MattiasH

Out of curiosity. Which spring rates did you start with and which did you change to?
I know that spring rates can't be directly copied to another car, because of several different parameters, but it is still interesting to see how it compares to other track day and race car spring rates.

Speedtoys

It's far from wont compare, a comparison will be difficult. Greg can explain more, it he wants to go into details.

GregBBRD

Yes, what I'm doing has probably nothing to do with any other 928 on the planet.

All 928's with stock upper mounting perches and stock length shock bodies are touching the bump stops at 140mm ride height in the front and 160mm in the rear.
What this means is that unless someone is running their car at stock ride heights (very unlikely, since almost everyone wants that "lowered look"), the spring rates installed on any of the lowered 928's becomes almost completely moot.
As soon as the suspension travels below ~120mm (front) the bump stop becomes the dominant part of the spring rate....and the spring is just there for the ride.

I completely re-engineered the upper spring perch and the mounts, to be able to get some usable linear spring rate with a lowered 928.....just because of the above issue.
My front bump stops do not even touch until the car is at 90mm (measured in the stock manner) and don't really contribute much to the spring rate until the front is below 80mm.
In other words, I'm using the bump stops much like Porsche intended, with the cars at stock ride heights, but I can lower the snot out of the car, before the bump stops "take over" the major part of the spring rate.

The advantage of this is that as the car rolls in a corner, the springs are allowed to control that roll in a linear manner.....making any roll induced understeer or oversteer virtually non-existent.
Super easy to drive and very predicable anywhere on the track.
There's never any "OMG moments" when the total total spring rate (actual spring rate, bump stop spring rate, and rubber "twist" spring rate all combined) goes to the moon, from body roll.

In terms of actual spring rate numbers, it's a work in progress.....and we will continue to increase it.

These pictures will give you some idea of where we are headed:


Highest "G" loading high speed corner on the "green" track on Friday AM with springs as delivered.

Highest G loading high speed corner on the "green" track on Saturday afternoon, after a large step in spring rate.






.

Rob Edwards

Another test and validation day yesterday:

Darklands

Sounds like an grown up 928! Maybe she eats expensive playthings on the track!

Well done, Greg!

GregBBRD

Like anything built out of so many "one off" custom pieces, it's a work in progress.
We went out to Thermal, yesterday, to fix the driver's seat "up and down" position switches and remove the bolster under the steering wheel. These two items will make the car much faster, as the seating position and knee position have been intolerable, for being able to turn fast laps.

The shop at Thermal also would not allow the car to run until an "oil leak" had been repaired.
The "oil leak" turned out to be brake fluid, which had apparently boiled out and sprayed out of the vent hole in the cap for the reservoir.
We changed the fluid from ATE 200 to Castrol SRF....that will fix that.
We also changed the alignment on the left front tire, as there was too much negative camber on the left front for all of Thermal's different race courses.

Many thanks to Rob Edwards, who has, long ago, added "master 928 mechanic" into his long list of life accomplishments.

Rob Edwards

And another four ~20 minute sessions of testing, on the big track configuration (~5 miles/lap) on Sunday, more suspension tweaks and additional ignition timing while monitoring temps and knocks.

GregBBRD

The owner isn't driving the car this week (there's a Thermal Club M2cs event this weekend and practice the rest of this week.)
So, the car is coming back here, so we can do another spring change. (Amazingly enough, there no alignment equipment at Thermal.)
And make some changes to a few other minor things.

IcemanG17

Sounds great.......but something about race 928's typically do........granted I had a little 4.5L semi-euro but man did it sound good at redline (lowly 6k)

GregBBRD

This car still has my three muffler, street exhaust on it...
Which makes it really quite fun, when watching the faces of the people in the hot pits.
It leaves, sounding like a Prius, but changes into that, at higher rpms and load.

Dave928S

Awesome sound!!!

GregBBRD

It's definitely a screamer, when "on the pipe".
We're going to remove the rear (most restrictive) muffler, this trip to the shop.

While the owner is not concerned about "lap times" (he frankly could care less), I quietly took some lap times, yesterday, when running the "long course" (5.1 miles.)
The owner has a collection of all of the BMW "M" cars ever made...including a couple of the modern BMW competition cars.
This "rock" of a 928 is with-in a couple of seconds of his best BMW's, on this course....running at 80% and not using the ABS, yet.

She's going to be very fast, me thinks.

Dave928S

Fantastic work Greg.

GregBBRD

Thanks Dave!
Since there was so many pieces that I had to "invent" to make this work like I wanted, it's been a long, tough road to get to this point.
And while it has been many years since I've been to any racetrack (by choice), it's kinda fun to be going out and dialing this monster in.

The engine has not been over 6800 rpms, to date....just breaking it in.
Counting days and laps, I'd guess that it has about 500 track miles on it.
I have an odometer reading when it left here....I'll have to take a look when it gets back here, this week.
But I'm guessing it is about time to "turn up the wick".

There's a huge amount of GT2/GT3 variants out there. One guy has 12 identical GT3's....all painted a different color, for his buddies to drive (they call the cars "Skittles".)
There's everything from 964 Singers, 993 GT2's, to the latest 992GT3 variants....literally hundreds of them.
This is the only 928.
It's great fun to watch the guys with all those trick late model Porsche rocket ships take note of this car and stand in the hot pits watching it go by.
And more than one has asked if this is one of one, or if I'd be interested in building another.

I'm not sure.
That may spoil the uniqueness, at this one track.

Jadz928

I for one just creamed my jeans. Count me in for seeing how this beast shakes out!