your forester has a FA 20F engine
the ascent has a FA 24 engine

All turbos are not equal

 

Hello and welcome @Postalleyblend. Subaru has apparently given our 2.4L engines a pretty conservative tune. Aftermarket tuning companies have found that there's really no benefit to running premium in these engines, so it's not even like Subaru recommends 87 knowing it holds the performance back. It seems that they've taken the deliberate approach of tuning the car specifically for 87 octane.

They may have gained some experience with the 2.0L engine in the Forester XT. They may have determined that premium didn't offer a big boost, or at least not enough to justify its use and associated cost increase for the consumer.

 

There are definitely design differences that allow the FA24 to operate very well on 87 octane fuel in comparison to the FA20DIT which recommended Premium. The additional displacement and differences in tuning make the FA24 feel much stronger in the low end than the FA20DIT which was a bit more peaky (but still ran out of breath up top).

The only time I was forced to use 87 octane in our 2014 Forester XT was not a fun experience. The motor pinged and knocked until the ECU adjusted things (likely pulled timing) to run more smoothly on the 87 octane fuel. At that point, power was greatly reduced and the fuel economy also suffered.

Long story short, 2 different engines (with some similarities) built for different purposes. I have been very impressed and happy with the way the FA24 performs using regular fuel.

 

^ There's been some great technical answers above, but the simple answer is that like everything else in this world, the technology has improved and continues to improve.

Rest assured that your new Ascent's engine will be just fine on 87 Octane. The manual doesn't lie, particularly as the manufacturer has their warranty to honor.

Our family's vehicle history has had its share of turbocharged Subarus: '05 WRX, '05 Legacy 2.5GT (5MT, one of the first in the midwest that was ECUTeK tuned), '09 and '13 Forester XTs, '16 and '19 WRXs.

Follow the manufacturer's recommendations, you;ll be just fine.

 

To be very clear, the higher octane fuel requirement that many older generation of Turbo engines (and some current vehicles) isn't because they are turbocharged. The reason is that those particular engines have designs that incorporate higher compression. Higher compression can cause pre-detonation of the fuel (knocking) and that is combated by high octane fuel which burns slower. Subaru engineered the Ascent's (and Outback XT's) 2.4L turbo to be able to run reliably on 87 or higher octane fuel without knocking. If someone really wants to kick things up a bit, Cobb does offer a high octane tune at which point one would have to buy and use premium fuel.

 

To be very clear, the higher octane fuel requirement that many older generation of Turbo engines (and some current vehicles) isn't because they are turbocharged. The reason is that those particular engines have designs that incorporate higher compression. Higher compression can cause pre-detonation of the fuel (knocking) and that is combated by high octane fuel which burns slower. Subaru engineered the Ascent's (and Outback XT's) 2.4L turbo to be able to run reliably on 87 or higher octane fuel without knocking. If someone really wants to kick things up a bit, Cobb does offer a high octane tune at which point one would have to buy and use premium fuel.

In terms of AFR, do you know what they change? Do they revise the tune for lower or higher rpm?

 

As @Jim_in_PA noted, the higher octane reduces knock with higher compression ratios. Since turbos under boost tend to knock, manufacturers have required higher octane to protect the engines from knock damage. As engine technology has advanced, the manufacturers have made other modifications to alleviate knock with standard octanes. Direct injection and intercoolers are major factors in allowing lower octane because the cooler fuel/air charge is less likely to pre-ignite.

 

Compression ratios and the type of fuel injection used are big factors in whether or not Premium fuel is required to prevent pre-ignition or knock in an engine. However back to the OP question, the FA20DIT in the 2014-2018 Forester XT and the FA24 in the Ascent have identical compression ratios (10.6:1) and both use direct injection.

There are many other design details in the engine, along with the tuning of the ECU (spark advance, fueling, boost pressure, etc.) that create the differences in fuel octane required.

 

An interesting aside on this topic (though still related): turbocharged engines typically have lower compression ratios than their naturally-aspirated counterparts because of the already-pressurized air entering the cylinder. The naturally-aspirated FA20D (in the BRZ) has a 12.5:1 compression ratio, whereas the turbocharged FA20F (in the Forester XT) and our FA24F have a 10.6:1 compression ratio.

The point about direct injection supporting lower octane is a great point. The port-injected FB25B has a 10.0:1 compression ratio and the newer direct-injected FB25D has a 12.0:1 compression ratio...both recommending regular octane.

Of course, there are a ton of variables at play, including other aspects of the tuning, maps, etc.

 

I do not know the answer to your question, Trey. But Cobb is here in the forum in the tuning discussion area...you could ask them directly.

Turbo And Tuning

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