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Comment by Scoundreller | Hacker News Reader

I just use a fraction of the recommended laundry detergent. Helps prolong clothing and colours too. Spot spray any actual stains.

But I also fear automotive durability is being lost in the name of fuel economy by using excessively thin oils. Not just in the motor (easy to change yourself, just ignore the label where they can’t recommend thicker oils), but also CV joints, bearings, transmissions and differentials.

I put 87 (US octane system) instead of the “required” 91 because the fuel mileage loss doesn’t make up for the added premium. But of course the manufacturer will never say you can do that because they’ll lose their fuel economy record.

It might actually matter with a turbo, but I don’t have one.

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> I put 87 (US octane system) instead of the “required” 91 because the fuel mileage loss doesn’t make up for the added premium.

Octane isn’t about gas mileage. It’s about compression. Lower octane gas is prone to auto-ignition at lower compression ratios. Running lower octane gas in an engine designed for higher risks catastrophic engine damage, as ignition can happen before the piston has reached peak compression.

Meanwhile there are (a decreasing number, probably to claim better fuel economy) that state “91 recommended” but don’t require it.

Reality is modern engines adjust timings to make up for lower than expected octane, because they have to deal with other situations that trigger it (e.g. elevation).

But running an engine designed to take advantage of a higher octane at lower octane will have a fuel economy hit. But appears to be a single digit percents difference while reducing unit fuel cost by over 15% where I am.

There are limits though, if you blow up your engine and they find 87 in the tank don't be surprised if the warranty is void.

Is there seriously an ability and reasonable chance that they could and would test the octane in your tank? That is surprising to me.

I believe some newer cars do store data that could indicate lower octane gas was put in.

But if you’re still under warranty and your engine blows up, it suggests a problem with the manufacturing itself. Exceptions could be if you put in E85 that’s not recommended as it has much lower energy and the engine needs to make a ton of adjustments for that which might be out of range.

I’m loooooooong past warranty here. Lots of suggestions of engine thermonuclear destructions of the country (on internet for putting lower octane than “required”), but so far so good.

> Reality is modern engines adjust timings to make up for lower than expected octane

Correct, but the manner in which many do this is reactive (i.e. piezoelectric knock-detectors), not proactive.

I put max octane in all the cars I own. I don't see a reason to play games with something like this.

Putting the highest-octane fuel in your car is just wasting money: there's zero benefit to it. You should only use the minimum octane recommended for that engine.

Peace of mind is a benefit.

I am not a long haul trucker. I put maybe 100 miles on my vehicles per month.

The cost of fuel is not relevant to me.

While it depends on brand, the detergents and other additives are often better in higher-octane fuels. They help keep fuel injectors and other parts of the fuel system clean, which reduces potential problems down the road (ha ha).

https://www.aaa.com/autorepair/articles/don't-confuse-gasoli...

> I put max octane in all the cars I own

Of what’s commercially available, that would be E85. I hope you’re not putting that into your E10 max vehicle.

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