The Octane Debate (one more time please)

[it's paid for]

Okay . . . so, I forgot the whole reason why there are three levels of octane gas and which one should be used in a particular situation.

I have a Chevy engine that has a very obvious behavior problem when low octane gas is used - we keep it filled up w/93 octane and it runs like a champ - no problems.

A friend of mine was explaining it this way . . . he said when the piston is nearing TDC on the fire stroke, sometimes low octane gas will pre-explode where if you had the same engine & used higher octane gas a burn would occur instead of an explosion . . . doesn't make sense to me, but he's been a mechanic for like a zillion years . . . so . . . what's up with octane?

Thanks,

IPF

[shepherd79]

never heard of that kind theory.
the way i see, if the car is made to run 87 octane, running 93 is just wait of money. the best upgrade is 89. IMO everything above on stock motors is waist of money.
i ran 3 full tanks of each gas last summer, and i didn't not see any diff between 89 and 93 except the higher price.

[1988starter]

never heard of that kind theory.
the way i see, if the car is made to run 87 octane, running 93 is just wait of money. the best upgrade is 89. IMO everything above on stock motors is waist of money.
i ran 3 full tanks of each gas last summer, and i didn't not see any diff between 89 and 93 except the higher price.

Over the summer I noticed I get about 1-3 mpg better on 93 I tried running 89 for a few weeks then 93.

[NXRacer]

talk to dibs about the reasoning behind octane. he's a chemist for a fuel company......

[Nuffice]

Okay . . . so, I forgot the whole reason why there are three levels of octane gas and which one should be used in a particular situation.

I have a Chevy engine that has a very obvious behavior problem when low octane gas is used - we keep it filled up w/93 octane and it runs like a champ - no problems.

A friend of mine was explaining it this way . . . he said when the piston is nearing TDC on the fire stroke, sometimes low octane gas will pre-explode where if you had the same engine & used higher octane gas a burn would occur instead of an explosion . . . doesn't make sense to me, but he's been a mechanic for like a zillion years . . . so . . . what's up with octane?

Thanks,



IPF

He's right.. The lower the octane rate, the earlier it will detonate. So if you have high compression car with low grade gas ( 87/89 ) it will denotate before TDC, which in return will damage your pistons.

[shepherd79]

Every car will see diff results with diff octane gas. your best bet would be to try them all, and make your own conclusion.

[BootMachine]

yes...as you squeeze the gas and air mix it will compress and get hot...so hot it can explode without a spark. When this happens you have two or more SMALL explosions in ONE combustion chamber and the shock waves hit eachother and cause BAD BAD BAD vibrations. You can hear the vibrations as KNOCKS!

Higher octane fuel does a few things.

1) You can squeeze it more before it will go boom (so you can control the boom with the ignition system)

2) It Burns slower than low octane gas so you will need to advance your timing so the spark hits farther UP in the compression stroke (farther away from TDC) so you allow for extra time for the fuel to burn

3) The overall volume of gas produced (size of the boom) is larger than low octane gas.

SO...advance the timing, up the compression, high octane fuel you will see results!

If your timing is set to low octane fuel you can actually kill HP by adding high octane fuel because it burns slower!

BIGGER BOOM.....BUT LONGER BURN TIME + added bonus of being able to squeeze it more without it exploding by itself!

Hope that helps!

[Dragonfly]

soooo if i adjustmy timeing ill get more bang with high octane gas?

[BootMachine]

YES CORRECT!

You gotta know how much though.

Here is what you want to have happen.

Piston goes down and sucks in air/fuel
hits BDC
starts to come back up
spark happens 1/2 way (not exactly but you know what I mean)
gas starts exploding
piston hits TDC
piston starts back down
AT THAT INSTANT
gas is finished exploding
piston hits BDC
exhaust valves open
back up to TDC

Advance the timing too much and your explosions will finish before the piston reaches TDC (bad bad bad..very bad..)

too little and you will have the explosion happening with the piston 1/2 way down on the down stroke and you will just get a little puff and no power.

You can tune when the spark happens by ear by listening for the engine knocking or you can put your forehead on the valve cover and feel for it..but the engine will be cold for that and you gotta tune a hot engine ..NOT a cold one!

