Originally Posted by royal flush

with 2 seperate gas tanks and i have about 12 gallons of 91 in the car--can i throw in 3.5 gallons of 100 and will it mix properly?

Originally Posted by SG Lou

Gas is gas so mixing isn't the problem. Do you have a formula to mix the 2 in proper proportions to get 93 octane or are you just guessing ?

Originally Posted by 8VETTE7

Eventually.....

You have to consider how the tanks in the C5 work. Each tank holds about 1/2 the total fuel capacity so about 9 gallons each. It the gauge is reading more than half then the drivers side tank is pretty full and the passenger tank well under 1/2 its capacity.

When you add fuel to the tank (fill port is on the drivers side tank) it gets pushed into the drivers side tank by the pressure from the station pump. Excess fuel in the tank, which is just starting to mix the two different octane fuels, flows over the cross over tube into the passenger tank. So the fuel in that tank which is ONLY the lower octane fuel further dilutes the octane mix. For a while the fuel in the drivers tank will be more concentrated with the higher octane fuel.

When you drive the car the electric fuel pump which is in the drivers tank supplies the engine and also sends a small stream of fuel to the jet siphon pump in the passenger tank. This creates a siphon action that transfers fuel from the passenger tank, which is more dilute than the fuel in the drivers tank, to the drivers tank in quantity greater than that being sent to the siphon pump. This transfer enhances the mixing of the 2 octanes. Excess fuel from the drivers tank will continue to flow back into the passenger tank until the fuel level in the passenger tank is depleted.

So bottom line, eventually the fuel will get mixed thoroughly. But initially the fuel in the drivers tank will contain more of the 100 octane fuel than will the passenger tank. Fun while the 100 octane remains not thoroughly mixed.............

Originally Posted by 8VETTE7

No way to know that I can think of except try and see what happens.......... I would think that upon the initial addition of the 100 octane fuel you will DEFINITELY feel the difference seat of the pants, but it could conceiveably be in spurts due to the initial incomplete mixing..

Originally Posted by 8VETTE7

Does the engine have mods and are you running a custom tune?

I have a magna charged LS1 and a custom dyno tune. I've never had a pinging issue here in Tucson which is usually only a few degrees cooler than Vegas. I do like to use the magna charger on the entrance ramps to the interstates so I am often up near red line in 2nd and 3rd gears until I have to lift........

Proper tune should be able to set fuel and spark to avoid detonation even for 91 octane fuel which is all I can find readily available here..... Long gone are the 97 octane Sunoco pump (branded Sunoco 260 and was a leaded fuel as I recall) fuel available back in the 70's.

Originally Posted by neutron82

hmm, I wouldn't expect to have an octane problem on a virtually stock car with 91 assuming the tune hasn't been tampered with... if you know someone with a datalogger that would be good to look at just for kicks... if you aren't seeing any knock then adding more octane isn't going to have many benefits... if you can find 93 at the pump sure that's not a bad idea but spending the money on race gas just to mix it on a lightly modded car is not very cost effective in my opinion

Originally Posted by royal flush

OK--tqoday i added 3.3 gallons of 100 to about 12 gallons of 91--drove around alittle bit--on the street in 3rd gear about 1/3 to 1/2 throttle -no ping--i think thats a good test right?

Originally Posted by Greg_E

I used to run 3 gallons of 100 octane in a tank in the hot weather. It seemed to run better and at the time it was cheap insurance. Unfortunately the price of the non-leaded 100 went up in price and never came down when gas prices fell. I always tried to get below a 1/4 tank and them put in the 100 octane. Then add the 91 until full. The mixing action of the 91 octane fuel being added will blend the two and as the fuel spills over to the passenger side it will at least be partially mixed. The jet pump will complete the job. I never notice any change in performance when I first drove after a fill up or after I drove it for a while.

Originally Posted by royal flush

I dont think performance is what its about--In hot temps 100+ heat--its this-- When your foot hits pedal to floor--Its faster than the computer can think to pull timing--you can hear pinging--its about safety to me-

Originally Posted by Greg_E

With 11:1 compression and some days near 100 degrees I did it for safety as well. The computer will pull timing if it detects knock but at that point knock is already occurring. Raising the octane a couple points in itself isn't going to make your car faster, but if it is preventing knock from occurring and causing the timing to be retarded then it has had a performance benefit.

IMO you shouldn't need to run 93 on a stock LS1.
If you are hearing knock on a stock LS1 engine when it's under a heavy load you might try running some Seafoam through the intake just prior to your next oil change. You could have some carbon buildup in your combustion chambers. Also check for oil being drawn into the intake manifold from the PCV system. If so you might want to get an oil catch can. Make sure you are running the correct heat range spark plugs and that they are properly gapped and in good condition.