Thread: Tuning and Octane

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I normally put 92 octane in my '69 mustang, but decided to be cheap and put 89 in it. As expected, the timing went off and the car was not happy. Is this just the gas, or is it possible to tune a carb for lower octane gas?

Your explanation is pretty vague. Assuming that you mean the car started pinging when you say "not happy", about the only thing you could do with the carb to affect that is to richen the mixture. That in turn leads to other undesireable operating issues so isn't the best solution and may not be effective enough anyway.

"As expected the timing went off...." leaves me wondering what you're really trying to say. Lots of things to guess, but better info would be more usefull.

Were you experiencing pinging? If so, you could back off the timing, as in not run it too advanced. Start backing it off and the pinging might/should go away.....depending on what engine mods you have done. You will lose some power, but the pinging generally will stop. I try to run my Ford as advanced as I can. When it starts to ping, I either have to put higher octane in it or back off the timing. The "pepiness" of the motor goes away too though. Your better off just running the premium if it has been happy all along.

maybe that 92 octane is part of the tune-up if runs good with it go with it

The timing was uneven and it was prone to stalling. Fuel efficiency also dropped significantly with 89 as oposed to 92. Is it possible to readjust the carb so that the same performance is achieved with lower actaine fuel?

Originally posted by gemcleod
The timing was uneven and it was prone to stalling. Fuel efficiency also dropped significantly with 89 as oposed to 92. Is it possible to readjust the carb so that the same performance is achieved with lower actaine fuel?

In a sense...maybe. But just back the timing 3-4 degrees and you should be good.

It may also have to do with the fact that the car is designed to run on leaded gas. Meahcnics tell me it is no problem using unleaded -- but it seems to me that they designed it to run on leaded for a reason.

Maybe I should just place a pound of lead ball bearings in the gas tank for safety!

heres something i've figured out over the years with my fords.... if your engine runs more efficient and has better power on the higher octane fuels, you really aren't saving money on the lower octane fuels, the more timing advance you can get out of your motor, the better gas mileage/power you can pull out of it.... if you want to try and run a lower octane fuel, than try using lucas octane booster, not the standard but instead the new stuff thats not street legal, costs like $5 a bottle and treats 25 gallons, the mustang has a 16 gallon tank so you really aren't going to be saving much on the cheaper octane only a few cents difference... might as well pay the 20-30 cents more per gallon, definately dont drive it around constantly pinging either.....

Remember that octane is the fuels resitance to "knock". Fuels with a high octane number have a slower burn time across the cylinder, thus the ability to run higher ignition timming. Lower octane numbers have a faster flame travel thru the cylinder.
Late model vehicles run lower octane fuels due to lower compression ratios, and lower emissions, thus less cylinder deposits.
Leaded fuels used the lead as a lubricant for the valve seats and to some extent the valve stems. Early heads quickly pound out the seats with non-leaded fuel as there is nothing to cushion the valve to the seat. Late model heads have hardened seats to help with the condition.