Gasoline Octane Requirements

[Ralph E.]

What would the gasoline octane requirement be for a 69 Corvette with a 350CI which has 11.0:1 CR?
Would pump gas premium octane be adequate?

[Mark E.]

Re: Gasoline Octane Requirements

The stock 350 with 11.0:1 CR in '69 is the L46 350HP.

Mike Furline, who has the '70 version of this engine, may be reading and have some thoughts...

My '70 LS-5 has 10.25:1 CR. In North Texas, premium is mostly 93 octane E10 - E15.

My ignition curve is 26* @ 2800RPM. In the winter/cool weather, it can take up to 12* initial during WOT. I use a variable vacuum advance (don't tell Duke), which allows me to dial-in part throttle advance. In the summer, it can take about 6* - 8* initial; I also reduce the part throttle vacuum advance rate a bit.

The difference may be due to the different seasonal gasoline blends the refineries brew up, the heat, or a bit of both.

I haven't tried, but I expect it would run better with more ethanol. The trade off is more fuel bowl evaporation (harder starts after a few days) and possibly more fuel system corrosion.

It runs a bit better on 100LL from the local airstrip. But not much better.

[Bill L.]

Re: Gasoline Octane Requirements

What would the gasoline octane requirement be for a 69 Corvette with a 350CI which has 11.0:1 CR?
Would pump gas premium octane be adequate?

I can only speak from my experience. Mine is a 70 L46 and frankly runs pretty poorly on today's 93 Octane. Yes I can tune it to run okay with 93 Octane. It runs and idles far better with a little more advance and a little 110 octane racing fuel. I typically add about 3 gallons per tank full. If I really want to have fun I will add about 5 gallons. I am lucky on that racing fuel is available 2 miles from the house.

Mine has not been apart except for intake, timing chain, and re-gasketing. Original carb and rebuilt distributor with stock advance can.

Bill

[Joe L.]

Re: Gasoline Octane Requirements

I can only speak from my experience. Mine is a 70 L46 and frankly runs pretty poorly on today's 93 Octane. Yes I can tune it to run okay with 93 Octane. It runs and idles far better with a little more advance and a little 110 octane racing fuel. I typically add about 3 gallons per tank full. If I really want to have fun I will add about 5 gallons. I am lucky on that racing fuel is available 2 miles from the house.

Mine has not been apart except for intake, timing chain, and re-gasketing. Original carb and rebuilt distributor with stock advance can.

Bill

Bill------

There are differences between 1969 L-46 and 1970 L-46. The 1969 L-46 used air injection reaction (AIR) for exhaust emission control. The 1970 L-46 used the controlled combustion system (CCS) for exhaust emission control. The latter involved changes to distributor advance and carburetion to achieve exhaust emissions control. I believe the CCS system might be more sensitive to octane.

By the way, the AIR system is a much better way to go from a performance perspective. Some folks mistakenly think that 1970 is better than 1969 due to the elimination of AIR from most engines. The opposite is true. AIR is better than CCS. That's why GM used it for all 1970-72 LT-1 and 1971 LS-6. I'm sure GM didn't like to use it since it's a whole lot more expensive than CCS.

[Duke W.]

Re: Gasoline Octane Requirements

Chevrolet's advertised CRs were on the optimistic side. Most L-46s left Flint closer to 10.5 than 11. Then it's a matter of what's been done to the engine in the last 47 years. Many have been rebuilt with "low compression". You can get a clue by measuring head gasket thickness with feeler gages. The OE head gasket was .018". Once you have an idea of what you have it's a matter of experimenting. Start off with premium and when the tank gets low, add a few gallons of mid-grade and test for detonation. If it doesn't try some regular. It really comes down to each individual case.

[Jeffrey S.]

Re: Gasoline Octane Requirements

Chevrolet's advertised CRs were on the optimistic side. Most L-46s left Flint closer to 10.5 than 11. Then it's a matter of what's been done to the engine in the last 47 years. Many have been rebuilt with "low compression". You can get a clue by measuring head gasket thickness with feeler gages. The OE head gasket was .018". Once you have an idea of what you have it's a matter of experimenting. Start off with premium and when the tank gets low, add a few gallons of mid-grade and test for detonation. If it doesn't try some regular. It really comes down to each individual case.

With much help from Duke, my '69 350/300 was rebuilt several years ago to 383 with a measured/true CR of 10.25:1. The advance map was optimized and the VAC runs on manifold vacuum (not ported). Long story short, the car runs beautifully with pump premium. In Detroit it is 93 octane, 10% ethanol. I would bet that the car would do fine with 89 octane but for the small amount of miles I put on the car each summer, the extra cost of 93 is not an issue.
Jeff

[Duke W.]

Re: Gasoline Octane Requirements

You forgot to mention that "cheater camshaft". I'd still love to see a chassis dyno test!

[Ralph E.]

Re: Gasoline Octane Requirements

Since I am thinking about purchasing a 1969 Corvette with a 350/350 L46 with 11.0:1 CR I am interested if the car can be driven using pump gas without any issues. Dukes suggestion to try the different octanes does not work for me since I would need to purchase the car to try the octanes out. Their must be someone here on this board which have 1969 Corvettes with L46 engine and could explain there experience using pump gas.

[Duke W.]

Re: Gasoline Octane Requirements

There aren't too many Flint/Tonawanda- built engines out there anymore, but anecdotal evidence indicates that most, especially small block, operate detonation-free on 91-93 PON pump gas, especially with the typically lazy OE centrifugal spark advance curve. Talk to the owner or dealer, and test drive the car. Load it up at low revs and listen for detonation. Also take a feeler gage set with you and measure head gasket thickness. That will give you a clue. Maybe the engine was rebuilt with low compression pistons and a thick head gasket. If the gasket is about .036" instead of the OE .018", even with OE-type domed pistons, the actual CR is probably only about 10:1. Also, converting to full time vacuum advance with a B22 VAC will reduce any tendency to detonate and enable a more aggressive centrifugal curve.

[Mike F.]

Re: Gasoline Octane Requirements

Since I am thinking about purchasing a 1969 Corvette with a 350/350 L46 with 11.0:1 CR I am interested if the car can be driven using pump gas without any issues. Dukes suggestion to try the different octanes does not work for me since I would need to purchase the car to try the octanes out. Their must be someone here on this board which have 1969 Corvettes with L46 engine and could explain there experience using pump gas.

The car will run fine on 93 oct pump gas.

[Michael W.]

Re: Gasoline Octane Requirements

Some cars run perfectly at factory timing specs using straight pump gas. Other cars need to have the ignition timing backed off a few degrees due to detonation issues when driven hard.

There's no car that can't be driven on pump gas. Beware those who spin myths.

[Mark E.]

Re: Gasoline Octane Requirements

Bill------

There are differences between 1969 L-46 and 1970 L-46. The 1969 L-46 used air injection reaction (AIR) for exhaust emission control. The 1970 L-46 used the controlled combustion system (CCS) for exhaust emission control. The latter involved changes to distributor advance and carburetion to achieve exhaust emissions control. I believe the CCS system might be more sensitive to octane.

By the way, the AIR system is a much better way to go from a performance perspective. Some folks mistakenly think that 1970 is better than 1969 due to the elimination of AIR from most engines. The opposite is true. AIR is better than CCS. That's why GM used it for all 1970-72 LT-1 and 1971 LS-6. I'm sure GM didn't like to use it since it's a whole lot more expensive than CCS.

AIR may perform better stock, but adds engine clutter. CCS involves much less clutter and the engine can be tuned for performance with virtually no visible changes.