BHP curves by RON - any rule of thumb ?

[cyrus1971]

Is there a mathematical way to derive BHP output for different RON fuels from a singular RON : BHP value. EG if you get 420 bhp from 98 Ron what would you get from 95 and 100 RON ? I know there are many variables air temp, air density, humidity, engine map etc. Please make some assumptions on them. Is there a rule of thumb for these things ?

Also does anyone know any Shell stations in the UK that sell V-Power fuel (or similar) which I believe is 100+ RON ? Cheers, Cyrus

[Raven Flyer]

The gains will depend upon the amount of advance that the ecu will allow. Most maps will limit this.

The difference between running safely on 95 and running at the edge of 100 could be over 6 deg. We used to work on 10bhp per degree of advance on a two litre engine, so pro rata this for a guide.

As you have said yourself though, too many variables for an accurate answer but I would guess a 996tt would be down more than 40bhp on 95 RON, if you were mad enough to put some in. Incidentally, if had to use 95 and then you limped to the next filling station without using any boost or engine load, the important parts of the map would not be overwritten and normal service would be resumed as soon as you filled up with good juice. Let it run for a minute before pulling off and you will still have full advance on the high load/high boost zones.

Vpower is not sold in the UK. The highest pump octane ratings are Optimax at 98.6 and Tesco at 99, although this may be Optimax rounded up to no decimal places.

Do not be tempted to use aviation 100ll. Its lead content will knacker you lambda sensor and its too slow burning to develop full power in a 7000+rpm engine.

[Marquis_Rex]

cyrus1971 said:

Is there a mathematical way to derive BHP output for different RON fuels from a singular RON : BHP value. EG if you get 420 bhp from 98 Ron what would you get from 95 and 100 RON ? I know there are many variables air temp, air density, humidity, engine map etc. Please make some assumptions on them. Is there a rule of thumb for these things ? Also does anyone know any Shell stations in the UK that sell V-Power fuel (or similar) which I believe is 100+ RON ? Cheers, Cyrus

No rules of thumb I'm afraid.
Aside from the many variables you mention- there's the fact that "ignition-loops" or how torque output changes with more ignition vary wildly depending on the type of engine.
A 996tt will have quite agressive igntion loops compared to a naturally aspirated engine- which means there is potentially more torque to be gained if the engines ECU can take advantage of the higher octane gas. If an engine is NOT knock limited at peak power, there will be no gain at peak power.
On the other end of the scale- if an engine is a relatively poor breathing 2 valver, low compression ratio, naturally aspirated, and not knock limited at peak power, you'll gain nothing at all at peak and your ignition loops will be very flat.