gas octane physics

[DAG]

^^^No0B^^^


I have never been given a stright answer, most the guys i talk to about gas quality and octane levels are fule injected 4bangers.

my question what makes an 2 stroke carb feed engine require a higher Octaine level? and why can you not run regular gas with that high proformance engine?

 

[unclehulka13]

higher compression=more prone to detonation.
You reduce detonation with higher octane.

granted, this all has been discussed many times before. I think you need to stick with what you have, no lameys, no total loss, no lightweight skis. Tune your beast, and go ride.

 

[eschberg]

Each level of octane burns at a higher point.

The lower the octane the lower flash point. Meaning, you are risking pre-igniting with so much compression and temperature. Its true for all gas-combustable engines... not just 2 stroke vs fuel injected.

Someone correct me if i'm wrong...

 

[DAG]

the reason i bring this up.. my dad will gladly pay for gass every now and agen, but he will only buy the cheap stuff... and when i bought my ski Donn said dont run any lower than 93, and 95-100 is what it should have..

and i can not give my dad a stright forward answer on why it requires higher octane

 

[unclehulka13]

if you're anywhere between 150-180, i would definitely run 93 octane. Anything less and you risk detonating. Or you could pay for your own gas, since you were talking about buying a lamey, 3 bux a gallon should be nothing for your rich ass :biggrin: .

 

[Mark44]

The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in your ski. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. So a "high-performance engine" has a higher compression ratio and requires higher-octane fuel.


Its all about compression guys
Mark44

 

[eschberg]

Read this or search howstuffworks.com for octane too... http://en.wikipedia.org/wiki/Octane_rating

 

[jetskiking]

You should tell your Dad that he can buy the gas now or buy an engine later.I remember sending my wife to get us all gas and told her 93 when she came back she had 30 gal of regular:banghead: Needless to say everyones car got some 2 stroke gas. She said she thought we just wanted it because it made the ski faster She did try though and now understands its so the skis wont blow up.

 

[sflsurfrider]

the reason i bring this up.. my dad will gladly pay for gass every now and agen, but he will only buy the cheap stuff... and when i bought my ski Donn said dont run any lower than 93, and 95-100 is what it should have..

and i can not give my dad a stright forward answer on why it requires higher octane

how old are you?

 

[Matt_E]

20, according to SBTToday profile.

 

[sflsurfrider]

does pops ride the SJ too?

 

[DAG]

how old are you?

ok i know i'm on this fourm way to much, so alittle bit about myself.
yeah i'm 20,
no, my dad does not ride the sj, he bought himself a 700XL.
we haev about 4-5 5gals gas cans and everyonce in awile when we have church or alot of family come down he just fills them all up. just to be nice b/c i let everyone ride the real jetski... so every time i just dump that 5gals of 83 into my car and go get 93... Oh yeah everyone looks at me like i'm crazy at the gas station...

PS: i'm not some snoby rich kid..:sneaky: i just put ALL my money into my SJ.
never go out at eat, never buy little peddy stuff, never go to the mall, exc..
I still have several gift cards from a few christmasses ago... might put them up on ebay

 

[Whtbread]

"PS: i'm not some snoby rich kid.. i just put ALL my money into my SJ.
never go out at eat, never buy little peddy stuff, never go to the mall, exc..
I still have several gift cards from a few christmasses ago... might put them up on ebay "

At least you have your priorities straight.

 

[SJBrit]

In actual fact, octane is put into fuel to make it LESS efficient i.e. less able to burn. It's heptane that actually burns. The octane rating tells you about the heptane/octane ratio - the higher the number, the more octane and therefore the less good the fuel. Seriously. When I say "less good" I mean that less of the volume of liquid you pour in your tank is actual fuel (heptane).

The problem with really good fuel (all heptane) is that it will self-detonate at high pressure, so in higher compression motors you need to add some octane in to "retard" the fuel a bit and wait for the spark to ignite the air/fuel mixture.

