Naturally aspirated and 87 octane...

[usptwins]

so why the question? that was rediculous. If you can/want to put 93 in your car, then why not just do it? This motor was made to use 93 and thats why its rated at 200 crank. That was a foolish question... Just put what it asks for and your all set. No need to wonder about something you don't/won't do.

[cruzinbill]

Originally Posted by joop1987

When you bought your FR-S/BRZ, the dealer probably put 87 in it and your car didn't blow up. The Mustang 5.0 pulls timing and runs on 87 fine; just less HP. Until we have the software available for logging, we just don't know. The logic might be there now.

I'm not sure if ECUTEK found the memory location where timing is stored. If they did, we can look at the difference between the exptected timing and actual. See if the timing is pulled with 87 octane once pinging is detected. The Turbo Subaru's have a software tool called Learning View that does this.

You must have some really shady dealers around you.... The Mustang also isnt running the same compression ratio nor the same type of fuel injection set up.

[joop1987]

Originally Posted by cruzinbill

You must have some really shady dealers around you.... The Mustang also isnt running the same compression ratio nor the same type of fuel injection set up.

If the Mustang can do it without direct injection, the FA20 should have no problem with 87.

All car dealerships says the put in premium, but they always put in the cheapest gas they can get.

[Turbowned]

Why 93 octane?

Because race car.

[TheRipler]

Short answer: Yes it will work, and you will lose power.

I know a guy who had his WRX tuned for 87. Thing is, he didn't tell the tuner before the session, just filled it up. Tuner was disappointed with the power he got, and probably spent longer than normal tuning. He was noticeably upset once the guy told him why it was so low. It was a good 87 map, but I recommend informing the tuner of your fuel choice before attempting.

[cruzinbill]

Originally Posted by joop1987

If the Mustang can do it without direct injection, the FA20 should have no problem with 87.

All car dealerships says the put in premium, but they always put in the cheapest gas they can get.

If thats what you want to believe thats fine. Most dealers arent like that tho. I saw mine filled up with prem, as have others. I was out walking around looking at used cars and one of the techs were over filling it up.

Also, if they are caught going against correct servicing specs it should be brought up to their supervisors.

[zooki]

Never mind guys, sorry for having a curious mind. Please go back to arguing over which air filter makes 1 more HP than another. I appreciate all the thoughtful replies.

[tech4pdx]

@zooki.. Not too long ago, I was being absent minded and told the gas station attendant to fill up with $15 regular (87 octane and here in Oregon you cannot pump your own gas). I didn't realize what I said until he gave me the receipt. Guess what happened to the motor? Nothing. It was fine. I just didn't go WOT until it all burned away. I'm nearly certain that Toyota/Subaru have already accounted for the fact that people might do this occasionally. I'm sure the motor can be tuned to run on a lower octane of gas, but (as you prob already know) you will make less power. The motor is high compression. If we were talking about a 8.5:1 motor, different story of course. I have also researched differences in behavior in this regard with iron vs aluminum heads.

Good thought experiment. Just not sure that any tuners would be willing to put much effort into an 87 tune on 12.5:1.

[Turbowned]

You can put 87 octane in the stock motor, but you shouldn't make a habit of it. A dealership that I used to work for would put regular gas in the engine for it's first fill-up. I can't say I agreed with it, but I'm sure it wouldn't do too much harm.

[wparsons]

Originally Posted by zooki

Well, here's my train of thought...if the aftermarket can get the electronics to cope with increased cylinder pressures of FI through tuning while still using 93 octane gasoline, why can't the engine be tuned to use 87 octane at the decreased cylinder pressures that a N/A motor has. It's not like 93 octane has any more power potential than 87, in fact it may have less power potential do do its slower burn rate. I think direct injection motors have a lot more leeway in the compression/octane relationship due to the way fuel is introduced into the cylinders. If you can get the same amount of power with lower octane gas, why wouldn't you? I'm talking about from the factory. I wouldn't spend the money on an aftermarket tune to get the motor to produce 200 HP on 87 octane. But I think the factory could do it. So, just to be clear. I am not looking for an aftermarket tune for 87 octane, I am fine with the car the way it is, I have no problem buying 93 octane either. This is just a thought experiment on what I've seen boosted motors doing with 12.5 to 1 compression and pump gas.

