I was talking to my marine mechanic brother in law last night, and he made an interesting comment about premium fuel. I asked him if I should be running premium in my LR3, and he said no, and that it would actually do more harm than good due to detention changes. My Audi is turbocharged and has carbon issues, so I think the premium is cleaner for that car, but I have no idea if the case made above makes any sense. FWIW the gas mileage delta from 87 to 89 seems negligible. Thoughts?

 

Not sure what engine you have, but my LR3 4.4 requires premium. A lesser grade results in lesser economy. Also there is a lot one needs to know about premium and other grades of gas. Its not necessarily cleaner, that is just marketing. It all comes down to ONE thing, octane rating and the compression of an engine. The higher the compression, the greater the octane needed. Racing fuels are generally 110. So putting premium in a vehicle that does not need it will result in NOTHING. But putting regular in a vehicle requiring higher octane WILL result in reduced performance and economy, the economy being a side-effect of the reduced performance. The knock sensors can hear a lot better than a human can, so even if one does not hear knocking it does not mean its not happening. Of course knocking in the long run can cause engine damage, but modern engines hold up better than older ones.

 

You want premium.

Any savings at the pump will disappear with worse gas mileage because the computer is pulling extra timing to combat the knock as well as you'll have less power.

Without premium you'll knock because the lower octane will prematurely detonate given the compression ratio. And eventually slap a piston into your valves, then you're paying thousands in repairs to save $2 at the pump

 

Is that really the case, the engine designers design for compatability and on some engines it is the lower rated fuel and not the premium type. I have a 4.0 litre V6 petrol, all documentation shows a 91 octane fuel is best for this engine as this relates to the design parameters. Yes you may get better mileage from premium but do you need it.

 

yes, because of compression ratio and engine design. "Octane" is how different fuels are rated based on combustion temperature, the higher the octane, the higher the burn point. The more compression you have in a motor, the higher the temperatures and pressure are in the combustion chamber.

Running 87 or 89 in a high compression ratio motor (like the 4.4 in the LR3 at 10.5:1) will cause it to predetonate and knock. The computer pulls timing to combat this, but only as a short term solution to allow you to safely get to your destination and fix it. It is not a permanent solution.

If you continue to drive with pulled timing and knock, the predetonation will win, you will slam a piston into your valves, you will ruin a head and potentially the entire engine costing yourself thousands in repairs...all in the name of saving $2 per fill up.

There's plenty of places to save some operating cost with an LR3, things that go into and effect how the engine runs isnt one of them.

 

So you're suggesting running 91 octane vs 89? I thought mid-grade fuel was sufficient.

 

OP - I'm still trying to figure out what your brother in law is referencing with the term 'detention changes'? What exactly does that mean in the terms of increased octane numbers?

As for your Audi having carbon issues, I'd be more interested to know if the model you have is Direct Injection or not...more than likely that's your issue and it has nothing to do with octane numbers.

 

If the ecu is smart enough to retard timing to compensate for lower octane; then how can continously running 87 octane harm the engine? Other than less power and mpg?

 

It not that it smart enough, its that it is listing for knock. So for it to retard timing, knock must happen. Engine knock is bad, very bad. Im not certain if there is live data for the knock sensors, but one way to test fuel grades would be to see if they are being tripped at all. But physics is physics.... so....

 

Yes, detonation(knock)is bad.
But, since it has knock sensors, then it's designed to retard the timing accordingly.
So then there should then be no detonation, and no problems.
Many D2 owners here have reported running 87 octane for years, without issue.