[Shovelman]

Why does my car need high octane gasoline?
Easy answer, to prevent preignition.

Next question, When does my car need high octane gasoline?
Not so simple answer; when the compression is at maximum psi.

When is that?
When there is a maximum of air in the chamber, that happens when there is no restriction in the intake tract and the boost is at maximum. When the throttle and intake valves are wide open and there is not a really high flow (high rpm)

Easier answer, when torque is maximum, because max compression causes max torque.

So when is that? At middling rpm and maximum throttle and maximum boost.

So if I am driving modestly, as most BMWs are most of the time (think X5) high octane is a waste of money.

Do I have this right?

[123Britt]

You can get away with using less than 93 Octane but a high detergent fuel being top tier is important .
.
Although I do 93 from either Shell/Texaco/Chevron/Costco
.
.

[3.0L]

Originally Posted by Shovelman

Why does my car need high octane gasoline?
Easy answer, to prevent preignition.

Next question, When does my car need high octane gasoline?
Not so simple answer; when the compression is at maximum psi.

When is that?
When there is a maximum of air in the chamber, that happens when there is no restriction in the intake tract and the boost is at maximum. When the throttle and intake valves are wide open and there is not a really high flow (high rpm)

Easier answer, when torque is maximum, because max compression causes max torque.

So when is that? At middling rpm and maximum throttle and maximum boost.

So if I am driving modestly, as most BMWs are most of the time (think X5) high octane is a waste of money.

Do I have this right?

You discovered a company 'save money' secret: your car really needs two gas tanks; one for 87 octane and one for 91/93 octane.

[inc235]

Only if you NEVER WOT or use more than 50% throttle. Yes, then it is waste in my opinion. But then, why did you even buy this vehicle to begin with?

[morphomeman]

By OP's reasoning, he does not need insurance either, since most of the time it does nothing and, if he drives carefully and is extremely lucky, he will never use it.
You need the high octane for that occasional moment when you have to (or want to) floor it from low rpm and the engine can take advantage of it.
The car calls for 91 octane. Where I live, you can get only 89 or 93. So maybe settle for 89 and feel a little better. Kind of like getting cheaper insurance.
Or OP can trade in the car for one that doesn't need any but the lowest octane fuel. I used to drive a Honda Fit (my daughter has it now). A great handling car, lots of fun with the MT. But it could not get out of its own way. You get what you pay for, at least some of the time.

[Ron Jeffries]

the prius is nice ...

[Rayscott]

91 here. Does it hurt or help to throw in some 93 on occasion?

[Maynard]

You are really misunderstanding this, but your post has some of the parts of it right. You don't 'need' good gas b/c the car will retard itself to compensate for your poor judgment. Congratulations - for less than $.75 a gallon you turned a BMW into a VW.

The octane helps at max cylinder pressures, not necessarily max power point (but usually the torque peak, which for us is about 3k rpm wide). At lower rpms damage is exaggerated, because there is more time for pre-ignition (and on our cars, the turbo is still stuffing max pressure in, hence the flat torque curves). This is more of a problem for turbos, because the low rpm and boosted charge means that the pre-ignition has a lot more force, more time to score pistons and deform rings. With crappy gas you are going to get pre-ignition, then timing and boost pulled back, across the range.

[USA-RET]

Never could understand paying $50+K for a car and the quibble about the price diff between the low and high octane gasoline. I know people that dive 10 miles to save $.02 a gallon ($.20 for 10 gallons)

A few $ more per tank overall for the high test is money well spent when your car requires it. Additionally stay w/ Top Tier gasoline for the cleaning agents it includes. More and more brands are now Top Tier sellers.

http://www.toptiergas.com/licensedbrands/

Giving up one $6 cup of Starbucks specialty coffee per week would surely cover that difference in most instances.

[USA-RET]

Originally Posted by Rayscott

91 here. Does it hurt or help to throw in some 93 on occasion?

I run 93 all the time in mine. Higher never hurts..lower does.

[Shovelman]

So yes I have it right but my driving style does hit max torque way too often to make low octane worth it. And yes I bought a sports car for a reason.

3.0 has it right also, two fuel tanks and even smarter injectors to know which to use. hmmm... this will not be on BMW's priority list anytime soon.

Now a company like GM might benefit from it when they really want to make those SUVs and trucks go fast and get great mileage. A smaller, high performance engine running low octane most of the time.

[Maynard]

Wouldn't cylinder deactivation do this much cheaper and w/o as many extra parts? Think how many gallons of regular you'd need to amortize the cost of an extra gas tank (but FWIW, a second/larger gas tank is my very top wish list item).

[msej449]

Over here in the UK, the sales pitch for Shell V-Power and BP Ultimate ACTIVE etc. isn't anything to do with the octane - it's the additives for engine cleaning. They happen to also be higher octane but most luxury/performance car owners are opting to use these fuels because they believe the claims that it's better for engine health, with a minor plus of a little extra mileage on each tank as well. The mileage claim is 'up to' 21 miles per tank, but the caveats indicate that's from new, over a long period, in comparison to a pretty neglected and maltreated engine.

[MarcoZandrini]

OP, your car has a turbocharger. The turbocharger (here after, "turbo") compresses the air that you engine is sucking in. Compressed air is always hot. That's why the car has an intercooler-to cool the hot compressed air. The engine then compresses the compressed air to increase engine power. That means the newly compressed air is very hot. Then, the engine sprays atomized gasoline into the hot, compressed air. Low octane gasoline is subject to "pre-ignition." Pre-ignition is basically the hot air and fuel mixture igniting without the sparkplug firing. This pre-ignition leads to pinging and knocking. This can damage the engine. The engine has knock sensors to determine when pinging and knocking occur. The engine management system (DME in BMWeze) retards the ignition and alters the how much fuel the injectors put into the cylinder. These decrease engine performance.
Gasoline with a higher octane rating is less prone to pinging and knocking. Using a higher octane gasoline allows the engine to operate as designed and get more energy out of each drop of gasoline.

