I was wondering for a stock 1968 396 what everyone's opinion's are on octance? I started working on this car using 87, then the more I thought about the high compression, I decided to switch to 91. That seemed to help the way it ran and idled, but this car has always seemed to run hot. After about 15 min. of idle time, it wants to puke out about a qt. of anti-freeze. I've installed a new thermostat, new radiator and cap, and it didn't seem to help. Once it gets rid of that antifreeze the first time, it doesn't do it again as long as you don't top it back off. A friend of mine says that this is fairly common. I just don't like having a qt. or more of air in the top of a big block's radiator. I was just wondering if this was tied to octane rating? Should I stay with the 91 octane and try to adjust my timing or plugs to make it run a bit cooler? Another good source told me I could back off my timing and put one step cooler plugs in. I just don't understand why it's necessary. I also put lead additive in the tank. That seems to be about a 50/50 argument good and bad idea. I think what I want is to stick with the 91 octane and the lead additive, but beyond that I hate to have to change much from stock. Anyway, let me know what you all have found from your past experiences. Thanks.