Fuel "Quality" vs Octane Rating

[SL8R]

Originally Posted By: Audi Junkie
I just don't want any regrets if I buy cheap super 93 over TT, mixed to 91-ish or Sunoco 91, which I get cheap from a rebate programme. My grocery shopping card is loaded to get $.60c off right now, usually less though.

The jist of the posts so far is that there IS NO difference in brand quality...?

All I care about is intake valve deposits on my DI turbo. I understand low grade gas may be to blame, when it flashes off the hot intake valves during VVT overlap.


Are you sure you understand how "direct injection" works?

The problem with intake valve deposits on the DI engines is due to the crankcase vapors that are recirculated by the PCV system, not due to low quality gas.

The fuel gets nowhere near the sites where the intake valve deposits form (unless you're hoping that it somehow survives the combustion process and is recirculated to the valves via the PCV system - highly unlikely).

If I was you I would worry about using an oil that has low volatility and will stand up to the fuel dilution.

It is brave to essentially void your warranty by installing the ecu tune on that engine.

 

[Audi Junkie]

Let me help you out.

While the valves undergo VVT action, there is an overlap where some of the fuel gets pulled back into intakes and it flashes off the hot valves.

There is no "warranty void", not only because I can switch between 4 different programmes right from my cruise stalk, one of them being stock, I can also lock it in with a code that locks the ecu with a firewall to any intrusion. It's like 1000-bit encryption, they can't see past the stock programme.

 

[SL8R]

Originally Posted By: Audi Junkie
Let me help you out.

While the valves undergo VVT action, there is an overlap where some of the fuel gets pulled back into intakes and it flashes off the hot valves.

There is no "warranty void", not only because I can switch between 4 different programmes right from my cruise stalk, one of them being stock, I can also lock it in with a code that locks the ecu with a firewall to any intrusion. It's like 1000-bit encryption, they can't see past the stock programme.


I must be missing something. When you say "overlap", are you talking about overlap between the intake and exhaust valves? If so, my understanding is that the overlap between the intake and exhaust is done in order to increase the cylinder scavenging effects by drawing intake air into the cylinder NOT by allowing cylinder gases to be "pulled back into intakes".

Do you have a link to a reputable resource that explains the VVT operation you're describing?

 

[Audi Junkie]

Not really. It's second-had from an expert though. Near the end RI_RS4's deposit thread, iirc. I think I've heard it from other sources too. I just don't want any regrets.

The oil situation is under control.

 

[Spyder7]

I may be wrong on this, but this is a good place to ask:

My car calls for "87 or higher." Price differences are negligible here, so I run 89. Went back to 87 and my mileage dropped and there was some tick, but I'm not sure if it was knock or lifter tick. 89 is in and all is well again.

By the way, this change was verified by Scangauge ECM readings. I don't know if the VVT-i can take advantage of higher octane and squeeze more out of it, though either way I will stick to 89. It did lead to wonder if the VVT-i and ECM is sophisticated enough to take advantage of 91. I can find out, by experimentation after this tank and objectively using the Scanguage, but looking for opinions here in the meantime.

-Spyder

 

[The Critic]

Originally Posted By: Spyder7
I may be wrong on this, but this is a good place to ask:

My car calls for "87 or higher." Price differences are negligible here, so I run 89. Went back to 87 and my mileage dropped and there was some tick, but I'm not sure if it was knock or lifter tick. 89 is in and all is well again.

By the way, this change was verified by Scangauge ECM readings. I don't know if the VVT-i can take advantage of higher octane and squeeze more out of it, though either way I will stick to 89. It did lead to wonder if the VVT-i and ECM is sophisticated enough to take advantage of 91. I can find out, by experimentation after this tank and objectively using the Scanguage, but looking for opinions here in the meantime.

-Spyder

Have you tried the test with 91? I'd be curious if the benefits stop with 89, or do they increase with 91.

 

[Ken2]

Originally Posted By: eljefino
Originally Posted By: getnpsi
Also would like to add that many pre-oil crisis engines had high compression using iron cylinder heads. The ignition systems were nothing great. They needed the octane to run even in light load conditions due to the 10-11-12:1 ratios, something that is getting to be more commonplace now but it took 40 years of technology and using better materials in everyday engines to get there.


+1 "regular" leaded gas was 89, why that grade stuck around.


A different method was used to calculate the octane number way back then. That was the Research Octane Number which gives a higher number. For many years the U.S. and Canada have used the average of the Research Octane Number and the Motor Octane Number. These are different tests under different conditions. RON gives a higher number; MON gives a lower number. (R+M)/2 is the number you see on the pump.