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Old 02-13-2022, 06:19 PM   #51 (permalink)
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I thought I'd mention a few more things about octane and compression ratios and fuel consuption.

Take the Mazda Skyactive-X SPCCI engine for an example. It has a compression ratio of 16.3:1 and yet runs better on 80 R+M/2 octane than anything higher, including 87 which is regular in most states. 91, or premium, is actually worse for it than regular. Of course it works on a different principle than traditional spark ignition engine, but the point is an engine can be designed to not only get good fuel mileage at high compression ratios and low octane, but it can actually be designed to need low octane.

A hybrid that needs premium fuel is still a bad design in my opinion.

Say two people buy 2022 Corollas. One buys a non-hybrid that costs $20,175, and a the other buys a hybrid Corolla that costs $23,750 (MSRP for both). According to the EPA, the hybrid gets a whopping 52mpg average but the non-hybrid only 33. Say the non-hybrid owner pays the current average for regular fuel at $3.48, but the hybrid owner pays $4.08 per gallon because he uses premium, which on average is 60 cents more than regular nationwide. Assuming all other costs are exactly the same, the hybrd owner will finally have spent the same amount of money on the car when both cars reach 133,000 miles (214,000km) each. Which, if they drive 12,000 miles per year, will take over 10 years for the hybrid owner to break even. Then after that the hybrid driver can actually start saving because of his decision, until he needs to replace the HV battery or special brake actuator or something similar.

But say both spent their money on regular fuel. Then in 93,000 miles (150,000km), less than 8 years, the hybrid owner will finally break even and can start saving.

And the Corolla is kind of an extreme example IMO. The hybrid version could have been more, as much as 20% more, not 17.7% more like in this case. And hybrids don't always get 57% better fuel mileage than their counterparts. If it only got 17% more but needed premium fuel, then even if the cars cost the same to purchase you would never save money if you paid for national average fuel prices. 30% better fuel mileage would mean you would have to drive nearly 200,000 miles just to break even if you use premium in the hybrid.

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Old 02-13-2022, 06:43 PM   #52 (permalink)
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Quote:
Originally Posted by Tahoe_Hybrid View Post
GM recommends you use premium fuel on the 6.0L or higher engines... they recommend it for a reason

btw this is an LS2 based engine. the 5.3 Flexfuel models also have profiles for higher octane fuels


if 91 oct did not work then why does AFM Hyper cycle the engine (between v8 and v4) when 87 octane is used?

when 91,93 oct you can actually accelerate in v4 with a mild up hill grade?
The reason GM recommends premium fuel in such engines is because such engines are not designed for economy. People don't buy vehicles with V8 6.0L engines to get good fuel economy. Such an engine is built for performance. They want to be able to haul around 10,000lb (sometimes more) trailers. they want to feel as though they got plenty of accelerator to spare. At this point fuel cost is the least of their worries. I personally wouldn't be bothered by needing to use premium fuel if I bought a Corvette, Silverado, Suburban, Tahoe or the like. I would buy one of those for what it can move or how fast it can move it, regardless of the fuel it needs.

This is like buying a Ferrari and asking if you can put cheap tires on it. That's not why you buy a Ferrari. Sorry, I hate to break it to you, but the Tahoe is not a vehicle you buy to get better fuel mileage in either. The hybrid system is a bit of a gimmick to sell cars. Hybrid systems in utilitarian vehicles are to give them more low-end torque. If you get slightly better fuel mileage then that's a small side benefit, nothing more.

In a car like my Prius things are completely different. The car feels like it has a boat anchor constantly attached. Towing a heavy trailer in the thing would be a nightmare. I can cram 5 people in it uncomfortably. But I didn't buy it as a people or cargo hauler or sports car or an off-road vehicle. It's a fuel sipper, that's what I bought it for. That's what people buy them for. And if I or any other Prius owner had to buy premium fuel, we'd be highly upset.

