L68 Engine �run on� after shut down

[David K.]

[Paul H.]

[Larry E.]

[Daniel S.]

[Duke W.]

[David K.]

[Larry E.]

David: If you try Av Gas 100LL it will run even better. If your engine was rebuilt with ORIGINAL SPECS it was
designed by Chevrolet to run on gas (High Test/and Lead). The ONLY GAS now available that is the closest to
the gas back then is Av Gas 100LL. The added benefit also is that Av Gas 100LL will last very long and not
deteriorate like the current gas that is available. Glad it worked out for you. Larry

[Duke W.]

O
( I had forgotten to mention in my previous post, that the engine rebuild included a hotter cam).

A "hotter cam" usually means longer duration and increased overlap, which may require a different vacuum advance due to the OE or OE replacement not passing the Two-Inch Rule. It's fully explained in my tuning seminar that you can find in the restoration documents "sticky".

Duke

[Duke W.]

David: The ONLY GAS now available that is the closest to
the gas back then is Av Gas 100LL.

Current 93 PON unleaded is equivalent in detonation resistance to 97-98 RON from the sixties. Many believe that sixties vintage premium was "100 octane", but that's a myth. The octane varied depending on altitude and temperature range, so during winter at high altitude, like Denver, it could be several points lower than Florida during the summer.

Avgas octane is derived using the "aviation method" octane rating system that yields very similar results as the "motor method" for mogas. The difference between MON and RON is in the range of 8-10, which means 100LL has a RON of 104-105, and the only production engine that needs that high octane number is L-88, which is 103 RON, minimum.

Duke

[Mark F.]

Current 93 PON unleaded is equivalent in detonation resistance to 97-98 RON from the sixties. Many believe that sixties vintage premium was "100 octane", but that's a myth. The octane varied depending on altitude and temperature range, so during winter at high altitude, like Denver, it could be several points lower than Florida during the summer. Avgas octane is derived using the "aviation method" octane rating system that yields very similar results as the "motor method" for mogas. The difference between MON and RON is in the range of 8-10, which means 100LL has a RON of 104-105, and the only production engine that needs that high octane number is L-88, which is 103 RON, minimum. Duke

Interesting, Duke...

So, are you saying my stock '67 L-68 should not need any octane booster product (or higher octane gas itself) if I run today's PON 93 ?
I'm in Pittsburgh PA (roughly 1,000 to 1,200 feet above sea level where I drive).

[David K.]

[Larry E.]

[Larry E.]

• AVgas is refined to a much higher standard than pump gas.
• AVgas is much cleaner than pump gas.
• AVgas tends to burn smoother than pump gas(probably because it is 100 octane)
• AVgas has some lead in it which is a lubricant (two stroke engines burn hotter and that extra lubrication can mean the difference between a running engine and a seized one. Also, with better heat dissipation, the engine running on AVgas may last a whole lot longer
• AVgas is the same whether you buy it in California, Alaska or Arizona. Pump gas is different from city to city and season to season.
• AVgas has a blue tint to it for easy viewing in a clear tank. Pump gas is a clear translucent.
• AVgas has a MUCH longer shelf life of several months compared to several weeks with pump gas.
• AVgas will not gum up if left for a long periods (The gumming of pump gas is from all the additives gone bad. AVgas has none of these additives)
• AVgas is an excellent cleaner.
• AVgas costs a little more than pump gas

BTW>No I am not a salesman for Av-Gas. Thanks for listening>Larry

[Duke W.]

Interesting, Duke...

So, are you saying my stock '67 L-68 should not need any octane booster product (or higher octane gas itself) if I run today's PON 93 ?
I'm in Pittsburgh PA (roughly 1,000 to 1,200 feet above sea level where I drive).

Correct. The advertised compression ratio is 10.25:1, but "as built" could be up to half a point less due to higher decks as the broach tools wore. Also, if the engine has been rebuilt it may have lower compression than OE.

I have a friend who had an original 16K mile '67 L-71. The heads had never been off. For years he lugged 5 gallon cans of avgas from a local airport to his garage, and the only reason he was able to get the avgas was he had a friend at the FSS. You can't drive the car up to the pump. Some, but not all, will allow you to pump avgas into cans. In California avgas is about two bucks higher per gallon than pump premium, which is currently close to five bucks per gallon.

I suggested he try some pump premium, which is only 91 PON in California,and at worst we might have to retard the initial timing a bit, which was set at the OE spec of 4 degrees, well below the 6-10 range required to achieve optimum WOT advance.

We had to drop it down to 2 degrees to eliminate the very slight detonation, but there was no SOTP loss of performance. We also converted the ported vacuum advance to full time to reduce the occurrence of hot running in low speed traffic.

A common myth is that vintage "high compression" engines need high octane race gas or avgas or some kind of "octane booster" many of which are snake oil. Part of the problem is the difference between the RON method used back in the day versus the PON method that has been in use since the seventies that few understand. Plus, few owners know the actual CR of their engine whether it's original or rebuilt, and some engines have been rebuilt over the decades whose compression is so low they can run detonation-free on 87 PON regular unleaded.

The bottom line is that, given all the variables, each engine is unique, and you can either believe the myth or get educated on octane ratings and do your own experiments to determine the minimum PON necessary to keep the engine out of detonation.

Duke

[Gary B.]

All,


One suggestion I would make for those interested in AVgas is trying UL94 before trying LL100. More and more local airports in CA have added unleaded AVgas to their offerings. Compared to 91 pump gas, unleaded 94 AVgas might be all that is needed for a given high compression motor. In addition, because of the lead content, LL100 AVgas is not legal for use in a street driven car, should that environmental reason influence your decision.





Gary