xtremeskier

"boosting octane on an engine that suffices on regular gas actually makes for worse combustion, since the fuel burns slower. peak cylinder pressure should occur around 10-15deg ATDC, and if the ECU is lighting it off at the same time it would normally, but it burns slower, you're applying less of the total combustion energy to the power stroke." From Import Post...

I thought boosting your octane, meant better and more complete combustion??
Please correct me if i'm wrong. smile.gif

Double_A

From my understanding that makes perfect sense.

Denisst99

it's right. .to much octane can actually lower performances

Double_A

Disregard my first statement I Tried to edit but i didnt get to it soon enough.

The only difference in gasoline with different occtane ratings is the amount of energy it takes to ignite it or start the chemical reaction. In no way does the octane rating affect power, gas mileage, better or cleaner combustion, or the amount of time the gasoline takes to burn.

An exaple would be boiling water. If you put a pot of water on the stove to boil thats about 40 degrees F. Its gonna take a lot of heat and time until it starts to boil. Now if you put a pot of water on the stove to boil thats about 120 degrees F. Its gonna start to boil in a lot less time than it took the cold water. But as you know once the both start to boil they boil the same. One isnt going to boil better than the other because it took less or more time till it got to the boiling point.

xtremeskier

Interesting. Ok then for example, let's say you are towing a trailer, higher octane levels must help, right? We have a 1996 Jeep Cherokee, 4.0L HO I6. By itself, 87 octane seems to be fine. But when we tow our trailer (around 3500 LBS), it would knock quite a bit, and also downshift a noticeable amount (usually when going up hills) when 87 octane was used. This was virtually eliminated though when 91 octane was used. There was no more knocking, and downshifting was reduced substantially. What i'm getting at is, if there is more load on the engine, higher octane levels must help then, right?

Double_A

Yes, if its an engine that at high load wont work on 87 to well. You could run a 600HP turbo engine on 87 if you kept the load down. Its just that in normal everyday driving conditions and even some extremes for you average 4 bangers, 87 will do the trick. Ive driven up some big ass mountains in New York that seemed almost vertical and i had no knocking what so ever. Now your Jeep had a motor about twice the size of the tibs and when towing your putting it under high load and when downshifting high RPMs cuasing even more heat. So with the extra load, high RPMs and bigger motor everythings is going to be hotter, inturn pre-igniting the fuel more often.

So to answer your question...yes. Higher octane will help when under high load such as when you hual your trailer. Jus as when ou turbo you car and are under boost. Everything is going to start running hotter so your gas will detonate or pre-ignite easier.

This site might be more help than I am being. Its very descriptive.
http://en.wikipedia.org/wiki/Octane_rating

Dmitry

Octan number shows how gas can resist detonation.

Detonation may ocur:

when compression ration is high
when boost is high
when there is too early ignitioin
when there is high loads
and ofcourse when octan level is low

there is no need in higher octan if your engine runs fine.

you may see some gains cause ECU will determine lower level of detonation on higher octan level and that why ECU may change a bit angle of ignition.

supercow

that makes sense but I don't think that applies to 87 vs. 93-94 octane.

87 has things like kerosene, diesel, etc. in it (transmix). so that's not helping. I think that would apply more to high octane fuel like 104 or 110.

Dmitry

It applies to all octans I think.

BTW different octan may burn with different temperature

DTN

high octaine is supposed to give your car that "rotten egg" smell out of the exhaust as well.