Yes, peak combus pressure may be reduced but Octane rating is all about how COMPLETELY a fuel burns ie. high octane fuel=slower flame front =more complete combustion=greater thermal energy created =more power especially in a turbo engine(according to BP petrochemical engineers comment I found on Yank site)
There are so many other factors governing detonation ie. intake temp etc; not just dropping the comp ratio. This is why the Dahlback racing golf 4 (www.Dahlbackracing.se) can run on +-112 octane fuel @ 3,6 bars boost and a 9.8:1 comp ratio. This is all very well
BUT I still seek that optimum mix ratio for toluene to race gas, someone must know it?

Feral
It does a mans heart good to realise that there are still people out there who remember that Ricardo really was the man behind the advances in combustion control, & since he was hampered with the low octane fuels of the day, he did it with science.
As to your last sentence, i couldn't possibly agree more.

Chrysler was a very early adopter of the Ricardo cylinder head design for their side valve 6 cylinder engines in about 1926 or so, & this design was a "high swirl" type. In more recent times, i have owned a brace of Moto Morini motorcycles. These were a brilliant V twin engine with a Herron head design. They also used a 'High Swirl" concept, but did it with clever port design as well as a squish chamber. The side valve engines did it with squish alone. Both of these engine designs were able to successfully run substantially more compression than other designs. With Chrysler, they were offering the optional "Red Head" with 6.5 to 1 at a time when others were struggling with slightly better than 5 to 1. The Morini's ran 10 to 1 in an air cooled motorcycle engine, & thats about 3 full points better than most could handle at the time on the same fuel.

Henry Ford refused to pay Ricardo for a licence to use his patent & as a result, model T's had 3.3 to 1 compression & the later Model A's were struggling to prevent detonation at about 4 to 1 with their open chamber heads.

Ricardo, the genius who's name deserves to be better known to automotive enthuisiasts everywhere.

I occasionally wonder if we could benefit from remembering his discoveries & incorporate the high swirl principals into our head modifications by trying to ensure that there is some squish chamber present. Not always easy, but worth a thought.
Chris

Toluene and xylene in fuel are usually only bandaids for high comp engines.

The calorific value of both toluene and xylene is low so when you use them to prevent detonation they also reduce the calorific value of the fuel. This means the peak combustion pressure is reduced and the mixture burns slower preventing detonation. The cost of this detonation resistance is reduced power due to the lower enrgy produced by each bang of the cylinder.

To make the same power as unadulterated hi octane fuels, you need more oxygen and more fuel to be burnt in the same time. This fuel mix is ideal in a turbo car where the amount of fuel burnt is easily controlled by bost presssure.

Often,a better solution is less compression and better finishing work on the combustion chambers and fire channel of the head. Good chamber design can make an engine burn more fuel efficiently and achieve better cylinder fill than mega compression. Another bonus of the lower compression engine and the resulting increased chamber efficiency is the reduced cylinder pumping losses from the engine.

There was a lot of work done on this subject in the 1920's - 1930's by Ricardo. He was the father of modern engines and fuels. His main work was done on compression ratios and how this effects fuel burn. This resulted in the development of knock ratings for a wide variety of fuels.

If you ever get a chance to read about him, (Ricardo) do so. It is an insight into the world of the engine design gods, and so much of what he found is very relevant today.

I used to run toluene with pump fuel 96 octane at a ratio of 5:1 for racing any more and you would end up holing a piston... That is 5 litre petrol to 1 litre of toluene...

Can any of you avid drag racers help me with fuel mixtures? I am running a race gas (102 oct) to toluene ratio (126 oct) ratio of 1:2 Am I on the right track. Here in South Africa our race gas is only 102 octane so we have to mix with toluene or xylene. Does anyone know the optimum mix ratio or better still does anyone know the ratio mix that the 4 cyl turbo F1 cars were using in the 80's. Please help-detonation sux!!