If I lived in Steven's Point, I'd fill 'er up with Point Beer. :lol: I drank quite a bit of that stuff in College. As I recall, it was "high-octane".



Kenny

 

Over the weekend, I have read hundreds of posts regarding fuel octane levels. When I picked up my 2000 B5 V6 variant in May, my local indie VW mechanic told me to fill it with AKI 89 octane. I am now convinced of the benefits of premium fuel and will soon make the switch from 89 to 93--we don't get 91 in Wisconsin. However, I still have some questions.

First off, let me try to summize what I have gathered from various discussions on this forum:
- The owner's manual states that 91

***Leaving this computer, will continue shortly from another terminal***

man he bookmarked a thread

 

Kenny, I'm sorry to tell you this, but the Point Brewery has a new brewmaster. Point Beer just isn't the same....



Haha, not really, Scott. I was kicked off a computer here at work, and saving what I had seemed like the best choice. My other options were to copy/paste what I typed into an email to myself (which I didn't really have time to do), or just retype the whole thing. Laziness prevailed.

 

lol

 

Here's a good post by Boris regarding the ECU, knocking, and timing: http://www.passatworld.com/forums/showpost.php?p=947986&postcount=12

 

Lower octane fuel reduces the car's power and efficiency partly because the engine will retard the ignition timing and partly because the turbocharger will have to limit its boost and reduce the compression ratio inside the cylinders. The engine internals are all iron, though, so a little knocking from time to time won't really hurt anything. If you haven't noticed knocking, it's unlikely that there's been enough of it to damage the engine.

 

knocking ---
it is from the gas pre-igniting from a hot spot on the piston from using bad or wrong octaine fuel.
the knock is a result of combustion happening before the piston is tdc, it is still on the upstroke.
here you read about it...
http://www.streetrodstuff.com/Articles/Engine/Detonation/index.php

 

My 2001 owner's manual and gas cap door clearly recommend 91 octane, but the sales person said we could use 89 octane. Of course, this is the same dealership that used bulk 5W-30 Castrol dino for free-and-worth-every-penny 1.8T oil changes.

I use 91 octane from a reputable supplier, such as Chevron or Shell, and don't lose sleep over gasoline. (We don't have 93 octane in California, and I am not convinced the Passat is designed to take advantage of it, anyway. My 2003 Dodge Stratus is designed for 87 octane gasoline, and that's what it gets.)

 

Hmmmmmmmmmm

Ask me about Toulene

oke:

 

My past experiance with the V6 (modified - intake, chip, UDPs: in SW FL) was to run 93.
Time of year didn't matter much, other than occasional "flutter" under heavy acceleration during summer months when the temperature and humidity were in the high 90's.

 

I get better throttle response and slightly better mileage with premium on my V6. The mileage is not quite the 10% difference that the 1.8T gave, but every little bit helps.

I doubt long term use of the 87 in a V6 is anything to worry about. If it was, we would have heard about it by now from multiple users.

 

I use 89 in my 1.8T 2001 with no problems and I have a 1 bar Wett chip. I use 93 when they have it sometimes but 89 works fine with an additive every 2,000 miles.

 

With the 1.8T, you are just wasting your money on 89. It will pull timing and power once it is warm. It will be like driving without the chip. The mileage will also drop nearly 10%.

 

Regarding that second point, I have read a few posts that state that the timing is being continuously adjusted. This boggles me. If the computer realizes that the fuel in use is causing the engine to knock, then why would it repeatedly attempt to correct the timing (and cause more knocking in the process) rather than remain at a retarded ignition setting until the engine is turned off. Then, at the next start-up, with the car not knowing what octane-rated fuel you may have just pumped into it, the engine will again adjust the timing for maximum performance.

Before predetonation actually occurs, the speed of the flame front in the chamber begins to rise. The ECU can measure the speed of the flame front ("deflagration speed") by measuring the time that elapses between the ignition firing and the subsequent vibration sensed by the knock sensors. In other words, the ECU can, and does, adjust timing to "walk the bleeding edge" of timing advance without having to wait for knocking/pre-detonation to actually occur.

As you've noted, the optimum of timing advance is affected by the temperature and pressure in the cylinder, the octane rating of the gasoline, and the air:fuel ratio. Also, timing advance is a function of RPM, since the goal is to have the ignited mixture in each cylinder reach maximum pressure just before the cylinder reaches top-dead-center. The number of variables, and the large range over which several of them can vary, means that the ECU has to do lots of ignition timing calculations anyway, regardless of whether the gasoline octane rating remains constant.

 

... In other words, the ECU can, and does, adjust timing to "walk the bleeding edge" of timing advance without having to wait for knocking/pre-detonation to actually occur.

... The number of variables, and the large range over which several of them can vary, means that the ECU has to do lots of ignition timing calculations anyway, regardless of whether the gasoline octane rating remains constant.

Thanks Pete, that was a very helpful post. You provided the best explanation that I have seen regarding what the ECU is doing and when and why it's doing it.