fuel for 10.25:1 motor?

Discussion in 'Small Block Tech' started by quick85, Feb 15, 2002.

  1. quick85

    quick85 wrench turner

    what do you guys run for fuel? no 99 octane here. found a 69 350 code RP. poston book shows good numbers until i looked at the fuel it needs!! pat myers chas, wva.
  2. flynbuick

    flynbuick Guest

    As I posted in another place earlier, my background is in airplanes. [My last drag strip appearance was in '68 in a 55 Chevy] Todays Avgas for piston aircraft is called 100 Low Lead[100ll]. It is dyed blue to alert us to contamination with Jet A which may cause an in flight failure. The name is misleading. If it were rated by the same method as auto fuel it would be a minimum of 112 octane and notwithstanding the name is highly leaded. An octane rating simply gives you a measure of protection against detonation for a given timing setting. More protection than is needed adds nothing to performance. If you want a more aggressive timing than your fuel would allow, then moving up to avgas will help avoid detonation. Todays avgas has a higher octane rating than the fuels available to autos during the glory years of the muscle cars. Incidently, my wife and I used to own a plane certified with two turbochargers and which required green avgas. On the auto octane scale it was 140-150 octane. There was a purple gas for the military rated higher. To my knowledge, neither are now produced.

    Jim Lore

    Jim Lore
  3. Mark Demko

    Mark Demko Well-Known Member

    You can run high compression(10 to 1) on 94 octane with no problems, BUT its all dependent on cam selection, operating temp, and carbon build up etc, etc. Im running at least 10 to 1 on my 350 with .005 deck height, heads milled .030 with Sterling pistons rated at 9.8 to 1 at .020 in the hole. Im running T/A's 310 cam that I can idle all the way down to 600 rpm in gear. I run 20 deg. initial timing with 36 total advance with and MSD timing computer. I dont experiance any ping on 94 octane with my combo. So it really is a matter of the combination as to what compression ratio you can run and tolerate 94 octane gas. If I were to run a stock 350 cam, theres no way my combo would tolerate 94 octane :jd: Mark
  4. Mike Atwood

    Mike Atwood The Green Machine

    I am running stock 350 10.25:1, and the only way I can run on 92-93 premium from the pump is by adding a product called "knockout". 104+ just does not work...unless you like using a gallon of it! :eek2: I don't know if they sell it anymore or not, but it works good. The last two years I have just been running the "racing fuel" that they sell a few blocks away with 106 octane. I usually mix that anywhere from 1:3 to 1:1 with 93 unleaded premium, depending on how good I want it running. 1:1 seems like it lets me pull just as hard as I want to and never pings .
    The fuel I use is purple in color......and smells "oh, so sweet" out the back end. :)
  5. quick85

    quick85 wrench turner

    stock 69 350

    thanks, for the tips . i started it today. i must have got a motor with the weakest valve springs in the lot or all my exhaust valves are sticking!i can push the valves open very easily!?! going to run compresion tests monday.its only running on 7 cyls. but still sounds good! pat myers chas, wva
  6. YellowLark

    YellowLark Well-Known Member

    My '66 340 has 10.25:1 CR and I have to mix 1 gallon of 110 leaded racing fuel to each 4 gallons of 93 octane pump gas to stop the pinging.

    MSD distributor (no vacuum advance) set at 12 initial 32 max.

    According to the Buick factory documentation, the engine was designed to run on 96 (by today's measure) octane fuel.
  7. rhedelius

    rhedelius Well-Known Member

    I have a 67 Skylark 340-4 with 10.25:1 CR. It has been upgraded with a Pertronix Ignitor, Pertronix FlameThrower coil (40K volt) and Jacobs Electronics plug wires. I run 89 pump gas here in Ca for everyday driving and don't experience ping unless I really get into it (floor it). On 92 it barely pings if I step on it. Right now the timing is pretty much stock so it may improve if I played with it a little. I've put almost 40K miles on it and it still runs great with 146K on it. I'm the 2nd owner and it was well maintained before I bought it and I have continued maintain it.
  8. YellowLark

    YellowLark Well-Known Member


    That's the lowest octane 340 driver I've yet heard of.

    Most other owners have my problem.

    Now, what is different about yours?

    If the ignition is basically similar, then what spark plugs are you using?

  9. rhedelius

    rhedelius Well-Known Member

    I just pulled one of my plugs out and it's a Champion RJ18YC. I gapped them at .040 instead of .035 because of the Pertronix unit. If I run the car with 87 I have to be real light footed as it pings real easy. With the 89 I can do my everyday driving with it as long as I don't get too carried away. I've run it like this since I bought it in the Spring of 97. I use the car often to commute long distances and to run around town.:TU: I'm sure glad I don't have to put in a bunch of additives as that would get real expensive and prohibit me from driving it on a regular basis. It's a great car and I hope to paint it soon. My 66 Skylark has the 340-2 (9:1) in it so it runs fine on 87. That may change with the 455 swap I have planned....
  10. YellowLark

    YellowLark Well-Known Member


    You have a 455 swap in mind? That's ironic because I'm planning to rebuild the 340 and use 9:1 pistons do avoid having to enhance the fuel.

    Because my car is a family legacy, I don't want to modify it too much (such as a 455). However, it would be more fun to play with if it was just any old car.
  11. rhedelius

    rhedelius Well-Known Member


    Yes, I do have a 455 swap planned for my 66. The previous owners never took care of the engine and even though it only has 76K on it, the oil turns real black shortly after changing it and it comsumes a fair amount of oil too. I like the 340-4 in my 67, it's a great little motor, I wish there was more available for it, but I picked up a 73 455 as I've always wanted a torque monster and figured it would be a fun cruiser in the 66 with the top down. It will be a mildly upgraded rebuild since I plan to drive and enjoy.

    I noticed you are using a 200-4R in your 66, how do you like it? I am planning on one for the 67 since I drive it so much. Did you have to do anything special to the crossmember? I know the Hot Rod article had to weld a support piece in theirs to be able to drop the pan. www.bowtieoverdrives.com has a crossmember made with a support platform, I may go that route. I was going to put an Auburn posi in the 8.2 rear with Richmond 3.55 gears to give me a little performance boost. The ST300 is a little bit of a slug, this way it would feel like I hopped up the 340 a little. The milege should improve too. :grin:

    Do you have any pics of your 66?
  12. YellowLark

    YellowLark Well-Known Member


    I had a local speed shop install the 200-4R last year based on an August 2000 Hot Rod article. Had to drill new frame holes, but used the original cross member.

    I had the shop owner show me that he could wrench the back nuts over the cross member. That was the only justification for the Chevelle piece, and it really wasn't necessary.

    Drove it 1700 miles to the BCA National last July, no problems, great mileage. Right now, it's leaking trans fluid, but the shop vows to keep fixing it until it stops.

    Don't have photos since I repainted it in 2000. Car is yellow with black painted top. Has base/clearcoat paint. Very shiny like a new Camaro.

    Currently have it jacked up in garage to install a dual master cylinder and Praise Dyno brakes. That means replacing every last brake component.

    When I get done in a few weeks, I will use my new digital camera and get some photos available.

    Harry Newman

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