Quote:
Originally Posted by frankchn
If an engine is rated for a certain performance with 93 octane gas, then running a 91 octane fuel through the system will be detrimental to performance.
Tuned engines do run boost at much higher than most manufacturers, so naturally they would be more susceptible to fuel quality issues. The engine themselves might not be able to retard timings enough to compensate for poorer octane ratings if they are running with extra boost.
That said, I think if BMW specifies that the minimum requirement to run the S63 engine is 91 octane, then using 91 octane gas is fine, even if the engine is not performing at the same rated horsepower as when using 93 octane gas.
I've not heard of any issues with knocking due to people putting in 91 gas rather than 93 gas in stock S63 engines, and I think that holds true for the new revision of the engine in the F90 M5 as well. Of course, if you tune, everything goes out the window.
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I think your mind is made up - which is fine. I did not say the car will knock on 91. I am saying the performance will be significantly degraded. For example - you will not go 10.9 at 129 MPH on 91 octane. Maybe an 11.4 or 11.5 at 122-123 MPH. If that isn't an issue for you - then all is well.