Hi,
I’ve recently moved-up from a 2.5l V6 vehicle to a 3.5l V6 vehicle. The manual is telling me the engine only takes premium gas. I’m not a total newbie to mechanics, I did most of the maintenance on my last car ( oil changes, PCV valve, spark plugs and wires, brakes) and I’ve read enough to know that knock sensors have greatly decreased the need for higher octane gas. There used to be a show “Lucille’s car care” or something like this and on one of these shows she explains octane and why it’s unnecessary to fuel-up with higher blends.
Am I really going to tear-up my engine if I pump-up with regular 87? Sorry, it just sounds kinda silly to me. But I’d welcome any knowledgeable replies. Thanks
With my understanding of octane, it’s not the size of the engine that matters, it’s the compression ratio. Higher octane gas means it resists spontaneous ignition at higher temperatures. That’s the reason that a higher compression engine needs higher octane gas: a lower octane gas will spontaneously ignite before the spark plug fires, due to the higher temperature induced by the higher compression.
It may be that knock sensors will reduce the knocking if you use a lower octane in a high compression engine, but I’d think that you’ll never-the-less be reducing your engine’s efficiency because the gas will still be spontaneously igniting prematurely.
If the manual says premium, I’d use premium. It isn’t because the engine has a larger displacement, though. Generally, a higher-compression engine will use higher octane fuel, although things like valve timing and combustion chamber characteristics also have some bearing on the octane requirement.
Although, as you say, the computer can alter timing to decrease or prevent damage, that’s not the best approach. The altered timing can decrease engine efficiency, can increase emissions, and might make your engine run hot.
It has nothing to do with displacement. If you have a high performance car that has a high compression ratio, then you might benefit from higher octane gas.
Knock sensors will kill your performance if you use low octane gas. They will adjust your ignition to keep your car from pre-igniting (knocking). So, to answer your question, lower octane gas will not kill your engine - just the performance.
I’ve did some research on this because my Mazdaspeed3 wants 91 octane fuel and I’m used to the cheap stuff.
Octane is the resistance of the fuel to (unintended) detonation. In a higher compression engine, the fuel can ignite without spark.
You can probably put 87 octane in your car, but it won’t run as well and definitely won’t be fast. I read an account of someone that drove a Mazdaspeed3 that was mistakenly filled with 87 octane gas, he said the power took a nosedive at 4,000 RPM, which was probably the computer safeguarding against the lower octane.
Now I’ve just got to find a gas station around here with 91 octane, the one I fueled up at last night had 87, 89, and 93. The 93 was 20 cents more expensive than the 89!
No, knock sensors reduce the consequences of not using high enough octane. They do not change the design parameters that call for a certain octane, and do not eliminate the consequences of not using what’s called for. They’re a back-up for occasional tight spots where the right octane is not available and for mechanical problems that induce knocking. They are not meant to be a way to select your own octane.
Maybe. Some engines are more sensitive to low octane than others. In the long run, though, consistently using too-low octane is likely to cause problems that will cost more to resolve than the money saved on gas on any engine. Ignore the recommendations at your peril.
And not meaning to be snarky, but this:
Premium Gas for a larger engine… Really?
…suggests you have a lot to learn on the subject. There is essentially no relationship between engine size and octane requirement.
As an anecdote (since I have no hard data) my truck with a 5.7 liter Hemi goes from a pig on the recommended 89 “midgrade” to an outright glutton when fed 87 octane. I was in a jam - nearly out of gas, and the gas station was out of premium and mid-grade.
The engine computer saw to it that there was no human-detectable knock, but acceleration was lousy. I was almost glad that the MPG was terrible (IIRC, it seemed liks about 10 MPG) so I could get the “bad” gas out. Actually, putting in a like amount of 91 set things right.
[thread=418888]Here[/thread] is an old thread discussing the use of low octane fuel in a high compression engine. There are several engineers and one automotive technician trainer weighing in; I think the latter steeds more authority than the former, but all are in agreement that using low octane fuel in a car engine calling for premium fuel is at best counterproductive and at worse may do damage to the engine and exhaust system. It’s true that most modern cars with variable valve timing will adjust timing to limit compression, but at significant cost to fuel efficiency, thus negating the ~5% savings you get from using regular unleaded instead of preimum.
In short, if the manual says use premium, you should use premium. That’s why they don’t just stick the notice in an obscure page in the manual, but also on the instrument panel and inside the fuel filler door.
gotpasswords, most of the time 89 octane is created exactly as you did in your truck; by blending the correct portions of 87 and 91. No harm, no foul, but next time your choices are between premium and regular, I’d go ahead and give yourself enough 91 RON to get to the next station.
Easy fix. Half 89, half 93. My wife’s car calls for 91 octane. If she doesn’t feel like mixing the gas at the pump, she will fill her car with 93 and at half a tank fill it with 89. Her car runs great doing this.
What’s the exact wording from your manual? Here’s the wording from my car (which is a 4.6L):
What strikes me most is, occasional light knocking is nothing to be concerned about. Also, am I understanding correctly the high-altitude instructions? Less octane is required to prevent pre-ignition, but yet the manual seems to say, don’t use less than 91 octane at high altitudes.
In any case, in my experience with my own vehicle and my own driving habits (i.e., YMMV), I have no significant difference in fuel consumption using 87 octane versus 91 octane. That’s to say, when I do the math, it’s still cheaper to use the 87. I also don’t seem to suffer noticeable lack of power; there’s enough torque there to accelerate as aggressively as I want.
Caveat: I posted my calculations somewhere here on another post, but that was back at about 2/gallon. Assuming premium still costs only .20 higher than regular, I may have to re-check my calculations.
What’s really killing my fuel economy right now is the downtown construction on I-94 causing all the damnable stop-and-go!
Some cars that call for Premium will run fine on Midgrade. Some. Not Regular, mind you, Midgrade, assuming your gas station even has it. Talk to your mechanic.
Depends on what you mean by midgrade. Here in VA I thought we had 87, 91, and 93, which would make 91 midgrade. However, the last 2 stations I’ve been to have had 87, 89, and 93.
Here, in Wisconsin, we have 87, 89, and 91/93 octane fuels available normally (some stations sell 100+ octane leaded race gas that ricers feel just peachy spending $5 a gallon on, but I digress…)
Last summer, on a trip to Bumscrew, Minnesota, I noticed that their “Regular” unleaded was 89 octane, “Mid-Grade” 91, and “Premium” 93. And it was no more expensive than its Wisconsin counterparts… What’s up with that?
IIRC, that’s because Minnesota law stipulates that all gasoline sold in-state is a 10% ethanol blend. Ethanol has a higher octane rating then regular gasoline, which accounts for the difference.
In other words, 89 octane gasoline in Minnesota is 90% standard 87 octane gas, and 10% ethanol. 91 Octane gas in MN is 90% standard 89 octane gas, and 10% ethanol, etc.
This is really one of the reasons I stayed away from some otherwise very appealing imports when recently shopping for a new car - they all required premium gas. This is mainly a function of the type of engine - high-revving, high compression DOHC engines like what’s in the TL and the G-35. Heck, I think even the top-end Camry and Accord want the good stuff now.
IMHO, now that GM is finally starting to invest in new engine technology, their engines seem to have the most desirable traits. The new direct injected 3.6L V-6 in the CTS will produce 302 hp on regular gas.
FWIW, we ended up getting an SUV (GMC Acadia). 275HP on regular from 3.6L, although the manual recommends mid-grade when towing. I was happy to get rid of my supercharged Grand Prix, which required 91 octane.
