Wat are the advantages Of using high octane gas?

[LawnBoy97]

I have a snapper hi vac and a craftsman pro "brushcutter." the lawn mower runs pretty good but I am mostly wondering about the trimmer. It starts in two pulls and runs pretty good, but it seems to smoke a lot as it gets warmed up and then when it drops to idle. It is around 10 years old and I was wondering if using a higher octane gas and maybe a semi synthetic blend oil would help it to perform at it's best. I was also wondering what high octane gas does ^in general. Like how it would change the performance of my mower. Thanks.

 

[DaveTN]

I don't use High Octane gas in small engines w/ low compression ratios. The reason for using High Octane is to prevent "Detonation" in High Compression engines. Trying to run low octane in a high compression engine will cause it to knock, sputter, kick and jerk and rattle like a jackass kicking in a tin stall. No real advantage to me in using high octane in a low compression engine. Compression ratio: volume difference between BDC (Bottom Dead Center of piston travel compared to TDC (Top Dead Center). In other words, the amount of volume compression you get from BDC to TDC or just before the spark plug fires expressed in a ratio. 10 to 1... 15 to 1... etc. Once you get into the higher numbers you're getting into the DIESEL range and thousands of degrees of temperature. Gas Law says that when you take a certain volume say at 50 degrees and compress it to half its volume, the TEMPERATURE DOUBLES! Doesn't take much piston travel and compression to multiply temperatures fast. Should you spray low octane gas into a high compression situation like that you will get a DETONATION rather than a CONTROLLED BURN and your engine will knock and cause damage if continued. Low compression engines, use low octane. High Compression engines use High Octane. That's about it.

 

[LawnBoy97]

Ok. So what are the high compression engines, because I was looking around on some forums and read that people were using the high octane gas ^in their 2 strokes. So then is it just not necessary Or could it damage the engine? Thanks.

 

[possum]

Well I just came in from shutting down a Briggs powered generator from a five hour run and it was running on 91 octane, has no high compression and no damage. Same gas I have been running for a few years now in all my small engines. It will not make an old engine a new one and neither will semi synthetic oils but both sure run nice.

 

[DaveTN]

Well I just came in from shutting down a Briggs powered generator from a five hour run and it was running on 91 octane, has no high compression and no damage. Same gas I have been running for a few years now in all my small engines. It will not make an old engine a new one and neither will semi synthetic oils but both sure run nice.

Using Low Octane, say 87 in a weedeater or pushmower won't make that much difference from what you're performance is now, but might get rid of some of that smoking due to better burn. The low Octane burns faster where the High Octane burns slower on the firing. Why not try the low octane for a few tanks and see if you have any better performance out of the weedeater/mower? I've never used high octane in a weedeater, not from fear of damage but only that the low octane works better in a low compression engine. Besides, you're just paying more money for less bang.

 

[John_in_TN]

Stihl has been reccomending 89 octane MINIMUM in their engines for several years now (in the Owners Manual). Locally, I have to buy at least the mid-grade gas to get 89 octaine.
Over the past several years, I have purchased new Stihl products from three different dealers. During the sales speel, all three of these dealers specifically reccomended using premium fuel and warned that use of Regular grade gas would void the warranty. They claimed the engines run cooler and burn more cleanly with the higher grade gas.

 

[ILENGINE]

All of the premium equipment makers recommend (require) 89-91 minimum. Sthil, Husky, dolmar, all run around 10:1 ratios, at least on their saws.

 

[tyjoja]

I don't care too much about the octane, I care that it doesn't have ethanol. And where I live the non ethanol available is higher octane.

 

[DaveTN]

All of the premium equipment makers recommend (require) 89-91 minimum. Sthil, Husky, dolmar, all run around 10:1 ratios, at least on their saws.

I stand corrected. Probably the slower burn on the mid to high octane would make it run cooler probably associated with the lean oil mixtures in these newer units. The old saws back years ago used something like a 20:1 oil/gas mixture. Nowdays you're up into the 50:1 That's a lean mix compared to the old saws. As far as ethanol I won't use it in a small engine.

 

[LawnBoy97]

So I guess I would be ok trying the mid grade gas with a semi synthetic oil at 50:1 ^in my 10+ year old craftsman weedwhacker, just to see what happens, or should I just keep doing what I am doing? I guess I am just a little confused.