Here's a rule of thumb. If the car is built for fuel mileage, it will run fine on regular fuel. If it needs premium, it is not a fuel mileage vehicle. That is not it's purpose. You can try to get better fuel mileage in it. But you're going against your goal as soon as you put premium in there. Unless premium really is about the same price as regular.

As a suggestion, I'd recommend looking at water injection. If increasing octane benefits fuel mileage, then using something as cheap as water could lower your overall costs. Either that or a methane conversion as methane has an octane of something like 120, IIRC.
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Old 02-13-2022, 07:16 PM   #53 (permalink)
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But you're going against your goal as soon as you put premium in there.
Clear premium included? In a 1971 economy vehicle included?
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Old 02-14-2022, 12:20 PM   #54 (permalink)
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Clear premium included? In a 1971 economy vehicle included?
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Old 02-15-2022, 01:17 PM   #55 (permalink)
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My 2015 Mazda3 has a 2.0 engine rated at 13.0:1 compression ratio. It does just fine with regular gasoline. No hint of a problem.

That has helped lead to a 4.6 cents-per-mile fuel cost over the ~40,000 miles I've driven it, which, admittedly, was before China's bat virus kicked us in the butt.
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Old 02-15-2022, 03:09 PM   #56 (permalink)
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Originally Posted by MeteorGray View Post
My 2015 Mazda3 has a 2.0 engine rated at 13.0:1 compression ratio. It does just fine with regular gasoline. No hint of a problem.

That has helped lead to a 4.6 cents-per-mile fuel cost over the ~40,000 miles I've driven it, which, admittedly, was before China's bat virus kicked us in the butt.
If GM would have continued manufacturing this drivetrain they could have gotten the best of both worlds on plain 87
2020 era engine controls are a world different from even 2015 let alone 10+ year old tech

A modern version of this would be smaller, lighter, more powerful and more fuel efficient.
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Old 02-15-2022, 07:58 PM   #57 (permalink)
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There are only two kinds of engines that need high octane. Ones that are being pushed to the limit in performance, and engines that are poorly designed.
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Old 03-04-2022, 03:18 AM   #58 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
The reason GM recommends premium fuel in such engines is because such engines are not designed for economy. People don't buy vehicles with V8 6.0L engines to get good fuel economy. Such an engine is built for performance. They want to be able to haul around 10,000lb (sometimes more) trailers. they want to feel as though they got plenty of accelerator to spare. At this point fuel cost is the least of their worries. I personally wouldn't be bothered by needing to use premium fuel if I bought a Corvette, Silverado, Suburban, Tahoe or the like. I would buy one of those for what it can move or how fast it can move it, regardless of the fuel it needs.

This is like buying a Ferrari and asking if you can put cheap tires on it. That's not why you buy a Ferrari. Sorry, I hate to break it to you, but the Tahoe is not a vehicle you buy to get better fuel mileage in either. The hybrid system is a bit of a gimmick to sell cars. Hybrid systems in utilitarian vehicles are to give them more low-end torque. If you get slightly better fuel mileage then that's a small side benefit, nothing more.

In a car like my Prius things are completely different. The car feels like it has a boat anchor constantly attached. Towing a heavy trailer in the thing would be a nightmare. I can cram 5 people in it uncomfortably. But I didn't buy it as a people or cargo hauler or sports car or an off-road vehicle. It's a fuel sipper, that's what I bought it for. That's what people buy them for. And if I or any other Prius owner had to buy premium fuel, we'd be highly upset.

Here's a rule of thumb. If the car is built for fuel mileage, it will run fine on regular fuel. If it needs premium, it is not a fuel mileage vehicle. That is not it's purpose. You can try to get better fuel mileage in it. But you're going against your goal as soon as you put premium in there. Unless premium really is about the same price as regular.

As a suggestion, I'd recommend looking at water injection. If increasing octane benefits fuel mileage, then using something as cheap as water could lower your overall costs. Either that or a methane conversion as methane has an octane of something like 120, IIRC.
the solution to any problem is adding more power

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