The engine is actually designed to work best with premium but can adjust to accommodate regular.
Because so many people would balk at buying a car that REQUIRES premium, they pull a marketing gimmick and say regular is recommended but if you use premium, the HP is improved. This pleases both crowds.
The programming is defaulted to accommodate premium fuel.
Then, if the majority of people do as expected and fill with regular, the “knock” sensor detects it and adjusts the timing to accommodate it. It suffers slightly in performance but is unlikely to be noticed by the cost conscious consumer.
Well put, TT. I would add that it would be easy for the programmer to use the knock sensor to detect regular gas, and then change the parameter map accordingly, i.e., the engine is not continually running up to a knock condition and backing off.
agree @TwinTurbo.
Motors aren’t what they used to be. with variable timing and variable ignition, motors can easily be made to adjust to the different octane ratings. It isn’t rocket science like it used to be. After all, we do have flex fuel cars and many are little different then cars that aren’t. Try it out.
M.B. With only 5 octane points to work with, they can’t increase cylinder pressures by enough to make much difference…90% (or more) of the time, it’s the throttle that is regulating cylinder pressures well below the point of detonation…Theoretically, efficiency increases as compression ratios are raised but, again, with only 5 octane points to play with, they can’t increase compression very much…
@Caddyman Tell that to the guy who was running 87 octane in his Lightning pickup (supercharged 5.4L) because he thought that it didn’t matter. He ended up with a hole in a piston, and because he didn’t use the required fuel, his warranty didn’t cover any repair work.
This happened at the Ford dealership I was working at, at the time. Needless to say, the Lightning’s owner wasn’t happy at all.
On my daily driver ( Supercharged Mustang GT), the ECU tune I have is for 93 octane, though the shop who did the dyno tuning said that 91 octane shouldn’t be an issue. Some once put 87 octane in it by accident, and there was noticeable predetonation, even under a modest load. So I had to drain the fuel tank, and then run up to the gas station and put some premium in a gas can and bring it back.
I absolutely concur that many cars wherein regular is “recommended” have the ability to adjust to operate on regular when the piezoelectric crystal in the knock sensor feels the shock of preignition and sends the ECU pulses.
The thing I’m having a hard time accepting is that engines designed to run on regular that’re operating properly can run better on premium… accepting, of course, that they can if they’re not running properly to begin with.
The tiny number of people who drive supercharged vehicles are motor-heads who religiously use the best, highest octane fuel they can find…But for the rest of us…
That’s changing, caddyman. Manufacturers are turning to boosting cylinder pressures with turbos and superchargers more and more to get acceptable performance out of smaller engines. I think that in a few more years every engine will be boosted.
The term “supercharger” immediately brings dragsters to mind, with their huge roots-type blowers, but remember that there are hairdrier-type (vane type) superchargers too. A “supercharger” is just an air pump driven by the crankshaft instead of the exhaust stream.
Even a supercharged engine can run regular if the compression ratio is lowered to accommodate the increased manifold pressure. Even engines tuned to need premium spend a lot of time with the manifold pressure so low that regular won’t detonate. So here’s my idea. Why don’t car makers use a dual fuel system? Instead of burning premium all the time, only burn it when the conditions demand it.
An idling engine doesn’t need premium, even if it needs premium at full throttle.
An engine throttled down to 10 horsepower on a car going 55 mph on a level road doesn’t need premium.
I envision two sets of fuel injector nozzles fed from two separate fuel systems and an ECU that shuts off the regular nozzles and switches to the premium nozzles at a certain manifold pressure. It wouldn’t have to be a sudden switchover, there could be a range of manifold pressure where both are delivering fuel, to make the switchover seamless.
Another benefit would be redundancy, If one system fails, you can limp home on the other system.
Yea I know! Some idiot would end up filling the premium tank with regular.