I owned a 2001 330Ci (E46 chassis) before I bought my M240i. I had a minor gas leak under the hood. I replaced the 12 injector o-rings (each injector has 2 o-rings). The business end of all 6 injectors were pristine when I removed them from the engine. Why? I've used nothing but TopTierGas in that car. I usually bought Shell V-Power. That's what I buy for my M240i.

Buy TopTierGas premium only!

[XutvJet]

Originally Posted by Maynard

The octane helps at max cylinder pressures, not necessarily max power point (but usually the torque peak, which for us is about 3k rpm wide). At lower rpms damage is exaggerated, because there is more time for pre-ignition (and on our cars, the turbo is still stuffing max pressure in, hence the flat torque curves). This is more of a problem for turbos, because the low rpm and boosted charge means that the pre-ignition has a lot more force, more time to score pistons and deform rings. With crappy gas you are going to get pre-ignition, then timing and boost pulled back, across the range.

Sounds like you're one of the few in the thread that actually understand the need for high octane fuel and that it's not really about full throttle and high rpms.

In these cars, the biggest risk is that twin scroll turbo. It spools up very quickly. In 6th gear and at 2,000rpms, even with 1/3 throttle application, you can generate nearly 70% boost within 1 second and generate near maximum torque. This can be a very dangerous situation because cylinder pressures and heat are extremely high all the while the motor is slowly laboring along because of the tall 6th gear. This creates a much larger window for heat soak, elevated and sustained pressure, preignition, and overall stress. It's called "high load" and can cause a thing in direct injection turbo motors called "low speed pre-ignition" (LSPI). In cars with slower reacting ECMs such as the Ford Ecoboosts motors, current gen WRX, Mazdaspeed 3, and some others, LSPI can destroy the motor. Luckily, BMW's ECM (or DME) is incredibly fast reacting and the short block is very stout thus LSPI isn't much of a risk.

Regardless, even under part throttle driving, a lot of you are using a lot more boost and load than you think. So when people say you don't need high octane fuel if you don't go full throttle often, they're quite wrong. You really need to be running at least 91 octane in these cars to have a good safety net. Also, running sub 91 octane will likely result in timing getting pulled and richer air fuel conditions to reduce detonation. All this will result in reduced fuel economy and a laggier motor.

[3.0L]

Originally Posted by USA-RET

Never could understand paying $50+K for a car and the quibble about the price diff between the low and high octane gasoline. I know people that drive 10 miles to save $.02 a gallon ($.20 for 10 gallons)

A few $ more per tank overall for the high test is money well spent when your car requires it. Additionally stay w/ Top Tier gasoline for the cleaning agents it includes. More and more brands are now Top Tier sellers.

http://www.toptiergas.com/licensedbrands/

Giving up one $6 cup of Starbucks specialty coffee per week would surely cover that difference in most instances.

^^^^ This!

[B_Rice]

You make less power and get worse gas mileage.

[pz619]

So what you wrote is laboratory correct. The real world is a little different. If you ever max your boost, and do so quickly, you'll want the higher octane gas which prevent pre-ignition as you noted. So while you could get away with this, let's look at it from a more practical standpoint.

Let's assume you fill up once a week and the cost difference is $2 per tank. That's $104/year. You only need one bad episode of pre-ignition before you are picking apart parts of your engine block off the highway. It may never happen, then again, it might happen. A new block is around $18k MSRP and that's assuming you install it yourself. As a general insurance policy, $104/year is pretty good deal to not have to deal with this problem while getting maximum performance and hp from your sports car.

[BEM-S4]

Originally Posted by pz619

So what you wrote is laboratory correct. The real world is a little different. If you ever max your boost, and do so quickly, you'll want the higher octane gas which prevent pre-ignition as you noted. So while you could get away with this, let's look at it from a more practical standpoint.

Let's assume you fill up once a week and the cost difference is $2 per tank. That's $104/year. You only need one bad episode of pre-ignition before you are picking apart parts of your engine block off the highway. It may never happen, then again, it might happen. A new block is around $18k MSRP and that's assuming you install it yourself. As a general insurance policy, $104/year is pretty good deal to not have to deal with this problem while getting maximum performance and hp from your sports car.

It's also under $10 a month for a maybe good / definitely not bad thing to do for your engine.

[Dylan86]

I've seen enough high mileage engines that have skimped out on premium fuel and synthetic oil, and can bet I will never use either in any of my vehicles. I often opt for 89oct instead of 87 in my Colorado, even though the stated minimum is 87oct on the gas door.. Same with oil, could use the generic 5w30, but run synthetic and it starts smooth with no racket, every time. I think the little things, go the length, at least for longer terms of ownership. If you lease, f'it, send it however you want, just mind the next CPO purchaser who may hate you (or BMW) for the rest of his or her life due to the leftover long term damage

[Rayscott]

So I have a 228ix 4 cylinder engine. I use 91 octane, should I be using 93 or is that more of an issue with 6 cylinder engines. I agree spending a little more for premium makes sense in terms of engine performance and long term reliability.

[Luftwaffe1O1]

The car is 50k, and you are worrying about saving pennies. When you are arguing whether its safe to run if you don't use full throttle etc.

Honestly, just keeping it in eco-pro, or changing your driving habbits significantly will save you more at the pump than running lower octane.

Its pointless to use 91+ in econoboxes, but why even risk it with our cars?