View Poll Results: Which octane fuel do you use?
87 octane (regular)
44
23.78%
89/90 octane (plus)
27
14.59%
92/93 octane (super)
97
52.43%
AV/ other or Allan 4 is a putz
17
9.19%
Voters: 185. You may not vote on this poll
Which octane fuel do you use?
04-11-2005, 09:50 PM
#34
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Re: Which octane fuel do you use?
Originally Posted by ThirdBird
It's just amazing how many people, even supposed motor-heads, have no clue what the real function of octane is. All you need to do is read a little and get the real scoop.
Compression, compression, compression. If you don't run a high compression engine, you don't need high octane. Octane prevents pre-ignition caused by compression. In a higher compression engine, the fuel/air may (will) explode under compression before the piston reaches the top of it's stroke. Imagine the stresses when that happens. That little "knock" you here is your piston being jammed back down on it's up-stroke. Not good! However, as many have stated above, if you don't need it, don't spend the money on it.
Compression, compression, compression. If you don't run a high compression engine, you don't need high octane. Octane prevents pre-ignition caused by compression. In a higher compression engine, the fuel/air may (will) explode under compression before the piston reaches the top of it's stroke. Imagine the stresses when that happens. That little "knock" you here is your piston being jammed back down on it's up-stroke. Not good! However, as many have stated above, if you don't need it, don't spend the money on it.
04-11-2005, 10:50 PM
#35
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Re: Which octane fuel do you use?
Originally Posted by Dean Ferry
Russ,
I use what Mercruiser recommends for the 502 MPIs, 87 Octane. Anything else, I'm told, is a waste of your money.
Dean
I use what Mercruiser recommends for the 502 MPIs, 87 Octane. Anything else, I'm told, is a waste of your money.
Dean
04-12-2005, 01:28 AM
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Re: Which octane fuel do you use?
Originally Posted by cuda
I agree that high compression is the main cause of pre-ignition, but high temps will also cause it.
Intake temp breaks out into two factors, the ambient air temperature and the amount of charge (boost) you run. If the ambient air temperature is already 90F and you run 15 psi, then the air intake temps are gonna be through the roof. Superheated intake temps, when mixed with fuel, leave the compression mixture no "room" when compressed, the mixture is already soaked with heat, so compressing it even a small amount causes it to preignite or "detonate" (basically turns your engine into a compression ignition aka diesel engine, very bad for pistons.).
If your engine is out of tune (timing) it will knock, or pre-ignite (this is where it becomes confusing, because pre-ignition doesn't exactly mean detonation, it just means the cylinder fires at the wrong time, which may or may not be due to detonation. If you run advanced timing, like 18 degrees plus, this means that the spark plug will spark and ignite the fuel 18 degrees BEFORE top dead center (TDC). So that the mixture ignites and pushes back on the piston at the optimal time (kinda like pushing a merry-go-round, if you wait to push the bar when it's right in front of you, it's not as effect as beggining your push slightly before it reaches you, to get the maximum effort out of it).
Compression is the amount that the engine "compresses" the mixture (imagine that
So if you have a high CR, then the fuel/air mixture will be superheated, nearly to the point of ignition. Hotter intake temperatures will obviously aggrivate this factor (heat soaking as I mentioned before).Lastly, like I stated earlier, engine load has a lot to do with this as well. If you have an engine in 4th gear going up hill on a 99F day with your foot to the floor, you will most likely hear that tell-tale little knocking sound (that's called engine knock or detonation), that the air/fuel mixture firing in opposition to the piston and creating massive amounts of force against the piston head and eventually, it's gonna cause a failure. Same is true of any engine, if you're in a boat and its 100F out and you're running 10psi of boost and you get on it and try to push the boat hard, it's not going to be good for your engine if you're running low ocatane fuel.
Octane rating is a rating which averages the MON and RON (won't get into this much) Mechanical/Research Octane numbers. Basically, they are an index indicating the fuel's chemical similarity to Iso-Octane. For simplicity, you can think of this as the fuel's likelihood to pre-ignite or it's "volatility" for simplisitic purposes. A higher octane fuel resists pre-ignition.
However, just as been shown in high end BMW's etc., if an engine is tuned to run 87 octane, that's what you should run. Running anything higher will not give you more power and actually MAY reduce power slightly (and reduce the money in your wallet in the process). In naturally aspirated engines, higher octane ratings generally equate to smoother engine operation, but not always, depending on the temps and CRs and timing (as I went through before).
04-12-2005, 05:39 AM
#38
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Re: Which octane fuel do you use?
WOW!!!! A question on OSO and consistent answers from everyone!!!! At least I'll feel comfortable running 87 from now on. Thanks for the input.
Russ C.
Russ C.
