What octane should I put in my '68 ??

[snowballfisher]

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.

[JohnZ]

91 octane should work fine. Your friend's advice is well-meaning, but he's misinformed; retarding timing will just make it run hotter. Make sure your vacuum advance can is working, and it's connected to full manifold vacuum, not "ported" vacuum, so it's fully-deployed at idle, for idle stability and good idle cooling. Don't "top off" the radiator - it'll just keep "puking" the excess coolant; the radiator must allow space for coolant expansion, which is why there's a "full cold" mark on the side tank about 3" down from the radiator cap. When it doesn't "puke" any more, leave it alone - that's its "happy level". Running colder plugs will just make it more likely to foul plugs around town; for normal driving, AC 45's or equivalent heat range in other plugs works fine - 43's or 44's will usually foul in around-town driving. Forget the additives - they don't do anything but eat the plating off the inside of your float bowl, and you don't need them unless you're pulling a trailer all day long at 100 mph.

[snowballfisher]

Well, thank you for all of the answers.  I really appreciate it.  There's so many different things that you hear out there.  I used to work at O'Reilly's and people were always asking me which fuel additives and lead subs. to buy and so on.  As many products as they carry to add too your existing fluids, it's mind boggling.  I don't think anyone has all the answers.  I do think that a lot of the stuff is just marketing gimmics.  I do have my vacuum advance hooked to full manifold vacuum.  Although I should probably check and see if it's functioning correctly.  I need to take a close look at the distributor and check things over.  I adjusted the points last summer, but I'm sure it could use a good tune-up.  I'm hearing quite a lot about the advance springs also.  I never have fooled with them, but I may try it on this one to see what kind of a gain I can get out of it.

[68rsssrag]

John

My L35 has a KD1 smog delete for Canada. I have my original Q jet mounted.

The vac advance takeoff is on the carb near the base (is this not venturi vacumn? )

Where should I take the vac advance line from? The only easy point is to go non original and tee off the rear booster / RS nipple, correct?

Arno

[JohnZ]

If you see normal vacuum at idle with a gauge at the carb nipple, that's a manifold vacuum source; if you don't see vacuum there at idle but you do when you open the throttle, it's a "ported" vacuum source. If you can't find a manifold vacuum source on the carb, you can use the fitting on the intake behind the carb.

Q-Jets don't have a "venturi vacuum" system - that's only found on Holleys with vacuum secondaries. Venturi vacuum" is created by mass airflow past a pickup tube in the main venturi, and is unrelated to manifold vacuum.

[68rsssrag]

Thanks John, I will test the port.

Arno

[Southerner]

Well, thank you for all of the answers.  I really appreciate it.  There's so many different things that you hear out there.  I used to work at O'Reilly's and people were always asking me which fuel additives and lead subs. to buy and so on.  As many products as they carry to add too your existing fluids, it's mind boggling.  I don't think anyone has all the answers.  I do think that a lot of the stuff is just marketing gimmics.  I do have my vacuum advance hooked to full manifold vacuum.  Although I should probably check and see if it's functioning correctly.  I need to take a close look at the distributor and check things over.  I adjusted the points last summer, but I'm sure it could use a good tune-up.  I'm hearing quite a lot about the advance springs also.  I never have fooled with them, but I may try it on this one to see what kind of a gain I can get out of it.

Why you have that distributor cap off grab the rotor and twist it, the springs should snap back. If they dont either the springs are weak or the centrifugal advance mechanism is seizing  up. This one problem has made engines temperamental and driven guys bananas over the years than anything else.