The best way to do it is to install an engine knock sensor. No knock...good..knocking..bad...that easy!

Any more complicated than that you are getting into like F1 shit where you actually measure everything and find the EXACT spot where to fire the fuel off!

Our ignition systems are not that accurate...use a knock sensor!

[BootMachine]

soooo if i adjustmy timeing ill get more bang with high octane gas?

dont forget...advance when the mixture is less explosive (high octane...bigger boom but slower boom)

retard when the mix is more explosive...like...high octane gas mixed with nitrous oxide or under 6 PSI from a turbo

[Blkblurr]

Bootmachine is correct on his description of octane differences and why it matters. The condition of the engine will make a differnece as well. The lower the compression the lower octane you can use without pre detonation.

[johnwc723]

its not like any stock honda accords (yet) or especially older ones like ours use high compression, i would say use the 87 unless you have a reason that you need to use higher octane (such as a crap load of engine mods that increase compression) in that case you could probably afford to spend the extra on gas because turboin anything (espcially our cars) aint cheap..

[it's paid for]

its not like any stock honda accords (yet) or especially older ones like ours use high compression, i would say use the 87 . . .

Yeah . . . great suggestion . . . my rings have almost 218K miles on 'em . . . and . . . for some reason . . . I doubt the compression is . . . up there where it yoosta be . . .

. . . I have a ring job scheduled for this summer . . . oh what fun . . . I can hardly wait!!!

. . . I was told to get the screw-in compression meter . . . Yes/no?

[BootMachine]

yes get the "screw in compression tester" I have one and it works great. I bought it for like 30 bones at Canadian Tire!

You guys would see gains if you added high octane fuel AND advanced the timing cause remember...the boom is bigger!

You get more bang for your suck!

[soljaboy2000]

what is a knock sensor and where are they sold?

[BootMachine]

what is a knock sensor and where are they sold?

en engine knock sensor detects the evil vibrations from pre-det or det. (knocking)

You can buy one from ANY good performance shop!

You will need to drill a hole in your block though!

[Ravenix]

i have perfect compression on all cylanders Weeeeeee, so what is a good octane to use in our cars i usually use regular and sometimes mid-grade, i dont really notice a difference.

[danronian]

I'm not sure if my timing is advanced, but when I use anything below 93 octane my car pings so much in fifth gear I can only use it at highway speeds. I think it is due to alot of carbon buildup also, even though I use fuel additive alot (160k mi.).

[Blkblurr]

Why are you in fifth gear at non highway speeds?

[Blkblurr]

Use 87 octane. Its all I've ever used in my SEi

[Busted_Blue]

how exactly do you detect knock or predetonation with the naked ear?

[Blkblurr]

You can hear it when you accellerate. Your engine makes a knocking sound. It's very obvious if your hearing is normal.

[Busted_Blue]

man i need a realllly quiet road then ehh?

[BootMachine]

man i need a realllly quiet road then ehh?

nah...I just rev it up and listen!

HOW DOES IT SOUND>>>>>hummm....

It sounds like someone is doing a drum roll on your engine block with wooden drum sticks with plastic tips....HARD!

The more your throttle is open the more you hear it....but only under load (unless its REALLY bad). So dont hold your engine at an RPM and listen...grab the throttle cable and put your ear close to the valve cover, plug the other one and VROOM, VROOM, VROOM.......pull the cable TIGHT (so wide open throttle) until your engine hits about 3500 / 4000 RPM and then snap it shut letting the RPMS drop back down to idle!

IF its REALLY BAD you will hear it no matter what!

[BootMachine]

I'm not sure if my timing is advanced, but when I use anything below 93 octane my car pings so much in fifth gear I can only use it at highway speeds. I think it is due to alot of carbon buildup also, even though I use fuel additive alot (160k mi.).

your timing is fucked then!

You should have no problem running low octane gas in the A20 engines on stock internals, compression and timing!

If you engine is suffering from pre-det due to carbon buildup it does not sound like pinging...it sounds like there is SAND in your engine when you go wide open throttle. It sounds like there are little grains of sand in there ticking around and grinding!

That is the sound of the explosions hitting eachother.

Carbon causes the pre-det because it stays hot between combustions (like the cherry on a cigar) and burns the gas before the plug can fire it!