91 - 92 octane pump gas in 11:1 engine.

[Ray G.]

Hello;
Petroleum experts ?

Please give your opinion on using 91 - 92 octane pump gas in 11:1 engines ?

We do have 110 octane available, but it is very expensive.

Thank you,
Ray

[Terry M.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Ray,
I'm sure you'll get a number of responses to your question so I'll start things off. I have a 1965 Fuelie which has 11:1 compression. I use only racing fuel in the car but it is not a daily driver nor driven often. It has factory 4.56 rear end gears as well and, so, not really cut out for highway cruising. Anyway, while racing fuel is expensive, it keeps better for longer durations and it certainly works well with the engine. If your car is driven often, you may want to find a lower cost alternative. Many enthusiasts use a mixture of aviation fuel and 93 octane pump gas. Others, just plain aviation fuel. Still others use 93 octane pump gas and refrain from hard running on the engine. Weather has an effect as well. Hot weather climates (like Arizona) will affect engine performance based on fuel quality. Higher octane fuels are less susceptible to vapor lock. Obviously, if you plan to drive far enough to need fuel, it is not likely you'll find racing fuel readily available. Not sure if this helps but I'm sure others will chime in.

Terry

[Joe L.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Hello;
Petroleum experts ?

Please give your opinion on using 91 - 92 octane pump gas in 11:1 engines ?

We do have 110 octane available, but it is very expensive.

Thank you,
Ray

Ray-----

Does it detonate (ping) significantly on 91-92 octane? If yes, you need a higher octane although not necessarily 110 octane. If no, you don't need a higher octane. It's as simple as that.

[Ray G.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Thank You.

[Duke W.]

Re: 91 - 92 octane pump gas in 11:1 engine.

How do you know the CR is 11:1? The fact of the matter is than most 11:1 engines left Flint closer to 10.5 due to high decks caused by broach tool wear. If the engine was rebuilt anytime from the seventies-on chances are it has "low compression pistons" because most thought that unleaded gasoline would never be higher than 91 RON. Rebuilders almost always use thick composition gaskets rather than the thin steel shim type OE gaskets.

You can get a handle on your actual CR by measuring head gasket thickness with feeler gages at the corners of the head-block interface. The OE small block steel shim gaskets were.018" thick and mid-'62 to '63 340/360 were double gasketed due to customer detonation complaints, but I figure that 99 percent of vintage Corvette engines have been rebuilt at this point in time.

Flexible borescopes are getting pretty common and can be used to observe the piston crowns to determine if they are domed or flat. A flattop piston with valve relief notches and a thick head gasket means the CR is barely 9:1 and the engine will probably run detonation-free on 87 PON unleaded regular, especially with the typically lazy OE spark advance map.

A recently rebuilt engine was assembled with flattop pistons with a 1.654" compression height instead of the normal 1.675". The owner requested 10:1, but it was barely 8:1. You can't trust anybody nowadays.

To get a handle on compression ratio and tuning issues go to the sticky section and open the restoration thread near the end. Use key words compression and tuning to find my compression ratio article and tuning seminar.

Measure head gasket and borescope the engine to get an idea of what you have. Then start doing some testing. If the highest octane pump gas allows detonation, make the spark advance map less aggressive . If no detonation then you might be able to use a lower octane pump gas.

I believe a lot of guys waste money on race gas and avgas when they really don't need it, but some engines do need higher than available pump gas octane, and blending in a fraction of higher octane will usually do the job.

A little detective work and a good test plan will pay dividends.

Duke

[Mark E.]

Re: 91 - 92 octane pump gas in 11:1 engine.

The above is all good advice. Ignition timing plays a major roll also. Start with disconnecting the vacuum advance and tune the centrifugal advance curve and initial timing to provide maximum advance with no detonation at WOT using your intended fuel. Then select a VAC that provides the best driveability with no detonation at part throttle. You can try different VAC cans or use a variable VAC.

With this method, you can probably get your car running well on premium fuel. I've done this successfully with engines with a stated CR of 9.5 to 11.0.

[Kent S.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Having owned two FI Corvettes and listening to advice from folks much smarter than me, I believe the use of race gas in those engines is due to fuel percolation rather than octane requirements. Today's pump gas - regardless of octane rating - has a significantly lower "boil off" point than yesterday's gas when these cars were new. This causes percolation of the fuel in the small spider tubes when hot and makes for a miserable driving experience. Race fuel solves that problem. As others have indicated, a carbureted motor should be able to run on today's 91-93 octane pump gas with no problems.

[Michael J.]

Re: 91 - 92 octane pump gas in 11:1 engine.

I have been using 91 octane ethanol free in my C2 fuelies for many years and have never had an issue. But I do live at 7000 feet, so I suspect that could have something to do with it.

[Duke W.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Then select a VAC that provides the best driveability with no detonation at part throttle. You can try different VAC cans or use a variable VAC.

There are over a DOZEN VACs for Delco single point/TI distributors, so where do you start?

I suggest reading the tuning seminar I previously referenced. You will learn of the dozen plus VACs for Delco/single point/TI distributors only THREE are the BEST FIT for any Corvette V-8, and you determine which one is the best fit by applying the Two-Inch Rule.

It's that simple and NO guesswork.

Duke

[Justin S.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Ray,
If you use Duke's great advice about using the bore scope you might even see the model number on the top of the piston and look up the specs.

[Tim S.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Hey there Ray!

Hope all is well with you. In response to your question, you MAY be able to get away with it. I have found some engines to be more tolerant of it. As you may know, each and every circumstance are a little different from one to the next. At the worst, you may have to be a little attentive to the overall timing, including how quickly it comes in.

After 40 to 60 years on these engines, who knows what all lurks within?
Tim

[Mark E.]

Re: 91 - 92 octane pump gas in 11:1 engine.

There are over a DOZEN VACs for Delco single point/TI distributors, so where do you start?

I suggest reading the tuning seminar I previously referenced. You will learn of the dozen plus VACs for Delco/single point/TI distributors only THREE are the BEST FIT for any Corvette V-8, and you determine which one is the best fit by applying the Two-Inch Rule.

It's that simple and NO guesswork.

Duke

Good point. I use a variable VAC, but if a more original look is wanted, it's good to know three choices are the most likely candidates. Ported vacuum works better with many applications; I'm not sure if/how the 2" rule applies in these cases.

[Eric P.]

Re: 91 - 92 octane pump gas in 11:1 engine.

Hi Ray,
No fuel expert here, just wanting to share my experience that may help you with your initial question.

The real issue as I see it is Ethanol blends vs, Ethanol free fuels, and then how octane ratings come into play. I am lucky to live in an area where I can buy ethanol free gasoline commonly called REC 90 or REC 91. There is also a small airport close by that sells 93- 94 Octane Ethanol free Aviation fuel. I mix some VP 110 race fuel with these ethanol free fuels from the pump to bring the octane rating up a few points to something like 96-98 octane (nothing too scientific) and my classic cars run great! It is just my opinion that if you can get away from the Ethanol blended fuel and get that octane rating in the mid 90 range like the gasoline that was available at the pump when these cars were built, your problem may be solved without running straight 110 Octane race fuel.

Good luck!
Eric

[Ray G.]