2.0 Premium vs. regular e85 gas stations in houston


The 2.0T will run forever on 87 because it has a low octane map in the ECM and the knock sensors will keep it here when running 87. The KR sensors are there to protect the engine and the "OK" to run the engine on 87 means it is safe and GM can state operating costs based upon the lower cost fuel. But there is a performance penalty because the engine controller has to retard timing so that cylinder pressure during the combustion event never becomes high enough that the mixture explodes rather than having a smooth fas burn across the volume of the air/fuel mix. Someone who has tuned the ECM will have seen the various tables and I suspect that part of the low octane map also calls for reduced turbo "boost"; in any case the retarded timing will result in longer spool up for the turbo under a call for a rapid increase in engine power.

Realistically I wouldn’t want to regularly operate a car knowing that its full power isn’t there if I need it and that is a decision you are making if you run any engine on lower than optimal octane fuel. The engine will protect itself by reducing power output so unless you just want a lower power ATS higher octane fuel is worth the slight increase in operating cost. The number one operating/ownership cost of a new ATS is depreciation and once you factor in depreciation, insurance, and maintenance costs the marginal difference between the lowest and a mid octane grade becomes a less significant part of operating cost.

But bottom line for me is when I need it I want the full vehicle power to be available. Yesterday I was driving 30 miles to a set of soccer fields where I was coaching my daughter’s team. I was in my ATS and left in plenty of time but on a rural 2 lane (marked 55 but typical speed is 62-65 MPH) I was behind a parade of 5 vehicles of "elderly folks returning from church". The group was moving at 44 MPH and were nose to tail so close that pulling in between members of the group while passing would be pretty close to trying to parallel park at highway speeds. An open stretch of road became available and I quickly passed all 5 and moved back into line at a rather excessive speed but necessary to get past a group of drivers who were a moving road block that did not allow any room between vehicles. Had the first one suddenly stopped for an animal they would have had a 5 car pileup which would still be pretty ugly at 44 MPH. When full power is needed it is there with the proper fuel, you certainly don’t always need it but having the power on tap when needed is the reason I never buy underpowered vehicles.

You run the recommended regular grade fuel in your ATS though correct….. being the 3.6?I have found on 87 that the 3.6 will sometimes have a very light knock under low RPM operation. I first noticed it going uphill at a state park in IL and at 30 MPH the transmission is programmed to shift up to a pretty high gear to save fuel but under those conditions it will have a slight but audible knock. Paddling down one range takes care of that but I often use 89 in the 3.6 and there is never any knock under those conditions. Realistically 87 works great in the 3.6 and you shouldn’t have any power loss under high throttle conditions where the normal ECM timing and mixture map takes care of things but I think it is purposefully tuned to run on the ragged edge at these lower speeds to do better on the EPA economy loop simulation. I never noticed it with the 3.6 in my 2008 CTS (which was the first gen 3.6 DI) but the tuning is clearly different in my 2014. One thing I like much better in the 2014 is the cold start behavior is much better behaved dropping steadily from fast down to curb idle as the front cats heat up. The 2008 would fast idle for a few seconds then try to drop several hundred RPMS and stumble badly before going back to high RPM. This behavior would repeat 2 or 3 times whenever the engine had fully cooled and was VERY annoying and I got in the habit of using remote start so that it acted more like a Cadillac and less like a 150,000 mile college beater car at startup.

My ATS and Corvette both have a nicely refined cold start and that annoying start strategy was the only thing that really bugged me about the 2008 version of the 3.6. But neither are as smooth as my 2018 GMC Sierra diesel which starts and immediately smoothly idles at 600 RPM with no stumble or surge from a cold start. But the diesel engine/Allison automatic transmission are nearly a $10,000 option cost over the base 6.0L gas/GM transmission combo so for that price it should have a smooth idle

For the 2.0T also realize that things will be different when it is running high sustained boost in high ambient temperature conditions. The charge cooler becomes less effective under these conditions leading to a hotter air charge entering the cylinders and detonation will occur now where it wouldn’t occur under brief high boost and/or cooler ambient air temperature.

GM has a factory authorized tune available for the LT4 engine used in the Z06 which substitutes a 100/93 octane map for the stock "91 required but won’t blow up with light operation on 89" map. The 100 octane map is there for track use where the supercharged engine was going into heavy knock retard as the charge cooler became less effective under sustained high boost. As timing is retarded the engine will run hotter so although the retarded timing saves the engine from piston and main bearing destroying detonation as it goes into heavier ignition retard it creates even more heat resulting in the need for ever increasing power killing timing reduction. Using 100 octane fuel allows full power to be produced without the increased heating from late timing.