I am driving a car designed to run on 87 octance gsoline. The regular gasoline (in the USA) now contains 10% ethanol. Now, some gas stations are selling the 85% alcohol blend. Could I use this in a pinch, or would it damage the engine?
Check owner’s manual. The vast majority of modern cars can run safely on E10 or E15 (10% and 15% of ethanol respectively) with no modifications, but to run on E85 it has to be designed that way.
Flex-fuel cars (as they are called) have metal fuel tanks (instead of the more common plastic ones) and upgraded fuel lines because prolonged exposure to alcohol can damage those parts. They also have different ECU mapping.
This wikipedia article lists some flex-fuel cars: List of flexible-fuel vehicles by car manufacturer - Wikipedia
I’m not sure what kind of pinch you’d be in where there was E85 but not regular gas avaliable… but in a serious “chased by an axe-murderer”-type situation, your car will run on it, possibly very poorly, but doing so habitually will sooner or later damage your fuel system and possibly the engine itself.
If your car isn’t flex-fuel, just don’t do it.
If you stick the E-85 nozzle in by accident once or twice, you’ll be okay. Your engine won’t knock, but like Dog says, your setting yourself up for long-term fuel system damage.
E-10 or E-15 is not a problem.
If you are being chased by GreasyJack’s axe murder and he has a big still full of moonshine (pretty close to E100) out back, you can dump that in the tank and take off. Otherwise, like the others said, if the car isn’t designed for E85, it’s not a good idea.
They started commonly adding ethanol to fuels in the 80’s, so cars made since then usually won’t suffer too much on E85. Cars made earlier than that can have worse problems, depending on what materials they used. Ethanol eats away at the aluminum and magnesium they sometimes used for engine parts, and eats away at cork gaskets and certain types of rubber and plastic. The damage does take a while, though, so you can get away from your axe murderer with no problem.
An old carburetor style car is going to run very poorly. A very simple adjustment to the air-fuel mixture (usually just a single set screw) will make it run a lot better. In fact, aside from a slight loss of power (E85 doesn’t contain as much energy per gallon as gasoline) you might not even notice the difference once you’ve got the air to fuel ratio just right. You don’t even need any fancy tools or measuring equipment to adjust it either. Just rev up the engine a bit while you turn the screw. When you get it smooth, you’re close enough (Side note - Try not to let the axe murderer catch up to you while you adjust the engine - blood in the carburetor doesn’t help). A carburetor style car will not adjust itself automatically. You can set it to run smoothly on gasoline or you can set it to run smoothly on E85. Whichever one you set it for will make it run poorly on the other.
A more modern car is going to try these adjustments automatically, but if the engine computer isn’t designed for E85, you may confuse the engine computer into thinking that it can’t properly control the engine. You’ll be able to limp along, but the car won’t run well. If you want to run on E85 more often than just in a dire emergency, you can reprogram the ECU on some older cars to handle it. You may need to replace certain parts like fuel lines and gaskets as well.
As was already mentioned, flex fuel cars can handle either fuel with no problem.
Let’s say I accidentally put the E-85 nozzle in and pump a gallon, realize my mistake, and pump in 15 more gallons that’s E-10. Is this going to be roughly the same as 16 gallons of E-15 and therefore OK? (If I did the math right, it’s close enough). Or is there more to it than that?