I'm having problems (serious) with the fuel system at the moment, but I'm still getting 18mpg in mixed driving. I use 91 octane, 0% Ethanol fuel... I believe the octane makes no difference in the face of a std. 10% octane rating, like FL fuel, which is 93 octane and 10% ethanol.

0% ethanol makes a BIG difference. However, once I ran 87 octane by accident, (10% ethanol) and the car did lose some power, as far as I could tell it ran fine though.

 

Lol well mikey i normally get shut down to lower case by forums, even though i type in upper. Because i have to run a adp program at work that only runs at upper case. Which is normally fine unless i hit the shift key(like i did for i) then it leaves me at all caps. Except for that 1 letter what yr vette?

 

Great thread. I've learned a lot. Previously, I assumed that octane had some relationship to stored energy and, therefore, more was better. But now I understand that octane is that which inhibits combustion. The goal being to only combust when there's a spark, and not because the engine is hot. Higher octane means less likelihood to combust due to heat which is not generated by the spark. I suppose the ideal gas from an octane standpoint would be that which only combusted at or above the spark temperature.

So, in octane, more is not necessarily better, especially if the engine is tuned to a certain level.

Thank you all for the education.

 

It's a '73 with a kinda '70 LT-1 engine. Surprises a lot of people when I hit the loud pedal.

Thank you for the thank you! if you want to read further, look up 'deflagration' in combination with 'detonation' and 'pre-ignition'. You'll see that all the myths fall apart pretty quickly.

 

this brings up a good point i forgot when we dyno'd the 580. On pump gas 93 octane(for which it was built). It dynos(engine dyno not chassis) at 853 hp. We ran 112 race gas in it and it pulled 844. The octane resists preignition and slows the flame front. So under pump gas it produces more power, useing a higher octane uneeded fuel all else being the same slowed the flame front and lost about 10hp. Now when sprayed its a chemical blower or high compression so the octane requirment goes up alot and the timing has to be pulled. I shoot for 2* pulled for every 50 hp of spray.
Mikey, youre vacuum advance is supposed to be on a ported vacuum source on the carb and not manifold. When your under cruise with high vacuum but throttle angle opening the ported vacuum source is not there, therefore you dont have vacuum advance when cruising. Now if you hook it up to manifold vacuum then yes you will have it pulling vacuum and increasing timing at cruise. But its not supposed to be on a non ported vac source. On my suburban with a 383, ( . 30 overbored 350 with a 400 crank) fuel injected with a 300 horse fogger with 8 plumbed in nozzles to each port. I run 38* advance on it as thats where it runs the fastest at the track. It is a computer controled dist. And 12* initial and curved up 38* it too pulls timing at rate of 2* per 50hp spray and i have the nitrous flowed curved through the accel ecu to go form a 25%hit off idle to 100% by 3000rpm with a timing curve to match. And i run a dedicated fuel system with a 1 gallon fuel cell filled with unleaded 108 race gas just for those times of need

 

Sorry for the continued hijack.

Ported vacuum was a smog reduction strategy which works at closed/idle throttle only, but kills performance. The ported vacuum ports on either a Holley or Qjet are fully exposed as soon as the throttle is 'touched'- even when on the high idle cam when using the choke. I run mine from the full vacuum post.

The stock '73 set up used an eletrical solenoid to inhibit vacuum except when in 3rd gear with an auto box or 3rd and 4th gear on a manual. Pretty crude set up. My original stuff is all in a box.