TSMB, I don’t really disagree. Is there a significant difference? Very likely not. My point, it’s free, then any slight difference would be to the benefit of the premium gas. I could go so far as to say if the knock sensor operated one less time with premium, there’s a difference. Is it significant? No, but when it’s free, why not?
I would use the higher-octane fuel if it’s at the same price point. Though I would check and make sure you don’t get less MPG out of the 93 octane fuel. If you do, I would revert to the 91.
To raise octane in fuel, oil companies use additives like Xylol and various other alcohols. These additives contain less “energy” for the same volume of liquid fuel as the “standard” lower-octane fuel. As a result, your mileage can suffer. Very similar to people getting less MPG using E85 in their tanks, though not as dramatic.
Re. the knock sensor(s) detecting higher-octane fuel and adapting to it: The problem with this idea is that knock sensors only detect knocks, not the formulation of the fuel. The info provided by the knock sensors is meant to be used in conjunction with other sensors, including those of a flex-fuel vehicle (if you have one) that actually determine what is in the tank.
If the PCM kept advancing timing to try and make the most of what fuel was in the tank without any actual way of determining what it was, I envision a situation where ignition timing is cranked up very high due to the owner using a tank of 93+ octane, then uses some cruddy gas and decides to beat on his car. The PCM has adapted to the high-octane gas and has ramped up the timing maps and cannot react in time to actually prevent damage. Or at the very least you end up with some temporary driveability issues as the PCM needs to throw away what it has learned and relearn what it can get away with. I expect your throttle response would be continually weird and you would experience surging as the PCM tries to continually remap the timing curve up and down to try and make the best use of the gasoline you have on board with not enough data points to work with.
But if a person pays a premium for an engine that needs premium,why cheap out and use substandard fuel in it?The knock sensor usually cuts it back the engine back to a level comparable to the standard engine,the old timers used to describe detenation as the valves rattling,use a lower compression large displacement engine if you want to run cheap fuel-Kevin
I agree, Kevin. It always puzzles me when I read of people that buy a high priced “premium” vehicle that requires premium fuel and then try to use regular to save a few bucks. Even if the engine is able to adjust, it’s adjusting to a sensor response (knock sensor) that should not be there, and it’s adjusting to a less than optimum operating condition.
Physics is easy to understand compared to peoples’ behaviors.
Well I’ve run my '06 Matrix about 100 miles on a tank of mostly 89 instead of 87 in weather ranging from 95-98F.
I know it’s too subjective to say that midrange torque seems a little less sluggish in stop-and-go traffic.
But what I’m sure about is the occasional single ping sound I’d get shifting through the gears is gone.
I drive in the city with the AC off and windows open so I can hear what the engine is doing very well going down residential streets next to rows of parked cars.
" The PCM has adapted to the high-octane gas and has ramped up the timing maps and cannot react in time to actually prevent damage."
I think the PCM would back off the timing after the first couple of pings.
These systems respond to changes in milliseconds.
PCMs/ECMs do not “adapt to high octane gas”. They’re programmed to adjust the spark advance and other operating parameters based upon the signals from the engine’s operating sensors to achieve an optimum compromise between performance, emissions, longevity, and efficiency. If they get a series of detection signals from the knock sensor, they then adjust to prevent damage, but if they do not they have no idea what fuel you’re running. They just-run-programs (respects to #5).
This implies xylol is an alcohol. It is not. It is another name for xylene. (FWIW, toluol is another name for toluene.) Xylene actually has about 10% more energy than gasoline on a volume basis.
@B.L.E. Oldsmobile tried something somewhat similar in the 50’s with their Jetfire turbo V8. It used a methanol/water injection system to keep pre detonation at bay, but the owner had to periodically refill the supply of methanol/water mix. As you might imagine it wasn’t a huge success.
@FoDaddy. Manufacturers are working on these systems again. There are significant potential advantages in cooling the hot charge coming out of a turbo- or supercharger with something. Methanol is good at this. It would have to be refilled regularly, but that’s easy enough. We’ll see. It has been researched for years.