Why Premium Gas in GM Ecotec 2.4L

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Recently bought a Polaris Slingshot which uses the GM 2.4L Ecotec motor and they
recommend 91 octane. Supposedly it has 10-1 compression; I don't know if
this is different from what GM used in the many cars they put this motor
in, but am wondering why 91 octane is needed?
 
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Polaris can recommend anything that they want.

You start out with "recommend 91 octane" and end with "91 octane is needed".

Recommend is different than needed.
 
GM recommended 91 octane or better for the 2.4L LE5 in Solstice, Sky, Cobalt/G5, and HHR applications.

The only applications to recommend 87 were Malibu, Aura, and Vue.

Being the Slingshot lifted the engine and trans from the Solstice/Sky, I'd image they're simply following right along with what GM recommended for the powertrain combo.
 
Try regular fuel and if it doesn't knock, you're good to go. (knock sensors will retard the spark) I'm sure there's more horsepower on tap with the higher octane. If you need the extra power, run 91.
 
I'd consider following the recommendation unless you're never going to drive in a spirited manner and just cruise around really slow.
 
I had a 96 Solstice and the manual said up to 91 octane. The truth was, it ran fine on 87, but had a little more power on 91. I could only tell the difference at high rpm. Otherwise, it felt and sounded exactly the same and ran perfectly on 87. I usually ran 87 and then on my way home on Friday, I'd fill up with 93 for the weekend to give it a shot in the arm for some fun runs on twisty roads.

Enjoy that Slingshot! It should be pretty quick with that powertrain. I loved my Solstice. The engine was great, the handling was awesome, and going through the gears on the 5 speed manual felt like a video game. Gosh I miss that car!
 
The octane requirements are just as much based on the calibration as the hardware. If the manufacturer recommends 91 or higher, the "tune" is likely optimized for 91 or higher.
 
Originally Posted By: mrsilv04
Polaris can recommend anything that they want.

You start out with "recommend 91 octane" and end with "91 octane is needed".

Recommend is different than needed.

Some car's owner manuals didn't say anything like "recommend" or "required" or "needed".

Honda S2000 manual just say "Premium unleaded gasoline, pump octane number of 91 or higher"

To some it is recommended, to others it is required. You can interpret that statement anyway you like.
 
Thanks folks for the feedback. I've been running mostly 89 and it does ok with that. The next grade in my area is 93. I think I'll try some 87 and see how it goes. I'm pretty much an easy driver.
 
If you can afford this expensive a toy, I think you can afford the put the correct octane in it
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My understanding is that you cant hear all knock and ping and without looking at the actual data can you know if its alright. And not all cars can pull enough spark to get rid of all knock.
 
Originally Posted By: Nate1979
My understanding is that you cant hear all knock and ping and without looking at the actual data can you know if its alright. And not all cars can pull enough spark to get rid of all knock.


This is absolutely correct. Knock tolerance is wildly different by application and simply cannot be generalized. Odds are if you actually hear knocking there is far more pre-ignition occurring than you will ever know.

Many newer cars simply will not detonate under almost any conditions, but that is simply a function of sensitive knock sensors.
 
My Burg calls for 87 octane...with 12.5 compression.
shocked.gif
Like, wow!

To the OP: it's a Slingshot...just run the good gas!
cool.gif
 
Originally Posted By: SteveSRT8
Originally Posted By: Nate1979
My understanding is that you cant hear all knock and ping and without looking at the actual data can you know if its alright. And not all cars can pull enough spark to get rid of all knock.


This is absolutely correct. Knock tolerance is wildly different by application and simply cannot be generalized. Odds are if you actually hear knocking there is far more pre-ignition occurring than you will ever know.

Many newer cars simply will not detonate under almost any conditions, but that is simply a function of sensitive knock sensors.


Yes, modern knock sensors and engine computers do a great job of reacting to the slightest knock. Back in the 1980s, you could gently tap the engine with a hammer, and the computer would retard the spark enough to bog down the engine. It would slowly advance spark back to normal. Now you have to use data logging or an oscilloscope to see what is happening when you tap the engine with a hammer.

In some Ecotec engines, there was no conventional knock sensor. Instead, the ignition coils would detect abnormal combustion somehow.

Some engines are programmed to ignore knock sensors under certain conditions. I remember mom's 1999 Honda CRV and 2002 Toyota Solara called for 87 octane fuel, but there would always be a few knocks at full throttle.

Anyway, you probably have an engine that will not be damaged by low octane fuel, but you will have reduced performance.

My dad's Pontiac G6 2.4 calls for 87 octane in the user's manual, so we never used anything else.
 
It can't possibly use that much gas...what are we talking about $20 a year for premium over regular?
 
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