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Old 12-16-2021, 09:51 AM   #41 (permalink)
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Originally Posted by Isaac Zachary View Post
Where octane is important is at higher engine loads, like when you have the pedal to the metal. But shifting from 4th to 3rd at part load because of using the recommended octane instead of high octane fuel for forced induction engines sounds fishy.
High engine load can occur at any rpm. Also, high engine load at low engine speed is more prone to pre-ignition because the air/fuel mixture has more time to heat up before the spark.

Given that these are hybrid vehicles also means that the ECU is always trying to run the engine moat efficiently, high load and low engine speed. While the vehicle is rated for 87 octane, it does benefit from more knock resistant fuels.

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Old 12-17-2021, 11:30 AM   #42 (permalink)
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I still don't get it.

How can I increase the CR of a stock 1972 1600cc air-cooled VW Super Beetle to 10:1, advance it to 45 BTDC (max), run 85 octane in it, lean the engine to run between 16:1 to 13:1, drive it at 65mph up long steep mountain passes with my "shift into highest gear at lowest speed possible and keep it floored" philosophy, and not get any pinging?

And then someone gets a V8 hybrid, drives it at 40mph on flat ground, and benefits from 91 octane to do that?

It makes zero sense to me.

It sounds like it's coked up with carbon and suffers from hot spots or has poor attomization. That or GM did a terrible job at designing the heads. Did the OP get a poor valve job or something and mess up the seat angles?
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Old 12-17-2021, 12:16 PM   #43 (permalink)
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Valid question, I don't have answers but maybe things to consider.
Static compression vs dynamic compression
operating temperature
elevation(which is going to directly affect that dynamic compression ratio)
carburetor vs fuel injection
Ambient temperature
Relative humidity

And others I'm sure.
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Old 12-18-2021, 06:07 PM   #44 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
Valid question, I don't have answers but maybe things to consider.
Static compression vs dynamic compression
operating temperature
elevation(which is going to directly affect that dynamic compression ratio)
carburetor vs fuel injection
Ambient temperature
Relative humidity

And others I'm sure.
That is a lot to consider. Static compression should be fairly high with a stock VW engine due to the stock low RPM cam.

Operating temperatures had head temps around 350F (verified with head temp sensor).

Elevation "affects" compression ratio. But you're also starting out with air that's warmer for the same amount of compression. So the air heats up similarly from the same amount of compression. And it's the heat, not the compression, that causes the air-fuel charge to detonate. (If you were to suddenly stretch out air at sea level to the same pressure as air at high altitude it would be considerably cooler.) A study in the 1980's revealed that when spark and AFR's are compensated for the change of altitude (which I did in the Bug), every 1,000 ft of altitude only allows for a decrease of around 0.2 R+M/2 octane. So technically at my altitude I should be using 85.5 R+M/2. https://www.sae.org/publications/tec...ontent/872160/

Fuel injection should help improve knock resistance due to better atomization.

Ambient temps aren't that high around here, so that could be part of it.

But the extremely dry air here should also increase knock probability IIRC.
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Old 02-11-2022, 07:46 PM   #45 (permalink)
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Quote:
Originally Posted by ksa8907 View Post
High engine load can occur at any rpm. Also, high engine load at low engine speed is more prone to pre-ignition because the air/fuel mixture has more time to heat up before the spark.

Given that these are hybrid vehicles also means that the ECU is always trying to run the engine moat efficiently, high load and low engine speed. While the vehicle is rated for 87 octane, it does benefit from more knock resistant fuels.
by the time the knock sensors pick up the issue you will need a new engine
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Old 02-11-2022, 09:50 PM   #46 (permalink)
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Originally Posted by Tahoe_Hybrid View Post
by the time the knock sensors pick up the issue you will need a new engine
What do you mean?

I've driven cars around with obvious engine knock and did not need a new engine.

