) and a particular roll axis inclination (line between front and rear roll centers.)

The rear roll center moves 1.6 times as fast as the front, so as you lower the car, the rear roll center goes lower significantly faster than the front--even when lowered exactly the same amount. So rake and roll axis inclination become independent of each other.

To compound the rake and roll axis inclination issue, the underbody aerodynamics is dependent on the rake and ride height. Lowering the car has increased the downforce on the car, but altered where the downforce is being applied. And as speed increases, the aero forces on the nose of the car increase, and the donwforce on the rear of the car increases, the car starts to wander. Once the nose comes down under braking the aCoF moves forward (to mid-car) and all seems OK-ish (stability wise).

So, I come back to:: you have two choices, lower the front or raise the rear. In the past, I have had more success, faster, by raising the rear.

The third choice of adding a splitter should still on the table.