It’s kind of a mess to balance, but maybe a simple way to do it might be to look at the amount of engineering time added with each point (or each three points, or each five points) of quality, and base the unlock dates on how much delay that adds to designing the engine under default engineering settings. So if a +15 in fuel systems adds six calendar-years of engineering time (or 288 ET, if I’m remembering the conversions right), then maybe you unlock electronic fuel injection six years earlier (or possibly nine or twelve - some multiple of six) if and only if you put in that +15. And the net result would be similar in effectiveness to +/-0 quality EFI built at the standard unlock date.
It makes things a lot more complicated, of course, but if seems like the complexity might be worth it, it might be fun to try. And from a UI perspective, quality-locked features might be greyed-out with a minimum quality indicated once they become provisionally available.
Let me make a counter-proposal, which adds a layer to Sandbox that we are adding to the Lite Campaign Mode.
When in the Sandbox Mode’s manager, you have a new button somewhere in the UI, which takes you to a tech pool distribution window. In that window there are 16 selectors, one for each section of the car and engine design to which you can distribute tech pool points. Every step up in tech pool costs more points to unlock. 5 tech pool in Bottom End costs you 5+4+3+2+1 = 15 points. In total, across your company you have something like 70 points to spend.
Whenever you export an engine or a car, a company tech profile would be exported with it so that the results are reproducible elsewhere.
That system would keep the game mechanics consistent across the game and give Sandbox Mode a new mechanic for interesting choices to compensate for the loss of the engineering and factory section (which will be tycoon mode only).
What about being able to change that amount by yourself while in the game? I assume that’s not going to be possible, but I think it should be since it’s a sandbox mode.
I’d rather have it either off or locked to something reasonably (you’d be able to choose that). You know what would happen with “choose yourself” it’s like giving children the cookie jar and say, yeah, take it to your room but you’re allowed to eat only one. The general flow of vomit later that day won’t be fun for either I wouldn’t mind having the free selection as an option though.
From my quick tries I think 70 is a reasonable number, higher gets too spammy 
