Good thread! I'm using cheap gas on my 2.4L. knock retard... I wonder if that's what I'm hearing? Happens randomly and almost sound like something hitting in the rear...

 

Here I buy 87 octane with 10% ethanol. There is 91 octane also with 10% ethanol available too, but being cheap as any Scottsman can lay claim to I don't buy it. The one station in town that had straight gasoline in one pump dug up the underground tank because the state thought it was leaking and has not replaced it.

When I was buying the non ethanol gas it seemed to me that the car got better MPG {wither it actually did or not is anyone's guess.}. I don't remember what the octane rating on that straight gas was anymore.

 

Accurately comparing fuel mileage between gasoline grades would be extremely difficult without special equipment and a very specific drive cycle over a "loop" course. If you're doing it simply by recording the miles driven and how much fuel it took on a fill up but you're not driving a calibrated drive loop, sorry...but your results will be inaccurate.

I have found though both chassis dyno testing a lot of on-road testing using HPTuners VCM Scanner that, if you run 87, on warm and hot days, at wide open throttle, the engine will detonate and the resulting knock sensor inputs will have the ECM retarding spark. When the spark is retarded, fuel mileage goes down but the the amount of time knock retard is preset is very short, so there won't be a practical difference.

That said, where there is a difference is in performance. You run on 87, then floor it, running up to or near the rev limit, you'll actually feel the KR.

My experience has been that running a mid grade gas will eliminate most knock retard in stock or near stock engines. As a result, I run 89 most of the time in our '07 2.4.

My ECM has "tune" in it but it's not a tune with an aggressive spark schedule. If I had that, I'd run 91.

With the calibration I wrote for our engine, the big changes are fan ons, the point at which PE comes in and the speed with which PE comes in. Finally, while the spark tables are still stock, I changed how quickly spark comes back in after the ECM stops sensing knock.

 

Thanks for the Tip Hib. :)

BTW, raising the dead, in the winter, is a new thing. hehe :)

 

My ride seems to be really be enjoying a steady diet of 100% real gas.

 

I have found that using mid grade fuel (89 octane) works just fine. Adding a high flow air filter(K&N or AEM) adds a mile or two to the MPG

 

Generally speaking, most cars get better mileage on 87, and some will lose a little on 91.
This is because the octane number relates to how resistant the fuel is to preigniting (ping/knock)). So 91 has more additives to prevent this, but gallon per gallon, the 87 has more energy in it, as it is not as diluted down as 91. Of course there are limits to just how little octane you can get away with, depending on the engine's compression ratio, tuning, etc.

 

So far I have Owned and Driven 8 different HHR's.

All were 2010 or 2011's and 7 had a 2.4L with auto trans.

The only fuel I have used in all of these is either Chevron 87 or Costco 87.

95% was Costco fuel.

Why ?
Because I have never encountered any engine pre ignition/Knock in any of them.
Also mostly Costco fuel because they are a High Volume Seller with an 87 octane fuel that has lots of high quality additives. They also maintain all their fuel stations to high standards.

All fuel was purchased in the Pacific N.West. area. Just out of Seattle.
Maybe our fuel blending here in this area is of a better quality. I don't know, but
I have tried 92 Octane several times and saw no noticeable difference.

Mileage on all has been over 30 mpg. hiway. Two trips being 34 mpg.
I don't bother with checking in City.
All HHR's were .. and are, Totally Stock.
Estimated miles driven total on all 8 would be around 95,000 miles.

I have done lots of driving in Mountainous areas Like Hurricane Ridge in the Olympic National Park. A non stop incline from Sea Level to 5000 ft.
When I feel like it I romp on it from zero to 70 mph.
Never have heard anything unusual.

I use Techron Plus and Bell fuel additives every once in a while to keep the fuel system in good working order. Also keep my AC Air Filter changed on a regular basis
In never let my fuel tank get lower then 1/3 full and in the winter never less then 1/2 full.
Very seldom need to run the Air Conditioner with our temps in the summer here averaging 75 degrees or less, but the strange thing is the 2 trips I made and got 34mpg average were with the Air Cond. on most all the time. Also went over 2 mountain passes, 3000 ft. from sea level each way.
Boogy up and coast down I guess.

Silverfox

 

do some research guys, 87 octane has more power potential (more explosive) than 91 or super. They put additives in the higher octane fuels to slow down the burn, which stops you from pinging, by making it less explosive. you should always get better economy with lower octane fuel. realize I didn't say performance because as we know pinging will retard timing and kill performance. the additives are why super is more expensive.