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87 vs 91 octane for better mpg?

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87 vs 91 octane for better mpg?

  #11  
Old 01-25-2008, 03:27 PM
Omnia Gloria Fugit
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Default Re: 87 vs 91 octane for better mpg?

The engine is tuned and designed to run on 87 octane! Octane is a NUMBER or rating that is used to express the resistance to uncontrolled pre-ignition, generally a slower combustion. If the engine is designed for a faster combustion (lower octane) then you will actually lose mpg by running a higher octane rating. (before you flame me this is for a layman's understanding of octane)
 
  #12  
Old 01-29-2008, 03:53 AM
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Default Re: 87 vs 91 octane for better mpg?

Using 93 octane in my insight
gives me more power during lean-burn.
That results in higher MPG.
 
  #13  
Old 01-29-2008, 05:44 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by rrrrrroger View Post
Using 93 octane in my insight
gives me more power during lean-burn.
That results in higher MPG.
You might consider updating your profile to show the vehicle and geographical area. Also, I didn't see a mileage record in the Insight database. Of course it isn't required but it often gives folks a clue about how Insights work.

What year? How many miles? How long have you had it?

Bob Wilson
 
  #14  
Old 01-29-2008, 06:40 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by Tim K View Post
The bottom line is pretty simple....

Use what the manufacturer recommends. They are telling you what the engine was designed to run on. Using anything else will likely have a negative effect on both your performance and fuel economy.

(I'm sure someone somewhere will post that in THEIR car THEY got better performance using a different fuel rating than recommended. In reality, your car will run best on what is recommended unless there is a problem with your vehicle...in which case the right action is to get it fixed not change fuel rating.)
This is all assuming the manufacturers are giving you a number from the engineering group. Marketing can always have a say in the number if they feel it is in the best interest of the company to market the car at a lower octane level. The engineering group will sign off saying it can be tuned to the lower octane level but actually can perform better at the higher octane level. I believe the Prius engine (atkinson cycle engine) is such a case.
 
  #15  
Old 01-29-2008, 07:49 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by livvie View Post
The engineering group will sign off saying it can be tuned to the lower octane level but actually can perform better at the higher octane level. I believe the Prius engine (atkinson cycle engine) is such a case.
The Atkinson type engine behaves the opposite of a high compresson race engine. The lowest octane available is the best choice. I use 85 octane in the Rocky Mountain states, and it works great when I drive a tank down to near sea level.

Octane is "anti-knock"... and the lower compression due to the atkinson cycle makes knocking almost impossible with even the lowest octane.

( I heard Poland sells 80 Octane, and Russia... 78 Octane... I'd like to try it some time... but it's pretty far for me to get some! ) -John

Numbers below from 85 Octane... 4,000 pound SUV too!
 
  #16  
Old 01-29-2008, 08:48 AM
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Default Re: 87 vs 91 octane for better mpg?

Are you sure about lower compression in an atkinson cycle? I could have sworn it had a higher compression than average.

I believe the Prius has:

Toyota Prius hybrid electric (front-wheel drive) with a compression ratio of 13.0:1

That's pretty high.
 
  #17  
Old 01-29-2008, 09:48 AM
SPL
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Default Re: 87 vs 91 octane for better mpg?

That's the "theoretical" compression ratio that you'd get if the closing of the intake valve wasn't delayed. But it is significantly delayed (by 32 degrees in the Camry Hybrid's 2.4-L ICE versus the non-hybrid's 2.4-L ICE). It's the expansion ratio that's represented by this number; the effective compression ratio is around 10:1, as in all low-octane ICEs. Calling it a "compression ratio" is what's throwing you off. The big expansion ratio is what gets more of the thermal energy out of the gases before they're discharged (hence the higher thermodynamic efficiency) — but it also breathes in less fuel per stroke, and so results in the lower power of the "Atkinson"-type ICE for its rated size. It's really more like a 2-L than a 2.4-L ICE power-wise.

You can see the comparison of these two engines on page EG-3 of the attached file from the 2007 Camry Hybrid "New Car Features Guide."

Stan
 
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  #18  
Old 01-29-2008, 10:10 AM
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Default Re: 87 vs 91 octane for better mpg?

Hmmm... cool. Can you explain the formula on how to take theoretical compression ratio and convert it to the "actual". I understand the delay, is it because the valve is open that the volume of the chamber is actually bigger therefore the compression ratio is smaller? So when cylinder is at TDC there is a valve (exhaust) that is still open?
 

Last edited by livvie; 01-29-2008 at 10:18 AM.
  #19  
Old 01-29-2008, 10:34 AM
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Default Re: 87 vs 91 octane for better mpg?

Originally Posted by livvie View Post
Hmmm... cool. Can you explain the formula on how to take theoretical compression ratio and convert it to the "actual". I understand the delay, is it because the valve is open that the volume of the chamber is actually bigger therefore the compression ratio is smaller? So when cylinder is at TDC there is a valve (exhaust) that is still open?
It is possible to work out the geometry but in the Prius community, we have access to the Toyota papers and maintenance manuals. The commonly believed values are:

8-to-1 - compression stroke
13-to-1 - expansion stroke

If you want to work out the geometry, the ratio should be roughly proportional to:

((intake_delay_close_in_degrees) / 180 degrees) * (max_cylinder_volume/min_cylinder_volume)

Due to the geometry of the connecting rod, there will be some variance but for a rough approximation, this should be close enough.

Bob Wilson
 
  #20  
Old 01-29-2008, 11:15 AM
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Default Re: 87 vs 91 octane for better mpg?

Using 93 octane in my insight
gives me more power during lean-burn.
That results in higher MPG.


.............because I can avoid using rich-burn when climbing gentle slopes (I left out that key piece).
Originally Posted by livvie View Post
Toyota Prius hybrid electric (front-wheel drive) with a compression ratio of 13.0:1
That's the compression-to-expansion ratio. Prius has an abnormally large expansion, but the same compression as any other car, so there won't be any pre-ignition of the fuel even with 85 octane.
 

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