Octane ratings and compression ratios.

[Dimebag]

Hi,

Can anyone explain why you need to run higher octane fuel when installing a turbo on a sled?

It has been my understanding that high compression engines need higher octane fuel to avoid detonation, right?

Also, people talk about that engines get better cooling with higher octane fuel, and that every engine with some mods on it should run higher octane fuel to compensate for these mods and get the most out of their engines.

Is there any truth to this at all?

Take my Raptor, Its a single cylinder 700cc fuel injected engine which is designed to run on regular fuel (according to the user manual). It has a compression ratio of 9.2:1. Then i change the air filter, exhaust and install a powercommander on it - which gains about 30% more hp from the engine.

People tell me I should run premium fuel on my Raptor after i did these mods.
Why is that? Again, is there any benefits at all with getting more expensive higher octane fuel?

Thanks.

 

[Daranello]

Engines make most power on the edge of detonation. Sometimes too much octain hurts power cause it burns at a higher temp.

 

[bottlerocket]

The engine is just a air pump. The more air/fuel you can get thru it the more power and heat it will generate. High compression motors are the same as low compression with boost. A low compression motor on boost will create as much or more compression then a natualy aspirated high compression moter as the boost is pushed in the motor with a hotter temp from compressing the air. High octane fuel burns better and also aids in the cooling process.

 

[Zakre]

I would not worry about high octane in the raptor, not needed even with those mods. The Power commander will compensate and add more fuel (to compensate for the better air flow you now have) This additional fuel will keep the Air Fuel ratio correct for your application assuming the PC is mapped correctly. Making the most power available.

Higher Octane fuel actually is harder to "burn" than regular, the idea is that regular octane fuel will ignite in the engine before the spark plug has fired causing detonation (during the compression stroke). The high octane resists the urge to burn longer and allows the engine to fire the fuel mixture at the correct time (instead of detonating early due to high compression or boost)

I hope this has made more sense to you.

 

[Dimebag]

High octane fuel burns better and also aids in the cooling process.

Higher Octane fuel actually is harder to "burn" than regular.

Thanks for your replies, but Im still confused. Hehe.
Does the high octane burn better, or is it harder to burn?

How does the high octane fuel aid in the cooling process?

Why does the new 2strokes need premium, while the older ones could run fine on regular? Higher compression ratio on the newer engines? or are there any other factors too?

Thanks again - the student.

 

[Zakre]

High octane fuel burns better and also aids in the cooling process.

Higher Octane fuel actually is harder to "burn" than regular.

Thanks for your replies, but Im still confused. Hehe.
Does the high octane burn better, or is it harder to burn?

How does the high octane fuel aid in the cooling process?

Why does the new 2strokes need premium, while the older ones could run fine on regular? Higher compression ratio on the newer engines? or are there any other factors too?

Thanks again - the student.

Its not about burning better , its about resisting the urge to spontaneously combust due to the compression ratio etc. High octane fuel can be compressed under hotter and faster conditions and still only fire when the spark plug goes off making for a clean quick burn making lots of power.

The new two strokes all have knock sensors now, they detect the detonation (fuel burning before it should) and retard the timing to counteract the detonation effect (this is hard on engines and should be avoided). Older 2 strokes don't have this sensor although some older ones did require higher octane then used a resistor switch to change the ignition characteristics to suit the lower/higher octane fuel.


Make any more sense?

 

[Dimebag]

I think i got it. Its not the easiest subject to understand. Especilly when you get different theories on the same subject.

Then what about the cooling process that bottlerocket mentioned?

From what i have been told, thats the most significant gain you get from running higher octane fuel (even if the engine is designed to run on regular).
That the engine supposely gets better cooling, and gets lets stress than it would on regular fuel.

I dont know if I believe in that part. I dont think so.

Would anyone care to explain that? Zakre

 

[Zakre]

I have never been told anything about any cooling nature of higher octane fuels.

There is less BTU of energy in higher octanes per unit, i.e. LPG .

So for at the pump High Octane fuel contains less heat units per litre that regular, the difference here is small but could be why the idea is out there?

 

[Dimebag]

That could be it. Maybe someone will chime in with more info. Its not the busiest time of the year on this forum..
There is so many myths surrounding the premium fuel, i almost cant believe it.

Thanks alot for all info. Now I feel a lot more comfortable with filling my Raptor with regular, rather than spending that extra money on premium, when it really should not be needed. My Nytro takes a fair share of premium fuel every weekend, so i dont feel sorry for the oil companies either

 

[yox185]

Octane is a fuels ability to "resist" burning or detonation. The higher the octane the more resistance to detonation. You need it in your turbo because use are raising the compression. Turbos "force feed" the engine air and fuel. Stuffing all that air into the cylinder raises compression. Raising compression raises the fuels vulnerability to pre-detonation.

 

[kinger]

87 octane is the HOTTEST fuel, it burns the hottest and fastest to ignite

91 burns SLOWER and slow to ignite

race fuel - burns SLOWER yet then 91 it takes a lot of compression, a lot of spark, to get it to ignite.

When fueling a engine you need to understand what its set up for. If you have a engine set up for 87 fuel and run 91 it will leave deposits in the engine of unburnt fuel because its a COOLER fuel and harder to burn. If its set up for 91 and you run 87 it will detonate and mechanically fail.

When you up the compression via engine changes or forcing air into the cylinder aka boost if you run 87 it will ignite before you want it to (aka on the compression stroke before the piston reaches preset timing point to light the plug) because 87 is so easy to light up this is called detonation. If you can imagine a cylinder where the piston is on the upward compression stroke LIGHTING and forcing the explosion against the direction it wants to go, that is when rods, pistons, or cranks break its rotating up and the combustion is forcing DOWN.

Remember fuel combusts when it is 'sqeezed' without a spark plug so you choose the appropriate fuel to NOT combust when 'squeezed' (aka compression) per the engine build then the spark plug ignites it WHEN you want and then power is created.

 

[Dimebag]

I got it.

I just wonder why people use premium in engines that are designed for regular.
I guess its because they have not read this topic yet.. And they believe that running higher octane fuel has a positive effect, even if the engine is supposed to run on regular fuel..

 

[bottlerocket]

I think i got it. Its not the easiest subject to understand. Especilly when you get different theories on the same subject.

Then what about the cooling process that bottlerocket mentioned?

From what i have been told, thats the most significant gain you get from running higher octane fuel (even if the engine is designed to run on regular).
That the engine supposely gets better cooling, and gets lets stress than it would on regular fuel.

I dont know if I believe in that part. I dont think so.

Would anyone care to explain that? Zakre

Its pretty much been answered but what I meant better fuel runs cooler is it is a fast explosion which doesnt built heat like pre- ignition/spark knock/detenation. Im meaning combustion temp which will relate to overall engine temp but detenation usaully will take a piston out long before you see a temp light. lol