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C3 Cam basics

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  • Rich Puleo

    C3 Cam basics

    Happy Holidays!
    Does anyone know how to determine the ratio of a rocker arm?
    Does anyone remember how to measure lobe lift in an assembled engine?

    I'm trying to narrow down a problem with a '68...but it has a 71 LT1 in it (actually from a Camaro). Apparently, it's a stock 9.0 compression, I have a consistant 130 psi on the compression test. The problem is it won't run well. I've got 7" of vacuum from the engine with a 650 Holley & a '68 cast iron intake (stock for 300/350 hp). My intention is to put the Rochester back on, but the carb mount is too short for the carb I have. The vacuum chamber in the front of the Rochester does not seal on this manifold. I would need to change the intake or carb, which involves more money that may not involve a fix. Also, there is a '67 style replacement fuel pump on this engine, the return line system has been removed. This is how I got the car. Now I'm trying to figure out what to do with it. I just want it to run decent enough to drive it as that's what I got it for.
    I need to measure the aftermarket cam that's in this thing to determine if the assembler put too big of a cam in it. 7" of vacuum with all vacuum lines disconnected & blocked off, so the cam is either too big or there is an internal vacuum leak (which I don't see). The heads appear stock but someone put roller tipped rockers in here. Which is another problem - the rockers for #1 are Comp Cams 1.52, stamped in the tops of the arms. The rest are stamped steel with no ID marks on them. They look like they're different going by size, but this is just a guess. I could try to match them at the speed shop during the week, but I'd like to try to lock down an answer today (Sunday) since it's my day off & this is fun.
    Thanks,
    Rich
  • Duke W.
    Beyond Control Poster
    • January 1, 1993
    • 15610

    #2
    Re: C3 Cam basics

    You can measure rocker ratio by zero lashing a valve and setting up two dial indicators - one on the rocker arm pushrod socket in line with the pushrod and the other on the valve retainer parallel to the valve axis.

    The rocker ratio will start out low and increase to a maximum at full lift. The OE rockers arms start at about 1.37:1 and peak at about 1.44.

    I NEVER recomend aftermarket rocker arms. They are of little value. Use OE rocker arms, and if your budget is tight there's nothing wrong with pulling a set off any junkyard engine.

    The above setup will also yield gross lobe lift (including the clearance ramps) which is .306/.323", inlet exhaust for the LT-1 cam.

    I would highly suspect that your engine has an aftermarket cam that has waaaaaaay too much overlap. The LT-1 cam should pull about 12" Hg. manifold vacuum at about 900 idle speed.

    Duke

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