Octane

[Anonymous]

I was watching something on TV last night about High Octane fuel.
Obviously higher compression means the need for higher octane.

But what I was wondering does lower compression with turbocharging increase the need for higher octane?

Timo61Ranchero

 

[66Sprint6]

From what Ive heard, when turbo charging, higher octane fuel help get rid of detonation. The higher the octane the more boost you can run without deadly detonation.
Matt

 

[BB6Mustang]

Higher octane fuel has a greater resistance to ignition and a slower burn. The only reason to use higher octane fuel is to prevent abnormal combustion (detonation, pre-ignition, etc...). Heat is the main cause of detonation and comes from many sources. Higher compression,advanced ignition timing, turbos, superchargers, and, to a lesser degree (no pun intended), underhood temperatures. Typical ways to prevent detonation are cold air intakes, intercoolers, retarded ignition timing, water injection, and of course higher octane fuel.

 

[Anonymous]

if you are running a turbo you need higher octane more than ever as it relies on boost to build compression. I once aske if i could run lower octane fuel in a turbo as it had lowere compression...

 

[Anonymous]

Funny thing about turbos...the answer to almost any question is, it depends!

The operator's manual for my turbocharged Eagle Talon specifies 91 octane fuel in order to achieve design performance specs, but caveats that with the admission that it will accept any octane unleaded gasoline, but at a lower performance level. The DSM forum guys have discovered numerous ways in which the ECU protects the engine under these circumstances, from enrichment to spark retard.

Of course, that engine has EFI. Our old hoopies can probably get away with cat-pee gasoline if the carby is jetted slightly rich and one runs a boost activated water/methanol injection system.

 

[Thad]

Question: What is your CR at atmosperic pressure (off boost) static and equivalent CR on boost, dynamic. A low compression engine at atmospheric is NOT a low compression engine on boost. So no matter what the off boost CR is the ON boost CR dictates octane required OR octane available limits usable boost. Alcohol/ water injection modify the effective octane rating of the air/ fuel mix.

Octane rating is a measurement of resistance to detonation. This resistance is because higher octane fuel burn faster not slower, it burn before temperature rise due to combustion and temperature rise due to compression can cause detonation. It does not resist ignition it resist detonation by burning before it can detonate.

Take black powder pour in a narrow line, light one end, it burns very rapidly to the other, temperature rise only due to combustion. Take the same amount of blackpowder confine in a small space then light, the sudden temperature rise will cause the entire amount to detonate. Temperature rise due to both combustion and compression.
Over simplification but the best I can do.

 

[StrangeRanger]

Higher octane fuels burn slower but ultimately may release more energy. To continue your gunpowder analogy, 87 octane is like 3031 and 93 octane is like 4350. You need the slower igniting fuel on high compression, agressively cammed motors to allow the greater ignition advance that they need to produce power. Run low octane fuels and too much burning would occur before TDC causing spark-knock. The related abilities of higher octane fuels to ignite and burn more slowly at hot sports and to resist auto-ignition in high compression engines is obviously a benefit too.

 

[Anonymous]

Also, the higher octane's BTU and slower burn can produce exhaust manifold heat increases. For turbos, this is an unexpected bonus. If your engine is unbalanced somewhat, due to uneven combustion on a cylinder-to-cylinder basis, higher octane is the only way to live with it as the compression goes up. Well, no, there is one other way - add exhaust gas back in (EGR) to slow the burn - this acts like an "octane booster" of sorts for controlling ping, but it does NOT add any power or BTU (heat), it actually cools the combustion process to reduce NOx production. Lowering this heat also lowers available power and reduces engine efficiency, hence the lower MPG on many carbed engines with EGR.

One interesting thing, though - running EGR seems to help keep sparkplugs cleaner, a phenomenon I've never understood.. :roll:

MarkP