Wow!! That explains it. A few years back, my wifes new 4cly Wrangler had piston slap going up a steep grade when using cheap fuel. So the engine must have been worn out at 1700 miles. Funny, but when she switched to premium fuel, per the Jeep Service Mgr, the slap went away. The better gas must have rebuilt the cylinder walls!! Thanks for your insight and knowledge.

 

Yeah, it must have. Or, in your impressive 32 years in the auto industry, you didn't learn the difference between piston slap and engine knock, which is what you described.

 

Slap / knock, I've heard it called both. Some say patato some say pototo. Some call it a cross tip while others call it a Phillips.

 

Potato/potato refer to the same thing. Cross tip/Phillips refer to the same thing. Engine knock and piston slap are by definition two totally different things.

 

Ok, you win. Where do I send the prize.

 

Fuel Octane, Choosing The Wrong Octane Will Cost You | What Could Be Greener

"What about using lower octane gasoline in a high octane engine?

In this situation, you will see negative effects that could be even worse. Using low octane fuel in a high octane engine will result in severely reduced performance because the engine will attempt to adjust to the lower octane gasoline. In extreme cases, or with prolonged use of low octane gasoline in these engines, pinging or pre-detonation can occur and can eventually destroy your engine. Pre-detonation causes very hot conditions in your engine and can melt sparkplug and pistons.

What fuel you use in your vehicle is important. Make sure you always follow the manufacturers recommendations. Using a fuel other than what the manufacturer specifies will in no way help you save money, gain power, or do anything other than cost you money."

 

As I have said, there are engines out there that MUST run 91 octane. If you run 87 in those, damage is very likely to occur (almost guaranteed).

The Volvo engines do not REQUIRE 91 octane. They are not "high octane engines" as referred to by that quote. They are tested using 91 octane, so the statement in the owner's manual is there so that driver's know that the performance numbers (power and fuel economy) are from tests done on 91. There is no requirement to run 91. Running 87 in a Volvo will do NO DAMAGE.

You made a big deal about how I am just a dumb mechanic and the engineers know more. Well what about the link I posted which had statements from Volvo about this? Or what about the statement in the owners manual about running 87 not having an affect on reliability? You can find generic information on the internet all day to support what you believe to be true. The fact is simply running 87 in a Volvo will not cause damage.

 

I have been a Front End General Sales Manager, Factory District Manager, and Factory Port Supervisor, helping to keep the Dealers Profitable so that the Techs would have Jobs. If they don't sell out the front door, they don't return to the back door for service. I've yet to see a Dealer Service Dept that could afford to stand alone, so yes, my 32 years was quite impressive!

 

That's great. Because the people on the sales end are definitely the ones who know about the technical stuff. That would explain why you think piston slap is engine knock.

 

EST6, I have not wasted my time reading any link that you posted. MY owners manual has no such statement, nor does the article that I linked to state that the 91 was a REQUIREMENT.

As I said above, you win. Just let me know where to send your prize.

 

I give up! I never equated low Octane Piston Slap to Knocking, You brought that up. I'm with rspi....You get the Prize!

 

rspi- You asked for some proof. I posted it but you didn't read it. It is directly from Volvo. You want to argue this but don't want to look at both sides of the arguement.

Your quote said for "high octane engines". That means am engine that must run on high octane. That would make it a requirement.

You are correct, your owner's manual does not have that statement. However, this question is about a newer car. Since you want to make this personal, I will. I know you like to jump around this site with your guesses and assumptions, but maybe you should not get involved when you don't actually know the answer. Not all Volvos are like the old ones you like. You throw in digs about what I make vs. an engineer. But who is a backyard mechanic to tell me what I do or do not know? As one of the top 90 Volvo mechanics in the US, I don't need to prove myself to you. I get paid well for what I do. I choose to be here to help others, not you. I don't know why you have such a problem with that.

Mike- there is no such thing as "low octane piston slap". Piston slap is a defined term. It is totally different than knock. You are using them interchangeably when they are not the same thing. You specifically said "piston slap is caused by low octane" and that is simply not true.

 

EST6: I will PM you with my personal info...

I use 91+ so I really don't need the info. Glad it's there for people that do.

 

I'm with Rspi.

If your manual says to run 91, run 91. Your engine will run happier and better. You can go cheaper but like Rspi said it's likely if you load it much you will end up pinging and the knock sensors will retard the timing to take the knock away and then you end up burning more fuel for the same power because you aren't able to take advantage of the spark advance possible with a higher octane fuel.

 

i just moved to N.C. from Mich. and I am seeing tons of 10% ethanol gas in the NC. area. I use the premium 91+ because it is the only grade with no ethanol in it, even then only 1 brand has none in it. In my past bmw 525 wagon and cadd. Escilade I got better gas mileage with premium ,just enough to offset the cost. so I run it
I hate this ethanol It tears up rubber hoses and plastic parts in the small engines ive seen. I read that ethanol attracts and hold water.
It cant be good to have that ethanol **** in my car!

 

As long as it's moving daily it's no problem. Just don't let it sit in there for a long time, like a few months. If you ever store the car find some gas without it or add Sta-Bil or something.

 

It cant be good to have that ethanol **** in my car!


Well, up north people pay good money to purchase gas line deicers like the product "Heat" and it's isopropanol which holds more water. In smaller amounts ethanol isn't a big deal and it does help the gas mixture burn cleaner. Your lungs are happier.
It also helps the gas go further because it's diluted. Not your car go further but we end up buying and using less crude.

Maybe it's harder on older equipment but the newer stuff is designed to run properly with it. Small engines too.

If you were ever in an area like the LA basin back in the 70's you'd think ethanol was sent by God.

 

Yeah, 10% ethanol is no problem. Volvo even has a bulletin about it being totally fine on any Volvo engine since 1976.

 

It's pretty clear to me you don't know the difference between piston slap and knocking. This is what you wrote: "Wow!! That explains it. A few years back, my wifes new 4cly Wrangler had piston slap going up a steep grade when using cheap fuel."

That would be knocking, also known as detonation, which is completely different from and has nothing to do with piston slap. Piston slap is caused by the pistons rocking in the cylinder bore.

Just read this entire thread and ES6T is right about everything.

The energy content of gasoline is the same regardless of the octane (higher octane fuel has very slightly lower energy content actually), so any mileage improvement with premium gas is a result of confirmation bias (wishful thinking) or driving with a lighter right foot. Yes, you can argue that the timing is retarded with lower octane fuel but that's only going to happen under high load operation. Most of the time the timing will be exactly the same with both 87 and 91 octane fuel.

Also, there's nothing inherently "better" about higher octane gas. Higher octane gas is more resistant to detonation allowing it to be used in engines with higher compression ratios, that's all. Some gasoline suppliers put a different additive pack in their premium gas, but that's at least as much a marketing gimmick as it is a real product improvement.

 

Huh? Knocking is "pre" detonation. Plain detonation is the actual explosion that takes place. And if you think there is no difference between 87 octane and 93 octane you'll never buy synthetic oil...