Reason my dealer suggests 89 octane gas rather than premium (more)...
Thread Starter
AudiWorld Super User
Joined: May 2000
Posts: 21,651
Likes: 0
Reason my dealer suggests 89 octane gas rather than premium (more)...
I was at my Audi dealer checking on some parts I ordered and happened to overhear a service manager explaining to an A6 owner why 89 octane gasoline is recommended (by the dealer) over 91 octane (premium) in 5-valve engines. The service manager explained that higher octane gasoline makes the 5-valve engines harder to start in cold weather conditions due to the higher combustion temperature of the fuel. He went onto say that it is fine to burn 89 octane gasoline year-round.
Anyone have any comments?
Anyone have any comments?
Elder Member
Joined: Mar 2000
Posts: 39,901
Likes: 1
92 Oct actually burns colder
I don't think temp matters much. At altitude, the the reduced oxygen in the air plus high octane can make a car hard to start.
The slower burning high octane gas actually burns colder than lower oct gas.
The slower burning high octane gas actually burns colder than lower oct gas.
AudiWorld Member
Joined: Mar 2001
Posts: 546
Likes: 0
The ECU will adapt to just about any octane - but I would follow the manual.
First, I don't see how switching from 91 to 89 octane can have any effect whatsoever on starting an engine - it is only a tiny difference. The engine ignites the fuel via a spark - not by high compression like in a diesel. Unless the cylinders are flooded, any fuel should ignite from the spark.
The higher (recommended) octane fuel will have less tendency to detonate under high load/pressure/temp, so the ECU is able to adjust the timing for better performance. 87 or 89 (especially under high boost, which you don't have) will cause the ECU to retard the timing. The car will still run well, and you may not even notice the difference.
Keep in mind, though, that the ECU can only detect detonation (and retard timing) after it has already happened. I cannot quantify the level of damage caused by momentary detonation. My suggestion is to stick with Premium fuel unless you are having the cold start problem that the dealer mentioned. Then you can try an experiment to see if you notice a difference.
The higher (recommended) octane fuel will have less tendency to detonate under high load/pressure/temp, so the ECU is able to adjust the timing for better performance. 87 or 89 (especially under high boost, which you don't have) will cause the ECU to retard the timing. The car will still run well, and you may not even notice the difference.
Keep in mind, though, that the ECU can only detect detonation (and retard timing) after it has already happened. I cannot quantify the level of damage caused by momentary detonation. My suggestion is to stick with Premium fuel unless you are having the cold start problem that the dealer mentioned. Then you can try an experiment to see if you notice a difference.
AudiWorld Expert
Joined: May 2000
Posts: 25,178
Likes: 0
My experience in hard starting on newer modern cars is....
That it is almost always a Fuel/Air ratio problem:
1. Vaccuum leak.
2. Temp sensor make the system think the car is still hot, and doesn't richen the mixture up enough.
3. throttle valve is not closing all the way.
Not bad for a shade tree guy - eh???
1. Vaccuum leak.
2. Temp sensor make the system think the car is still hot, and doesn't richen the mixture up enough.
3. throttle valve is not closing all the way.
Not bad for a shade tree guy - eh???
New Member
Joined: Aug 2000
Posts: 189
Likes: 0
My experiences......
I hve burned every thing from 87 -93 octane. Here is what I have noticed....
The lower octane fuels below 89 result in poorer gas mileage. 89 - 91 results in the best fuel economy and 91 seems to give the best power to throtle response. I have even gone so far as to calculate MPG to Price Per Gal. for the different octanes. The end result (depending on you gas prices) is that 91 octane gives the best MPG and power with 89 running a very close second. As far as dollars in your pocket the trade off between the two is alomost nill if the cost is is less than $.05 per gal. All this is IMO.
The lower octane fuels below 89 result in poorer gas mileage. 89 - 91 results in the best fuel economy and 91 seems to give the best power to throtle response. I have even gone so far as to calculate MPG to Price Per Gal. for the different octanes. The end result (depending on you gas prices) is that 91 octane gives the best MPG and power with 89 running a very close second. As far as dollars in your pocket the trade off between the two is alomost nill if the cost is is less than $.05 per gal. All this is IMO.
Thread
Thread Starter
Forum
Replies
Last Post
Dumb gas question: Since octane rating is supposed to be percentage of octane, how is there 105, etc
ATH
S4 / RS4 (B5 Platform) Discussion
2
May 19, 2004 10:36 PM
Milpitas2.7T
S4 / RS4 (B5 Platform) Discussion
27
Nov 4, 2001 03:44 AM
Audi juice
A4 (B5 Platform) Discussion
5
Oct 29, 2001 08:37 PM