New cars have a knock sensor (or sensors) for a reason. It would be kind of silly ff the reason they are there were to simply verify the engine blew up.
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Old 02-12-2022, 03:08 AM   #47 (permalink)
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Originally Posted by Isaac Zachary View Post
I don't see why lower octane would cause it to downshift unless it's severly affected by something else.

On the other hand, it kind of defeats the purpose of getting better fuel mileage if you have to pay for premium fuel.

Octane shouldn't be much of a difference at lower engine loads. At lower loads engine timing and power output have more to do with just finding the right ballance. At idle you should be able to advance the timing as much as you want and not cause any detonation.

Where octane is important is at higher engine loads, like when you have the pedal to the metal. But shifting from 4th to 3rd at part load because of using the recommended octane instead of high octane fuel for forced induction engines sounds fishy.
premium is only 4.58 when they have the weekly discount... it's only 0.10c more then regular... all the cars have been switched to premium... t
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Old 02-12-2022, 03:14 AM   #48 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
What do you mean?

I've driven cars around with obvious engine knock and did not need a new engine.

New cars have a knock sensor (or sensors) for a reason. It would be kind of silly ff the reason they are there were to simply verify the engine blew up.
relaying on sensors is ridiculous the v8 engines should be getting premium anyways 10.7:1 compression ratio

the cost per mile is less
23 with 91
0.199 /mile

18.5 with 87
0.24

Last edited by Tahoe_Hybrid; 02-12-2022 at 03:24 AM..
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Old 02-12-2022, 07:01 AM   #49 (permalink)
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10.7:1 still doesn't mean it needs premium in my opinion. I don't try to second guess manufacturers and their recommendations. So many other things affect detonation that the compression ratio is meaningless without taking every other part of the engine design into account.

For an example, one thing I did on my Bug was create a .030" piston to head "quench" space which lowers the chances for detonation, which is why some call it "mechanical octane." Modern engines have an even tighter quench and that is even slightly wedged to maximize its effect. Another factor is fuel atomization and head designs that keep the fuel atomized also greatly reduce detonation. 10:1 is high CR for a carbureted car. It isn't for a modern fuel injected one. Then there's also the cam profile factor. Many modern fuel-efficient engines create an atkinson cycle effect with their cam profiles. As a result, the actual dynamic compression ratio can be far lower than the measured static compression ratio.

At any rate, if it's only 10 cents more expensive, then by all means do what you want to do. Premium is 90 cents more expensive here. That's almost a dollar more per gallon, or nearly a third more expensive. It is even a pain to have to pay more for our mid-grade, which is 87 octane or regular everywhere else, every time I have to drive out of state. I can nearly reach the coast on one tank and wouldn't want to do it with 81 to 85 octane.
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Old 02-13-2022, 05:04 PM   #50 (permalink)
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Quote:
Originally Posted by Isaac Zachary View Post
10.7:1 still doesn't mean it needs premium in my opinion. I don't try to second guess manufacturers and their recommendations. So many other things affect detonation that the compression ratio is meaningless without taking every other part of the engine design into account.

For an example, one thing I did on my Bug was create a .030" piston to head "quench" space which lowers the chances for detonation, which is why some call it "mechanical octane." Modern engines have an even tighter quench and that is even slightly wedged to maximize its effect. Another factor is fuel atomization and head designs that keep the fuel atomized also greatly reduce detonation. 10:1 is high CR for a carbureted car. It isn't for a modern fuel injected one. Then there's also the cam profile factor. Many modern fuel-efficient engines create an atkinson cycle effect with their cam profiles. As a result, the actual dynamic compression ratio can be far lower than the measured static compression ratio.

At any rate, if it's only 10 cents more expensive, then by all means do what you want to do. Premium is 90 cents more expensive here. That's almost a dollar more per gallon, or nearly a third more expensive. It is even a pain to have to pay more for our mid-grade, which is 87 octane or regular everywhere else, every time I have to drive out of state. I can nearly reach the coast on one tank and wouldn't want to do it with 81 to 85 octane.
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?


Last edited by Tahoe_Hybrid; 02-13-2022 at 05:14 PM..
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