New SBC350 questions

Discussion in 'Engine Topic' started by jtherrien, Sep 30, 2016.

  1. jtherrien

    jtherrien Member

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    Just purchased and swapped in a "racing build" SBC350 into my 78 LT. Little did I know what this meant.

    Supposedly compression is upwards of 10.5:1. I've been told to use racing gas only- 110 octane- by the builder. Is this necessary, or can I get away with 91? Or perhaps use additives (which I know is frowned upon?) I know that using the wrong octane can cause detonation problems so I'd rather avoid the death of my new engine before it makes it out of the driveway.

    If it makes any difference, the build is as follows: forged factory crank, forged piston heads and rods, mild cam, ported and polished 76cc heads, copper head gaskets, Gen2 intake manifold.

    Better yet, the old quadrajet that came with it was crap. I'm (temporarily) running a 500cfm Rochester dualjet. Do I risk running too lean on this carb? On a regular 350 street application 518 is decent, 650 for racing...again, I've been told this is a "racing build" and that it absolutely needs a bigger carb immediately.
     
  2. Crusty75

    Crusty75 New Member

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    Do you have any info on the Pistons and the casting number of the heads? 10.5:1 is an awful lot of compression for a 76cc head. The fact that it has a mild cam and a Quadrajet tells me that this probably isn't a race motor unless it was purpose built for a specific class of racing. I wouldn't be surprised if it actually turned out to be 9:1

    To answer your question, you'd probably need an additive with 91 octane to run at 10.5:1 based off of the motor you describe. If this really is a race motor, I'd throw a Holley 650 on there, a Quadrajet probably wouldn't work good, let alone a dualjet
     
  3. grasmo

    grasmo Veteran Member

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    I have almost 11:1 and have no trouble with pump gas.
     
  4. grasmo

    grasmo Veteran Member

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    are the heads iron or aluminum? iron will possibly be more sensitive but I have used them with 11:1+ on pump gas, 93 octane.
     
  5. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Like others, I'm kind of doubting those numbers are accurate. A whole bunch of us running pump gas on 10:1 and more engines. Put 5 gal of premium non ethanol, and see what it does. You can always bring the timing down a bit, and like you said, use an addatitive.
     
  6. jtherrien

    jtherrien Member

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    The casting numbers I don't have off hand and the car is not where I am right now. I believe they are iron, they were 76cc heads off of a 400 supposedly. The engine was built up with all forged parts, that's all I know. The builder was a friend of a friend, so it's tough to get info. I was told to use Cam2 gas. I will inquire again as to specific specs.

    As for the Quadrajet, it was a spare "just to get it started". The engine had a Holley 225GPH fuel pump on it, and he was planning on putting a dual carb setup on it. I've taken that pump off and replaced with an OEM one for obvious reasons.

    Thank you all for the responses. I figured these were all bench racing numbers. I'll give some premium octane a shot and see how it goes.
     
  7. jtherrien

    jtherrien Member

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    Cam2 is the generic name for Sunoco's Standard 110 Octane gas.
    I just need to move the car across town
     
  8. G72Zed

    G72Zed Veteran Member

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    JT, there might be a reason why the "builder" said to use 110 octane. If its a "race" build or some sort of "spec" engine, a 10.5 CR, large 76cc chambers along with a "mild" cam might point to domed pistons, and could have high cylinder "cranking" pressure due to early intake valve closing.

    You could always test this with a compresion tester and see what it cranks over at. If it comes in around 180-200psi, you should be ok with 91-93 pump gas.

    Comp. ratio numbers alone can be misleading. I worked on 2 diff 350 sbc at once, one hot street engine, one "race" engine, alum head, "looked" the same,both were @ 10.25 compression ratio & roller cams.

    Street engine cranked 190psi, used 91 octane.

    The "race" engine cranked 245 psi, 109 race gas, no pump fuel with that engine.
     
  9. Damon

    Damon Veteran Member

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    I'm with G72- compression test FIRST. If it's cranking over 200 PSI, yes, you are probably in the danger zone as far as using pump gas.

    I doubt it's got that much compression with an old set of 76cc smogger heads on top, but you never know what somebody will screw together.
     
  10. cadillac_al

    cadillac_al Veteran Member

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    Yours would be the only "race build" using 400 heads. I'll bet it is a standard flat top piston rebuild. I don't usually believe half of what people tell me. Good luck with it though.
     

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