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Would you run 87 octane if was FREE???

8337 Views 28 Replies 16 Participants Last post by  skirmich
My wife has gained the "perk" of free gas at her job. Unfortunately, it is only 87 octane.
Our 2015 MDX has always had top tier brands of 93 octane, but I hate to pass up FREE!
Around here we have 87/89/93 octane choices.
She usually burns about one tank per week. It would save us about $50 weekly.
Is there an octane booster that she can add at fill up that would allow us to take advantage of this new opportunity?
Your opinions are appreciated.
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^^^The air is less dense at altitude, thus less compressed air/fuel mixture which would dramatically reduce the likelihood of detonation. Meanwhile, back at sea level, if you run 87 the computer pulls timing so that the spark arrives milliseconds later than if you use premium, there will be zero damage to the engine. Octane is simply the measure of the fuels resistance to ignition, so to a point the higher the octane the more timing (earlier arrival of the spark to ignite the fuel) the engine can tolerate to build a nice stable flame front for maximum power. With 87 the spark arrives late so less flame front, so you might notice reduced power and reduce fuel mileage, but the engine management software is going to manage things at a safe level.
That's a nice summary of the "back of the napkin" theory, but things get a bit more complex when you recognize that the fuel/air ratio matters, with lean mixtures ( less fuel vapor relative to air ) being more likely to detonate at any given level of compression. And it's the mass ratio that matters, not the volume ratio, so that's where things get interesting in adjusting for the more dilute air at high elevation. Don't forget the proportion of oxygen in air doesn't change with elevation, all the constituent gases become more dilute. ( But other added gasses, such as water vapor, will reduce the relative amount of oxygen in air at any elevation... this is a manifestation of the law of partial
pressures ). And ambient temperature matters for a variety of reasons.
But all of this blather aside, at the end of the day it's the metering systems of the air and fuel intake systems that determine how much air and fuel enter the combustion chamber before the valves close and the piston starts compressing the mixture. So it's not really so obvious that lower ambient air pressure will produce an inherently less explosive mixture upon compression in a modern internal combustion engine.
But it makes a good story if you want to sell cheap fuel for a tidy profit at higher elevation, doesn't it?
Anyway, I completely agree that advanced computerized engine controls are capable of dealing with fuels that that have a slightly higher propensity for premature and uncontrolled ignition. Within limits. But there is always someone who wants to make a quick buck by pushing those limits too far.
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Haha, your bigger napkin offers more room for "blather" but the simple fact remains that high altitudes have the same effect of physically lowering the compression ratio of the engine thus reducing the octane requirements. Why do you think vehicles lose power at altitude? Why do you think planes are pressurized? Power loss is not because of fuel or air metering limitations, it's because air pressure is lower at altitude and when the throttle opens less (but still carefully metered) air/fuel mixture is entering the engine resulting in reduced power (3-4% per 1000' elevation is the general rule).

Back on topic, it's a combination of compression ratio AND spark advance curve tables used in the MDX that result in the 'recommendation' (not requirement) of premium. The spark table adjusts to 87, zero damage will result if it's a top tier fuel. This topic is like a "which oil is best" thread, it's a road to nowhere.
Compression ratio is determined by the geometry of the combustion chamber ind stroke of the piston, but reduced ambient pressure would presumably result in reduced maximum pressure of the fuel/air mixture in the chamber during the compression stroke if no ignition happened. But as noted, that's only one of the factors involved. Interesting discussion, but as with many theoretical discussions it is woefully lacking in supporting data.
All I know is my metabolic engine is woefully lacking at high elevation. :flushed:
I drove my 1969 Mustang to the top of Pikes Peak (14,114 feet) once - boy was the decrease in power noticeable. I haven't done it in the MDX or with any fuel injected engine to see how noticeable of a difference there is. It was so long ago that I drove up Pikes Peak I don't remember if the vehicle pinged or not but I remember it pinging like crazy on Mexican gas even though I bought their highest octane despite the vehicle only requiring regular in the USA due to itnot being a high compression engine.

And yes - the compression ratio of an engine is fixed and is based on the mechanics of the piston in the cylinder. In the case of high altitude the compression ratio would be the same as at sea level but there'd be less air being compressed - less density as it enters the cylinder, compressed at the same ratio as at sea level but to a correspondingly lower pressure at full compression, which means less air, which means less power. The only way to overcome this is with a supercharger or turbocharger which puts air into the cylinder under pressure rather than ambient pressure.

But I think most of these discussions are best left to the manufacturer recommendations - people who generally know far more about these areas than we do even if we're engineers of some variety. The distinction of 'recommended' versus 'required' is an important one here and it can be double checked by paying attention to any pinging.
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Absolutely. Been doing it for 2 years and 30,000 miles.
5 MDXs 87 octane in all never any issues! Its your money waste it if you like.
5 MDXs 87 octane in all never any issues! Its your money waste it if you like.

I tried a tank or three of both rides...every time the MPG took about a 10-15% hit...Along with less top end power..

No idea how the 3rd gen responds...I am sure the power would be down, or else premium wouldn’t be the recommended fuel. ( I am assuming it is)

Some Manufacturers have gone as far to issue two HP numbers based on fuel, but the MPG sticker is going to based on the recommended fuel...
^ It is Recommended on the 3G.. Acura wont even post HP numbers for Regular Gas use. The HP Specs on the data sheet is using PREMIUM ONLY so its bound to lose power on Regular for sure.
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