<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tommy36998:
Hey what octane is everybody using. Manufacture recommends 87%. I've used 90%octane in my lexus and mercedes as required. Is using 90% a waste in the hummer </div></BLOCKQUOTE>
No way will I get into an Octane discussion...
But my views as it pertains to the H3.
High performance, high-compression engines such as those used on Vettes are programmed to use high octane fuel; however, their programming will allow for regular fuel, and the computer will retard the timing until knock disappears. If knock cannot be controlled, then the engine light will come on and knock sensor codes will be set.
Non-high performance, low compression engines are designed for regular fuel. If you use premium, the computer will not really know since there will be no engine knock in the beginning. However, since the engine is low compression, chances are, the fuel will cause a carbon buildup on the top of the pistons, thus effectively raising the compression. This raise in compression will now cause knock if regular fuel is used. This process has to happen over a great number of miles, and buildups do not always occur, it is Dependant upon the engine (Saturn's original engines were notorious for carbon buildup if premium fuel was used...one reason why we researched top head cleaning chemicals and found Techron the best on the market).
Now along comes GM's new breed of engine, non-high performance, high compression. The engine used in the H3 is considered a high compression engine; 10:1. So, I wondered if the computer could advance timing if a higher grade of fuel was used. I should have figured out the answer, but I didn't, and the answer is no.
The knock sensor is calibrated to pick up knock, signal the computer, which in turn will retard timing to stop knock; with the retarding of the timing changing the power curve. So, if regular fuel does not cause any knock, the sensor will not detect any knock, so the engine controller leaves well enough alone.
It made so much sense I was pissed I didn't realize the answer. Now, if you are in a location where regular fuel allows for a small knock and the sensor and computer slightly retard timing, then yes, jumping to a mid-grade will probably help. (Some will say that in high-altitude areas, mid-grade will help. Never really noticed in any of my other vehicles when driving I-70 through CO. My Jeep was a dog no matter the altitude or the fuel, and the Vette already used the highest grade I could find. HP does decrease as you gain altitude.)
A small amount of knock is not harmful to the engine. So, is the increase in price (.10 here from regular to mid-grade) worth the money?
There is a Citgo station near where I work that sells mid-grade at the same price as regular many times during the month. So, for two tankfuls I used mid-grade, and by the seat-of-the-pants testing I did, I noticed no improvement in pickup, or in noticeable fuel mileage. (Two-tenths increase in one tankful, one-tenth decrease in the second tankful. But weather conditions were not monitored or the time I allowed the H3 to warm up in the AM before planting my rear end on the cold leather seat.)
So, I have not really answered the question, but for the price difference, I will continue to burn regular. (I hate using Citgo since it is owned by that slim bag in Venezuela, so after my test, I no longer visit that station.)