OC­TANE RAT­INGS

Performance Vauxhall - - TECH -

Oc­tane rat­ings are re­ferred to in one of three main units of mea­sure­ment: Re­search Oc­tane Num­ber (RON), Mo­tor Oc­tane Num­ber (MON) and Anti Knock In­dex (AKI). Most of the world uses RON rat­ings, yet USA and Cana­dian driv­ers use fuel rated by AKI. This is es­sen­tially a mix of RON and MON. Due to the fact the MON test is much tougher than that of RON (the for­mer award­ing fuels an eight to ten-point lower rat­ing than RON), the over­all rat­ing of AKI is close to five points lower than RON. Got it?!

In sim­ple terms, the higher the oc­tane rat­ing, the more re­sis­tant the fuel is to what span­ner­wield­ers re­fer to as knock, det­o­na­tion or pre-ig­ni­tion. Ba­si­cally, this is the point at which the heat and/or pres­sure in the engine cause the air/fuel mix­ture to ig­nite be­fore it’s sup­posed to. Pre-ig­ni­tion can cause se­ri­ous harm to an engine, some­times re­sult­ing in cat­a­strophic me­chan­i­cal fail­ure. Melted pis­tons, blown head gas­kets… you name ‘em, pre-ig­ni­tion has caused ‘em!

It’s de­sir­able to use fuel with a high oc­tane rat­ing when work­ing with petrol en­gines, but con­trary to pop­u­lar be­lief, un­less the engine is mapped for higher oc­tane fuel (many mod­ern sports cars utilise fuel man­age­ment soft­ware to this ef­fect), power won’t be in­creased by any sig­nif­i­cant amount. Con­versely, if an engine has been mapped to take ad­van­tage of higher oc­tane fuel than what it is you’re pay­ing for, then you’re en­cour­ag­ing de­creased per­for­mance and com­plaints re­gard­ing re­li­a­bil­ity. As we’ve said pre­vi­ously in Per­for­mance

Vaux­hall, don’t think that run­ning your Grif­fin on your lo­cal su­per­mar­ket’s un­leaded petrol and then only top­ping up with higher oc­tane fuel the morn­ing of your dyno day is go­ing to bring you big bhp.

Us­ing higher oc­tane fuels al­lows you to in­crease an engine’s re­sis­tance to pre-ig­ni­tion, thereby en­abling the use of higher com­pres­sion ra­tios, higher boost pres­sures and more ad­vanced ig­ni­tion tim­ing, all of which help your mod­i­fied Vaux­hall’s engine to pro­duce more power.

It’s also worth not­ing that while sim­ply adding higher oc­tane fuel won’t pro­vide mean­ing­ful power in­creases un­less the host engine has been mapped ac­cord­ingly, po­tent petrol can help with re­li­a­bil­ity by in­creas­ing the ‘buf­fer’ be­fore pre-ig­ni­tion kicks in. It’s with this in mind driv­ers of­ten add oc­tane booster or higher oc­tane fuels when us­ing their cars at the track, where an engine is sub­jected to pro­longed bouts of ag­gres­sive use.

If you’re go­ing to re­tire your car from the road, be sure to use up leftover fuel in the tank

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