octane and boost levels
filed in 87 Supra
Question: Not all turbo cars use knock sensors, but most do. I have been doing some experimentation with turbo boost pressures relative to octane ratings as indicated on the pump. My Audi is supposed to run on “87 octane or better” with the factory setup, around 7 psi boost. In fact, it runs fine with 86 or 87 octane, starts more easily and gets better mileage with the 86/87 low octane stuff vs premimum (93) fuel. The mileage increase with low octane fuel vs premimum fuel has been noted here by others. I am using an aftermarket knock indicator to monitor knock levels and a self-modified adjustable wastegate to vary the boost. (kids, don’t try this at home!) The table below summarises the results of my testing to date:
86 ~7 psi boost without knocking 86/87 ~7-8 psi boost max without knocking 89 ~8-9 psi boost max without knocking 92 ~12 psi boost without knocking, possibly higher 94 14+ psi boost without knocking, no knocking ever noted with 94 (more testing needs to be done with 89, 92 & 94 octane fuels) (all fuels are Sunoco for repeatability)
It seems to be approximately 1 psi additional boost possible without preignition per additional octane number. What have others observed? Indicated boost pressures +/- 1 psi due to inaccuracy of stock boost guage calibrated in BAR, 0-2 BAR limits. Knock at WOT at highway-type seeds is much more observable in 4th and 5th gears. This is where more modern knock- sensing dynamic ignition timing control would be a real advantage. Also, with dynamic ignition control there would be less need (none?) to manually adjust the boost via the wastegate for different fuel qualities to acheive maximum possible performance potential with the fuel in use. The advantage to the “manual” system I use is that you *know* what is really going on under the hood and you also *know* that when setup properly you are getting all the performance possible given a particular fuel quality and that the “black box” is not retarding your timing and reducing maximum performance potential. I would like to have a display to indicate in realtime exactly what the computer is doing, how much advance/ retard is being applied and why, knock sensed, boost pressure, inlet charge temperature and all other monitored operating paramaters that control ignition timing. Oh well. My computer just monitors boost pressure, charge inlet temp and WOT position and all I can monitor is actual knock levels.
You mentioned in your article: In fact, it runs fine with 86 or 87 octane, starts more easily and gets better mileage with the 86/87 low octane stuff vs premimum (93) fuel. The mileage increase with low octane fuel vs premimum fuel has been noted here by others.
I have experienced exactly the opposite on my 1987 Toy Turbo Supra.. the higher the octane, the better the mileage. The Supra uses dual knock-sensors, one at the top of the cylinder and one near the bottom. If you consider what the knock sensor is supposed to do, better mileage should be the result (ie: advance the ignition to the point where knock JUST starts to occur). I can’t understand why lower octane would produce better mileage.. any thoughts? I am concerned about how your supra’s knock sensor works. _I_ would design a knock sensor to RETARD timing which would be set in advance to have MAXIMUM ADVANCE FOR THE SPECIFIED FUEL. A good system should never induce pinging to decide where to back off. Pinging destroys engines. Severe pinging destroys engines in seconds. What octane is recomended for your supra? If it’s 87 and up and you have to use 90-93 octane you shoud think about having you timing checked unless: 1) You constantly use high amounts of boost. 2) You are willing to pay the price in fuel and WANT your timing in advance of the factory specs for additional power availible through the use of high octane in your turbo.
Want to give your own answer? Want to comment?