93 Octane?

[serialk11r]

Speaking of pulling timing, I wonder if newer cars have ECUs that will adjust valve timing before adjusting ignition timing...Hyundai for example seems to not lose fuel economy on 87, which probably wouldn't be the case if they were relying solely on retarded ignition timing to prevent knock.

At any rate with gas prices these days, premium vs. regular difference is getting effectively smaller.

[Sasquachulator]

Damn 93 is not widely available here, but there are a few pumps that have it scattered around the city.

I'd like to use 93 if at all possible.

[86fanatic]

Real question I would be asking is what does 91 vs. 93 look like to WHP numbers. With a low power light car like this, every single horsepower counts. I assume the official 0-60 is done with 93 octane.

I only have 91 available in my area as well.

[blu_]

It won't make less hp unless it has to pull timing.


If you live in an area with only 91 its just something you have to accept if you want to own these kind of cars. I understand why people worry about it, but its really not that big of a deal when you think it through. And given that you cant do a thing about it, there isnt much of a point of worrying.

The only thing you can do is get a tune at some point when you mod the car that is optimized for the gas you always use.

[Want.FR-S]

IIRC WRX and STI both have the same requirement of asking for 93 AKI but it seems like people have been using 91 AKI premium without issue. Bear in mind that in Japan, 98 RON (93 AKI) is their premium gas and some stations even provide 100 RON as well. So just use 91 AKI if that is only available.

[ashtray]

Re: "hearing" knock. I still think the driver shouldn't hear audible knock - the sensor should detect slight knock and start adjusting timing. There shouldn't be 5 seconds of rattle from under the hood before the engine reacts.

Re: hp. On my WRX, stage 1 tuning, I use the 91 octane map. It's rated at 5hp less than the 93 map. Extrapolating you might say we're losing 3-4hp on the BRZ, but I don't think you can make a direct comparison between the two engines. And 3-4hp is probably the variance from one identical engine to the next anyways, and likely not noticeable.

[SkullWorks]

E85 requires about 30% more fuel to achieve it's stoichiometric ratio.

You usually need a larger Fuel pump and injectors just to tune at the stock power levels,

The rubber liner in the fuel hoses got revised to deal with E85 and in general the INSIDE of any new fuel injection hose is compatible with Ethanol,

HOWEVER the outside of the hoses aren't all compatible, and the intank hoses tend to degrade rapidly with above E10 concentrations of ethanol in fuel.

I am anticipating it will be a popular mod to convert the cars to run on E85 in NA form to take full advantage of the compression and DI.

After i finish the DI injectors and pumps that is

[blu_]

Originally Posted by ashtray

Re: "hearing" knock. I still think the driver shouldn't hear audible knock - the sensor should detect slight knock and start adjusting timing. There shouldn't be 5 seconds of rattle from under the hood before the engine reacts.

Re: hp. On my WRX, stage 1 tuning, I use the 91 octane map. It's rated at 5hp less than the 93 map. Extrapolating you might say we're losing 3-4hp on the BRZ, but I don't think you can make a direct comparison between the two engines. And 3-4hp is probably the variance from one identical engine to the next anyways, and likely not noticeable.

Yes, if you are hearing audible knock at high RPM and throttle, at least on my STI, you have probably already done damage to your engine. Today's computers are very sensitive, so much so that road vibration can even cause phantom knock that can pull timing temporarily. Older engines weren't tuned nearly as aggressive, so knock wasnt as big of an issue.

As far as running a 91 stage 1 map, you have less hp because the timing map has been retarded in the tune. You are basically not making as much EXTRA hp in those maps. But on a stock map there are larger tolerances and as long as the computer isnt pulling timing you should make the same amount of power as if you were running 93 (at least Im pretty sure). Running high octane fuel in my old car wouldn't make the car feel fast, it would just feel a bit smoother.

