10:1 compression.... 87 octane???

[Dominic]

My Subaru 2.5i is normally aspirated with computer controlled ignition, variable valve lift, and MPFI injection. The owner's manual states "87-octane" which is what I've been running with seemingly no problems.

That said... the CR is 10:1 on this engine, which makes me feel funny when I put 87-octane in it. Does anyone here agree with me that I should probably run 89 in it at least, or should I assume the Japan guys retarded the timing enough to run 87 and that 89 would be a complete waste of money?

Just stirring the pot...

 

[dwendt44]

Does the Subaru have a knock sensor?
The static compression ratio may 10:1, but the running CR is likely less.
The computer on newer cars adjusts valve timing and ignition timing to compensate. Newer engine designs (combustion chambers) can run higher compression without ping or knock on lower octaine fuel.
If it runs well on 87, run 87.
You gain nothing and may lose something by running higher octaine than what's called for.

Exception: If you are out in the 'boonies' and the only available fuel is 'Fred's Quick Stop', I'd go for a little higher grade.
My 2¢

 

[Steve S]

What you have to try is to run 4 or 5 full tanks of the high octane and see if the gas mileage increases.

 

[98LSC32V]

I would use 91-93 octane if it was my car...

 

[Quest]

LOL!

My fit is at 10.4:1 compression ratio and it calls for 87 octane only.

tried 92~94 before, no change in performance and mileage.

if yours calls for 87, I would use 87.

 

[racer12306]

im with quest, if it says 87 is good then run 87

 

[Vilan]

My Metro is 10:1 and calls for 87 octane. I wouldn't worry about it.

 

[PandaBear]

Same for my corolla, 10:1 compression and 87, but a 9.8:1 Integra GSR spec 91. You have to look at the ignition timing too, because the corolla has I think 12deg advance and Integra has 18deg advance.

 

[Throckmorton]

Same goes for an Accord V6.

 

[Steelhead]

buying the top octane gas at a station especially one out in the boonies is risky business.
slow turnover of fuel in top octane tank can cause fuel and additive breakdown after just 2>3 months, especially the ethanol
plus there is a higher risk of water condensation
most high volume stations sell 10 to 1 regular to mid and top octane grades

if the engine manuf. recommends 87 octane fuel, run it and use a fuel injector cleaner. far safer and more cost effective

 

[ARB1977]

Use what it says as stated before.

 

[cmhj]

Having built engines, there's one thing many overlook, which allows low octane in a "hi compression ratio" engine.

Fuel injection alone will allow a higher compression ratio due to a better atomization process of the air/fuel mixture. Even without fuel injection a better atomization process has been achieved on many engines thru better/altered intake designs.

The bottom line is 87 octane now works on many engines with no longer term effects.

 

[Dominic]

Interesting, I knew the dynamic CR had a lot to do with it but wow 10.4:1 in the fit and 87? Amazing.

 

[Junior]

Aluminum heads and lower temp thermostats help to lower cylinder temperatures. The T-stat on my Honda v-6 is 180 vs the typical 192.

 

[Drew99GT]

My Corolla is around 10 to 1; I run 85 octane!

 

[Black Bart]

The CR on a engine can be 10-1 on one and run 87 octane just fine but that don't mean that all with 10-1 CR can because the camshaft determines the DCR.
Using static compression ratio as a guide to what fuel will work is useless you must consider the profile of the camshaft.

 

[mechtech2]

The car is detuned from it's max possible if you can safely run 87 in it.
In other words, a compromised program in the computer.

 

[Bamaro]

Compression ratio is only one part of determining required octane for a well maintained car. Timing is another key factor. I would use what the manufacturer recommends.

 

[Quest]

used to be back in the good old mechanical distributor days with centrifugal mechanical timing advance, higher octane gas is needed if you want to maintain stock ignition settings on a stock engine over time due to increased tendency to pre-ignite as the engine accumulates mileage.

With the advancement in computer control and ignition mapping, many late model EFI engine designs can pretty much do away w/o the need for higher octane gas than necessary for your computer will constantly compensate for the ignition mapping across the entire rpm spectrum.

So, don't be surprised if your new vehicle purchased over the past several years comes with high compression ratio (over 10:1) and yet only calls for 87 octane rating. This is because the computer is taking on the task of dynamically mapping/constantly adjusting the ignition timing to accommodate it's needs. That's why for some of us cars, fueling with high octane than what's being called for doesn't give any added benefits (feels exactly the same as 87), other than burning a bigger hole in your pocket.

My 2c's worth.

 

[Autodarken]

10:1 is nothing...I've been running nothing but 87 octane fuel in every recreational machine I owned for 20 years now. And even 20 years ago, 10:1 in a motorcycle was common. The last 10 years 11:1, and 12:1 are very common.

The last 4cy I owned had 12:1 compression, made 110hp at the crank, and hardly ever ran below 4-5k rpm, redline was 15k.

My current bike, an Aprillia Tuono (V-twin 1000), runs a 11.8 compression ratio and produces a claimed 139hp at the crank. Again, I run nothing but 87 ocatane, cause I cant get it to knock or ping even if I try.

So, what I'm saying is, a ratio of 10:1 these days is becoming conservative. Design Engineers have found methods to avoid pre-ignition/knock/ping.