Chris, I had considered "anti-dive" geometry as a source of investigation but this is a very momentary, transitionary, and very high-speed specific thing I am feeling. I still believe my setup values are the source and it's not a deeper foundational chassis design issue.
Once I have eliminated the toe change idea I will go to what I think is the real problem. I still am running 900-pound springs on the rear, 750F, and the original QA-1 shocks. The shocks are at the functional limit of their adjustable damping range with both compression and rebound settings almost at the full hard limits. I think more R rebound and F compression would be my first thing to do if I could.
AND I really can't add any rear wing (I'm at 8-9 degrees of AOA ) because of high-speed download suspension compression. Again too light springs. The reason I have not done this is the car is really good on slower tracks with 110-120mph straights and slower speed corners.
So in the end I will need to more than likely, re-valve or replace shocks, go to about 1050R/ 850F springs, and add rear wing AOA. Maybe I need two sets of shocks and springs. Then just swap them out for the track I will run at. Lots of work and money but, it is what it is.
Rodger, I don't think my car makes anything like the downforce that it would take to cause F1-style porpoising. It's not an up and down thing, it's more of a rear-steer side-to-side feeling as the suspension aero unloads. And it is a feeling and it only lasts a second or two. Not a real controllability problem that requires a lot of steering correction. But it's there and it does bug me a bit.
I'll get it.............I like this stuff and it's why I bought my SLC in the first place, so I could enjoy the journey of the development of the car. Kind of a poor man's version of what it must be like to learn how to race a real prototype.