[EvoFanatic]

Originally Posted by Dave-ROR

The smallest wheel size that fits the rotor and caliper assembly = the brake size. So any rotor and caliper that won't fit under a 14" wheel but will fit under a 15" wheel is a 15" rotor..

Totally wish that's how measurement system worked here. It makes so much more sense.

[Baldeagle]

Just curious. How is it that Mazda’s new SKYACTIV-G 2.0-liter engine uses a 12.0:1 compression ratio and requires only 87 octane fuel and the Subaru 2.0 liter engine uses a 12.5:1 compression ratio and requires 93 octane? (The Mazda 2.0-liter engine produces 148 lb-ft of torque compared to 151 lb-ft for the Subaru. Power output is not that different.)

[chulooz]

Originally Posted by Baldeagle

Just curious. How is it that Mazda’s new SKYACTIV-G 2.0-liter engine uses a 12.0:1 compression ratio and requires only 87 octane fuel and the Subaru 2.0 liter engine uses a 12.5:1 compression ratio and requires 93 octane? (The Mazda 2.0-liter engine produces 148 lb-ft of torque compared to 151 lb-ft for the Subaru. Power output is not that different.)

Just missing ~50hp

I dont know what kind of voodoo Mazda is using for skyactiv but it's got some nice perks for sure.

Please can we just drop the fuel requirement concerns; its tuned for premium and that's that.

[Enemies]

Like almost 20% less horsepower... But, not that different!

That's like saying Hyundai Genesis Coupe weight and FR-S/BRZ weight... Not that different!

[Baldeagle]

I understand the engines are not identical, but as stated - close. From a few quick calculations, I can conclude the shape of the torque curve is different. We know that the Subaru engine generates 151 lb-ft @ 6,400rpm. We also know that the Mazda engine produces 155 hp @ 6,000 rpm. From that we can calculate that the Mazda engine must also produce 135 lb-ft @6,000rpm. At high rpm, the boxer engine has more torque. (But then again, at 4,000 rpm the Mazda engine produces 148 lb-ft while the boxer engine probably produces about 135 lb-ft. So its a give and take. )

But can anyone help me understand why one engine with a 12.0:1 compression ratio is fine on 87 octane fuel and why another engine with a 12.5:1 ratio requires 93 octane. Does the answer lie in valve size, valve angle, camshaft profiles, cylinder bore, etc.? (And if the concept of efficiency enters the equation, why does the heavier, larger and less aerodynamic Mazda3 get 39mpg hwy compared to the estimated 30 mpg hwy for the BRZ?)


I realize these are technical questions. Perhaps I should have selected an existing thread in the “engine” section of the forum rather than the “general” forum to post these questions. But since it is primarily an octane related question, I selected this thread.


EDIT: Could it be that the Mazda engine use “straight” direct injection and the Subaru engine uses a combination of direct and port injection? Could the direct/port combination provide less cylinder cooling than straight direct injection? If cylinder temps are higher, it would contribute to pre-detonation and the need for higher octane. Any thoughts on that?

[old greg]

Originally Posted by chulooz

Just missing ~50hp

Originally Posted by Enemies

Like almost 20% less horsepower

The difference in HP is purely due to RPM. In terms of BMEP (the standard yardstick for pissing contests between engineers) the two engines are very evenly matched.

Originally Posted by Baldeagle

But can anyone help me understand why one engine with a 12.0:1 compression ratio is fine on 87 octane fuel and why another engine with a 12.5:1 ratio requires 93 octane.

Mazda went to great lengths to increase the knock resistance of their engines.

Originally Posted by Baldeagle

And if the concept of efficiency enters the equation, why does the heavier, larger and less aerodynamic Mazda3 get 39mpg hwy compared to the estimated 30 mpg hwy for the BRZ?

Taller 6th gear (probably) and lower bsfc at low engine speeds. The same characteristics that allow the Subaru motor to make more torque at high rpm than the Mazda make it less efficient at low rpm.