Compression ratio; fuel octane

[jake75]

I just read that the compression ratio of the Vibe engine is 10:1. I thought that high compression engines required higher octane than 87. On the other hand, always read that use of any higher octance than recommended was a waste of money, and might even cause damage. Any advantge to using higher octane in the Vibe? Any anticipated damage caused by use of higher octane fuel?

[Jahntassa]

I know the GT requires the higher octane, just like my old Ford Probe GT.. anything less than 92 octane will start the engine knocking..

[joatmon]

The base engine has a lower compression ratio and does not need higher octane gas. There's a couple of threads that talk about the effect of running higher octane in the base engine, such as thread 4643 thread 4845 and thread 177 Most people feel that running higher octane fuel in the base engine is a waste of money and may result in an increase in engine deposits. Some feel that the automatic knock sensor would detect the higher octane, advance the timing and give more power. I don't think any Viber has done a scientific analysis (with dynomometer results and internal engine inspection) to settle the question absolutely. I'm cheap and run 87 in my base.If you have the GT, then you need high octane because of the higher compression ratio. Using the lower octane fuel will either give you knocking, or maybe the timing will be retarded and you'll lose power.

[ragingfish]

quote:Using the lower octane fuel will either give you knocking, or maybe the timing will be retarded and you'll lose power.I believe I read somewhere the more the car retards the timing, the worse it is for the engine over a long period of time?In other words, I think if you continue to run 87/89 octane over several months, it can cause premature wear of engine components, yes/no?

[joatmon]

All I can be sure of is that I have a base engine, not the GT, and the owner's manual says to use 87 octane in it, and that's what I've been using. If I had a GT, I'd use premium. I don't know which engine jake75 has. It might be better to move back over to http://forums.genvibe.com/zerothread?id=4643 and consolidate the discussion.

[ragingfish]

quote:It might be better to move back over to http://forums.genvibe.com/zerothread?id=4643 and consolidate the discussion.Agreed.

[NovaResource]

quote:I just read that the compression ratio of the Vibe engine is 10:1. I thought that high compression engines required higher octane than 87.The base is 10:1, the GT is 11.5:1. Compression ratio is only one factor to octane requirement. Cylinder pressure is another. Things like overlap (having both the intake and exhaust valves open at the same time) lowers cylinder pressures and allows higher compression engine run on lower octane. Also, the cylinder head chamber design has and effect.quote:On the other hand, always read that use of any higher octance than recommended was a waste of money, and might even cause damage.No damage really, just a waste of money. The only real damage comes from using lower than required octane. That's 87 minimum on the base, 91 minimum on a GT.quote:Any advantge to using higher octane in the Vibe?Only on a modified Vibe (supercharger, turbo, etc). On a stock Vibe (even one with a CAI) there is no real advantage.quote:Any anticipated damage caused by use of higher octane fuel?See my above comment.

[ragingfish]

I'm not sure they even sell gas lower than 87 anymore.Some places consider 89 as regular...but I've never seen lower than 87.

[NovaResource]

I have seen 85-octane "Economy" gas.

[ragingfish]

quote:I have seen 85-octane "Economy" gas.As usual, I stand corrected.

[AKLGT]

I have a GT and never find any octane higher than 90 here. is that going to damage my engine over the long run?

[NovaResource]

quote:I have a GT and never find any octane higher than 90 here. is that going to damage my engine over the long run?Probably not. Your colder climate will help offset the slightly lower octane.

[dmitri]

nah its not going to hurt it. the higher octane will keep your engine cleaner though

[NovaResource]

quote:the higher octane will keep your engine cleaner thoughOctane has nothing to do with keeping your engine clean. Octane has to do with a fuels ability to resist pre-ignition.While most higher octane fuels are slightly cleaner than lower octane fuels, all fuels are required to meet minimun standards on keeping engines clean.

[Yoda]

good info so far - only thing I can add is that you don't really need as much octane to prevent knocking at higher altitudes... http://www.chevron.com/prodserv/fuels/g ... tane.shtml Not being familiar with Alaska, I'm wondering what kind of altitudes you see there. I know last time I was in CO, they also sold 85 octane as regular unleaded at all the gas stations.