It isn't cylinder pressures that require higher octane, it's the amount of spark advance that requires higher octane. Yes increasing boost will create more opportunity for knock/detonation, but that can be fixed by pulling a bit of timing back.

It isn't an issue on boosted cars because they're adding a lot more power already, so pulling timing back to avoid detonation isn't an issue.

On an N/A engine you would have to pull timing to prevent detonation, but without the boost there's no way to get that power back. If you were to run an N/A engine on the timing tables from a boosted tune you would be losing quite a lot of power across the board.

The stock ECU will pull timing back if it detects any knock, so it's "safe" to run it on 87 octane, but you'll get worse mileage and less power since the ECU isn't running on the optimal maps.

[xwd]

It's absolutely cylinder pressures which require higher octane...

Spark advance can cause detonation, but the reason you have to pull back timing is based on cylinder pressure, whether it's a NA or FI application.

87 octane fuel doesn't burn correctly with higher cylinder pressures and spark, it explodes, which causes detonation. To say you could tune a car for 87 octane and get the same amount of power as a car tuned on 93 is completely incorrect.

[tech4pdx]

I always thought that the relationship of spark advance to cylinder pressure was inverse.

[zooki]

Originally Posted by dwx

It's absolutely cylinder pressures which require higher octane...
Spark advance can cause detonation, but the reason you have to pull back timing is based on cylinder pressure, whether it's a NA or FI application.
87 octane fuel doesn't burn correctly with higher cylinder pressures and spark, it explodes, which causes detonation. To say you could tune a car for 87 octane and get the same amount of power as a car tuned on 93 is completely incorrect.

I'm wondering how much direct injection changes this. Just a few years ago you would have never thought we could boost a 12.5 to 1 compression motor with 5-8 lbs. of boost on 93 octane, but yet here we are, doing it reliably. I would guess(just a guess) that a DI motor can use a lot less spark timing since the ability to control the injector timing and precise amount of fuel injected seems to help control detonation in a big way. I have been trying to find info on how spark timing relates to fuel injector timing on a DI motor in the context of crank degrees. Plus the advances in combustion chamber design that promotes fast, even burns. But I'm still looking for that info. Anyway, I was just curious, and thought I'd throw it out here. I guess I should have expected some of the replies I got....

[wheelhaus]

Originally Posted by zooki

Never mind guys, sorry for having a curious mind. Please go back to arguing over which air filter makes 1 more HP than another. I appreciate all the thoughtful replies.

lol I feel your pain... Trying to get a technical thought discussion going on the internet is... painful.

I don't have the answer but I thought I'd help clear up some confusion.
I too don't quite understand all the intricacies of engineering and tuning a modern DI internal combustion engine, but I get what the OP is saying. Most of you boneheads need to read the question and stop staring at your own colons.

If a high compression engine requires 93 octane at stock power levels @ 12.5:1 CR, but 93 octane is also stable enough for bolt on FI at 12.5:1 CR, why isn't the inverse true? IF an OEM engine were designed at 12.5:1 CR boosted and required 93oct, wouldn't we expect to be able to run lower octanes if it were changed to NA and produced less power?

It's an exercise in engineering thought, the question is not "hai guyz will it go OK on 87? derp? And the answer is not "OMG RTFM NOOB".

Anyways, it's obvious that high compression and high heat require more stable octane ratings, but the (perfectly legitimate) question still stands: How much has DI changed this train if thought? Could DI allow us enough finite control of what's happening in the cylinder to improve efficiencies and net the same power from lower octanes?