Results 1 to 9 of 9

Thread: Do low loads ever need high octane?

  1. #1
    Potential Tuner
    Join Date
    Mar 2017
    Posts
    3

    Do low loads ever need high octane?

    Has anybody done any analysis of knock and timing on low-octane fuels?

    I'm thinking in particular about the low-load region that characterizes the way my parents drive: Throttle never exceeds 50%, and engine speed never exceeds 60% of redline. So they have a factory engine that specifies "premium" fuel. But I'm suspecting that with loads like that they will never see a condition where the cylinders get hot enough to knock even on "regular" gasoline. (In which case, I should tell them to save their money and only buy regular gas.) Is there any evidence to support or refute my suspicion?

  2. #2
    Tuning Addict 5FDP's Avatar
    Join Date
    May 2012
    Location
    Rogers, MN
    Posts
    13,533
    It's probably not likely they'd see much knock and timing reduction with the way they drive but if they had to step on it, performance would be less than if running the proper fuel grade. I mean it's only like 5-6 bucks per tank for most fill ups depending on where you live, it's like not even worth it to run a 87 octane gas when you aren't saving very much money.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
    Potential Tuner
    Join Date
    Mar 2017
    Posts
    3
    Right: the cost is easy enough to see. But there's this perennial debate about whether to buy high-octane when indicated. If the advantage is literally zero for a driver profile like I describe, I think that's noteworthy.

    And this is not hyperbole: I honestly believe they have never pressed an accelerator past 50%, nor do they tow or carry heavy loads. Of course, the optimal course of action in this case would be to give them an engine with half the capacity and remap the throttle. But they buy these higher-end cars that don't offer the nice trims with the smaller engines....

  4. #4
    Senior Tuner mbray01's Avatar
    Join Date
    Dec 2013
    Location
    Slidell, La.
    Posts
    1,015
    This can be a slippery slope with this scenario. You are actually creating high loads when you throttle in enough to accelerate, but not enough to downshift to a passing gear. You actually fall into areas that tend to be high timing areas that are utilized for better fuel mileage, and ultimately result in a lot of pinging. Sure your pistons will tolerate a ton more pinging here than under full throttle at 6500 rpms, however I would still stick with the premium fuel.
    Michael Bray
    Rusty Knuckle Garage
    Slidell, Louisiana
    20yr Master Tech.
    Advanced Level Specialist
    Custom Car Fabrication, Customization, High Performance.
    GM World Class Technician
    Shop Owner

  5. #5
    Advanced Tuner
    Join Date
    Jan 2009
    Location
    Spokane, WA
    Posts
    480
    Simple. Try it and see if it knocks. Like ^ said, there is a point where you are opening the throttle to not even 50% and still be at 100% load, up into the 90+kpa areas. But also keep in mind, even if it doesn't knock, vehicles just run better on the higher quality fuels. That's really the key...

    Think of octane as knock retard. Nothing more. If it knocks, it either needs more octane, less timing or less heat (lower engine temp, etc). Engines need heat to create power, call it BTU's. If your engine/tuning is setup to require x btu's, and the octane you are giving is only good for x btu's, then it will knock. ...trying to simplify it without getting too technical or involved...

  6. #6
    Potential Tuner
    Join Date
    Mar 2017
    Posts
    3
    I'm tempted to try to find an OBD logger so I can test this out. (Any recommendations?) I thought that if any of these premium factory engines are actually unable to attenuate knock by adjusting timing they'll illuminate the check engine light.

    So (my understanding – please correct if wrong!) what actually happens when somebody fills with 87 octane is that at some operating points the ECU might begin to detect knock and will begin to retard timing. At that point they're getting reduced fuel economy (and lower maximum power). But the question that really interests me is how often this occurs for drivers like I described, and how much fuel does the engine "waste" compensating for the reduced knock resistance?

  7. #7
    Advanced Tuner
    Join Date
    Jan 2009
    Location
    Spokane, WA
    Posts
    480
    Like we said above, RPM doesn't dictate octane. It's easy to have 100% load on an engine down at 2k RPM, or even lower. Just because it's not in power enrichment mode, doesn't mean it's not loaded. Look at the MAP sensor values. Anything over about 75 kpa...

  8. #8
    Advanced Tuner 68Camaro's Avatar
    Join Date
    Mar 2011
    Location
    Van Buren, AR
    Posts
    326
    what vehicle do they have?

    I have found several of the factory platforms that will benefit with 91 octane and also several platforms that would knock if the knock sensors weren't set up as sensitive as they are (even on 91).

    light throttle pulling a grade on the interstate with the converter locked you will be amazed how much load this will actually put on the engine - and this is where the majority of my tuning work is done to achieve fuel mileage and responsiveness

  9. #9
    Advanced Tuner RandomEnthusiast's Avatar
    Join Date
    Feb 2016
    Location
    Texas
    Posts
    333
    Granted mine is oldschool obd1. But if you take an engine and lug it at say 1500 rpm converter locked with 60+ percent throttle going up a hill in a 4k+ vehicle weight then yeah. 30 degrees of timing is not your friend. I have retuned my whole timing table around this area to stop knock because even with 93 octane and a 180* thermostat I was pinging like crazy. Now the newer pcms should have less problems with this but you never know.