The fuel companies to a great job at marketing high octane fuel as "premium" and charging more for it, so much so that the ricers will fill their tanks with it thinking it makes their cars go faster. More octane is good, right? NO!! They're charging you more for fuel WITH LESS ACTUAL FUEL IN IT!! More octane, less heptane. The fact is that a high performance motor needs the octane in there to prevent it from blowing itself up, so sure, high octane means more power, but it's about the motor, not the fuel.

 

[Whtbread]

In actual fact, octane is put into fuel to make it LESS efficient i.e. less able to burn. It's heptane that actually burns. The octane rating tells you about the heptane/octane ratio - the higher the number, the more octane and therefore the less good the fuel. Seriously. When I say "less good" I mean that less of the volume of liquid you pour in your tank is actual fuel (heptane).

The problem with really good fuel (all heptane) is that it will self-detonate at high pressure, so in higher compression motors you need to add some octane in to "retard" the fuel a bit and wait for the spark to ignite the air/fuel mixture.

The fuel companies to a great job at marketing high octane fuel as "premium" and charging more for it, so much so that the ricers will fill their tanks with it thinking it makes their cars go faster. More octane is good, right? NO!! They're charging you more for fuel WITH LESS ACTUAL FUEL IN IT!! More octane, less heptane. The fact is that a high performance motor needs the octane in there to prevent it from blowing itself up, so sure, high octane means more power, but it's about the motor, not the fuel.

I learn something new everyday. Good post

 

[DAG]

In actual fact, octane is put into fuel to make it LESS efficient i.e. less able to burn. It's heptane that actually burns. The octane rating tells you about the heptane/octane ratio - the higher the number, the more octane and therefore the less good the fuel. Seriously. When I say "less good" I mean that less of the volume of liquid you pour in your tank is actual fuel (heptane).

The problem with really good fuel (all heptane) is that it will self-detonate at high pressure, so in higher compression motors you need to add some octane in to "retard" the fuel a bit and wait for the spark to ignite the air/fuel mixture.

The fuel companies to a great job at marketing high octane fuel as "premium" and charging more for it, so much so that the ricers will fill their tanks with it thinking it makes their cars go faster. More octane is good, right? NO!! They're charging you more for fuel WITH LESS ACTUAL FUEL IN IT!! More octane, less heptane. The fact is that a high performance motor needs the octane in there to prevent it from blowing itself up, so sure, high octane means more power, but it's about the motor, not the fuel.

WOW, very good post. thanks

 

[SJBrit]

By the way, there is a way that a car can make more power with high octane fuel, but it's a bit arse backwards: If you have a motor which needs high octane to stop detonation, then it will have a knock-sensor which retards the timing if it detects deto. That means that if you put low octane fuel in, the motor will automatically compensate and you'll lose power (quite a bit too).

So, that kind of motor will actually perform better on high octane fuel, but not because the quality of the fuel is better. What's really happening is that the motor is operating at its full potential on the high octane, and holding itself back if you put the cheap stuff in.

 

[wildman326]

My understanding is that the higher the octane the cooler it burns. I was told that if you think of the fuel as having a "burn time" (small but still measureable) the higher octane burns longer so that it is less likely to flash and therefore won't make the engine burn as hot. So you are less likely to deto. The higher the octane the longer the burn, so you can advance you ignition. You effectively start the burn earlier knowing that it will burn longer (and cooler). So, if you advance your timing too much and run low octane fuel you can deto more easily.

I think what I've typed goes along with everything else here . . .

If that's not correct please fill me in? :biggthumpup:

 

[keefer]

I think that the discussion of octane is more about the fuels resistance to self ignite due to temperature and compression. The way I understand the several sources I have read, the fuels energy per volume depends on its composition. One high octane fuel will burn hotter or cooler than the next depending on how it is formulated. I personally would want a fuel that is octane rated high enough to eliminate deto, burn clean, and have a high energy per volume rating. I.E. "More bang for your buck but not too soon" ! :bigeyes:

 

[SJBrit]

Here's a good write-up if you want more details:

http://www.answers.com/topic/octane-rating-1