So if I get e85 at the pump, what’s the octane of the gas they’re diluting the e100 with?
For example, to make e40, I’d use “e85” + some amount of e10 gasoline… does it matter if it’s 87 or 93?
Those are two very different questions from my perspective.
If you take E98 from a pale, you can mix it with whatever you want. You can mix it with 93 and get a very high quality high octane blend all the way down to 40%.
If you get E85 or E54 from the pump, what they cut that ethanol with is unlikely 93 octane. Your guess is as good as mine. I haven't been able to find any reliable information about it to answer but I have observed instances where octane was questionable in something testing at 60 or 70%.
*Edit* I reread your question. Yes it does matter if you blend down with 87 or 93. If you use a lower octane, it will lower the end resulting octane rating. Does it matter so much that you need to worry about it? Well, that depends on what you're trying to achieve. If you're setup doesn't need a lot of octane to avoid knocking, then no, it doesn't really matter. If you're trying to get as much octane as possible because your setup needs it to not knock, then it does matter, a lot.