Topping off fuel?

[regalfriend]

97.5 GS When I fill the gas, after the nozzle clicks off I can usually get about 3 more gallons. That will fill it all the way up to the neck. car was never hit in the rear. This happens at all gas stations. That rules out over sensitive fuel nozzle. I never get any codes. I know some cars it is not good to top off. It may set a code. Is there any harm in topping off to the max. The reason I do it is because my milage is not so good (mostly due to lead foot). Also in my state I can only find 91 octane. Is this ok? There is a place that has 100 octane. I dont mind spending the $$$ to mix it w/ 91 to get a higher octane.... My math is not good. If I need 15 gallons total, how much 100 octane should I put in with 91 octane to get 93 octane?

[GTP Dad]

I think just about every car ever made can get additional fuel in the tank when the nozzle kicks off the first time. If you are planning on driving for a moderate distance once you fill up, I see no reason why topping off the tank is harmful. I have been doing it for years especially if I am going to be doing some moderate distance afterward. If you plan to let the car sit for a few days it might not be a good idea to fill the tube all the way as it may cause some issues like leaking out the cap if parked sideways.

As for the octane issue my math isn't very good either but based on my calculations if you add 3 gallons of 100 octane to 12 gallons of 91 octane you should be pretty close to the desired octane reading. However, why spend the extra money. If you are getting a lot of KR it would be worth the extra money but if not stick with the 91 octane and you shouldn't hurt anything.

[johnsonace]

GTP Dad:
Just curious, what's KR?
regalfriend, based on my experience, 91 should be octane plenty OK. My 97.5 GS has never seen anything but 87 octane for 145K. I get 22mpg city & 27 interstate, but I rarely peel rubber.

[BNaylor]

Originally Posted by johnsonace

Just curious, what's KR? My 97.5 GS has never seen anything but 87 octane for 145K. I get 22mpg city & 27 interstate, but I rarely peel rubber.

87 octane??? Have you ever run Autotap or a scan gauge capable of reading KR - knock retard. You'll be surprised to see how much you get running regular unleaded even if you drive like an old lady. In a worse case scenario I might run mid-grade in my Regal GS but I have one range colder spark plugs. I ran a test using my Autotap program and laptop when the Katrina ordeal caused high gas prices. Under moderate acceleration I've seen 2-3 degrees KR on regular unleaded. At WOT over 5 degrees of burst KR.

Here is some interesting info for you.

L67 SII 3800 engines require 91 octane premium gas or better per the GM owner's manual. However, 87 may be used only in emergency situations for a short period of time. The primary concern on a L67 engine is knock retard (KR) due to the supercharger, especially in high boost conditions. If KR is detected the PCM will retard igniton timing degrading performance.

Here are some interesting facts:

For every 1 degree on a L67 SII 3800 you will lose approximately 2 - 3 horsepower.

For every degree of knock retard it takes .8 seconds recovery time.

"Once the PCM has retarded timing sufficiently to reduce knock below the currently detected peak level, a changeable parameter in the PCM governs how quickly the overall ignition timing can be restored to normal levels. The engine could see a peak of 15 degrees of KR from which the originating detonation may immediately disappear. However, the PCM will not instantly restore timing to pre-detonation levels. Instead, the PCM cautiously and conservatively restores ignition timing at a rate of 0.8 degrees per second. In the event of a 15 degree KR event, it would take nearly 19 seconds for the ignition timing to be restored to pre-KR levels. By the time your car sees full power again, the race is already over. This 'time' that the PCM takes to restore the ignition timing is called the Recovery Rate. The Recovery Rate will continue in this slow fashion until KR reaches zero, KR increases back above the current recovery value, or the throttle is released."

FAQ: Why is knock retard (KR) bad?

"Due to the retardation of the ignition timing, KR causes the vehicle to lose substantial power. More importantly, though, the flame front collisions are EXTREMELY harmful to the pistons. These highly volatile areas in the cylinder can cause stress cracks in your piston, which will eventually give way causing an entire CHUNK of your piston to lift right off and begin banging around inside the cylinder. This is why when the spark plug is removed after such an event, the plug end is bent all the way over. The broken piston can be VERY expensive to fix if you are not capable of doing the work yourself. DON'T EVER DISABLE YOUR KNOCK SENSORS. It takes less than 3ms to damage your engine due to knock."

[regalfriend]

On my GS there is a sticker on the fuel door that says "Premium Fuel Only". Gm would'nt put it there unless its absolutely needed. It increases the overall operating expense for the lifetime of the car. I dont have an AutoTap to check KR. At times it does feel a little slower. In Nevada 91 is the new Premium. I've always used 93 up until a few monthes ago. Once I was nearly empty and put about 5 gallons of 100 octane only. I think there was a slight differencein performance at WOT.

[BNaylor]

Yeah I agree Regalfriend, better safe than sorry later.

In West Texas we are at lower octane levels by one point because of the altitude and then they rip us off with 5% ethanol added to our gas so called oxygenated from October - March. I'd like to see if I can get well over 200K miles or more with my Regal GS. But depending on the gas prices this summer I may use mid-grade and just baby it. I don't need to hot rod it.