The eternal question of premium versus regular gas

I’ve been the happy owner of an '03 BMW 503i for about a year. I’ve generally been running regular gas in it despite mfgr recommendation to use premium; after I bought the car I can’t afford premium! there has been no knocking, loss of power, etc. A friend recently said that I’m still doing damage to the engine, mostly in the form of excessive carbon build-up in the pistons. Opinions/advice?

You have no knocking because of the knock sensor is retarding the timing advance to prevent knocking. The other by-product is reduced mileage and carbon build-up. The carbon build-up can take a bit to get to the point of damaging the engine, but will eventually. Basically, excessive carbon build-up will cause the knock sensor to retard the timing even more, and eventually to the point it cannot any more. Then, you’ll hear the valves pinging away like a diesel. Also, the carbon build-up will gunk up the inside of the engine, and damage sensors. These sensors can be expensive to replace, and make you wish you didn’t try to save $0.30/gal.

This is a progressive problem, similar to high-blood pressure or high-cholesterol. You won’t feel the effects right away, but, over time, will take it’s toll.

Also, switching back will not fix the problem right away. But, just like quitting smoking, will prevent additional damage, and may clear up over time.

I’m a little confused. You own a BMW, which brings premium prices for repair, maintenance and insurance…and premium fuel prices. Everything about owning a BMW costs extra and you are worried about paying 30 cents more for gas, when gas costs $4+? If you don’t want to pay to ride, I recommend you by a vehicle that expects less.

Premium is not likely costing you anything. Switching to regular will cost you some power and mileage. So you may actually end up paying less for high octane (I hate that word premium as it seems to indicate a matter of quality and octane is NOT a measure of quality, that word premium really means additional cost.) because it will reduce the amount of fuel you buy.

I’m pretty uninformed about cars generally and I don’t like to spend money unncessarily - how confuisng is that?! But I know that my knowledge in this area is limited which is why I go to places like Car Talk for info (not personal judgments!). Keep in mind that putting premium gas in a car not designed for it (like the Camry’s etc that I drove before) is a waste of money. Add to that the fact that putting regular in the BMW doesn’t have any immediate or obvious effects. That all adds up to a reasonable question as to whether I’m wasting money or not. Based on other more informative replies, now I understand that I should be using premium for this car. But it’s question of not spending money unncessarily, not a question of not “wanting to pay to ride.” $4 a gallon gas, which we didn’t have when I bought this car a year ago, is also an issue. There are people who are generally cost-conscious and financially conservative that also drive premium cars - they’re not mutually exclusive.

There is no one size fits all in regards to your question. Some vehicles will accept or tolerate a lower octane gas just fine and others will not. The critical thing is if you hear any pinging and/or notice any mileage drop-off.
Your friend is incorrect about this excessive carbon build-up theory.

I’d be the first to tell you to up the octane rating if there was a hint of a problem but many times any pinging that occurs may be caused by a faulty EGR system; either a defective part of clogged EGR passages. The EGR system is constantly full of burned exhaust gas and will soot up just like a fireplace chimney.

For what it’s worth 91 octane is recommended for my Lincon Mark and it’s been on a steady diet of 87 octane for years. Many of the rural gas stations here do not even carry 89 octane anymore much less 91.
I maintain my Mark to the hilt and inspect the spark plugs every 15k miles as plug inspection is the best way of knowing what is going on inside a running engine. They burn perfectly with zero problems.
That Mark also has 224k miles on it right now and a compression test several weeks back while replacing the plugs showed 185 PSI on all cylinders. Can’t get any better than that.

I’ve also made a number of trips to CO and used 85 octane in the mountains with never a hiccup. The car gets 27 MPG on 91 octane on the flat and gets 27 MPG on 85 octane in the mountains. It’s a wash.
As I said, every case is different but if the car does not ping, suffer power loss, loss of mileage, etc. then I don’t see a problem even over the long term.
My SAAB Turbo (200k+ miles) does not ping or suffer a power/economy loss either. Even when nailed to the floor at full boost nary a hiccup on 87.

(You might also do a net search on how octane in the U.S. is figured and compare that to the European rating. Same gas, different number. This even occurred in WWII with aviation gas used in military planes. The U.S. octane rating of Avgas was 130 or so and the Germans used 100 octane. Same gas, different figuring in reverse.)
That’s just my two cents anyway.

I’m pretty uninformed about cars generally and I don’t like to spend money unncessarily - how confuisng is that?!

Very confusing…especially since you bought a BMW. Not the MOST PRACTICAL car you can buy.

Add to that the fact that putting regular in the BMW doesn’t have any immediate or obvious effects

As long as the knock sensor is working then there shouldn’t be any engine damage…but if it quits working…then you could end up putting a new engine in there. The car should be experiencing less performance and gas mileage. The way the knock sensor works is if it detects knocking it retards the timing…thus giving you less performance and gas mileage. By using premium the timing won’t be retarded and the performance and mileage will be what it was designed to do.