Here's something, although I don't know how well it actually answers your question.
"I'm sure you remember that I own a 2.24v, wHich I love. Since I wrote last time a lot of things happened. One of them was really bad. I will not fully explain now, what happened, because it is a very long issue, but I beg you to give this advice to all Biturbo owners on your pages, that 2-litre engines up to 01/92, cannot, must not, use unleaded fuel.
Worst still if you use 100 octane fuel. Bad if use other level of octane. Maserati never explained or cared about this issue, and people like me who drive there cars, I still do 30,000 kms per year, end up suffering the consequences.
Even if you use additives the problem will happen!! When I used Shell V-Power 98 ron with Shell Millenium additive, the problem was there but it only showed after 40,000 kms or so. I know this because I did almost 220,000 kms with my 4.24v, and started to use unleaded fuel in the last 20,000 kms or so, then I sold the car."