Relationship Between Fuel Octane and Power

[The Critic]

Originally Posted By: Shannow
Nissan may (and I say may) have specified premium as the fuel for the life of the engine, but found in the real world that octane creep is lower than estimated, and then revised their recommendation to take advantage of that.

That sounds like a possibility, but this doesn't explain why members on the Nissan Murano forum have noticed reduced power and economy with 87 octane compared to 91/93. The difference, supposedly, is not subtle, but in fact rather noticeable.

Perhaps the next step for me is to go drive a 07 Altima with 91 octane in the tank and compared the two "butt dynos?"

 

[Shannow]

workmate has an I-Phone that has a "dyno" application in it.

Might replace the butt.

 

[The Critic]

Originally Posted By: Shannow
workmate has an I-Phone that has a "dyno" application in it.

Might replace the butt.

jeez, this is too easy.

If only finding an 07-09 Altima SE that has consistently been fed Premium is that easy.

 

[ZeeOSix]

Originally Posted By: CaspianM

The main factor for an engine to be required to run on high octane is higher compression ratio as the low grade fuel would pre ignite under the high compression and combined with heat the engine would knock.


The shape of the combustion chamber, piston crown and spark plug location can also have a bearing on how much octane is required for a given CR also.

It could be that Nissan has changed the physical configuration of the head and tuning of the engine to obtain the same peak HP output with lower octane fuel.

My previous research on this found that the heat energy between 87, 89 and 92 octane (ie "pump gas") is virtually the same. The higher octane rating just means it's harder to ignite the air/fuel mixture ... but once it's ignited the flame speed and energy output is basically the same between all pump gas octanes.

Of course if an engine starts to "ping" with 87 octane and the knock sensor retards engine timing then there will be a power decrease accordingly. If you can get an engine to not ping on 87 without de-tuning via ECU reaction to ping, then the engine would theoretically not put out any more power if running 92 octane.

 

[Vikas]

Is the dyno application a freebie on the iPhone?

 

[IndyIan]

From what I've read dynolicious is supposed to be the best one, but its not free. It seems the iphones actually have better accelerometers in them than the G-tech and produces better results.
I should download it sometime but haven't gotten around to it. The trick in comparing cars is to test two cars on the same piece of road. I find most "flat" roads here aren't that flat.

 

[wapacz]

Originally Posted By: ZeeOSix
Originally Posted By: CaspianM

The main factor for an engine to be required to run on high octane is higher compression ratio as the low grade fuel would pre ignite under the high compression and combined with heat the engine would knock.


The shape of the combustion chamber, piston crown and spark plug location can also have a bearing on how much octane is required for a given CR also.

It could be that Nissan has changed the physical configuration of the head and tuning of the engine to obtain the same peak HP output with lower octane fuel.

My previous research on this found that the heat energy between 87, 89 and 92 octane (ie "pump gas") is virtually the same. The higher octane rating just means it's harder to ignite the air/fuel mixture ... but once it's ignited the flame speed and energy output is basically the same between all pump gas octanes.

Of course if an engine starts to "ping" with 87 octane and the knock sensor retards engine timing then there will be a power decrease accordingly. If you can get an engine to not ping on 87 without de-tuning via ECU reaction to ping, then the engine would theoretically not put out any more power if running 92 octane.


yeah its my understanding its not really compression that dictates the octane that most be used. Rather its is the hot spots formed in the cylinder and head during the compression stroke. That can ignite the fuel if they get hot enough. With that idea you can change the design of the heads cylinders and even spark plug to have a uniform dissipation of heat and a higher compression ratio. Then there are some neat things like oil squirts that shoot at the bottom of the cylinder to remove heat and with direct injection the phase change of the gasoline from liquid to gas to cool down the cylinder.

 

[mechtech2]

Carbon build up was much greater years ago.
A LOT more.
But, it would come off MUCH more readily with an Italian tune up.
Modern deposits from fuel are very hard, and build up slowly.
With today's higher compression ratios, any deposits increase the CR at a faster, non linear rate.

BTW, Pennzoil Ultra must be great for this, as their advertising picture shows it cleans a piston's top!