69lqqker

Hello,
I need advice as to how much octane or what type of fuel to use in a spec'd out L88 clone with 12.5:1 compression. Please Help!! I am living at a higher elevation of 2375 Ft above sea level.

The GM factory rating is 103 research octane or 95 pump octane. I have been told that the actual minimum requirement is an average of the two @ 99 octane. Would this be correct for a 12:1 engine?

I have seen 3 options & don't know which would be better or most cost effective.

1. Use racing octane booster with current premium fuel. The highest octane fuel that I have seen here at my local pump here in Canada in 92. The racing octane booster that I have seen say it will raise the octane level up 6 to 8 points. The bottle also says for high 12:1 compression engines, to add 2 bottles to a 20gal tank of fuel. Bottle also reads as "Lead Lubricant" & "Decarbonizer"
The problem I have with octane booster is that I have been told that continual use will cause deposits.

2.Use 110 octane racing fuel that is readily available here but extremely expensive here at $9.50USD/ gal. I believe this is rated at low lead, but am not sure yet.
Would this be to much octane for continual usage for a 12.5:1 compression engine? Should I mix with the 92 premium grade, and if so, to what octane reading? Or just use the 110 octane outright.
Any disadvantages of using thisa 110 racing fuel?

3. use of aviation fuel. I have not really looked into this yet, so I am not sure what the local octane rating would be yet. Also if it is leaded or low lead rated.
Any disadvantages of using avgas for this requirement? I have read that it may wear out the carb or internal components?

How much octane is to much. I have read an L88 article where the owner was using 130 octane avgas.

Any other recommendations as to what type of fuel to use on a 12.5:1 compression engine?

Any opinion as to which method above (1,2 or 3) may be better to use?

I have read many articles & archived forums. With all the conflicting advice out there, I am really confused now.

Thanks for you help & comments. If anyone has any in depth comments, please email myself at kevtech@powersurfr.com

SWCDuke

Octane number can be very confusing. There are two primary octane test methods - motor method and research method, the motor method being the more severe, but the research method is considered more appropriate for automotive use. The "pump" octane number (PON) is the arithmetic average of the two. The arithmetic difference between the Research Octane Number (RON) and Motor Octane Number (MON) is called the fuel's "sensitivity". Typical sensitivity is about 8, so a 103 RON would be about 99 PON, which you will never find.

Be careful when people (or magazines) start throwing around octane numbers. The first thing you want to ask is: Research, Motor, or some other method? If they give you a blank stare, they have their head in a dark place.

Dual rated aviation fuels are rated under "rich" and "lean" mixture conditions, and the "rich" rating was applicable to highly boosted recips of yore, which ran up to 50 percent excess fuel (sometimes augmented with water/alcohol injection) during takeoff and initial climbout to help keep the engines out of detonation. For automotive use, the lean rating would be applicable. I'm not sure what method is used to rate current aviation gasolines. I believe it's Research, but not sure, so the octane ratings may not be directly comparable. Find a knowledgeable person at your local FBO to learn what the aviation number is, or you could contact the aviation fuel manfacturer.

I would suggest a suitable fuel blend to achieve detonation free operation and avoid the use of commercial octane boosters. You should endeavor to obtain a fuel or make your own blend that is at least 103 RON/95 MON, so first you must understand what the marketer is quoting. RON? MON? PON? Some other method? Once you know what you are buying, then it's a matter of experimention. Chances are you will be able to use a blend of the 110 octane leaded racing gasoline (I assume this is RON) and unleaded pump premium. I'd suggest starting out with a small quantity of a 50/50 blend. If it detonates, increase the percentage of the racing gasoline until you achieve a blend that supports detonation free operation.

In any event, it's going to be expensive. The L-88 was developed specifically for racing on the highest octane racing fuels available, and that is still the case. It was never intended to be driven on streets and highways.

Duke

jimgessner

A agree with Duke. In my old racing days, we used to bring home 10 and 20 gallon containers of 108 or 114 Racing gas. ( These are RON numbers. )
I have found over the years, that by adding 25% race fuel to 75% unleaded 92 , a very good low lead 98 or 99 can be achieved. This works great for a 11:1 365hp or old fuel injected pop up piston motor. But I really do not think I would want to run the risk of anything less than race fuel for a 12.5 to 1 L-88 race motor. To many $$ to not insure . The old adage is " if you want to play, you have to pay "
Have fun.
Jim

67435animal

You need racing fuel, which is the only real source of octane. The additives are a huge waste of money.

In my 435, I mix one gallon of 116 octane leaded to 2 gallons of SU. Expensive but the car thrives on it.

I suggest you do the same. Since you are in Alberta, there is not shortage of gas; hopefully, racing fuel is readily available.

Bob

69lqqker

This may be a dumb question, but what preformance difference may I see using straight or mixed race fuel compaired to straight premium.

67435animal

I don't think you need straight racing fuel, but, it depends on the octane rating of the fuel, the compression of your engine and the engine timing.

I can only relate to my own situation. If I try to run straight SU, the car will not idle, and detonates like crazy. It wants better fuel. I experimented and found that 1 (116 octane) to 2 (SU 93) was perfect, giving me about 101 octane. BTW, I pay $5.00 USD per gallon for the 116 leaded.

You will have to try to find that point where your engine pings a little and then adjust. My timing is set at 12 degrees of advance.

Believe me, when the octane is where the engine wants it, you will not believe how strong it runs. Your engine might start best with a 1-1 mix of SU 92 and the 110 racing fuel giving you about 101 octane. Try to set your timing initially at about 10-12 degrees advanced. These engines like that. Cut back a little on the timing and or the fuel mixture until you are happy and the engine is happy.

And, yes, the octane boosters leave a rust like deposit on the plugs and in your upper cylinders.
Good luck,

Bob

PS-it costs to play but, what fun!


[Modified by 67435animal, 6:13 PM 9/30/2001]

SWCDuke

The issue is detonation, not performance. You need a fuel of sufficient octane to keep the engine out of significant detonation under all operating conditions. Detonation certainly kills power, but sustained heavy detonation will destroy pistons, valves, and heads.

If the engine is well set up and tuned and runs without significant detonation it should perform well.

BTW, if this is an "L-88" clone, you were very ill advised to build it with the original L-88 compression ratio, unless you were building a dedicated race car.
Also, the L-88 cam was designed to be run with headers and open exhaust. The backpressure from any kind of steet exhaust system will signficantly detract from the engines output and response with a proper racing exhaust system.

The L-88 may be revered by Corvette enthusiasts, but it is totally inpractical for the street. It was, and is, a pure racing engine designed to perform best between about 4500 and 7000 revs and won't really provide very satisfying street performance.

Duke

63Corvette

Um...................I would use the highest octane RACING gas you can buy!
But, hey? that's just my opinion.